Document:

UNITED STATES

 

 

 

ARC
PHOTOVOLTAICS CENTRE OF EXCELLENCE 2010/11 ANNUAL

REPORT, pp 61-99

ARC PHOTOVOLTAICSCENTRE OF EXCELLENCE ANNUAL REPORT,

2012, pp 169-224

 

 

 

Article referenced as support for the following
sections:

Page 8 & 9: Figure and Text on Tandem Solar Cells

4.5 THIRD GENERATION STRAND -ADVANCED CONCEPTS

	
  

 	
  

 
	
 University
 Staff:

 
	
 •

 	
 A/Prof.
 Gavin Conibeer (group leader)

 
	
 •

 	
 Dr
 Richard Corkish

 
	
 •

 	
 Prof.
 Martin Green

 
	
  

 	
  

 
	
 Senior
 Research Fellows:

 
	
 •

 	
 Dr.
 DirkKÖnig

 
	
  

 	
  

 
	
 Research
 Fellows:

 
	
 •

 	
 Dr
 Shujuan Huang

 
	
 •

 	
 Dr.
 Ivan Perez-Wurfl

 
	
  

 	
  

 
	
 Lecturers:

 
	
 •

 	
 Dr
 Santosh Shrestha

 
	
  

 	
  

 
	
 Postdoctoral
 Fellows:

 
	
 •

 	
 Dr
 Supriya Pillai (part time)

 
	
 •

 	
 Dr
 Xiaojing Jeana” Hao

 
	
 •

 	
 Dr
 Sangwook Park (to Apr 2010)

 
	
  

 	
  

 
	
 Professional
 officers:

 
	
 •

 	
 DrTom
 Puzzer (part time)

 
	
 •

 	
 Dr
 Patrick Campbell (shared with Thin Film)

 
	
 •

 	
 Mark
 Griffin (shared with Thin Film)

 
	
  

 	
  

 
	
 Research
 Associates:

 
	
 •

 	
 Dr
 DidierDebuf (adjunct fellow)

 
	
 •

 	
 Yidan
 Huang

 
	
 •

 	
 Lei
 “Adrian” Shi (from Sept 2009)

 
	
  

 	
  

 
	
 Higher
 Degree Students:

 
	
 •

 	
 LaraTreiber

 
	
 •

 	
 Pasquale
 Aliberti

 
	
 •

 	
 Yong-Heng
 So

 
	
 •

 	
 Robert
 Patterson

 
	
 •

 	
 Binesh
 Puthen Veettil

 
	
 •

 	
 Chao-Yang
 “Jeff” Tsao

 
	
 •

 	
 Bo
 Zhang

 
	
 •

 	
 Andy
 Hsieh

 
	
 •

 	
 DaweiDi

 
	
 •

 	
 Zhenyu “Wayne” Wan

 
	
 •

 	
 Craig
 Johnson

 
	
 •

 	
 Sammy
 Lee

 
	
 •

 	
 Haixiang
 Zhang

 
	
  

 	
  

 
	
 Casual
 Staff:

 
	
 James Rudd (part time)

 
	
  

 
	
 Visiting
 Researchers:

 
	
 •

 	
 Dr.Yukiko
 Kamikawa (AIST, Tsukuba, Japan, June 2009 – Dec 2012)

 
	
 •

 	
 Prof.
 Bingqing Zhou (Inner Mongolia Normal University, China, Sept 2009 – Aug 2010)

 
	
 •

 	
 Dr.
 Mallar Ray (Bengal Eng. and Sci. University, May 2010)

 
	
  

 	
  

 
	
 Undergraduate
 Students:

 
	
  

 
	
 4th
 Year Thesis Projects:

 
	
 •

 	
 Yu
 Feng

 
	
 •

 	
 Tao
 Zhan

 
	
 •

 	
 Chen
 Pan

 
	
 •

 	
 Xi Li

 
	
 •

 	
 YaoYao

 
	
 •

 	
 Linzhi
 Ma

 
	
 •

 	
 Lin
 Dong

 
	
  

 	
  

 
	
 Masters
 Project:

 
	
 Chandraprasad Ramachandran

 
	
  

 
	
 Visiting
 Practicum students:

 
	
 •

 	
 Stephan
 Michard (RWTH Aachen, Germany, Aug 2009 – Feb 2010)

 
	
 •

 	
 DolfTimmerman
 (Amsterdam University, Oct – Dec 2010)

 

20 nm

abstract

There
has been approximately equal work on the two major projects in Third Generation in 2010. These are the
Group IV nanostructure tandem cells project - the “all-Si” tandem cell - and the Hot Carrier
solar cell project, with its continuing funding from GCEP (Global Climate and Energy
Project). There has also been further work on Up-conversion and on Plasmonics.

The Si
nanostructure work has seen increased understanding of the mechanisms for transport and quantum
confinement. More sophisticated modelling of both has been tied more directly to improved interpretation of experimental results. This has led to establishment of a predictive ‘equivalent circuit modeller’ which will allow optimisation of Si QD device parameters to maximise transport and
performance in the photovoltaic devices. Improved models for the understanding of doping effects in these materials have also been established. Work on alternate matrices for Si quantum dots, in both silicon
nitride and carbide, has seen development of composite structures which have improved transport in the growth
direction, whilst maintaining quantum confinement in the plane, but which also increase the uniformity of QD sizes. Work on Ge nanostructures has also improved with high electrical p-type conductivity established for Ge quantum dots and
excellent pseudo single crystalline growth quality for Ge quantum wells in a nitride
matrix. Heterojunction photovoltaic devices
combining the advantages of two of these different material types are now being
investigated.

Hot
Carrier cells have seen very significant improvement in demonstrated resonance in energy selective
contacts using Si nanostructure layers, as well as a development of 2 and 3D modelling of
transport in these structures. Modelling of Hot Carrier efficiencies continues to get more
sophisticated with application to real material systems such as III-nitrides and
inclusion of Auger processes which become significant at high carrier concentrations. Work on
absorbers has allowed
modelling of the phononic properties of a range of bulk materials, in
conjunction with
time resolved photoluminescence measurement of carrier cooling in some of these materials. Also
modelling of coherent nanoparticle nanostructures, which emulate the phononic properties
required, has developed into direct application to structures grown directly. These
structures include the

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Loss processes in a standard
solar cell: (1) non-absorption of below band gap photons; (2) lattice
thermalisation loss; (3) and (4) junction and contact voltage losses; (5)
recombination loss.

FIGURE 4.5.1

colloidal
dispersion of Si nanoparticles, which has seen coherent arrays successfully grown, and growth of III-V
quantum dot structures with collaborators, with modelling and characterisation indicating their
usefulness as absorbers. The parameters needed for a real hot carrier solar
cell are now
better defined, and candidates for its structure are now being modelled in more
detail. This
will feed into fabrication and characterisation of some candidate structures in
the near future.

The
up-conversion project in 2009 reported the approach of use of
porous-Si as a host material for an Erbium upconverter. This has the advantage of much greater
versatility in fabrication and electrochemical doping with Er, compared to other host phosphors.
Work in 2010 has further demonstrated up-conversion of below band gap photons. The ease of
control of porous-Si growth at different refractive indices has also been exploited to create fully
integrated multiple Distributed Bragg Reflectors. These allow a tuning of the forbidden photonic band such that
regions just outside the bandgap (which have enhanced density of photonic
states) lie at the 1500nm absorption window of Er. This effectively concentrates light into this absorbance
window and boosts up-conversion quadratically. Enhancements of up to 80 are possible with
multiple levels, with a consequent narrowing of the wavelength range enhanced. Enhancements of
40 are very practical. This represents a big potential improvement for up-conversion efficiency.

Plasmonics
has previously been applied to both frst generation and second generation Si
cells with very significant enhancements in absorbance of near band gap photons.
Work in 2010 has applied plasmonic silver nanoparticles to nanostructured Si QD layers. Again
significant enhancements of photoluminescence (reciprocal with absorption) have been absorbed, up
to 16x for wavelengths just shorter than the effective band gap. Also application to the rear
of the layer is seen to be better than the front because the reflection of the silver particles
themselves is beneficial rather than parasitic. Just as with thin film second
generation cells,
light trapping by these non-texturing methods is very important for third generation devices,
which consist of very thin layers
of material.

This
progress in all the main Third Generation project areas is improving understanding and allowing optimisation of
modelling, structures and devices. Developments to come in the next year will see significant
advancement in these areas, with excellent prospects for good demonstration devices.

4.5.1 third generation Photovoltaics

The
“Third Generation” photovoltaic approach is to achieve high
efficiency whilst still using “thin flm” second generation deposition methods.
The concept
is to do this with only a small increase in areal costs and to use abundant and
non-toxic materials
and hence reduce the cost per Watt peak [4.5.1]. Thus these “third generation”
technologies will
be compatible with large scale implementation of photovoltaics. The aim is to decrease costs to
well below US$0.50/W, towards US$0.20/W or better, by dramatically increasing
efficiencies but maintaining the economic and environmental cost advantages of
thin film deposition techniques (see Fig. 4.1.3 showing the three PV generations) [4.5.1, 4.5.2].
To achieve such efficiency improvements such devices aim to circumvent the
Shockley-Queisser limit for single band gap devices that limits efficiencies to
the
“Present limit” indicated in Fig. 4.1.3 of either 31% or 41% (depending on
concentration ratio). This requires multiple energy threshold devices such as
the tandem or multi-colour solar cell. The Third Generation Strand is
investigating several approaches to achieve such multiple energy threshold device [4.5.1, 4.5.3].

The two
most important power loss mechanisms in single-band gap cells are the inability to
absorb photons with energy less than the band gap (1 in Fig. 4.5.1), and
thermalisation of photon energy exceeding the band gap, (2 in Fig. 4.5.1). These two mechanisms alone
amount to the loss of about half of the incident solar energy in solar cell conversion to
electricity. Multiple energy threshold

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A tandem photovoltaic cell
using quantum confined QDs or QWs to engineer the band gap of the top cell and
potentially also the lower cells. Short wavelength light is absorbed in the top
cell and longer wavelengths in lower cells, thus boosting the overall voltage
generated and hence efficiency.

FIGURE 4.5.2

approaches
can utilise some of this lost energy. Such approaches do not in fact disprove
the validity of the Shockley-Queisser limit, rather they avoid it by the exploitation
of more than one energy level for which the limit does not apply. The limit
which does apply is the thermodynamic limit shown in Fig. 4.1.3, of 68.2% or
86.8% (again depending on
concentration).

In the
Third Generation Strand, we aim to introduce multiple energy levels by
fabricating a tandem cell based on silicon and its oxides, nitrides and carbides using reduced dimension
silicon nanostructures to engineer the band gap of an upper cell material. We
are aiming to collect photo-generated carriers before they thermalise in the “Hot Carrier” solar
cell. Also
we are investigating absorption of two below bandgap photons to produce an
electron-hole pair
in the cell by up-conversion in a layer behind the Si cell using erbium doped
host materials. In order to optimise the requisite properties, all these structures are
likely to be thin hence maximising absorption of light in thin structures
through light trapping is very important. Hence we are also investigating
localised surface plasmon enhanced coupling of light into these Third Generation devices.

4.5.2 Si nanostructure solar cells

4.5.2.1 the ‘‘all-Si’’ tandem cell

Researchers:

Shujuan
Huang, Ivan Perez-Wurfl, DirkKÖnig, Tom Puzzer, Xiaojing Hao, Sangwook Park, Bo Zhang, Dawei Di,
Yong-Heng So, Zhenyu Wan, Sammy Lee, Yidan Huang, Gavin Conibeer, Martin Green

We are
developing a material based on Si (or other group IV) quantum dot
(QD) or quantum well (QW) nanostructures, from which we can engineer a wider band gap material
to be used in tandem photovoltaic cell element(s) positioned above a thin flm bulk Si cell, see Fig. 4.5.2.

Previously
we have demonstrated the ability to fabricate materials which exhibit a blue shift in
the effective
band gap as the QD or QW size is reduced, using photouminescence [4.5.4] and absorption
[4.5.5] data [4.5.6]. A thin film deposition of a self-organised QD
nanostructure is achieved through a sputtered multi-layer of alternating Si rich
material and stoichiometric dielectric [4.5.4]. On annealing the excess Si
precipitates into small nanocrystals which are limited in size by the layer thickness,
thus giving reasonable size uniformity, as first demonstrated by Zacharias
[4.5.7]. Demonstration of doping of these layers with both phosphorus and boron to create a
rectifying p-n junction has resulted in devices with a photovoltaic open circuit voltage of 490mV [4.5.6, 4.5.7, 4.5.8].

Formation
of Si (or Ge or Sn) QDs through layered thin film deposition of Si rich material which crystallises into
uniform sized QDs on annealing.

A cell
based entirely of Si, or other group IV elements, and their dielectric compounds with other abundant elements
(i.e. silicon oxide, nitride or carbide) fabricated with thin film techniques, is advantageous in terms of
potential for large scale manufacturability and in long term availability of its
constituents. Such thin film implementation implies low temperature deposition without melt processing, it hence also involves imperfect crystallisation with high defect densities. Hence
devices must be thin to limit recombination due to their short diffusion lengths, which in turn
means they must have high absorption
coefficients.

For
photovoltaic applications, nanocrystal materials may allow the fabrication
of higher band gap solar cells that can be used as tandem cell elements on top of normal Si cells
[4.5.11, 4.5.12]. For an AM1.5 solar spectrum the optimal band gap of the top cell required to
maximize conversion efficiency is 1.7 to 1.8eV for a 2-cell tandem with a Si
bottom cell
[4.5.13]. To date, considerable work has been completed on the growth and characterization of Si
nanocrystals
embedded in oxide [4.5.7, 4.5.14] and nitride [4.5.15, 4.5.16] dielectric
matrices. However, little has been reported on the experimental

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FMG scheme employed for
finding electronic states and the confined energy.

FIGURE 4.5.5

Multilayer deposition of
alternating Si rich dielectric and stoichimetric dielectric in layers of a few
nm. On annealing the Si precipitates out to form small nanocrystals of a size
determined by the layer thickness. Nanocrystal or quantum dot size is therefore
uniform.

FIGURE 4.5.3

Bulk band alignments between
silicon and its carbide, nitride and oxide. Tunnelling probability between QDs
separated by d depends
exponentially on the square root of the barrier height (ΔE1/2)
multiplied by d (e.g. [4.5.19]
p244).

FIGURE 4.5.4

properties
of Si nanocrystals embedded in SiC matrix [4,5,17]. These are of particular interest for
application in photovoltaic devices because of an expected significant increase in carrier transport due to a
decrease in the barrier height between adjacent nanocrystals [4.5.18]. As a result,
sufficient carrier mobility can be obtained to satisfy device fabrication requirements.

4.5.2.2 Fabrication of Si QD nanostructures

Thin
film techniques are used for nanostructure fabrication. These
include sputtering and plasma enhanced chemical vapour deposition (PECVD). The deposition is a
variation of the multi-layer alternating ‘stoichiometric dielectric / Si rich dielectric’ process,
shown in Fig. 4.5.3, followed by an anneal during which Si nanocrystals precipitate limited in
size by the Si rich layer thickness [4.5.12, 4.5.7]. The most successful and hence most
commonly used
technique is sputtering, because of its large amount of control over deposition material,
deposition rate and abruptness of layers. A multi-target remote plasma sputtering machine with two independent RF power
supplies as well
as dditional DC power supplies is used
in this work.

RF
magnetron sputtering is used to deposit alternating layers of SiO2 and SRO of thicknesses down
to 2nm. [SRO refers
to Si rich oxide, formed by co-sputtering Si and SiO2 .] Deposition of multi-layers,
consisting typically
of 20 to 50 bi-layers, is followed by an anneal in N2 from 1050 to 11500C. During
the anneal the
excess silicon in the SRO layer precipitates to form Si nanocrystals between the stoichiometric oxide layers.

For Si
QDs in SiO2 the precipitation occurs according to the following:

Precipitation
of excess Si from Si rich dielectrics in SiNx and SiC follows a similar
crystallisation reaction as Si precipitates from the amorphous matrix. The
techniques has also been applied to growth of Sn and Ge QDs in SiNx in SiO2.
Ge quantum dots can be precipitated at substantially lower temperature, as discussed below.

4.5.2.2.1 carrier tunnelling transport in Si QD superlattices

Transport
properties are expected to depend on the matrix in which the silicon quantum dots are embedded. As shown in
Fig. 4.5.4 different matrices produce different transport barriers between the Si dot
and the matrix, with tunnelling probability heavily dependent on the height of this barrier. Si3N4
and SiC give lower barriers than SiO2 allowing larger dot spacing for a
given tunnelling current.

The
results suggest that dots in a SiO2 matrix would have to be separated by
no more than 1-2 nm of matrix, while they could be separated by more than 4 nm of SiC. Fluctuations
in spacing and size of the dots can be investigated using similar calculations. It is also found that the
calculated Bloch mobilities do not depend strongly on variations in the dot spacing but do depend
strongly on dot size within the QD material [4.5.18]. Hence, transport between dots can be
significantly increased by using alternative matrices with a lower barrier height, ∆E. For the same tunnelling current
the spacing of QDs can increase for oxide to nitride to carbide matrix.

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4.5.2.2.2 modelling of quantum confinement and tunnelling
transport

Researchers:

Binesh Puthen-Veettil, Robert
Patterson, Dirk KÖnig

Band gap engineering for
these Si nanostructure materials requires a large number of almost identical
quantum dots (QDs), evenly distributed in a dielectric layer. As the proximity
of dots increases, confined wavefunctions in the QDs interact with one another
to form mini-bands. It is also seen that higher Si content in the Si rich layer
gives rise to bigger QDs, due to the tendency of QDs to merge together and form
bigger dots during annealing [4.5.12, 4.5.20]. If the dielectric layer is a
good diffusion barrier for Si (such as the nitride layer in Si/Si3N4
or Si3N4 interlayer samples, see sections 4.5.2.3.1 and
4.5.2.3.2), this effect is strongly reduced. For relatively thick layers in
which diffusion is not constrained, the shapes of these QDs can differ greatly
from an ideal spherical shape. Calculation of confined energy in QD structures
is of interest since their electronic and optical properties can be determined
by determining the confined energy levels in the structure.

Since these calculations are
extremely difficult to perform using ab-initio methods due to the large memory
and computation time requirements, we have developed a model to quantitatively
analyse electronic states and their interactions in a QD array, in the
framework of the effective mass approximation (EMA) calibrated by ab-initio DFT
calculations. This model is realized by solving the three dimensional time
independent discretized SchrÖdinger’s equation in the EMA, employing a modified
Full Multi Grid method (FMG) [4.5.21] to overcome the difficulty of
computational intensity. By using exact solution methods to solve the Eigen
equation for the coarsest grid, the non convergence problems [4.5.22] arising
from oscillatory eigenvalues are eliminated. Since initial guesses of the
solution are very close to the approximate solution, this method proves to be
much faster than other iterative methods. The flow diagram for this process is
shown in Fig. 4.5.5.

In order to demonstrate the
conformity of results from this model with experimental results, we compared it
with experimental results for Silicon QDs in SiO2 dielectric grown
using sputtering methods and characterized using photoluminescence (PL)
[4.5.20]. Figure 4.5.6 shows the dependency of confined energy on the size of
QDs for Si/SiO2 QD structures. Confined energy increases
exponentially as QD size decreases. Comparison shows that for larger QDs
experimental and theoretical values are similar, but for smaller dots the
experimental results for the confined energy shows a lower degree of
confinement than the theoretical result. This difference can be attributed to
interface defects between Si and SiO2 and to defects within the SiO2
matrix but near the interface. For QDs below a diameter of about 4 nm these
interface effects have a larger influence on the confined energy level. This is
because the ground state electron density, which is concentrated at the centre
of the QD, is closer to these interface defects for the smaller QDs and hence more
strongly influenced.

As an extreme example of
irregularly shaped QD, we have investigated electronic states in aribitrarily
shaped QDs. Here we have used the “horseshoe” morphology [4.5.24], formed by
lateral growth of Si QD inside SiO2 dielectric shown in Figure 4.5.7
(left). These states are found to be very different from the expected
electronic energy levels that are associated with spherical QDs as shown in
Fig. 4.5.7 (right). The degeneracy in spherical QDs (3 in first excited level
and 5 in second excited level) is lost due to the asymmetric shape. The
deviation from the expected confinement level becomes significant at higher
confined levels.

This thus indicates the
importance of knowing the regime of QD growth relevant for the material
conditions. As shown in the TEM tomography of Section 4.5.2.2.3, under
appropriate conditions of Si density and annealing, a spherical morphology can
be maintained and QD merging avoided, thus giving greater control over the
confined energy levels.

Simulation results -
multigrid method and Density Functional Theory (DFT) calculations [4.5.23]
compared with the experimental results for Silicon QDs in SiO2
dielectric.

FIGURE 4.5.6

First three modes of
electronic wave-functions in a “horseshoe” shaped QD with high surface to
volume ratio (far left). The ground state is located near the centre of the dot
and is approximately in a spherical shape with zero nodes. The number of nodes
increases with the increase in the energy level. Energy levels associated with
a horseshoe shaped QD compared with that of a 6nm diameter spherical QD (left).
The two morphologies give energies which are most similar for their ground
states, with the horseshoe shape giving significantly lower energies than
spherical for higher confined levels. The degeneracy in energy levels is lost
in the horseshoe shaped dot.

FIGURE 4.5.7

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Energy filtered TEM
tomography images of Si QDs in a SiO2 matrix formed as multilayers
(left), and top view of one Si QD layer (right).

FIGURE 4.5.8

The spectra fittings of the R and T
data. The inset shows the corresponding employed optical model in
the fitting of a 20 bilayer structure composted of 5 nm SRN / 3 nm Si3N4.

FIGURE 4.5.9

Real (ε1) and imaginary
(ε2) parts of the complex dielectric function of Si QDs samples with
various sizes obtained from spectral fittings.

FIGURE 4.5.10

4.5.2.2.3 3D Ef-tEm tomography on Si QDs embedded in oxide
matrix formed as multilayers (collaboration with cornell university)

Researchers:

Xiaojing
Hao, Shujuan Huang, Lena Fitting Kourkoutis (Cornell University), Ivan Perez-Wurfl, Tom Puzzer

A
significant challenge to the design of nano-scale materials and devices arises
from the difficulty of characterising complex three-dimensional structures on small
length scales, which is nonetheless critical in understanding the material performance. Whilst a number of techniques,
such as
conventional transmission electron
microscopy, provide sufficient two-dimensional (2-D) resolution, they have insufficient depth sensitivity to obtain internal three-dimensional (3-D) structure. Electron tomography can be applied to obtain such information by using high-angle dark-feld scanning transmission electron microscopic (STEM) tomography and energy filtered transmission electron microscopy (EFTEM) tomography. We have applied this EFTEM tomography to investigate Si QDs
embedded in an oxide matrix formed
as multilayers, in order to better
understand the dot size, shape and
distribution for further optimization of
Si QD device performance. Fig. 4.5.8 illustrates the ability of electron tomography to reconstruct the morphology and internal structure of a Si QD nanostructure. Fig. 4.5.8 (left) shows the aligned
“on-axis” reconstructed EFTEM tomography,
and Fig. 4.5.8 (right) shows a top-view of the Si QD size and shape
distribution in one Si QD layer. With such information, we can confirm that inside the Si QD layer, most of the Si QDs are reasonably
spherical and well separated from
each other. This indicates that
individual QDs have not merged to form larger particles under these conditions. This is important in designing the optimum concentrations to give minimum QD spacing whilst maintaining individual
QDs.

4.5.2.3. Different materials for QD nanostructures

As
described in section 4.5.2.2.1, different matrices are useful to modify
both the tunnelling probability between adjacent quantum dots and the energy levels in the quantum
dots themselves. This approach has been carried out for both Si QDs in SiNx and in
SiC. It is also possible to have asymmetric superstructures in which quantum confinement and hence enhanced band
gap is maintained in the plane, but in which tunnelling probability is maximised in the
direction normal to the plane such that transport of carriers to contacts is maximised. Interlayers use for such
asymmetric structures can also act as diffusion barriers enhancing size uniformity of QDs.

Alternative
group IV materials such as Ge offer the possibility of lower temperature precipitation of nanostructures and the potential for band gaps lower than that of silicon should these be
required for tandem cell elements
under a silicon cell. We have
investigated both QDs and QWs of Ge in either SiO2 or SiNx
matrices.

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(a) Normalized PL spectra of
Si QDs samples (S3-S5) with various sizes. (b) Comparison of band gap expansion
(ΔEg) of Si QD as a function of QD size.

FIGURE 4.5.11

4.5.2.3.1 Silicon QDs embedded in silicon nitride matrix

Researchers:

Yong-Heng
So, Shujuan Huang

 (i) Size-dependent optical properties of Si QDs in Si-rich
nitride/Si3N4 superlattice

An
approach similar to spectroscopic ellipsometry analysis has been
proposed to determine the size dependent optical properties of silicon quantum dots (Si QDs) in Si-rich
nitride/silicon nitride superlattice structure (SRN/Si3N4-SL)
[4.5.26]. The optical
properties of Si QDs are modelled using the Tauc-Lorentz (TL) model and Bruggeman effective medium
approximation that can yield the energy bandgap of the Si QDs based on spectral ftting of the reflection
(R) and transmission (T ) of characterized samples, as shown in Fig. 4.5.9. A
four-phase
optical model as shown in the inset of Fig. 4.5.9 was employed for the spectral fitting.

Figure
4.5.10 shows that the dielectric functions of Si QDs are strongly
size dependent. Sample S3, S4 and S5 correspond to SRN/Si3N4
thicknesses of 3/3 nm, 4/3 nm and 5/3 nm, respectively. The suppressed imaginary
dielectric function of Si QDs exhibit a broad peak centred between transition energies E1
and E2 of bulk crystalline Si and which blue shift towards E2
as the QD size reduces.

Figure
4.5.11(b) shows that the band gap expansion indicated by the TL model when the size of Si QD reduces is
in good agreement with PL measurements (Fig. 4.5.11(a)). The bandgap expansion with the
reduction of Si QD size is well supported by the ab-initio calculations of confined energy levels from [4.5.25].

 (ii) n-type conductivity of nanostructured thin film composed
of antimony-doped Si nanocrystals in silicon nitride matrix

Highly
conductive thin films composed of antimony (Sb)-doped Si
nanocrystals (Si-NCs) embedded in Si3N4 matrix were prepared by a
co-sputtering technique.
Results from structural characterizations suggest that doping with Sb concentration of 0.54 at. % has negligible
affect on the crystallization properties of Si-rich nitride (SRN) films. The X-ray Photoelectron (XPS) data
of Fig. 4.5.12(a) and (b) show the Si 2p and
Sb 3d3/2 spectra of
the undoped and Sb-doped SRN films. The slight asymmetric Sb 3d3/2 peak was analyzed using a
curve-fitting routine and could be decomposed into two components, δ1
(537.0 eV) and δ2 (537.8 eV) as shown in Fig. 4.5.12(c). The two
components δ1 and δ2 (0.8 eV apart) can be assigned to
neutral Sb and positively charged
donors, respectively.

Thus,
the appearance of δ2 suggests the presence of Sb-Si bonds which
implies that Sb stoms were either incorporated within the Si-NCs or located at the interface between the
NCs and Si3N4. matrix.

XPS spectra of (a) Si 2p and (b) Sb 3d3/2 for the undoped and 0.54 at.% Sb-doped
Si-NC. (c) Fitting of Sb 3d3/2
spectrum by two peaks separated by 0.8 eV in energy (δ1 and
δ2).

FIGURE 4.5.12

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(a) Conductivities measured
throughout the entire voltage range. (b) ln(σ) vs 1/T1/2 plot
from 120 to 220 K, for the 0.54 at.% Sb-doped sample. The solid line is least
square fit to the data. (c) Temperature-dependent conductivities for the
undoped and 0.54 at.% Sb-doped samples.

FIGURE 4.5.13

The
temperature dependent conductivity data of Fig. 4.5.13(a) indicate that the conductivity of
0.54 at.%
Sb-doped SRN film (2.8X10-2 S/cm) is six orders of magnitude higher than that for undoped material (7.3X10-8
S/cm), which could be attributed to an increase in carrier concentration. Furthermore, n-type
electrical behaviour with carrier concentration of 2.4X1016 cm-3
and mobility of
2.94 cm2/V s in the doped films as observed from Hall measurements was
attributed to free carrier generation due to the effective Sb doping.

The
temperature-dependent conductivities of both undoped and Sb-doped samples are shown in Fig. 4.5.13(b). As
can be seen, doping of the Si-NCs strongly influences the electronic transport properties of the films.
Arrhenius-like temperature dependence is observed in the T range between 220 and 320 K,
attributable to thermally activated conduction. From the slope of the Arrhenius
plot, we
found that the activation energy EA
decreases to 0.182
eV for the Sb-doped Si-NCs film, suggesting effective n-type doping of the
Si-NCs. Nevertheless, the extracted EA
is much larger than that of bulk Si with the same dopant. One possible explanation for
the large EA observed would be the deeper
donor level
expected in Si-NCs. Also, it is possible that the number of free carriers
available for conduction may be limited due to a trap density distributed within the bandgap that
is comparable to the doping
density.

Interestingly,
the conductivity for the Sb-doped sample at T < 220 K deviates from Arrhenius behavior and is best
described by the expression, σ = σ0 exp [-(T0/T)1/2] as illustrated in Fig. 4.5.13(c). The charge transport
mechanism can be explained well by
a percolation-hopping model.

4.5.2.3.2 Si QDs in SiO2/Si3N4 hybrid
matrix

Reserachers:

Dawei
Di, Ivan Perez-Wurfl, Gavin Conibeer

To
improve the current transport properties in the vertical direction and to
obtain better size control of Si quantum dots, we proposed a new design based on Si QDs embedded in a
SiO2/Si3N4 hybrid matrix [4.5.27, 4.5.28]. By
replacing the SiO2 tunnel barriers with the Si3N4 layers, the
new material manages to constrain the growth of doped Si quantum dots effectively and
enhances the apparent band gap. Also electrical characterisation on Si QD/c-Si hetero-interface test
structures indicates the new material possesses improved vertical carrier transport properties.

Doped
and undoped samples with different barrier dielectrics (SiO2 and Si3N4)
are compared. Samples were fabricated using the co-sputtering multilayer technique with
layers containing excess Si consisting of SRO for both materials, with an SRO thickness of 4 nm.
Samples were either undoped or co-sputtered with either P2O5 or B
for n- and p-type doping respectively. After annealing at 11000C the crystalline
properties were characterised by X-ray diffraction (XRD), as shown in Fig. 4.5.14. For samples with Si3N4
barriers, the measured Si NC sizes are 4.3 nm when undoped, 3.5 nm when B doped and 5.0 nm when P2O5
doped. For samples with SiO2 barriers, grain sizes are 7.7 nm when undoped, 7.2 nm when B
doped, and 15 nm when P2O5 doped. The Si NC sizes vary significantly
although all samples have the same silicon-to-oxygen ratio in the SRO layers and the same as-deposited layer thickness.

It can
be observed from the experiment that samples with Si3N4
barriers contain generally smaller Si QDs than samples with SiO2
barriers. This
is because Si3N4 a denser and stiffer material than SiO2,
acts as a better diffusion barrier than SiO2 [4.5.27]. It could also be
thermodynamically related to the interface free energy between

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XRD patterns of (a) samples with SiO2 barriers, (b) samples
with Si3N4 barriers.

FIGURE 4.5.14

Vertical current density
measurements on (a) B doped samples, (b) P2O5 doped
samples. Insets show how barrier type and NC dimension influence the vertical
current transport process (showing dominant effect only in each case, not to
scale).

FIGURE 4.5.15

the Si
NC and the matrix dielectrics during the growth of the Si NCs. As a result, Si3N4
is able to constrain
the growth of Si NCs very effectively, within ±1 nm of the intended diameter (i.e. 4 nm, the deposition thickness
of SRO). In contrast, Si NCs with SiO2 barriers are significantly
larger than the
as-deposited thickness of SRO - over 180% larger for undoped and B doped and 375% for P2O5
doped.
Hence, it can be noted that samples with phosphorus (P2O5) dopants
tend to form larger sized Si NCs than undoped samples, while introduction of boron doping tends to
suppress this size increase and slightly reduces NC size. This phenomenon is more pronounced for
samples with SiO2 barriers (Fig. 4.5.14 (a)) than those with Si3N4
barriers (Fig. 4.5.14 (b)).

Si3N4
barriers have lower band gaps (5.3 eV) than SiO2 (9 eV) therefore they should
be more transparent
to charge carrier transport because of the larger tunnelling probability for electrons and holes. To verify this
hypothesis in practice, we compared the current transport of SiO2 barrier
structures with Si3N4 barrier structures via the measurement of
vertical currents under a bias
voltage.

The
current-voltage characteristics of these test structures are shown in
Fig. 4.5.15. It should be noted that these samples are not PV devices. Therefore,
the dark currents measured are primarily affected by the materials’ conductivities and are not related to carrier
recombination. It can be clearly seen from the measurement that for B doped
materials, Si3N4 barriers result in a current enhancement of
approximately an order of magnitude over those with SiO2 barriers (Fig.
4.5.15 (a)).
This can be qualitatively explained by [4.5.19]:

 (4.5.1)

where Te is the tunnelling
probability between quantum dots, d is
the barrier thickness or the separation between dots, m* is the effective mass of electron, ħ is the reduced Plank constant,
Δ E is the energy difference between
the conduction band edge of the barrier material and the conduction band edge of the confined QDs.

Although
the difference in Si NC sizes would play an important role in the
tunnelling event, the variation of Δ E is
still the dominant factor for B doping. On the other hand, for the P2O5
doping, samples with Si3N4 and SiO2 barriers have similar
currents (Fig. 4.5.15 (b)). These apparently contradictory results can be explained by
referring back to the XRD results. The P2O5 doped sample
with SiO2 barriers contains Si NCs with an average size of 15 nm,
this suggests that a single Si NC is physically penetrating three thin layers
on average (inset of Fig. 4.5.15 (b)).

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(a) Raman spectra of SRC
samples after RTA and (b) the same samples with additional furnace annealing.
(Bulk Si wafer peak at 520.5 cm-1 for reference.)

FIGURE 4.5.16

This
greatly reduces the number of barrier layers that the carriers need to tunnel
through. This size effect becomes so significant that it effectively competes with and largely
cancels the increased conductivity due to lower ΔE
for the Si3N4 barrier material.

This
result is very promising for both QD size control and increased vertical currnt
transport in these nitide barrier interlayer Si QD materials.

4.5.2.3.3 Silicon QD nanocrystals embedded in silicon carbide
matrix

Researchers:

Zhenyu
Wan, Shujuan Huang, Gavin Conibeer

Silicon
carbide (SiC) has a lower barrier height than either Si3N4 or SiO2.
Therefore it seems likely that it
should offer a higher tunnelling probability between
QDs and hence a higher conductivity matrix than either. In previous work, we
have demonstrated that both Si and
SiC have been crystallised by high
temperature annealing of a single
thick Si-rich SiC (SRC) layer or of a Si1-xCx/ SiC multilayer structure [4.5.28, 4.5.29]. We believe that the formation of β-SiC
nanocrystals may hinder the formation of Si QDs. Also they may cause current leakage via the SiC grain boundary traps to increase the shunt current in the solar cell. Therefore, the work in 2009
focused on studying the mechanism of
the crystallisation of SiC and
optimising the materials by comparing the
annealing methods of rapid thermal annealing
(RTA) and conventional furnace annealing.
We found that RTA annealed samples revealed
a better degree of crystallisation of Si nanocrystals with a smaller residue of
amorphous Si [4.5.30]. The work in
2010 has been to further improve the
structure by reducing the stress caused
by RTA and by replacing SiC barrier layers with thin silicon nitride layers in order to suppress the growth
of SiC crystals.

 (i) Study of Rta induced stress

Two
different annealing processes have been applied on all SRC amorphous samples: furnace annealing at 1100oC/1hr
and RTA at 1100oC/30 sec. After annealing, Si and SiC nano-crystals were clearly observed in TEM
and XRD. The peak positions of Raman spectra are down-shifted to lower wavenumbers for RTA
samples (Δ=5.4cm-1 from bulk Si) as compared to furnace annealed samples (Δ=2cm-1
from bulk Si), indicating greater tensile stress in the Si nano-crystals, as shown in Fig.
4.5.16 (a).
This is because of a different thermal expansion coefficient for Si and SiC
crystals and the fast temperature ramping rate in RTA. In order to release the stress in RTA samples
an additional annealing process was carried out. Raman analysis indicates that after an additional
furnace anneal at 11000C for 30 min, all samples with different Si concentrations could release most of
their residual stress, as shown in Fig. 4.5.16 (b). However, an additional RTA
could not
release residual stress in low Si concentration samples due to insufficient duration [4.5.31].

 (ii) Si QDs embedded in Sic matrix with Si3N4.
barrier layers

Si3N4
is also considered preferable to SiO2 in term of carrier transport.
It has been proven as a good diffusion barrier to suppress inter-diffusion
between silicon rich layers in multilayer structures during annealing [4.5.27]. In
2010, we have successfully introduced ultra-thin Si3N4
(UT-SiN) barriers (0.2nm-2.0nm) into a Si-NC in SiC matrix structure using sputtering followed by
RTA. Crystallisation of the SiC matrix has been greatly suppressed and a clear layered superlattice
structure can be observed as shown in the TEM of Fig. 4.5.17. The nanocrystal size of all samples was
calculated using XRD peak analysis to quantitatively investigate the optimum
thickness of the Si3N4 barrier layer required to produce confined
crystalline Si-QDs, as shown in Fig. 4.5.18. It is seen that as the Si3N4 barrier layer thickness increases, the
β-SiC peaks decrease, such that β-SiC-NCs almost disappear when the Si3N4 barrier thickness is over
0.8nm.

HRTEM image of annealed 2.0nm
UT-Si3N4 sample. The maintenance of a layered structure
and the presence of Si-QDs can be seen.

FIGURE 4.5.17

XRD analysis and
corresponding grain sizes from the Scherrer formula for all the Si3N4 barrier
thicknesses. Insert: Si (111) and SiC (111) peaks of each sample.

FIGURE 4.5.18

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Absorption edge fitting for
UT-SiN2.0 and UT-SiN0.2 samples respectively, (insert: band gap energy for
different Si3N4 thicknesses).

FIGURE 4.5.19

Temperature-depentent conductitity of different Si2N4 barrier
thickness.

FIGURE 4.5.20

Optical
characterization has been performed to investigate quantum confinement effects for different nanocrystal
sizes. Optical analysis reveals a blue shift in the strong absorption edge, consistent
with quantum confinement effects in small Si NCs matching qualitatively calculated results using effective mass theory
from other groups, as shown in
Fig. 4.5.19.

Temperature
dependent I-V measurements were carried out to investigate the carrier transport mechanism. The
conductivity is best described by the expression, σ = σ0 exp [-(T0/T)1/2]
when the Si3N4 barrier thickness is
greater than 0.5 nm, as illustrated in Fig. 4.5.20. The charge transport mechanism can be
explained well by the percolation-hopping model, similar to the result found in Section
4.5.2.3.1 for Sb doped SiQDs in Si3N4. Finally, we conclude that in future, a 0.5-0.8 nm thick layer of Si3N4
barrier layer would be optimum to achieve both good Si-NC confinement and minimal film
resistivity for a candidate material for photovoltaic applications.

4.5.2.3.4 germanium Nanostructures

Although
the main focus has been on Si nanocrystals during the last decade, other group IV
nanocrystals
have also been studied. Since Ge has smaller electron and hole effective masses and a
larger dielectric constant than Si, the excitonic Bohr radius of bulk Ge is
larger than that of Si. This leads to a more prominent quantum confinement effect in Ge NCs. Furthermore
the lower melting point of Ge at 938.3oC implies that Ge NCs should be able to form at lower
temperatures than Si NCs. This is indeed a significant advantage both for processing compatibility and for
processing costs, although set against this are the greater cost and lower abundance of Ge as compared to Si.

4.5.2.3.4.1 Quantum Dots in Sio2

Researchers:

Santosh Shrestha,
Bo Zhang, Pasquale Aliberti, Gavin Conibeer

 (i) fabrication of germanium quantum dots

The
current work builds on earlier work on Ge nanocrystals carried out in our group
[4.5.32]. The new work uses a different approach to annealing of the samples which gives
a wider range of control of Ge NC formation. Ge NC samples have been fabricated by RF
magnetron sputtering using a combination of a combination target comprising a fused quartz disc and
high purity Ge strips. Sputtering of the Ge-rich oxide (GeRO) layer is achieved by sputtering
with just an Ar background, whereas sputtering of the oxide layers, GeO2/
SiO2, is performed by reactive sputtering with O2. These layers are
alternately deposited to obtain the desired multilayered structure. A thick oxide capping
layer is deposited to prevent oxidation of the GeRO layers during subsequent annealing. Post deposition annealing of
the samples was performed under a low pressure (in-situ in the growth chamber) during which Ge
NCs are formed. Results

Cross-section HRTEM image of
a typical sample containing GeRO layers between GeO2/SiO2
layers following annealing.

FIGURE 4.5.21

(a) Raman spectra and (b) GIXRD image of the as-deposited and annealed
films.

FIGURE 4.5.22

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Raman spectra of the samples annealed for different durations.

FIGURE 4.5.23

Raman spectra of the samples annealed at different temperatures.

FIGURE 4.5.24

Room temperature PL of a multilayer film containing Ge NCs in GeO2/SiO2.

FIGURE 4.5.25

Raman spectra from Ge-NCs in single layer samples grown at different
substrate temperatures.

FIGURE 4.5.26

from
structural characterization using TEM, Raman, XRD and PL have been obtained.

Figure
4.5.21 shows a high resolution TEM image of a section of a typical sample containing
GeRO layers between GeO2/ SiO2. The sample was annealed at 6500C. Ge nanocrystals of about
5nm diameter are
evident. Although there is a slight variation, the NCs appear to be spherical in shape. The clear lattice
fringes observed in the TEM image give direct evidence of the formation of the Ge NCs. It should be noted that the
distance between the lattice fringes is 3.3 A, which is consistent with the lattice spacing
of the {111} planes of the Ge diamond structure.

Fig.
4.5.22 (a) shows Raman spectra of the as-deposited and the annealed samples, including those for bulk Ge as a
reference. For these samples, the sputtering times for each GeRO layer and GeO2/SiO2
layer were 8 minutes and 6 minutes, respectively, and the post deposition annealing was performed
at 6500C for 40 minutes. A broad hump at around 270 cm-1 is observed in the spectra of the as-deposited film which
is attributed to the non-crystalline Ge phase. But in the annealed film, it is replaced
by a sharp peak
at 300.5 cm-1, which is very close to the Ge-Ge optical phonon mode for bulk Ge (300.2 cm-1),
indicating the formation of Ge NCs with good crystallinity. Peak broadening and an
asymmetric shoulder on the lower frequency side can be interpreted by the model
of the optical phonon confinement effect in nanocrystals [4.5.33]. However, it seems that in our Ge system
the high
frequency side of the Raman peak does not show a shift to lower phonon frequencies which is usually the case for NCs.

Figure
4.5.22 (b) shows GIXRD patterns for this sample. In the as-deposited film there are no
obvious peaks but only two broad bands at around 2θ = 260 and 2θ = 490. After annealing,
the sample shows
three sharp peaks at 27.120, 45.130 o and 53.210, corresponding to the groups of planes
{111}, {220} and {311} of crystalline Ge, respectively. This observation confirms good
crystallinity of the Ge phase in the film and agrees well with our Raman results. The average size
of the Ge NCs, calculated from the {111} peak broadening using the Scherer
equation, is about 4.5 nm. This value is slightly smaller than that
estimated from the HRTEM image. The difference in Ge NCs sizes obtained from
these two
methods may possibly be due to spatial non-uniformity of the Ge NCs size; with TEM probing a
much smaller sample region compared to XRD measurement. In addition, the penetration depth of the incident X-rays is
larger than the thickness of our film, thus information obtained from XRD is averaged throughout the whole film.

The
crystallisation of Ge NCs with annealing duration has been investigated with Raman spectroscopy. Fig. 4.5.23
shows Raman spectra for identical multilayer samples with each GeRO and GeO2/SiO2
layer deposited for 6 minutes. The samples were annealed at 6850C for different durations as indicated
in the diagram. The crystallization of Ge is found to take place within the first few minutes of
annealing. However, the noticeable broad hump, which is attributed to small NCs, suggests an early
stage of the crystallization process at this annealing duration. As annealing duration increases,
Raman peaks become sharper and narrower indicating an increase in Ge crystallinity. It is also
observed that the peaks show negligible difference for annealing durations longer than 10 minutes.

The
average size of NCs, as calculated from XRD data, increases with
annealing duration. This is consistent with the Raman results and agrees well with the growth
dynamics by diffusion of neighbouring Ge atoms. Most importantly, the increase in the NCs size
begins to level off after 15 minutes of annealing in this particular case. This
is similar
to the size confinement effect observed for the growth of Si NCs in SiO2 matrix
[4.5.7, 4.5.34]. In our superlattice structure, Ge NCs are confined within the GeRO layers
between GeO2/SiO2 layers,

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A High resolution TEM image of multilayered Ge NCs in SiO2
matrix. 

The sample was grown at 3800C

FIGURE 4.5.27

which
work as barrier layers to the crystal growth. Hence, Ge tends to
precipitate with a diameter approximately equal to the thickness of the GeRO layers.
This control mechanism is more significant for thin layers with thickness of a
few nanometers, within which there is 2D rather than a 3D growth.

Figure
4.5.24 shows the Raman spectra of the samples annealed at different temperatures for 40 minutes, which is enough
to complete the growth process as discussed above. As expected, the small nanocrystalline
asymmetric hump is reduced with the increase in annealing temperature within
the range
of temperatures studied here, just as in the case of annealing durations. The results also
illustrate the formation of Ge NCs in SiO2 matrix at a temperature as low as 620
0C.

A sample
with the identical process sequence as those for structural characterizations was used
for photoluminescence measurements, except that the film was deposited on quartz. Fig. 4.5.25
shows results
of room-temperature PL measurement. The PL spectrum consists of a single broad band
centred at 1.77eV (corresponding to a wavelength of 700 nm) which can be fitted
with three Gaussian distributions.
Measurement on GeO2/SiO2 flm deposited on quartz under similar conditions did not show any observable PL in the range of wavelengths concerned. Thus the PL signal from
the multilayer sample can be
attributed to Ge NCs in the GeRO
layers. Furthermore, a blue shift of PL energy with NCs size has been shown in previous work [4.5.32]. We tentatively consider that this is
due to the band gap increase induced
by the quantum confinement effect in Ge NCs [4.5.35].

 (ii) Low temperature growth of germanium quantum dots

In this section,
a low temperature growth of Ge nanocrystals is discussed. This is advantageous because of the potential
for reduced processing cost and suitability for cheaper substrates such as soda-lime glass.
Samples were grown with RF magnetron sputtering as described earlier, except in this case the
substrate was heated during the film growth and no post annealing process was used [4.5.36].

Figure
4.5.26 shows Raman spectra for a single layer GeRO film deposited at
different temperatures as indicated in the diagram. For clarity, the graph has been divided into three
different regimes. In regime I, the samples grown below 3500C are shown. In this
case, the spectra show no Raman peaks related to nanocrystalline Ge, instead broad bands
centred
around 280 cm1 are observed. The bands are shifted toward higher frequency by about 10 cm1 compared with that
of amorphous Ge (α-Ge) located at around 270 cm 1. This indicates that α-Ge
coexists
with very small particles (12 nm) [4.5.37].

For the
sample grown at Tg = 3200C, the observation of a hump at 280 cm 1
as well as a shoulder at 298 cm-1 implies an increase in the
number and size of small particles. The rapid growth of Ge NCs is found to occur in
Regime II, in which samples grown at temperatures between 3500C and 4000C are shown.
The Raman peak corresponding to the TO phonon mode of the crystalline Ge (c-Ge), near 300.4 cm-1, appears
at Tg = 3500C and becomes sharper with increase in the growth temperature. This range of temperature
is very close to the onset temperature for Ge crystallisation reported in [4.5.38]. However, when
the growth temperature Tg reaches 4200C (Regime III), a drastic degradation
of Ge-Ge peak intensity and shape is observed. These phenomena indicate the
absence of Ge crystallinity in
this temperature range.

Based
on the Raman results discussed above, it is expected that the growth
temperature window is between 350oC and 420oC, in which Ge atoms or clusters can accumulate
and eventually grow into highly crystallised nanocrystals. These temperatures are much lower than the
usual post-deposition annealing temperatures used for Ge NCs fabrication.

A HRTEM
image of a multilayered sample consisting of alternate layers of GeRO and GeO2/SiO2
films is shown in Fig. 4.5.27. In this case the sample was grown at Tg =
380oC without further post-deposition thermal treatment. The TEM image shows close to
spherical Ge NCs with fairly uniform size, separated

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Schematic diagram of a Ge-NC/n:c-Si HJ diode.

FIGURE 4.5.28

(a) The semi-logarithmic plot
of dark I-V curves of a typical i:Ge-NC/ n:c-Si HJ device in both polarities at
room temperature. (b) The measured suns-Voc characteristics of the
HJ device. The red solid circle indicates the 1-sun point.

FIGURE 4.5.29

by the
barrier layers. The Ge NC sizes are in the range of 4.8 - 5.8 nm which is consistent with
the average size ( 4.8 nm) determined from XRD. The image in the inset shows clear lattice fringes
inside one of the Ge-NCs (indicated by the arrow). The fringes are consistent with {111}
planes of the Ge diamond
structure.

 (iii) Electrical properties of ge-Nc thin films

Ge-NC
thin films about 250 nm 300 nm were deposited. The coverage of Ge was 20% of the area of the
composite target. XPS has shown that this resulted in a Ge atomic concentration of 35% in the films. Undoped, Ga-doped and
Sb-doped Ge targets
were used. During the sputtering process the substrates were intentionally heated up to 380 oC for
in-situ growth of Ge-NCs. Finally, RTA treatments at different temperatures, including
650oC, 700oC, 750oC, 800oC were carried out in nitrogen [4.5.39].

The
current conduction characteristics of i:Ge-NC thin films were measured
using HP4140B pA Meter/ DC voltage source with tri-axial wires to eliminate
noise. The relatively high conductivity and carrier concentration in the
i:Ge-NC thin films is quite surprising, taking into consideration that they
were not intentionally doped. Temperature dependent measurement revealed a lnσ OC T-1relationship, suggesting
a thermally activated nearest hopping conduction
mechanism in these films. The carrier transport was considered to occur at the
surface state energy level and a theoretical calculation predicts that the
density of surface states of Ge can provide sufficient free holes to explain
the observed conductivity. RTA treatments further increase the flm conductivity without changing much the structural properties of nanocrystals. This improvement
was tentatively attributed to the modification
of surface structure of NCs and reduction
of oxygen-deficiency-related defects in the SiO2 matrix. The effect of incorporating moderate amounts of Ga and Sb dopants was also investigated. The doped films exhibit similar conduction properties to the intrinsic films,
which means the films were still
dominated by surface state induced hole conduction and that the dopants were
not effectively activated. This is not surprising if one realizes the screening
of shallow dopants in NCs due to the
increase of binding energy and ionization
energy. This effect together with the inherent hole generation effect can make
it very challenging to produce n-type Ge-NC thin films.

 (iv) Ge-nc/c-Si Heterojunction Devices

Heterojunction
(HJ) devices employing Ge-NC thin flms on lightly doped n-type crystalline silicon substrates with impurity concentration of 1 x 1015 cm3 (n:c-Si) were fabricated to evaluate the compatibility
of the nanostructured thin film and investigate
the design parameters required for its application
in photovoltaic devices. The schematic diagram
of the HJ device is illustrated in Fig. 4.5.28. The total area of the device is 1 cm2. The thickness of the Ge-NC layer is about 250 nm. The Ge-NC thin films were post-annealed by RTA at 800oC in order to achieve lower film resistivity and higher
carrier concentration. The Al front
fingers and rear contact were deposited by thermal evaporation. No passivation
or sintering process was performed on the devices.

Figure
4.5.29 shows the dark I-V curves and suns VOC of a typical i:Ge-NC/n:c-Si HJ device at room temperature. The device
shows good current rectification of three orders of magnitude at ± 1.5 V. Since both front and
rear electrodes were ohmic contacts, the rectification effect can be attributed to
the junction. The apparent photovoltage was detected from the illuminated device and the 1-sun
VOCwas found to be 314
mV. The best fitting to the experimental data using a two diode model predicts an effective ideality
factor of 1.01 throughout the entire injection range, which is indicative of a dominating bulk and
surface recombination in the heterojunction device. Whilst this HJ device does not demonstrate
photovoltaic behaviour of the Ge NC material it does show good rectification and is

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XRD spectra of Ge QDs of
various Ge content in Ge-rich nitride layer with a fixed substrate temperature
of 600 0C (left) and TEM image of 40 vol% Ge self-assembled at 600 0C (right).

FIGURE 4.5.30

encouraging
as a starting point for using Ge-NC in true photovoltaic applications.

4.5.2.3.4.2 germanium QW and QD nanostructures embedded in
silicon nitride matrix

Researchers:

Sammy
Lee, Jian Chen, Shujuan Huang, Martin Green

As
discussed above in section 4.5.2.3.1, a silicon nitride matrix (Si3N4)
should increase tunneling probability and hence increase conductivity of Ge
nanostructure materials. In addition a nitride will strongly suppress the
possibility of oxidation of Ge. Ge QDs and QWs were fabricated by co-sputtering followed by
post-annealing and self-organisation by substrate heating.

 (i) ge QDs embedded in silicon nitride matrix

Using a
very similar approach to our previous work on Ge QDs in oxide and Sn
QDs in nitride matrices, alternating layers of germanium-rich nitride (GRN) layers
and Si3N4 layer were alternately co-sputtered on a heated substrate of
either a silicon wafer or quartz slide. The Ge QDs crystallise due to minimisation of surface
energy and the Si3N4 layer deposited on top of the GRN layer truncates
crystal growth.
The temperature of the substrate and the Ge content in GRN layers were the dominant factors
in
controlling the size of the NCs. The structure of the NCs was studied by
GIXRD, Raman spectroscopy and TEM, and the optical properties were studied with transmittance and
reflectance measurements analysed
using a Tauc-plot.

Figure 4.5.30 shows the XRD spectra of
samples of various Ge content in GRN. The Bragg
peaks of
{111}, {220}
and {311} planes sharpen with increasing Ge content, indicating the increase of the nanocrystal
sizes. Using
the Scherrer equation the size of the nanocrystals was estimated as shown in Table 1,
with sizes in agreement with the NC size observed in TEM images.

Figure 4.5.31 summarises the band gap
engineering trend with the size of Ge QDs. The
absorption edge shows a tendency to shift to high photon energy when Ge
nanocrystal size decreases. The absorption coefficient
was calculated by a simplified equation;

where T and R
are the transmission and reflection and d is the thickness
of the film. With decrease in Ge content in the GRN layer, and hence in the diameter of the QDs, the
absorption edge appears to increase by up to 1.0 eV, as estimated with the Tauc-plot method.
However, these data should be considered as indicative of the trend only as there is error in
quantitative values for the shift
in absorption edge due to uncertainty in
extrapolating back to the energy axis.

 (ii) ge QWs with silicon nitride barriers

With a
similar approach to the Ge QD fabrication, Ge QWs were fabricated
by co-sputtering of alternating GRN and Si3N4 layers and followed by furnace annealing or RTA.
The Ge content in GRN was higher in this case in order to give continuous layers. It was varied
between 60vol% to 100vol% and the annealing temperature in N2 was also
varied from
6000C to 9000C for 1 hour. The structural and optical properties were studied
by GIXRD, Raman spectroscopy, TEM and transmission and reflectance measurements.

Fig.
4.5.32 shows the continuous Ge layer with its QW-like-structure between
amorphous Si3N4 barriers. Highly ordered lattice planes can
clearly be
seen in these Ge QW, indicating formation of large single crystal type
structures. Based on Raman and XRD characterisation, the crystallisation temperature shows a
significant dependence on the thickness of either Ge QW or Si3N4 barrier

	
  

 	
  

 	
  

 
	
 Ge vol%

 	
 ∆(2q) of (111)

 	
 NC Size (nm)

 
	 

 	 

 	 

 
	
 30

 	
 5.62

 	
 NA

 
	
 40

 	
 3.85

 	
 2.22

 
	
 50

 	
 2.41

 	
 3.54

 
	
 60

 	
 2.38

 	
 3.59

 
	
 70

 	
 1.92

 	
 4.45

 

Ge NC size estimation from XRD.

TABLE 4.5.1

Absorption edge observed from transmission and reflectance measurements.

FIGURE 4.5.31.

TEM image of the sample containing 60 vol% Ge and furnace annealed at
9000C

FIGURE 4.5.32

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Schematic diagram of the of
the fabricated inter-digitated Si QD in SiO2 devices, with B and P
doping to create the p-n junctions and illustrating the effect of current
crowding due to high lateral resitnce to the back contact.

FIGURE 4.5.33

layers,
varying from 7000C to 9000C. The absorption coefficient estimated
from the transmittance and reflectance measurements also shows an apparent increase in band gap by
up to 1.1eV when the thickness of
the Ge QWs is 1.5nm.

Both Ge
QDs and QWs nanostructures in Si3N4 show evidence of band gap
enhancement. Further work will investigate electrical properties and the possibility of device fabrication.

4.5.2.4 Silicon nanocrystal devices on quartz substrates

Researchers:

Ivan
Perez-Wurfl, Xiaojing Hao, Dawei Dai, Adrian Shi

As
previously reported in the 2008 and 2009 annual reports devices have been
fabricated using the SiQD in SiO2 materials, with p-n junction formation using B or P
of multilayers, respectively [4.5.9, 4.5.10, 4.5.40]. The fabricated p-n diodes consisted of sputtered
alternating layers of SiO2 and SRO onto quartz substrates with in-situ boron
and phosphorus doping. The top B doped bi-layers were selectively etched to
create isolated p-type mesas and to access the buried P doped bi-layers. Aluminium
contacts were deposited by evaporation, patterned and sintered to create ohmic contacts on both p and
n-type layers. The fabricated interdigitated solar cells have an effective area of up to 0.12cm2.
The devices exhibit rectification and a photovoltaic response with Voc up to
493 mV, but with
as yet very small currents and bad fill factors. These are partly due to the very high resistance of the material and in
particular to the relatively long lateral paths to contacts at the back contact, necessitated by groth on
an insulating quartz substrate. The device structure with appropriate contacting is shown
schematically in Fig. 4.5.33. This also shows the current crowding effect which
results
from the high lateral resistance.

4.5.2.4.1 Equivalent circuit model of nanocrystal devices on
quartz substrates

In
order to better understand the limitation imposed by the device on the solar cell
performance, it is necessary to find a good equivalent circuit model specific to our devices. The high resistivity of
the base layer makes it imperative to consider the two dimensional effects of current flow
[4.5.41]. In order to analyse the I-V characteristics
of these diodes we first generalised the model to include any number of diodes in series. The series
connection of diodes is used to explain the ideality factors higher than two normally observed in our
structures. We believe this is a reasonable model as the ideality factor
observed is
almost independent of the diode current. This type of behaviour has also been
observed in multi quantum well laser diodes [4.5.42], where it has been proven
to arise from an unintended series combination of diodes. Based on this series combination of diodes, it
is possible to obtain an expression relating temperature dependent I-V measurements to the band gap [4.5.9]. It is
further possible to linearise the expression around an average measuring
temperature, Tavg, as follows:

 (4.5.2)

where k is Boltzmann’s constant, q is the electron charge, Tnom is the temperature at
which the saturation
current, /o/, is defined, ni is the diode ideality factor, and αiis the saturation current
temperature
exponent. Notice that this equation shows that the I-V
characteristics are related to a sum of band gaps.

As the
current flows from the base contact to the emitter, a linear voltage drop along
the base and under the diode isolation mesa causes an exponential change in
the diode current. This crowding of the current at the edge of the diode

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mesa,
depicted in Fig. 4.5.33, can be modelled adding a current dependence on the series resistance. This series
resistance, Rs, can be
expressed
as the sum of a current independent, Rext
and a current dependent series resistance Rint arising from current crowding:

 (4.5.3)

The value
of this resistance can be found by numerically solving the following transcendental equation:

 (4.5.4)

Within
this mathematical framework it is then possible to extract the value of the series resistance, remove its
effect from the measured /-V characteristics
to extract the actual ideality of the diode current. We normally observe an ideality factor of 3. Based on the
proposed model of at least two diodes in series, without any assumptions, it is only
possible to establish that the value of the band gap extracted from temperature
dependent l-V measurements corresponds to a sum of the band gaps (or activation
energies) of these diodes.

An
interesting behaviour observed in these devices was the apparent lack of correspondence
between the dark and light /-V characteristics.
The series resistance extracted from the diode dark l-V characteristic is too small to explain the limited short circuit current
as well as the low fill factor measured under a simulated 1-sun condition. A more complete circuit
model is necessary to explain this discrepancy. We have proposed a model where the observed behaviour
is due to two distinct areas in the fabricated devices. The photocurrent is produced only in a small
area of the device, this area being proportional to a fraction of the normalized diode area. In a Spice
circuit model this area is given a value smaller than 1, that we denote as
fraction. This
will be a fitting parameter to reproduce the measured dark and illuminated /-V characteristics. Figure 4.5.34 shows the
circuit proposed.

A
relatively large percentage of the device area is responsible for the
measured characteristics in the dark. The current in this area is caused by the
diffusion
of minority carriers caused by the applied voltage (V1 in Fig. 4.5.34) from the p or n side to
the
opposite region. This current is expected to be large due to the low lifetime of the minority carriers (mostly
recombination current in the depletion region). The observed series resistance will be proportional to
the total series resistance, Rtot, and inversely proportional to the area,
R2=Rtot/ (1-fraction), where the
diffusion current occurs. The dark l-V behaviour
is modelled by the top branch of the circuit depicted in Fig. 4.5.34. D_QD1 and D_Sch represent the series
connection of diodes whose band gaps are extracted using Eq. (4.5.2). The series resistance, R2, has the
temperature and current dependence detailed in Eqs. (4.5.3) and (4.5.4).

Only a
small part of the diode area may have a large enough lifetime to
produce a photocurrent. As this photocurrent flows only through this fractional area, the series
resistance is inversely proportional to this fraction: R1=Rtot/fraction.
Since the photocurrent, Iph, flows through R1, the illuminated /-V characteristics are limited by a larger
resistance than
that observed in the dark condition, as long as the fraction of the diode area is smaller than one half. The simulations
depicted in Fig. 4.5.35 show the reduction of lSC as the fraction, f, is varied from 99% to 1% of the total
diode area.

With
the circuit model described, it will be possible to extract complementary
information from dark and
illuminated l-V measurements.

In view
of these simulations, it is clear that the electrical characterisation of our
devices needs to take into consideration previously overlooked limitations. For
example, great care should be taken when interpreting the Quantum Efficiency extracted from a spectral
response measurement as the assumed condition of short circuit current may be incorrect even
if the device is externally short circuited (internally, the diode may be forward biased). Moreover, as the current is proportional to the photon flux,
and the flux is generally different at each wavelength tested, the internal bias of the device can be different at each point of the spectrum investigated.

4.5.2.4.2 Demonstrator program for Si nanostructure devices

Using
the approach described in Section 4.5.2.4.1, a device simulator has been developed
in an MS Excel spreadsheet. The Demonstrator is a 2D simulator of the diodes fabricated from Si QD
nanostructures. It is based on a simple diode model with a few fitting parameters and
uses the method in section 0 and in [4.5.9].

	
  

 	
  

 
	
 There
 are three areas which the user can modify:

 
	
  

 
	
 1.

 	
 Measured
 parameters: these come from measured properties of the fabricated diodes. The base resistivity
 and its temperature dependence
 are measured. The device geometry is known. The default values are
 representative ones for the devices.

 
	
  

 	
  

 
	
 2.

 	
 Jo,
 ideality factor, band gap and XTO can be varied to give flexibility in ftting to a specific
 diode properties.

 
	
  

 	
  

 
	
 3.

 	
 The
 variation parameters are Current and Temperature.

 
	
  

 	
  

 
	
 The
 outputs are:

 
	
  

 
	
 1.

 	
 The/-V curves for the range of temperatures
 chosen, in both log-lin and
 lin-lin output.

 
	
  

 	
  

 
	
 2.

 	
 The variation
 of the ideality factor with current for various temperatures.

 
	
  

 	
  

 
	
 3.

 	
 The
 series resistance with temperature.

 
	
  

 	
  

 
	
 4.

 	
 The
 extracted band gap, using either a calculation incorporating a series
 resistance correction or with both a series resistance and a current modification term.

 

Equivalent Spice circuit representation of the fabricated devices.

FIGURE 4.5.34

Spice simulations of 1-sun
I-V characteristics based on the circuit depicted in Fig. 4.5.34. The fraction,
f, represents the normalized area of the diode where the photocurrent is
produced.

FIGURE 4.5.35

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Dark I-V curves.

FIGURE 4.5.36

Local ideality factor.

FIGURE 4.5.37

Some
example outputs, using the default parameter settings, follow:

Figure
4.5.36 shows that a reasonable diode characteristic is shown, with good turn-on and rectification.

Figure
4.5.37 shows the increase in the ideality factor with current is as expected with a diode transiting from
domination by radiative recombination (n=1) to being dominated by Shockley Read Hall
recombination (n=2). But the large regime in which n=3 and then region where it goes to much higher
values, are indicative of a non-physical aspect to the model. The most likely explanation is
that rather than two diodes in series there are actually several, probably a variable number in
different parts of the device, the combination of which lead to a composite ideality factor.

The
demonstrator is useful as an iterative tool used to simulate the real
measured data from a device. The parameters used for such a fit then define the
values of the circuit elements in the EC model. Attachment of physical meaning
to these elements will then require further development and interface with the EMA/quantum
mechanical model.

4.5.2.5 Doping in Si QD nanostructures

Fabrication
of a PV device from the Si QD materials requires a control of work function such as to
allow separation
of photogenerated electron-hole pairs. Methods by which this can be achieved include fabrication of a grown or diffused p-n
junction or p-i-n junction with the Si QD
multilayers as the i-region. These
require careful control of the work functions of the p and n-regions. P-n
junction devices have been fabricated and doping demonstrated. Progress on the
theory explaining these doping
effects using conventional dopants and the possibility of other
modulation doping approaches has been made.

As
discussed in Section 4.5.2.4, rectifying p-n and p-i-n structures can be
fabricated with ‘conventional’ doping by incorporation of B and P during sputtering growth [4.5.9,
4.5.10]. Also, formation of p- and n-type materials is clearly demonstrated in the
Si QD nanostructure materials with Si nitride interlayers doped with either P
or B discussed in Section 4.5.2.3.2; in the Sb doped n-type Si QDs in Si3N4
in Section 4.5.2.3.1; in the p-type GeQD in SiO2 material in section 4.5.2.3.4; and
in MOS type devices doped with either P or B by a diffusion anneal discussed in [4.5.43].

However,
the doping mechanisms taking place in these structures are not well understood.
Theoretical
work has shown that direct doping of the QDs is prevented by segregation of
impurity atoms
from the perfectly crystalline QDs [4.5.44, 4.5.23]. Experimentally this is
supported by data on the free electron density in Si nanocrystals using Electron
Paramagnetic Resonance (EPR), which show that 95% of P atoms are segregated to the surface of the
nanocrystals and that their contribution to doping is at least an order of magnitude
lower than the atomic concentration [4.5.45]. Further evidence that doping causes electronic changes comes
from the quenching of luminescence on the incorporation of P (or Au) in Si nanostructures
[4.5.46] and also from a small enhancement of luminescence observed at low P doping levels followed
by quenching at higher levels [4.5.47]. This latter being explained by the passivation of QD surface
states by low levels of P increasing luminescence and then the saturation of this mechanism by excess
P. This also explains the non-monotonic behaviour of activation energy seen in P doped nanostructures [4.5.10].

If
direct doping of the QDs is not occurring, then another possibility is
modulation doping of the matrix SiO2, as is commonly used in
III-V nanostructures;
but this is ruled out because of the very high ionisation energies of about 5eV that would be required for the
resultant deep defects in a dielectric matrix [4.5.48]. These clearly will not occur at room
temperature, or even at higher temperatures.

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Two models for doping of Si
QD nanostructures. (a) An extended sub-oxide region surrounding the Si QDs
allows doping by P or B at relatively shallow levels. These then can provide
carriers to be captured by the QDs through a modulation doping mechanism. (b)
QD size enhancement (suppression) by P (B) doping results in a type I
heterojunction between P and B doped material. This facilitates carrier
separation due to the much greater mobility of electrons - the Dember effect
[4.5.49].

FIGURE 4.5.40

4.5.2.5.1 Doping mechanisms in Si QD nanostructures

Researchers:

Dawei
Di, Xiaoming Hao, Ivan Perez-Wurfl, Gavin Conibeer

As there
is clear evidence for doping by P and B, a third possibility is that it is the sub-oxide QD/
matrix interface region, with its shallower doping defect levels, that is
doped by the P or B atoms, thus providing free carriers to be captured by the QDs [4.5.49]. As shown in
Fig. 4.5.38, ab-initio calculation of the levels associated with B atoms associated directly with the
interface region on the surface of a Si QD indicate that there are no shallow or deep levels introduced by the
B within the effective band gap (HOMO-LUMO gap). The reason is that the strong affinity of B for
O results in very strong B-O bonds, effectively splitting any available OMOs
and UMOs a
long way apart and well inside the already existing density of states.

However,
this approach considers the interface between Si QD and SiO2 matrix to be very
abrupt. In
fact there is highly likely to be an extended transition region of a sub-oxide
around the QD. If this region is a sufficiently extended silicon sub-oxide (SiOX)
it could provide flat Bloch bands able to be doped in a pseudo bulk-like regime. In order that B and P doping
levels are shallow enough to be ionised in such a region, x would need to be less than
about 0.5. Such a region would be amorphous and hence not able to be modelled with ab-initio methods. But the presence
of such regions can be determined from absorption measurements.

There is
some support for this theory in the absorption data for Si QDs in SiO2
[4.5.5] shown in Fig. 4.5.39. This shows a region of strong absorption with an absorption edge
which blue shifts as the O to Si ratio increases, which translates to an increase in QD size. But
there is also an additional weakly absorbing tail which extends well into the effective band gap
region. This tail is likely to be due to an amorphous region around the Si QDs. An explanation for the band
gap of this being larger than bulk Si is if this region consists of sub-oxide
material. If this region were doped with B or P, it could provide free
carriers to be captured by the QDs in a modulation doping mechanism, thus giving rise
to the p- or n-type behaviour observed. (This is illustrated schematically in Fig. 4.5.40
(a).)

An
alternative explanation for the rectifying character of the junctions obtained
can be
based on the modification to the Si QD crystalisation discussed in Section 4.5.2.3.2, for
P and B doping of Si QDs in oxide with nitride interlayers [4.5.49]. The fact that P (B) doped material produces
larger (smaller) QDs means that its confined energy levels and hence effective band gap
will be smaller (larger) than undoped material under otherwise similar conditions. Thus a junction between a P and a B doped
region will actually
be a type-I heterojunction between small and large band gap materials, respectively, as
illustrated in Fig. 4.5.40 (b).

It is
also very likely that the mobility of electrons in these materials, whilst not
big, will still be much greater than that of holes. Hence photogenerated electrons near the
junction would experience a drift field sweeping them into the P doped material
with its lower conduction band edge, whilst holes with their very limited
mobility would be immobile. This would result in electrons accumulating in the P

Ab-initio calculation using
GAUSSIAN: (a) the density of states for a Si83/84OH62
oxide terminated QD with and without a B atom at the interface; (b) the optimised
Si QD, consisting of Si83B-OH62, with the B atom at top
centre (in pink).

FIGURE 4.5.38

Room temperature absorption
co-efficient of annealed SiOx/ SiO2 multilayer films with
various x (1100oC, 1hour). The inset is an estimate of the approximate optical
band gap (filled squares: strong absorption; empty circles: absorption tail)
[4.5.5].

FIGURE 4.5.39

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Spatial distribution of 4s
and 3d electronic states of Sc (upper) and energetic arrangement of 4s and 3d
states (lower).

FIGURE 4.5.41

Electronic density of states
(DOS) of Si10 QD in 2 ML SiO2 (dotted black), fully
oxidised nanocrystal (grey) and Si10 QD in 2 ML SiO2 with
Sc replacing Si atom in the SiO2 shell at the outermost position
(top). DOS of Sc-doped approximants split into a (spin up, red) and b (spin
down, blue) DOS because of non-even number of electrons within the approximant
due to acceptor state of Sc. The b-highest occupied molecular orbital (HOMO)
1193 indicated in magenta can take up 0.07 electrons from the Si10
QD over two 2 ML (6 A) of perfect SiO2.
The p-lowest unoccupied MO (LUMO) 1194 shown in green can take up a
much bigger proportion of an electron if the Si QD is slightly bigger (e.g. Si35,
dQD = 11A). The Sc-doped nanocrystals are shown with the
iso-density plots of 1.35X10-3 e/A3 for p-HOMO 1193
(lower left) and p-LUMO 1194 (lower right). The atoms of the Si10 QD
and the Sc atom are highlighted in cyan. Si atoms in SiO2 are grey,
O atoms are red, H-atoms terminating the outermost O-bonds are white. All DFT
computations carried out with a Hartree-Fock 3-21G(d) optimisation and
B3LYP/6-31G(d) electronic structure calculation [4.5.23].

FIGURE 4.5.42

material
and being collected in the external circuit as if from an n-type
material, with electrons injected back into the device through the B doped material where they would
recombine with the immobile holes, thus making the B material appear p-type. This qualitatively
mirrors the behaviour observed, but will require corroboration. As it relies on
the difference in electron and hole mobilities, it is essentially the same as
the Dember effect, which is known to produce a photovoltaic effect through carrier separation,
albeit not very efficiently.

It is
also possible that both the sub-oxide doping effect and the mismatched
QD size effects operate in the device. The fact that both mechanisms act in the same direction is
fortuitous as both could therefore contribute to the observed rectifying
photovoltaic behaviour. Differentiation between these or a determination of the relative strengths
will require further study on the effects of doping, interlayers and band
alignments. These will also further optimisation of the devices and lead to better photovoltaic
performance from these Si QD nanostructure
devices.

4.5.2.5.2 ab-initio modelling of modulation doping
possibilities in Si QDs nanostructures

Researchers:

Dirk
KÖnig, James Rudd, Daniel Hiller (IMTEK, University of Freiburg, Germany)

As
discussed in Section 4.5.2.5, direct doping of QDs by dopants is
thermodynamically very unlikely. The alternative of modulation doping of the SiO2
matrix is also not feasible due to the strong anionic nature of O leading to
very deep ionisation energies [4.5.48]. We have concentrated recently on possible candidates for more
efficienct acceptor doping of Si QDs. Modulation acceptors must have an energetic position below
the confined hole level, they must not introduce defect levels in the QD band gap if located at
the interface as an active dopant and as a completely saturated foreign atom,
and they must have one single oxidation number. The transition metal scandium
(Sc) fulfils the
last two requirements. It also has a very suitable valence state
configuration which consists of a full (doubly occupied) 4s shell and one
electron in
the 3d1 state located within the
4s shell. On the energy scale, the 4s electrons have a higher binding energy as compared to the
3d1 and 3d2 states. This provides the 3d1 and 3d2
states with an electrostatic screening which results in a higher binding energy of these states. Thereby
an unoccupied 3d2 state exists at a high binding energy which is able to take up an electron from a
nearby source such as a Si QD, see
Fig. 4.5.41.

Ab-initio
Density Functional Theory calculations on a Si10 QD in 2 mono layers (MLs) of
SiO2 have been carried out, with a Si atom at the outermost position with full O
termination replaced by a Sc atom. The Si10 atom cluster is the smallest
size which still
behaves as a Si QD [4.5.23]. Such a nanoparticle allows us to directly observe the electronic
structure of Sc as a modulation acceptor in SiO2 in the proximity of a Si QD, see
Fig. 4.5.42.

Figure
4.5.42 shows that 0.07 electrons stemming from the Si10 QD (dQD = 7.3A) are localized at the Sc atom over
two 2 ML (6 A) of perfect SiO2. Technological relevant QD sizes start at dQD
≈ 20A, with
a much lesser degree of quantum confinement as compared to a 7.3A QD. It is thus reasonable to
assume that
a Si QD with dQD ≥ 20A will be
positively ionized
to a much greater extent, ensuring that holes are majority carriers in a Si QD super lattice
(SL)
embedded in SiO2. Experimental
verification of this doping mechanism is being carried out. The difference to
conventional acceptor modulation doping as used in III-V electronic devices is the way the Si or Ge QDs are
ionised by Sc in SiO2. As ‘Sc induces a deep acceptor level in SiO2 it cannot be thermally ionized from the SiO2 valence band. Instead, the Sc acceptor state takes up the
electron directly
from the initially occupied state at a lower binding energy presented by a nearby QD, see Fig. 4.5.43. This requires the
Sc acceptor to be within 50A of the Si QD in order to exploit field emission

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Acceptor modulation doping:
Conventional example with GaS QD in AlAs matrix and electron thermally emitted
into acceptor (Mg), such that holes are captured by the QD (left). In SiO2,
charge transfer occurs directly as the Sc state is too deep within the band gap
to be thermally ionized. Instead, the acceptor state is ionised directly from
the QD by feld emission of an electron into the Sc acceptor state.

FIGURE 4.5.43

which
is a consequence of the energy difference between the QD HOMO and the Sc acceptor state.

Acceptor
modulation doping of SiO2 can be accomplished by
incorporation of a small amount of Sc, say one monolayer of Sc, into the SiO2
barriers between
the SRO QD array layers. During the segregation anneal, Sc is incorporated into the SiO2 matrix
and thereby activated as an acceptor. Ab-initio calculation indicates that Sc attached to the Si QD does not
create any levels within the HOMO-LUMO gap of a Si84(OH)64
QD. We can therefore
assume that a Sc atoms bonded onto a Si QD does not result in a detrimental change in its electronic structure.
This concept is now being investigated
experimentally.

4.5.2.6 Summary of group iv nanostructures for tandem cell
elements

In 2010,
significant progress has been made on understanding the transfer of growth parameters of Si QDs to other
matrices. The use of anisotropic structures both to provide diffusion barriers to excessive QD growth
and to provide higher conductivity paths in the transport direction, has shown the validity of
these approaches both for Si3N4 and for SiC matrices.
Knowledge of Ge QD nanostructures has improved significantly with clear p-type behaviour of
Ge QD s in SiO2 and very successful growth of Ge QWs in Si3N4
matrix. The increase
in sophistication of modelling of confined energy in these structures now matches reasonably well with experimental data.

Improved
modelling of confined levels in irregular shaped particles also matches well with experimental data of
other groups. Higher level device modelling is now able to describe the anomalous effects
due to high lateral series resistance in devices and consequent current crowding. This is now
being used to optimise nanostructure growth and device design, so as to push beyond the current
open circuit voltages of 490mV. Part of this is an understanding of doping. This
has progressed with greater knowledge and control over the effects on QSD size of
introduction of
P and B dopants, and modelling of possible doping mechanisms based on modified
modulation doping and these size effects. Alternative modulation dopants are
being modelled with scandium as an acceptor looking like a strong candidate. Further work
will build on this increased understanding of anisoptropic structures, growth
of different
materials and understanding of doping to improve materials and devices further.

4.5.3 hot carrier cells

Researchers:

Shujuan
Huang, Santosh Shreshtha, Dirk KÖnig, Robert Patterson, Pasquale Aliberti, Binesh Puthen Veettil,
Lara Treiber, Ivan Perez-Wurfl, Andy Hsieh, Yu Feng, James Rudd, Stephan Michard, Martin Green, Gavin Conibeer

Hot
carrier solar cells offer the possibility of very high efficiencies
(limiting efficiency above 65% for unconcentrated illumination) but with a structure that
could be conceptually simple compared to other very high efficiency PV devices - such as multi-junction
monolithic tandem cells. For this reason, the approach lends itself to ‘thin film’ deposition techniques,
with their attendant low costs in materials and energy usage and facility to use abundant, non-toxic elements.

An ideal
Hot Carrier cell would absorb a wide range of photon energies and
extract a large fraction of the energy to give very high efficiencies by
extracting ‘hot’ carriers before they thermalise to the band edges. Hence an
important property of a hot carrier cell is to slow the rate of carrier cooling
to allow hot carriers to be collected whilst they are still at elevated
energies (“hot”), and thus allowing higher voltages to be achieved from the cell and hence higher efficiency.
A Hot Carrier cell must also only allow extraction of carriers from the device through
contacts which accept only a very narrow range of energies (energy selective contacts or

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Band diagram of the Hot Carrier cell. The device has four stringent
requirements:

FIGURE 4.5.44

(a) Schematic representation of energy and
particle fluxes interactions used in the model (Particle fluxes - full line
arrow, energy fluxes- dotted line arrow). (b)
HCSC efficiency as a function of carrier extraction energy level.
Parameters used are: thermalisation time = 100 ps, concentration = 1000,
lattice temperature = 300K and absorber layer thickness = 50 nm. [J, F and
E are current density and
particle and energy fluxes as denoted by subscripts for A absorption, E emission and IA Auger
processes.]

FIGURE 4.5.45

Carriers transmission probability versus energy for (a) ideal ESC, (b)
non-ideal ESC.

FIGURE 4.5.46

ESCs).
This is necessary in order to prevent cold carriers in the contact
from cooling the hot carriers, i.e. the increase in entropy on carrier
extraction is minimized
[4.5.50]. The limiting efficiency for the hot carrier cell is over 65% at 1 sun and 85% at maximum concentration -
very close to the limits for an infinite number of energy levels [4.5.1, 4.5.51, 4.5.52]. Fig. 4.5.44 is a
schematic band diagram of a Hot Carrier cell illustrating these two requirements.

	
  

 	
  

 
	
 a)

 	
 To
 absorb a wide range of photon energies;

 
	
  

 	
  

 
	
 b)

 	
 To
 slow the rate of photogenerated carrier cooling in the absorber;

 
	
  

 	
  

 
	
 c)

 	
 To
 extract these ‘hot carriers’ over a narrow range of energies, such that excess
 carrier energy is not lost
 to the cold contacts;

 
	
  

 	
  

 
	
 d)

 	
 To
 allow efficient renormalisation of carrier energy via carrier-carrier
 scattering.

 

In 2010
modelling of Hot Carrier efficiencies has progressed with implementation of real material
properties to give more realistic efficiencies for InN which include Auger processes and more
realistic contact structures. Significant progress has been made on demonstrating
resonance in double barrier selective energy structures. Further work on triple
barrier double Si QW structures has been carried out for rectifying ESCs. This is complemented by improvements in 2/3D
modelling of transport in these ESC structures. For absorbers, modelling of
nanocrystals superlattice arrays has bee applied to real material systems. The growth of such
systems in both III-V QD superlattices with collaborators and with colloidal
Langmuir-Blodgett dispersion of Si nanocrsystals has produced structures which are now being characterised
for their modulation
of phononic properties. Also meausurement of carrier cooling rates has been
extended to other large phononic gap bulk materials including InN, demonstrating the
importance of material
quality. Design of structures for hot carrier cells which should be practical and realisable
has developed, with the device properties more carefully specified and plans for fabricating such
structures in real devices.

4.5.3.1 modelling of hot carrier Solar cell Efficiency

Researchers:

Pasquale Aliberti, Yu Feng,
Santosh Shrestha, Gavin Conibeer, Martin Green

Collaboration with:

Yasuhiko Takeda (Toyota
Central Research Laboratories, Nagoya)

Previous
work was focused on developing a new model to calculate limiting efficiency of
a real HCSC based on an Indium Nitride (InN) absorber layer. InN has been chosen as a
potential material because of its narrow electronic band gap for absorption of a wide range of photon
energies, whilst also having a wide phonon band gap. This is good for
suppression of
phonon decay, and hence for slowing carrier cooling [4.5.53, 4.5.54].

Calculation
of limiting efficiency was performed taking into account real optical and electronic properties of InN,
removing most of the ideality assumptions used in other models [4.5.51, 4.5.52, 4.5.55, 4.5.56]. The
detailed band structure

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(a) HCSC efficiency as a
function of extraction width of ESCs for different extraction energies ΔE.
(b) HCSC efficiency as a function of extraction energy ΔE for different
ESCs energy width. Thermalisation time is 100 ps, lattice temperature is 300 K.
Absorber layer thickness is 50 nm.

FIGURE 4.5.47

of
wurtzite bulk InN has been considered in performing computation of carrier densities, pseudo-Fermi potentials
and II-AR time constants [4.5.57]. Results have been calculated considering ideal energy selective
contacts for the HCSC, which means that contacts have a very high conductivity and a discrete energy
transmission level.

Recently
the limiting efficiency for the hot carrier InN solar cell has been
calculated considering non-ideal ESCs. In this case the carriers are not extracted on a single
energy level, but in a finite energy window. Calculations have been performed taking into account
contact resistance and entropy generation
effects.

The
flux of current travelling through the ESCs towards the cold metal
electrodes can be described using
the following relation.

 (4.5.5)

The
current density in this case is proportional to the occupation
probability at the two sides of the ESC. Equation (4.5.5) has been derived assuming no
correlation
of energy of electrons in three different directions as shown in (4.5.6). This assumption is
acceptable if there is a parabolic dispersion relation at minimum energy point
along the three different directions.

 (4.5.6)

Based
on the energy and carrier conservation, Δμ and TC at steady state are
calculated.

 (4.5.7)

[quantities
as defined in Fig. 4.5.45 (a).]

The
maximum efficiency has been found for a ΔE between 1.15 eV and 1.2
eV with a transmission energy window δE of 0.02 eV. The value of limiting efficiency is
39.6% compared to 43.6% calculated in the previous section using ideal ESCs. The drop in efficiency is mostly
due to the decrease of open circuit voltage related to the decreased extraction level, equation (4.5.7).
This is partially compensated by an increase in extracted current due to increased II rate.

Figure
4.5.47 (a) shows calculated efficiency as a function of for several
values of extraction energy. In all the curves two different trends can be identified. If the value of δE
is too close to zero, the efficiency is very low due to low carrier extraction,
thus a very small value of short circuit current. The conductivity of the contact in this
case is indefinitely large. Enlarging δE, the number of carriers available for extraction increases,
improving JSC, and so the maximum efficiency. In general the efficiency peak
has been
found for values of δE from 0.02 eV to 0.1 eV depending on the extraction energy ΔE. For
the configurations which show higher efficiencies, ΔE < 1.35 eV, the optimum
value of δE goes from 0.02 eV to 0.05 eV. This optimum value for δE of between
is very
close to kTRT. This represents the
variation in energy in the contacts such that approximately the kTRT. will inevitably be lost anyway by carriers
thermalising within the contacts. Thus it sets a lower limit on a reasonable δE. Therefore this result indicates that the transmission energy range has
to be very small and confirms once again the high selectivity requirements of ESCs for HCSC [4.5.58].

In Fig.
4.5.47 (b) the value of maximum efficiency as a function of ΔE is
reported for different values of δE. It can be observed that for small transmission energy window the
extraction energy which allows maximum efficiency is lower compared to the one calculated using ideal
ESCs. This effect is related to the higher occupancy at lower energies, which increases the value of JSCfor contacts with a small transmission window.

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Peak at 1.205eV of the
conductance vs. energy plot for different values of σ for configurational
disorder (left). Conductance vs. energy plot with peak at 1.573 eV for
different values of σ for morphological disorder (right). Conductance
given in units of 2e2/h.

FIGURE 4.5.48

Planar view of the double
barrier structure formed by Si QDs in SiO2 matrix, with SiC barriers
of 2nm width (left). Mean diameter of QDs is 1.8nm. I-V characteristics for the
double barrier structure with SiO2 and SiC barriers at temperature
10K and 300K (right).

FIGURE 4.5.49

4.5.3.2 Energy Selective contacts

The
requirement for a narrow range of contact energies can be met by an energy selective contact
(ESC) based on double barrier resonant tunnelling. Tunnelling to the confined energy levels
in a quantum
dot layer embedded between two dielectric barrier layers, can give a conductance
sharply peaked at the line up of the Fermi level on the ‘hot’ absorber side
of the contact with the QD confined energy level. Conductance both below this energy and above it
should be very significantly lower. This is the basis of the current work on double
barrier resonant tunnelling ESCs.

4.5.3.2.1 modelling of QD structures

Researchers

Binesh
Puthen-Veettil, Dirk KÖnig, Gavin
Conibeer

We
developed a robust 2 dimensional model for describing the transport properties through quantum dot structures
and have used this model to understand the filtering characteristics of Energy Selective Contacts
(ESCs). In this way we are able to compute the effective filtering in 2 dimensions by running numerical
simulations. The model is developed from a discretized SchrÖdinger equation by
considering the sample volume as a collection of discrete points and using an
effective mass approximation method over the entire volume.

During
fabrication of the quantum dots in a dielectric matrix for selective energy contacts,
different kinds of irregularities can be present in the structure, the major
disorders being configurational (disorders in the position of the dots) and morphological
(disorders in the size of the dots). The extent to which configurational and
morphological disorders determine the electrical properties of the overall
structure is investigated using simulation runs of the model. The disorders are assumed to follow a normal
distribution from the mean position and size. The results show the outcome of an average
of 1000 simulation runs with different standard deviation (σ) values.

Figure
4.5.48 shows the simulation results for resonance in 2.6nm Si dot in SiO2 matrix under different orders of
configurational disorders. As the disorders increases from σ =0 to σ =1 the
conductance decreases by 53%, but the resonant energy remains the same at 1.205eV. This shows the confined energy in
the QDs does not change as their size is fixed but the effective filtering reduces dramatically.
Figure 4.5.49 shows the simulation results for a 2.2 nm Si dot in a SiO2 matrix under different
orders of morphological disorders. As the disorder increases from σ =0 to
σ = 1, the conductance decreases by 60% and the resonant peak remains
the same at 1.573eV. But the morphological disorders cause major impact compared to
configurational disorders because of the widening of the energy selection
window. This is due to the distribution in size of the QDs, since QDs with
different sizes have different resonant energies which are slightly different
from the mean resonant
energy, the average of them all increase the spread of the resonant peak thus reducing the
efficiency of the double barrier structure as energy selective contacts.

A planar
representation of the double barrier structure is shown as in Fig. 4.5.49 (left). The
barriers are
usually high band gap dielectric materials like SiO2, Si3N4
or SiC. SiC barriers have advantages over SiO2 barriers that SiC
barriers in double barrier structure makes a very good diffusion barrier for
silicon during processing, which can yield a

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Principle of oa-IV, shown for
electron injection into the sample (left). If free carriers obtain additional
energy by a massive photon flux significantly above the band gap, a smaller
bias field is required to reach a resonant condition. Sample stage of oa-IV
(right); wavelength range indicated. Every 50 nm spectral range has a photon
flux of approximately 50 Suns.

FIGURE 4.5.50

structure
with uniform QD size in the transport direction thus ensuring sharp resonances at the
resonant energies of the structure. Since the conductance of the structure is more in the case of
SiC
barriers because of the lower barrier height of SiC, thicker barriers can be used to give the same conductance as SiO2,
for which the very thin barriers are difficult to fabricate.

Figure
4.5.49 (right) shows the I-V characteristics for the double barrier QD
structure with SiO2 dielectric and SiC barriers at temperature 10K and 300K. As can be seen, the
structures with SiC potential barriers have higher current density at both low
temperature and at room temperature. As the temperature increases, the current density as well
as the
width of the resonant peak increases. Also, the negative differential
resistances appear at slightly lower voltage in structures with SiC barrier
than that in structures with SiO2 barrier. This is due to the increased leakage and
thus the lower confinement energy
for the SiC.

4.5.3.2.2 characterization of Energy Selective contacts using
‘optically assisted I-v’

Researchers:

Dirk
KÖnig, Stephan Michard (RWTH Aachen, Germany), Binesh Puthen-Veettil

Collaboration with:

Daniel
Hiller (IMTEK, University of Freiburg, Germany)

Optically
assisted IV (oa-IV) is a new characterisation method which
investigates an energy selective contact (ESC) fabricated on a Hot Carrier Absorber material with optical
excitations in a pre-defined wave length range. If a very large optical generation rate Gopt
exists adjacent to the ESC, it can probe this HC population immediately next to the contact.
Alternatively, if the HC population within the absorber is known, the ESC can be tested for its
energy selectivity. The basic principle is to provide free carriers with
additional energy by optical means, see Fig. 4.5.50. The resonant level of the ESC can then be reached by carriers at
a lower bias feld,
shifting the tunnelling resonance of the ESC - here a QD array - to lower bias voltages Vblas.

Samples
based on Si were provided by the University of Freiburg, see Fig. 4.5.51. Si is very
far from being an ideal Hot Carrier Absorber due to the complete lack of a gap between optical and acoustic
phonon modes and
hence no propensity to slow carrier cooling. Furthermore, Si has an indirect band gap
in the spectral
range of optical excitation which renders the interpretation of measured data rather
complex due to the absorption coefficient asi changing over two orders
of magnitude. However, for initial proof of concept it is adequate providing the excitation of hot
electron-hole pairs in the material is sufficiently large and collection can be made from very close to the ESC.

A Xenon
(Xe) arc lamp is a continuous light source with one of the most
constant photon fluxes Fhn in the spectral range 1000 to 500 nm. Nevertheless, there are significant
deviations in particular in the long wavelength region (> 800 nm). We therefore
compare oa-IV curves which have the same Vbias for the current
transition from forward to reverse direction, Vbias (j ® 0), indicating a
constant carrier product (constant quasi-Fermi level positions) at the ESC. Under these
conditions, the current density as a function of quasi-Fermi level position can be
assumed to be identical for these curves and this current density can be
treated as a constant offset for
each Vbias value.

For
liquid nitrogen (LN2, T = 77 K) measurements, resonant tunnelling
behaviour (negative differential resistance - NDR) was detected for all spectral ranges. With the method
mentioned above, only the oa-IV curves from 800 - 750 nm and 650 -

Sample structure for oa-IV
(upper). Contact area on QD array is 1 mm2, with ≈ 1010
QDs. Optical generation rate Gopt within ballistic transport range to the ESC and its
relative change
for photon wave length intervals used in oa-IV (middle). For these, the absorption coefficient of
c-Si aSi and the
photon flux of the Xe arc lamp Fhn
were used (lower). Full green rectangles show range of measurements which can
be compared. Dashed green rectangles
show measurements which cannot be
compared, but approach the high
optical injection limit. Please note
breaks on left ordinate in both
graphs.

FIGURE 4.5.51

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Schematic representation of time resolved photoluminescence arrangement.

FIGURE 4.5.53

oa-IV results of a typical
sample with optimum barrier thicknesses and QD size at LN2
temperature (upper) and at room temperature (lower). Curves in the wave length
intervals from 800 to 600 nm can be compared due to their identical offset of Vbias
to absolute zero Vbias., showing the free carrier product to be
constant. Curves for 550 - 250 nm and 1000 to 500 nm show maximum optical
injection limit. Both curve groups printed in bold.

FIGURE 4.5.52

600 nm
with Fhn ≈ 50 Suns for each interval could be
directly compared
to each other. In addition, we evaluated the curves with optical excitations in the range of 1000 - 500 nm (Fhn. ≈ 600 Suns) and 550 - 250 nm (Fhn ≈ 140 Suns). The former is near the practical high
concentration limit for solar cells of 1000 Suns.

As shown
in Fig. 4.5.52, for the curves in the 800 to 600 nm region at LN2
temperature, the potential difference between Vbias,. (j ®0) and the average voltage of the resonance (VRes,avg
) shows a decrease
of 48 mV with increasing photon energy (decreasing λ range). As carrier densities (quasi-Fermi
level positions) are identical for these curves, this potential drop corresponds to an increase in free hole
energy of 48
meV. As this energy difference is equal to the effective kT, it can be interpreted as an increase in carrier temperature
of DT = 560 K when going from 800 - 750 nm (1.55 -
1.65 eV) to 650 - 600 nm (1.91 - 2.06 eV). Even at Fhn. ≈ 50 Suns, this
effect is
small. This is due to Si being a very unsuitable absorber material, further corroborated by the
ratio of peak to valley current densities (quality factor of NDR - QFNDR)
dropping from 2.2 to 1.5. Si does not have a phononic band gap so that optical phonons undergo a rapid decay
which tremendously accelerates
carrier cooling.

For the
1000 - 500 nm and 550 - 250 nm ranges, the QFNDR increases significantly to
4.1 and 3.2, respectively.
The potential difference Vbias (j ® 0) -VRes.avg is
shifted by 365 and 200 mV, respectively; a quantitative comparison of both curves to each
other or to the curves in the spectral range from 800 - 600 nm is thus not
feasible. However, it shows that the energy selectivity may increase under high concentration Fhn.

For all
oa-IV curves, a tunnelling feature for electron extraction can be seen at 760
mV forward bias. However, the background current density is much higher than the tunnelling portion of the
current density in this forward bias range,
so no NDR can be seen.

At room
temperature (T = 300 K), NDR was detected only for λ ranges ≤ 600 nm, while the
impact of resonant electron injection is still visible for curves in the 800 - 600 nm
range. The background current density in the bias range around the tunnelling event increases by about
an order of magnitude when going from LN2 to room temperature. This
is due to thermal activation of trap states and increased thermal scattering during carrier transport through the QD
array. It has the same effect on NDR suppression as explained above for electron injection under
forward bias.

The QD
arrays were processed by segregation anneals, comprising some 1010 QDs as
ESCs under
the contact pad. It is very encouraging to see resonant tunnelling transport
even at room temperature
in a material system which was not grown epitaxially. At LN2 temperature (77 K),
an increase in carrier temperature of DT = 560 K was detected when increasing
the photon energy from 1.6 ± 0.05 eV to 1.985 ± 0.075 eV under a constant photon flux of Fhn ≈ 50 Suns. More suitable Hot Carrier
Absorber materials will be investigated.

4.5.3.3 hot carrier absorbers: slowing of carrier cooling

Researchers:

Robert
Patterson, Gavin Conibeer, Santosh Shreshtha, Dirk KÖnig, Pasquale Aliberti, Shujuan Huang, Yukio
Kamikawa, Lara Treiber, Martin
Green 

Carrier
cooling in a semiconductor proceeds predominantly by carriers scattering their energy
with optical phonons. This builds up a non-equilibrium ‘hot’ population of optical phonons which, if it remains hot, will drive a reverse reaction

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to
re-heat the carrier population, thus slowing further carrier cooling. Therefore
the critical factor is the mechanism by which these optical phonons decay into
acoustic phonons, or heat in the lattice. The principal mechanism by which this
can occur
is the Klemens mechanism, in which the optical phonon decays into two acoustic phonons of half its energy and of
equal and opposite momenta
[4.5.59].

The
build up of emitted optical phonons is strongly peaked at zone centre both for compound semiconductor due to the
FrÖlich interaction and for elemental semiconducotrs due to the deformation potential
interaction. The strong coupling of the FrÖlich interaction also means that high energy optical
phonons are also constrained to near zone centre even if parabolicty of the
bands is no
longer valid [4.5.60]. This zone centre optical phonon population determines that the dominant optical phonon decay mechanism is this pure Klemens decay.

4.5.3.3.1 Suppression of phonon decay
in bulk materials

In some
bulk semiconductors, with a large difference in their anion and cation masses,
there can be a large gap between the highest acoustic phonon energy and the
lowest optical phonon energy, possibly large enough to block operation of this Klemens
mechanism, which can be termed a ‘phononic band gap’. Work previously presented [4.5.61] using a simple
1D force constant model and complemented by high accuracy DFT computation [4.5.62]
indicated that GaN, InN and InP all have large phononic band gaps, which are
close to those found experimentally [4.5.63]. We are using time resolved
photoluminescence (tr-PL) to investigate the carrier cooling rates in these
materials.

4.5.3.3.2 time
resolved photoluminescence measurements of bulk phononic band gap materials

Researchers

Pasquale
Aliberti, Dirk KÖnig, Santosh Shrestha, Gavin Conibeer, Martin Green

Collaboration with:

Raphael
Clady, Murad Tayebjee, Tim Schmidt (University of Sydney), Nicholas Ekins-Daukes (Imperial College)

The
potential efficiency boost, which can be achieved by Hot Carrier solar cells, is directly related to the
possibility of extracting high energy carriers from the absorber layer before thermalisation, increasing
the voltage and hence the conversion efficiency. The poor conversion efficiency of photons
with energies above the band gap of the absorber is the main loss mechanism in conventional single
junction solar cells. The investigation of thermalisation time constants of hot carriers is a crucial
step towards the engineering of Hot Carrier cells. The efficiency of an InN
based hot carrier solar cell has been calculated using a complex theoretical
model, see Section 4.5.3.1. I was found that the limiting efficiency is strongly
related to hot carriers relaxation velocity in the absorber [4.5.57].

A
comparison of femtosecond time resolved photoluminescence (tr-PL) spectroscopy
between InP and GaAs was reported in last year’s annual report and is also now
published [4.5.64]. This showed a distinctly longer carrier cooling time constant for the wide
phononic gap InP as compared to almost zero phononic gap of GaAs. It also showed further
evidence for excitation into higher side valleys for both GaAs and InP for appropriate excitation
wavelengths. In 2010 the hot carrier cooling in InN has been investigated using
tr-PL. The wide gap between optical and acoustic branches in the InN phononic
dispersion relation
(wider than that for InP) prevents the Klemens decay of optical phonon into acoustic phonons. This can lead to
slower carrier cooling due to “Hot Phonon Effect” [4.5.65]. The decay of hot
carriers for different excitation wavelengths InN has been investigated.

Tr-PL
experiments have been performed on InN samples using the measurement configuration shown in
Fig. 4.5.53. In this technique a laser pulse acts as a switching gate relating the
photoluminescence signal to the time domain. The PL signal is collected from the sample, after a femtosecond
laser excitation, and focused in a non linear crystal. The gate signal is generated
from the same laser and is focused on the same crystal after passing through an optical delay
stage. The signal is detected using a monochromator and a PMT Our system
configuration provides 150 fs pulses with tunable wavelength over a range of 256 nm
(4.84 eV) to 2.6 μm (0.48 eV).

Figure
4.5.54 is a three dimensional representation of PL as a function of
time for all the probed wavelengths. It can be observed that the PL sharply rises when the carriers
are photo-excited by the laser pulse. The fast decay of the PL shows the thermalisation of
carriers towards respective band edges. The decay is faster for highly energetic carriers
compared to carriers closer to the bandgap. Thus the carrier population quickly degenerates
towards the band edges during the thermalisation process. In InN the thermalisation is most probably
due to
interaction of highly energetic electrons and holes with LO phonons [4.5.66].

To
investigate the rate of the carrier cooling process, the effective temperature
of the carrier population has been calculated fitting the high energy tail of the PL
spectrum for every single time during the cooling transient. The PL has been ftted assuming that
carriers form a Boltzmann-like distribution in a femtosecond time scale using the following equation.

L(ε) represents
the measured PL intensity at energy ε, α(ε) is the measured sample
absorption coefficient,
EGis the InN energy
gap, 0.7 eV in this

3D representation of InN time resolved PL data.

FIGURE 4.5.54

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Some images of the structure
as grown and modelled. (Left) the near-perfect stacking of the structure is
shown. (Centre) in-plane InAs QD arrangement. (Right) simple hexagonal
superlattice structure showing stacking. The QDs are similar to flattened disks
with in plane dimensions of 4050 nm and heights of 7 nm.

FIGURE 4.5.57

Carrier relaxation curves for
InN. The photoexcited carrier density is
1.5 x 1019 cm3. The blue dashed curve is a single
exponential fit.

FIGURE 4.5.55

Schematic of the simple
hexagonal superlattice of InAs QD disks, showing their relative dimensions. The
red square indicates orthorhombic repeat unit for the simple hexagonal system.

FIGURE 4.5.56

case,
and kB is the Boltzmann constant. TC is the
fitted parameter and represents the hot carrier temperature. Figure 4.5.55 shows the carrier
temperature transient, which seems to follow an exponential behaviour quite well (blue line - ft).

The
fitting of the calculated temperature data has been performed using a single exponential.

Here
τTH represents the carriers thermalisation time constant,
whereas C and K are two constant parameters. The fitted value for τTH
is 7 ps. This long
cooling constant can be attributed to the hot phonon effect due to the long lifetime of the
A1(LO) phonon
[4.5.64,4.5.66]. This hot carrier relaxation velocity is still too high to achieve a
considerable efficiency
gain in an InN hot carrier solar cell. However, it has been demonstrated that,
for InN, the
carrier cooling velocity is strictly related to the quality of the material
and slower carrier cooling constants compared to the ones calculated in this
chapter have been reported in the literature [4.5.67].

4.5.3.3.3 Nanostructures for absorbers

Nanostructures
offer the possibility of modification of the phonon dispersion of a composite
material. III-V compounds or indeed most of the cubic and hexagonal compounds can
be considered as very fne nanostructures consisting of ‘quantum dots’ of only one atom (say I
n) in a matrix (say N) with only one atom separating each ‘QD’ and arranged in two interpenetrating
fcc lattices. Modelling of the 1D phonon dispersion in this way gives a close agreement with the phonon
dispersion for zinc-blende InN extracted from real measured data for wurtzite material.

Similar
‘phonon band gaps’ should appear in good quality nanostructure superlattices, through coherent Bragg reflection
of modes such that gaps in the superlattice dispersion open up [4.5.61]. There is a close analogy with photonic structures in which modulation of
the refractive index in a
periodic system
opens up gaps of disallowed photon energies. Here
modulation of the ease with which phonons are transmitted (the acoustic impedance) opens up gaps of disallowed phonon energies.

4.5.3.3.3.1 force constant modelling of iii-v QD materials by
Sk growth

Researchers:

R.
Patterson, Y. Kamikawa, G. Conibeer

Collaboration with:

Yoshitaka
Okada - University of Tokyo

3D
force constant modelling, using the reasonable assumption of simple
harmonic motion of atoms in a matrix around their rest or lowest energy position, reveals such
phononic gaps [4.5.68]. The model calculates longitudinal and transverse modes and can be used to
calculate dispersions in a variety of symmetry directions and for different combinations of QD
sub-lattice structure and super-lattice
structure.

III-V
Stranski-Krastinov grown QD arrays of InAs in InGaAs and InGaAlAs
matrices are fabricated at the University of Tokyo using MBE. We are
investigating these
for evidence of phonon dispersion modulation and potential slowed carrier cooling. In order to understand
the expected phonon dispersions these are being modeeled using the 3D force constant technique.

Lattice
matched and strain compensated material pairs that may produce large phonon bandgaps are of
interest. Previous iterations in the design of these structures indicated the
importance of separating “light” and “heavy” atoms to different parts of the nanostructure. Initially,
the lightest atom in the system, As, was present in both the QD and the matrix. This meant that
the reduced mass of both regions (proportional to the sum of the inverses

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Dispersion curves for InAs
QDs in a simple hexagonal SL. The matrix is
In0.5Ga0.5xAlxAs with (x = 0.4). There
is a large phonon bandgap present in the system due to the presence of Al. The
gap seems very tolerant to Al location. (Far left): <100>, (Centre):
<110>, (Left): <111> directions.

FIGURE 4.5.58

Densities of states (DOS) for
the dispersion curve immediately to the left. Note small changes in the DOS
could be due to pseudo-random locations of the Ga and Al particles or to
differences in crystalline direction. (Far left): <100>, (Centre):
<110>, (Left): <111> directions respectively.

FIGURE 4.5.59

of each
atomic mass) was very similar as the light element dominates in this case.

On this
iteration structures with an In0.5 Ga0.5-x Alx As matrix (with x=0.4) and InAs QDs were grown. Significant Al content
was introduced into these structures with the expectation that this light element, segregated to
the matrix material, might produce appreciable phonon bandgaps. A schematic of the
structure is shown in Fig. 4.5.56. Some images derived from characterisation of the structure are
presented in Fig. 4.5.57. The superlattice of QDs has a simple hexagonal structure. Extraordinary
periodic out-of-plane stacking is achievable and largely defect free structures can be grown
on the order of microns.

Force
constant modelling of this structure predicts an appreciable phonon
bandgap, as shown in Fig. 4.5.58 and Fig. 4.5.59. This bandgap is due almost entirely to the
presence of the Al in the system. A small bandgap is present due to the mass
difference between In, Ga and As, but it is less than a quarter of the size
shown in Fig. 4.5.58 and Fig. 4.5.59. Due to computational constraints the size of the QDs is
quite small, only about a nanometre in diameter. While actual sizes for these structures are too
computationally intensive to model without extreme effort, recent modelling with gradually
increasing size suggests that the dispersion relations scale linearly with
size. That is,
once the discrete distances (bond lengths) are small relative to the superlattice unit cell
dimension, the dispersion should look exactly the same when scaled such that
the relative dimensions are preserved. This will be investigated in greater
detail in further work.

4.5.3.3.3.2 fabrication and characterisation of highly
ordered nanoparticle arrays for hot carrier absorber

Researchers:

Lara
Treiber, Shujuan Huang, Gavin
Conibeer

As
demonstrated by the work on modelling
phonon modulation, periodic core-shell QD
arrays offer a way to significantly
change the phonon modulation in a
superlattice because the core and
shell can be of materials of very
different force constant, directly leading to a strong phonon confinement. Deposition methods are being investigated to fabricate such highly ordered QD arrays.

The
Langmuir-Blodgett (LB) technique fabricates thin films from
colloidal dispersions of quantum dots (QDs). The LB technique allows for controllable, uniform film formation
and subsequent transfer onto a solid substrate, such as quartz [4.5.69]. Through
surface passivation of the QDs, interparticle spacing can also be controlled. Figure 4.5.60 illustrates the
set-up of the LB apparatus and the
functionalized QDs.

In 2010
work has focused on fabrication of periodic thin films of Si
nanoparticles (NPs) provided by the University of Minnesota. The Si NPs under
investigation are surface passivated with dodecane (C12H26),
a long chain hydrocarbon rendering the surface hydrophobic. The particles were dispersed
in chloroform and investigated using TEM, UV-vis spectroscopy and
photoluminescence (PL). Using TEM imaging the average diameter was found to be
3.0 ±0.5 nm. Fig. 4.5.61 shows a droplet of solution (dispersed in chloroform)
and the inset highlights the lattice fringes confirming the presence of silicon
- a 3.14 A
lattice spacing corresponding to the {111} planes.

Langmuir-Blodgett set-up for flm fabrication with surface functionalized
Si NPs.

FIGURE 4.5.60

(Upper) TEM images of Si NPs
dried on a grid. (Lower) TEM image of Si NP monolayer deposited at 50 mN/m of SP

FIGURE 4.5.61

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(Left) Optical absorption of
Si monolayers deposited at various surface pressures (SP). (Right) PL of Si NP
films at varying SP, as compared to the Si NP solution.

FIGURE 4.5.62

Conceptual Integrated Device

FIGURE 4.5.63

Monolayer
structures of Si NPs have been successfully deposited using the LB method and characterisation has been
on-going using TEM, UV-vis, PL and Raman spectroscopy. Figure 4.5.61 (right) illustrates TEM image of
the Si films fabricated at 50 mN/m surface pressures (SP) of the water trough. As noted, high contrast
is difficult to achieve due to the small density difference of Si and the
carbon-passivating chain. As a result, to further quantify the coverage and
regularity, computer recognition software is required; on which work is on-going. It is
difficult to see for this reason, but Fig. 4.5.61 (right) appears to show a dense
packing of Si NPs.

UV-Vis
spectroscopy was used to study the optical absorption of the films.
As illustrated in Fig. 4.5.62 (left) for single layers of Si NPs at varying compression surface
pressures (SP), the absorption clearly increases when SP increases,
corresponding to higher packing densities of Si NPs. The PL spectra of the above films show
narrow and symmetric peaks at 1.6 1.7 eV, as shown in Fig. 4.5.62 (right), which are in good
agreement with the estimated bandgap using the ‘effective mass approximation’. The red shift in emission
energy from Si NP films as compared to the solution corresponds to the electronic
energy transfer from small particles to larger ones when they are closely packed [4.5.70].

Investigation
of the electronic and phononic properties of the monolayer and multilayer structures are on-going
using low-wavenumber Raman and time resolve PL spectroscopy.

4.5.3.3.3.3 implementation of nanostructures for hot carrier
cells

In order
to use a nanoparticle array as a Hot Carrier absorber, and hence
utilise slowed carrier cooling due to modified phonon dispersion, then a way to fabricate a complete
structure with ESCs must be established. A structure in which nanoparticles are
arranged in a uniform 3D array should give the required phononic band gap. The
degree to which this needs to be ordered is still under investigation. Certainly a large
difference between masses is beneficial between the nanoparticles and the matrix and this can be
best achieved in an array of ‘core shell QDs’ in which the core and shell have very different acoustic
impedance in order to promote coherent reflection and hence confinement of phonons [4.5.61]. The
matrix in which such an array is embedded needs to allow transport of carriers to
contacts and also electron-electron and preferably hole-hole scattering to
renormalise carrier energies. Such a structure should also have a narrow electronic band gap so
as to absorb a wide range of photon energies. This combination of properties is challenging but not
mutually exclusive because phononic properties are largely independent of electronic properties.
Energy selective contacts are also required for such a structure. These would most likely be arranged
at the top and bottom of the absorber. (A hole contact might not need to be selective because the
hole population only contains a small fraction of the hot carrier energy and
hence thermalisation of holes is less important.) These ESCs would be QD or QW
double barrier structures. (Addition of an extra layer to give double QD/QW triple
barrier structures should also give rectification at the contact.) Such a
QD array / double barrier QD ESC structure is shown schematically in Fig.
4.5.63.

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Physical picture of the ND HC
Solar Cell (upper) with a carrier diffusion feld (core/shell thickness ratio
not to scale), embedded into a wide band gap solar cell [4.5.71]. Iso-valent
impurities provide an energy selective level for electrons. Band diagram of the
ND HC Solar Cell matched to the physical structure (lower). Direct generation
of hot exciton (1), energy selective extraction of hot electron from Hot
Carrier absorber (2), thermionic emission/diffusion of holes over the barrier
(3), electron-electron-scattering with renormalization of carrier energies and
possible energy loss by phonon emission (4), free carrier re-absorption (5),
impact ionization (6) and carrier generation in the wide band gap matrix (7).

FIGURE 4.5.64

Nanoparticle cell design incorporating EScs and absorber

Researcher:

Dirk
KÖnig

A full
Hot Carrier cell requires control of optical, electronic and phononic
properties in the same structure. One possible approach is the ‘nano-dot (ND)
Hot Carrier cell’ embedded into the i-region of a wide band gap solar cell as shown in Fig.
4.5.64.

Hot
Carrier solar cells must have a continuous electronic density of states (DOS)
to maximise maximum photon absorption and elastic electron-electron scattering. The
latter refills the depleted electronic energy levels from which electrons are
extracted through the ESC. On the other hand, if they do not have a phononic
band gap themselves the NDs must not be much bigger than the ballistic mean free path in
order to prevent inelastic scattering by which carriers cool down rapidly. However, optical phonons
can be confined in the nanocrystals as mentioned in Section 4.5.3.3.3 by arranging for a
mismatch between optical phonon energies in the two materials. Core shell nanocrystals can be used
to realise this [4.5.61, 4.5.62]. In addition the shell can be doped with iso-valent impurities
which form split-off subbands within the electronic band gap of the material, such as boron replacing
gallium in gallium nitride (GaN) [4.5.71]. Such an ESC would only work for one carrier type as
co-doping of a thin nano-shell appears not to be technologically feasible. For some III-V compounds with a
very high effective mass ratio between holes and electrons such that most of the excess energy during
optical generation is taken up by the electron, thermalisation of holes does not represent a large loss in
energy. Hence thermionic emission of holes over a low barrier can be used to transport holes into
the bulk of the device to be collected by the junction with little loss in efficiency.

4.5.3.3.4 hot carrier absorber: choice of materials

Researchers:

G.
Conibeer, R. Patterson, P. Aliberti, S. Huang, Y. Kamikawa, D. KÖnig,
Binesh Puthen-Veettil, S.Shrestha,
M.A. Green

The
properties required for a good hot carrier absorber material are
listed below in order of
priority.

	
  

 	
  

 
	
 1.

 	
 Large
 phononic band gap (EO(min) - ELA) - in order to suppress
 Klemens decay of optical phonons this must be at least as large as the maximum acoustic phonon
 energy. Hence a large mass difference (or large force constant difference) between
 constituent elements is required
 [4.5.61].

 
	
  

 	
  

 
	
 2.

 	
 Narrow
 optical phonon energy dispersion (ELO -EO(min)) -
 in order to minimise the loss of energy to TO phonons by Ridley decay. This requires a high symmetry atomic
 or nano-structure, preferably cubic with degenerate optical phonon energies at zone
 centre.

 
	
  

 	
  

 
	
 3.

 	
 Small
 electronic band gap - to allow a wide range of photon absorption. This should be less
 than 1eV. For 1 sun concentration the optimum is 0.7eV - as a band gap below this energy gives no advantage in
 the balance between absorption and emission. As the concentration ratio increases this
 optimum band gap decreases to zero at maximum concentration [4.5.1].

 
	
  

 	
  

 
	
 4.

 	
 A
 small LO optical phonon energy (ELO). This reduces the amount of
 energy lost per LO phonon emission, requiring a greater number to be emitted for a given
 energy loss [4.5.62]. However, it is difficult to have both a small ELO and the large phononic
 gap required in 1. This condition requires further investigation, but does still argue for a small ELO.

 
	
  

 	
  

 
	
 5.

 	
 An
 absolute small maximum acoustic phonon energy (ELA). This maximises the
 phononic gap if ELO is also small. A small ELA requires a large
 mass for
 the heavy atom and/or small force constant for its bonds.

 
	
  

 	
  

 
	
 6.

 	
 Good
 renormalisation rates in the material, i.e. good e-e and h-h scattering (e=electron,

 

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Use of the periodic table to
analyse possible analogue compounds of InN based on atomic mass combination and
electro-negativity.

FIGURE 4.5.65

	
  

 	
  

 
	
  

 	
 h=hole).
 This requires a reasonable DOS at all energies above E , or at least only very
 narrow gaps between energy levels, ≤ kT300K. This in turn requires a good
 overlap of
 wavefunction for carriers through the material, i.e. poor electronic confinement or a reasonable
 conductor. This condition is met in all inorganic semiconducotrs quite easily, with e-e
 scattering rates of less than 100fs for reasonable carrier concentrations. It may not be met in organic semiconductors or in nanostructures with large
 barrier heights.

 
	
  

 	
  

 
	
 7.

 	
 Good
 carrier transport in order to allow transport of hot carriers to the
 contacts. This is similar to 6, except that it only need be in the
 direction of the contacts, probably the z growth direction. A reasonably low
 resistance is probably good.

 
	
  

 	
  

 
	
 8.

 	
 It
 should be possible to make good quality, highly ordered, low
 defect material. Preferably it should be easy and cheap to do this.

 
	
  

 	
  

 
	
 9.

 	
 Earth
 abundant and readily available and processable constituent elements and processes.

 
	
  

 	
  

 
	
 10.

 	
 No or
 low toxicity of elements, compounds and processes.

 
	
  

 	
  

 
	
  

 	
 [(ELO
 , EO(min) & ELA are the maximum optical, minimum optical and
 maximum acoustic phonon energies respectively.]

 

InN has
most of these properties, except 4, 8 & 9, and is therefore a
good model material for a hot
carrier cell absorber.

Analogues of InN

As InN
is a model material, but has the problems of abundance and bad material quality,
another approach is to use analogues of InN to attempt to emulate its near ideal properties. These
analogues can be II-IV-nitride compounds, large mass anion III-Vs, group IV compounds/alloys or
nanostructures.

II-IV-Vs: ZnSnN; ZnPbN; HgSnN; HgPbN

With
reference to Fig 4.5.65, it can be seen that replacement of In on the
III sub-lattice with II-IV compounds is analogous and is now quite widely being investigated
in the Cu2ZnSnS4 analogue to CuInS2 [4.5.72].

ZnGeN
can be fabricated [4.5.73] and is most directly analogous with Si and GaAs. However, its band gap is large at
1.9eV. It also has a small calculated phononic band gap [4.5.74]. ZnSnN has a smaller electronic
gap (1 eV) and larger calculated phononic gap [4.5.74]. It is however difficult to fabricate,
and also its phononic gap is not as large as the acoustic phonon energy making it difficult to
block Klemens decay completely. HgSnN or HgPbN should both have smaller Eg and larger phononic
gaps. These
materials have not yet been fabricated [4.5.75].

Large mass cation:

The Bi
and Sb compounds have large predicted phononic gaps and Bi is a relatively abundant material, with
only low toxicity [4.5.75]. BiB has the largest phononic gap but AlBi, Bi2S5, Bi2O3
(bismuthine) are also attractive. Similarly
SbB has a large predicted phononic
gap. That for AlSb is the same size as
the acoustic phonon energy and its band gap is 1.5eV, making it marginal as an absorber material and
similar to InP.

Group IIIA III-Vs

LaN and
YN both have large phononic gaps whilst that for ScN is too small.

The
Lanthanides can also form III-Vs. ErN and other RE nitrides can be grown by MBE. The phononic band gaps of the
Er compounds are predicted to be large, because of the heavy Er cation, but its
discrete energy levels make it not useful as an absorber, although the combination of properties
in a nanostructure could be
advantageous.

Group IV
alloy/compounds:

All of
the combinations Si/Sn, Ge/C or Sn/C look attractive with large gaps predicted in 1D models. However being
all group IVs they only form weak compounds. Unfortunately SiC, whether 3C, 4H or 6H, has too narrow a
phononic gap. Nonetheless GeC does form a compound and is of significant interest [4.5.76].

There
are also several other inherent advantages of group IV compounds/alloys all of
which are associated with the four valence electrons of the group
IVs which result in predominantly covalent bonding:

	
  

 	
  

 
	
 a)

 	
 The
 elements form completely covalently bonded crystals primarily in a diamond structure (tetragonal
 is also possible as in βSn). However for group IV compounds the decreasing
 electronegativity down the group results in partially ionic bonding. This is not strong in
 SiC and whilst it tends to give co-ordination numbers of 4, can nonetheless result in several
 allotropes of decreasing symmetry: 4c, 4h, 6h. However, as the difference in
 period increases for the as yet theoretical group IV compounds, so too does the difference
 in electronegativity and hence also the bond ionicity and the degree of order. For a
 hot carrier absorber this is ideal because it is just such a large difference in the
 period which is needed to give the large mass difference and hence large phononic
 gaps. All of GeC, SnSi, SnC (and the Pb compounds) have computed phononic gaps large enough to block Klemens
 decay, and should also tend to form ordered diamond structure compounds.

 
	
  

 	
  

 
	
 b)

 	
 Because
 of their covalent bonding, the group IV elements have relatively small
 electronic band gaps as compared to their more ionic III-V and much more
 ionic II-VI analogues
 in the same period: e.g. Sn 0.15eV, InSb 0.4eV, CdTe 1.5eV. In fact to achieve approximately
 the same electronic band gap one must go down one period from group IV to III-V and down another period from III-V to
 II-VI: e.g. Si 1.1eV, GaAs 1.45eV, CdTe 1.5eV. This means that for group IV compounds
 there is greater scope for large mass difference compounds

 

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Cross-sectional electron micrograph of a 30-bilayer porous silicon
distributed Bragg reflector.

FIGURE 4.5.66

	
  

 	
  

 
	
  

 	
 whilst
 still maintaining small electronic band gaps. A small band gap of course being important for
 broadband absorption in an absorber - property 3 in the desirable properties for hot
 carrier absorbers listed above.

 
	
  

 	
  

 
	
 c)

 	
 The
 smaller Eg would tend to be for the larger mass compounds
 of Pb or Sn. Which, to give large mass difference, would be compounded with Si or
 Ge. This trend towards the lower periods of group IV also means that the maximum
 optical phonon and maximum acoustic phonon energies will be smaller for a given mass
 ratio - the desirable
 properties 4 and 5.

 
	
  

 	
  

 
	
 d)

 	
 Furthermore,
 unlike most groups, the group IV elements remain abundant for the higher mass
 number elements - desirable property 9. Property 10 is also satisfied because the
 group IVs have low toxicity.

 

Nanostructures:

As
discussed in section 4.5.3.3.3 QD nanostructures can be viewed in the same way as compounds. Their
phononic properties can be estimated from consideration of their combination force
constants. Hence it is possible to ‘engineer’ phononic properties in a wider range of
nanostructure combinations. Of the materials discussed above the Group IVs lend themselves most
readily to formation of nanostructures instead of compounds due to their predominantly
covalent bonding, which allows variation in the coordination number. Therefore the
nanostructure approaches of section 4.5.3.3.3 are consistent with a similar description as analogues
of InN, whether it be III-V QDs, colloidally dispersed QDs or for core shell QDs.

4.5.3.3.5 Summary of hot carrier solar cell research

2010
has seen significant development in most areas of Hot Carrier solar cell work. The modelling of
efficiencies not only now includes real material parameters for highly promising absorber
materials such as InN, but also is now extended to Energy Selective contacts of finite width.
The transport across such contacts is further modelled for a range of QD matrix
combinations. Work on ESCs has seen further more detailed demonstration of the necessary
resonance in double barrier resonant tunnelling structures, with additional evidence
for hot carrier populations, albeit very small, from illuminated I-V measurements with the ‘optically
assisted I-V technique. Measurement of InN with time-resolved PL has indicated some
evidence for slowed carrier cooling, further corroborating the importance of a large
phonon band gap to block optical phonon decay, but also highlights the
importance of material quality. Modelling of nanostructures for absorber materials has
focussed on real III-V QD structures, showing phonon bandgaps which will soon be measured in phonon dispersion measurements.
Progress on the Langmuir-Blodgett approach to ordered nanoparticle arrays has seen development of ordered single layer
arrays of Si nanoparticles. The potential application of nanostructures to fully integrated
devices has started to be investigated conceptually, with various designs considered.
Similarly the possibility of absorber materials which are analogous to InN is also being
investigated. These many aspects of Hot Carrier cells will see further development and
consolidation in 2011 with recent success in significant additional funding.

4.5.4 up-conversion

Researchers:

Craig
Johnson, Gavin Conibeer

Collaboration with:

Peter
Reece (Physics, UNSW)

Up-conversion in novel silicon-based materials

Up-conversion
(UC) in erbium-doped phosphor compounds (particularly NaYF4:Er) has been shown to be a
promising means of enhancing the sub-band-gap spectral response of conventional Si solar cells without modification of the
electrical properties of the cell [4.5.77]. In this scheme, a layer containing the phosphor is applied
to the rear of a high-efficiency bifacial cell. After absorbing two long-wavelength (1500nm)
photons - which are transmitted by the cell - the excited Er ions can relax by
emitting a photon with an energy greater than the Si band gap, thereby increasing
the current that can be extracted from
the cell.

While
phosphors have demonstrated high-efficiency UC behaviour, their use presents
particular challenges with regard to fabrication and cost. Our work in the last year has focused on the
development of Er-doped porous Si (PSi:Er) as an alternative UC material. PSi is
unique in that its porosity -and hence the material refractive index - can be varied as a function
of depth, allowing for the elaboration of high-quality monolithic Si optical structures such
as distributed Bragg reflectors (DBRs). A cross-sectional electron micrograph of such a structure
is shown in Fig. 4.5.66. Its porous substructure also allows for deep infusion of dopant
atoms via techniques such as
electroplating.

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Simulated reflectivity
spectra of PSi DBRs with varying numbers of bilayers; layer thicknesses were
adjusted to fix the short-wavelength stop band edge at 1550nm (marked with red
dashed vertical line).

FIGURE 4.5.68

Band structure in the
neighborhood of the fundamental band gap of an infinite 1D photonic crystal
(left) and group velocity in the material (right).

FIGURE 4.5.67

We have
previously reported room-temperature UC luminescence from Er-doped nanoporous Si when excited at very high
power densities with a 1550nm laser. In the past year we have focused on increasing the efficiency of this
UC luminescence by modifying the
PSi structure as described above.

The
dependence of the efficiency of the UC process on irradiance is
non-linear in the range of “sun-like” incident power. That is, the concentration
of the
incident radiation into a small Er-doped region results in greater efficiency
than for non-concentrated radiation over a proportionally large-area device. Typically,
concentration is achieved using lenses or non-imaging optics, but we have examined an analogous
electromagnetic field enhancement in the vicinity of optically-active Er ions by the excitation
of slow-light modes in these PSi:Er DBRs.

Field enhancement in 1D PSi:Er photonic crystals

Though
their use preceded the advent of the photonic crystal concept, DBRs can be described as one-dimensional
photonic crystals. The regular variation of the optical thickness of multiple alternating layers
results in wavelength-dependent interference behaviour manifesting as discrete bands of allowed and disallowed
electromagnetic modes, as shown in the left panel of Fig. 4.5.67. The “edge” of a photonic
band is a spectral region in which the effect of coherent scattering processes
changes abruptly between transmission and reflection of incident photons as determined by the superposition of
Bloch waves. Insofar as the group velocity vg
of an incident wave can be said to transition between “positive” and
“negative” values
- implying energy propagation into and out of the structure, respectively, in bands of strong
transmission and strong reflection - there exists an inversion point at which vg is zero, corresponding to a
standing wave established by the superposition of scattered Bloch
components. As can be seen in the right panel of Fig. 4.5.67, group velocity is
drastically suppressed
across a broad region near the zero-point. For steady-state equilibrium conditions,
this slowing of energy propagation requires a proportional increase in energy density, that is, an enhancement of the
electromagnetic field strength within the structure. It can be shown that the efficiency increase
for a two-step UC process is proportional to (c/nvg
- 1)I 0,
where c is the speed of light in vacuum, n is the average refractive index of the multilayer material,
and /0" is the incident field intensity [4.5.78]. In this way the
slow-light mode acts to augment the interaction that converts energy from the
field into atomic potential energy, resulting in a boost in the efficiency of the
process until saturation is
reached.

Simulation of field enhancement in 1D Pc structures

Using a
1D transfer matrix calculation, we simulated the reflectivity
characteristics of a series of PSi DBRs with 10, 20, 30 and 40 high-/low-porosity bilayers.
As shown in
Fig. 4.5.68, the reflectivity characteristic of a DBR contains a wide photonic stop-band. The structural parameters
were tuned slightly in each case as the number of bilayers increased to produce a
short-wavelength band edge minimum at 1550nm, the excitation wavelength of
interest (marked with a dashed
red line).

Maximum
transmission of 1550nm light into the structure is clearly required for efficient
coupling into
the Er ions. From Fig. 4.5.68 it is evident that an increased number of layers “compresses” the reflectivity
characteristic, resulting in a steeper

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Intensity-valued mode profile
in DBRs with varying numbers of high/low-porosity bilayers for a leading band
edge fxed at 1550nm. All intensities are normalized to the incident field
intensity and each plot is independently color-scaled to show maximum detail.
The air and substrate interfaces with the DBR are indicated by the dashed black
(upper) and red (lower) lines, respectively

FIGURE 4.5.69

band
edge and a narrower transmission window between the stop band and the neighboring interference fringe.

Figure
4.5.69 shows the calculated field intensity profile for these DBRs in
the spectral region between 1500 and 1600nm. The results have been normalized to the
incident field intensity and each plot is independently colour-scaled to show maximum detail.
They clearly demonstrate that considerable field enhancement is possible
throughout the depth of such a multilayer structure, with peak relative
intensities of more than 11 for the 30-bilayer structure and more than 18 for
the 40-bilayer
structure. However, it is also clear that the regions of enhancement are increasingly narrowly-concentrated as the
number of bilayers increases. This is due to the additional interference fringes in the
reflectivity characteristic of the DBR that arise with additional layers, as is apparent from
Fig. 4.5.69.
Calculations for more than 40 layers show that enhancements of up to 80 are possible for a large number of bilayers
though the peak narrows to less than 1nm wide for 100 bilayers.

Conclusion

These
simulations show that drastic field intensity enhancement is in
principle achievable in Er-doped PSi DBRs. Our current work is focused on the fabrication of suitable
samples by anodic chemical etching, electroplating, and high-temperature annealing.
Angular-dependent room temperature photoluminescence studies are underway to probe the effect of the
photonic band edge on the efficiency of UC in the optically-active Er ions within
the structures. This Si-based approach to efficient up-conversion is
conceptually promising and by optimising our fabrication techniques to improve the quality of our
optical structures we expect it will prove to be valuable work.

4.5.5 Plasmonics for 3rd generation structures

Researcher:

Supriya
Pillai

Plasmonics
is the study of the interaction of light on a thin metal dielectric interface resulting in the collective
oscillation of the free electrons in the metal. This gives rise to exciting properties that are very different
from the bulk metal. It is an emerging area of technology for photovoltaic applications with potential for light
management at the nanoscale. Ever since our
proof-of concept results were
published in 2007 [4.5.79], there has
been a huge increase in interest in incorporating
nanoparticles on solar cells due to their demonstrated light trapping
properties. The large optical polarisation
associated with the nanoparticles results in a strong enhancement of the electric field around the vicinity of the
particles along with strong
scattering. This optical property is being utilised for improving light
trapping in thin-flm solar cells.

The
scattering or near field enhancement is strongest at the surface plasmon resonance (SPR) frequency and this can be
tuned by changing the size, shape, dielectric medium around the particles. SPR tunability offers
control in manipulating light in the regime where the absorption of the solar cells are weak. Our
recent work (see section 4.4 Thin Film) has shown the benefits of having the particles
on the rear as opposed to previous work with front located particles [4.5.80]. By having
the particles
on the back of the cell any potential loss due to absorption in the metal or losses due to sub-resonance
photocurrent suppression resulting from destructive interference of the scattered and transmitted light into
the semiconductor layer can be avoided [4.5.81]. An additional advantage is that
because the scattering plasmonics layer is electrically decoupled from the cell, the
antireflection layer on the front and the plasmons

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Schematic representation
showing an example of surface plasmon resonance tunability by varying the of
size of the metal nanoparticles for wavelength dependent light trapping in a
tandem solar cell.

FIGURE 4.5.70

can be
optimised independently of each other. Our recent work has also made
use of an additional reflecting layer (paint) along with the nanoparticle layer on the rear side
of the cell to maximise the randomisation of light and to ensure no light is lost [4.5.80].

In this
study we focus our interest on the use of plasmonics for low-dimensional structures like
quantum dot structures for possible applications in tandem solar cells. PL measurements were used
to characterise the samples. The analysis is based on the reciprocity of light that a good
absorber can be a good emitter and the proof-of concept established with EL measurements in an earlier study
[4.5.82]. Hence the PL enhancement would be synonymous to an increase in photocurrent for an optimised QD cell. The tunability of plasmon
resonance allows the possibility of increasing the photocurrent of each cell in
a tandem cell configuration. Optimising the nanoparticle parameters can help achieve wavelength dependent light trapping which will
be an encouraging step towards increasing
the performance of a tandem solar
cell. Figure 4.5.70 shows one way of
achieving the SPR tunability by varying
the size of the nanoparticles in a tandem cell configuration.

The
sample under study is a 4nm single layer silicon QD structure in an oxide
matrix with a 6nm capping oxide layer with an emission wavelength of 930nm (1.3eV). Mass
thicknesses of 10, 14, 18 and 22 nm silver were deposited and the particles annealed at 2000C in nitrogen for
an hour. A 532nm, 10mW Nd:YAG laser was used to illuminate the samples for both the front
(incident on QD layer) and rear side (incident on the quartz slide) locations
of the silver
nanoparticles and the PL plots studied. PL measurements were carried out on
these samples and
the effect of different size particles were investigated both before and after
the deposition of the
nanoparticles.

The
self assembly techniques of nanoparticle fabrication give a varied size and shape
distribution very suitable for a broadband response. This is particularly noted for
particles corresponding to thicker Ag layers as can be seen from the PL plots in
Fig. 4.5.71. The red-shift of the enhancement peaks with larger particles
are also clearly evident. More details of this work has been reported in
[4.5.83].

Rear
located nanoparticles perform better than front located nanoparticles, consistent with the results from thin-film
cells. We believe this is because of the change in scattering cross-section of the nanoparticles due to
the changes in the driving feld and also due to the change in the mode of
excitation (from air for front located nanoparticles and from Si for rear
located nanoparticles) for the two different locations on the samples. More work need to be done to
better understand the mechanism. Further work will also concentrate on using alternate
approaches for tuning the resonance position close to the band-edge (emission wavelength for
PL) to increase the scattering properties and hence absorption in the weakly absorbing range.

4.5.6 concluding remarks for the third generation section

In 2010
work has proceeded significantly in all the areas of Third
Generation research, with improved fabrication and characterisation of materials and complexity of modelling
which together give an overall better understanding and optimisation of devices.

Group IV
based nanostructure materials have seen significant improvement in device design. With interlayers, in both SiNx
and SiC matrices, used to effectively control both the uniformity of size of Si QDs and carrier transport
in the growth direction. Modelling now allows the confined energy levels of complex QD shapes to
be calculated and their overlap to form mini-bands estimated in more detail. At the same
tiome improved characterisation is giving a much clearer idea on the shape and distributions of Si QDs
and is now allowing choice of growth parameters to give uniform size spherical QDs
to be grown. Ge nanostructure materials have seen improved control of p-type conductivity in

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PL plots from a 4nm single
layer QD structure in an oxide matrix with a 6nm capping oxide layer (right
axis) along with the corresponding enhancement plots (left axis) with silver
nanoparticles corresponding to 10, 14, 18 and 22 nm mass thickness of silver
for (a) silver nanoparticles on the front and (b) silver nanoparticles on the
rear.

FIGURE 4.5.71

Ge QDs
and very highly crystalline growth of Ge QWs. Homojunction Si nanostructure devices have demonstrated
improved conduction, there has been continued demonstration of rectifying
properties for heterojunctions with c-Si and heterojunctions between two nanostructured materials are now
starting to exploit the advantages of
both.

Hot
carrier cells have seen further development of efficiency modelling
which now not only includes material parameters such as Auger coefficients for real
promising absorber materials such as InN, but also includes the effects of real
non-ideal energy selective contacts. There has been significant improvement in measurement of negative differential
resistance in Si QD based energy selective contacts is now allowing and this overlaps well with
continuing improvement in modelling the transport through such structures in various matrices.
Carrier cooling measurements on the candidate absorber material InN have shown evidence for phonon
bandgaps building on the slowed carrier cooling seen in InP previously. Modelling of
analogues of these materials in nanostructures now predicts the phonon properties of rea lIII-V QD arrays
grown by collaborators. In addition colloidally dispersed Si nanoparticles have now been fabricated
as coherent single layer nanoparticle arrays and are progressing towards multi-layer structures.
Theoretical work on the potential materials for absorbers with controlled phon dispersion and
hence slowed carrier cooling has progressed on approapratie analogues for InN and devices
involving various nanostructure configurations are being analysed conceptually.

Up-conversion
has seen a further significant development of the use of porous-Si as a host for
Er up converting species. The porosity of porous-Si can be controlled to give
layers of alternating refractive index which can be used as distributed Bragg reflectors to modifiy
the photonic density of states such that there is strong enhancement of photonic modes at the critical Er
absorption at 1500nm. This has been demonstrated and can lead directly to large
increase in the up-conversion efficiency, thus tackling one of the key problems
of rare earth up-conversion
that it is inherently non-linear and dependent on concentration.

Plasmonics,
whic has previously been shown to enhance emission and absorption from both 1st
and 2nd
generation cells, has now been applied to the Si annostructured layers of 3rd
generatrion materials. This has shown significant enhancement in luminescence
demonstrating the strong potential to achieve local concentration with a global 1 sun illumination. This is
particularly useful for several of the non-linear 3rd generation approaches.

The
development of all the 3rd generation projects in 2010 now allows
much greater understanding of the materials and devices. Work in 2011 will see
consolidation of this into improved devices. Several new areas of funding
will contribute to this and also allow development of new project areas.

References:

	
  

 	
  

 
	
 4.5.1

 	
 M. A. Green, “Third
 Generation Photovoltaics: Ultra-High Efficiency at Low Cost”
 (Springer-Verlag, 2003).

 
	
  

 	
  

 
	
 4.5.2

 	
 J. Nelson, “The Physics of
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 4.5.3

 	
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 4.5.5

 	
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 4.5.6

 	
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 4.5.7

 	
 M. Zacharias, J. Heitmann,
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 4.5.8

 	
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ANNUAL REPORT

	
  

 	
  

 
	
 4.5.9

 	
 I. Perez-Wurfl, X. Hao, A.
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 applications,”Applied Physics Letters, 95 (2009) 153506.

 
	
  

 	
  

 
	
 4.5.10

 	
 XJ. Hao, E-C. Cho, G.
 Scardera, Y.S. Shen, E. Bellet-Amalric, D. Bellet, G. Conibeer, M.A. Green,
 “Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem
 solar cells”, Solar Energy Materials and Solar Cells, 93 (2009) 1524.

 
	
  

 	
  

 
	
 4.5.11

 	
 G.
 Conibeer, M.A. Green, R. Corkish, Y.-H. Cho, E.-C. Cho, C.-W. Jiang, T.
 Fangsuwannarak, E. Pink, Y. Huang, T. Puzzer, T. Trupke, B. Richards, A.
 Shalav, K.-L. Lin, Thin
 Solid Films 511/512 (2006) 654.

 
	
  

 	
  

 
	
 4.5.12

 	
 E.-C. Cho, Y.H. Cho, T.
 Trupke, R. Corkish, G. Conibeer and M.A. Green, Proc. 19th European
 Photovoltaic Solar Energy Conference, Paris, (June 2004), p. 235.

 
	
  

 	
  

 
	
 4.5.13

 	
 F.
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 4.5.14

 	
 T. Arguirov,T. Mchedlidze,
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 4.5.15

 	
 T.-W. Kim, C.-H. Cho, B.-H.
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 4.5.16

 	
 Y-H. Cho, E.-C. Cho, Y
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 Photovoltaic Solar Energy Conference, Barcelona, Spain, June 6-10, 2005,
 p.47.

 
	
  

 	
  

 
	
 4.5.17

 	
 Y Kurokawa, S. Miyajima, A.
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 4.5.18

 	
 C.-W.
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 4.5.19

 	
 K. Boer, “Survey of
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 4.5.20

 	
 Aliberti, P., et al.,
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 selective contacts applications”, Solar Energy Materials and Solar Cells.
 94(11): p. 1936-1941 (2010).

 
	
  

 	
  

 
	
 4.5.21

 	
 E.A.B.Cole, “Mathematical
 and Numerical Modelling of Heterostructure Semiconductor Devices: From Theory
 to Programming”. 2009: Springer. 406.

 
	
  

 	
  

 
	
 4.5.22

 	
 Grinstein, F.F., H. Rabitz,
 and A. Askar, “The Multigrid Method for Accelerated Solution of the
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 51: p. 423-443.

 
	
  

 	
  

 
	
 4.5.23

 	
 D. KÖnig, J. Rudd,
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 silicon quantum dots”, Physical Review B, 2008. 78(3): p. 035339.

 
	
  

 	
  

 
	
 4.5.24

 	
 Yurtsever, A., M. Weyland,
 and D.A. Muller, “Three-dimensional imaging of nonspherical silicon
 nanoparticles embedded in silicon oxide by plasmon tomography”, Applied
 Physics Letters, 2006. 89(15): p. 151920-151920-3.

 
	
  

 	
  

 
	
 4.5.25

 	
 L.-W. Wang and A. Zunger,
 J. Phys. Chem. 98, 2158 (1994).

 
	
  

 	
  

 
	
 4.5.26

 	
 Y-H. So, M.A. Green, G.
 Conibeer, S. Huang, A. Gentle, “Size dependent optical properties of Si
 quantum dots in Si-rich nitride/Si3N4 superlattice synthesized by magnetron
 sputtering”, Applied Physics Letters, accepted Feb 2011.

 
	
  

 	
  

 
	
 4.5.27

 	
 D. Di, I. Perez-Wurfl, G.
 Conibeer, M.A. Green, Solar Energy
 Materials & Solar Cells 94 (2010) 2238-2243.

 
	
  

 	
  

 
	
 4.5.28

 	
 G. Conibeer, M.A. Green, D.
 Konig, I. Perez-Wurfl, S. Huang, X. Hao,
 D. Di, L. Shi, S. Shrestha, B. Puthen-Veettil, Y So, B. Zhang and Z.
 Wan, “Silicon quantum dot based solar cells: addressing the issues of doping,
 voltage and current transport”, Progress in Photovoltaics: Res. Appl. 18
 (2010) In press, Published online in Wiley Online Library (wileyonlinelibrary.com).
 DOI: 10.1002/pip.1045, accepted Sept 2010

 
	
  

 	
  

 
	
 4.5.28

 	
 D. Song, E. Cho, Y Cho, G.
 Conibeer, Y Huang, S. Huang, MA. Green, “Evolution of Si (and SiC)
 nanocrystal precipitation in SiC matrix”, Thin Solid Films 516 (2008)
 3824-3830, doi:10.1016/j. tsf.2007.06.150

 
	
  

 	
  

 
	
 4.5.29

 	
 D. Song, E-C. Cho, G.
 Conibeer, Y Huang, C. Flynn, MA. Green, “Structural characterization of
 annealed Si1-xCx/SiC multilayers targeting formation of Si nanocrystals in a
 SiC matrix”, Journal of Applied Physics, 103 (2008) 083544.

 
	
  

 	
  

 
	
 4.5.30

 	
 Z. Wan, S. Huang, M.A.
 Green, G. Conibeer, Physica Status Solidi C, “Residual stress study of
 silicon quantum dot in silicon carbide matrix by Raman measurement”, EXCON
 ‘10 Proceedings special issue, accepted September 2010.

 
	
  

 	
  

 
	
 4.5.31

 	
 Zhenyu Wan, Shujuan Huang,
 Martin Green and Gavin Conibeer, Rapid thermal annealing and crystallization
 mechanisms study of silicon nanocrystal in silicon carbide matrix, Nanoscale
 Research Letters, 7541289134788882, accepted January 2011.

 
	
  

 	
  

 
	
 4.5.32

 	
 F. Gao, M.A. Green, G.
 Conibeer, E-C. Cho, Y Huang, I. Perez-Wurfl, C. Flynn, “Fabrication of
 multilayered Ge nanocrystals by magnetron sputtering and annealing”,
 Nanotechnology, 19 (2008) 455611.

 
	
  

 	
  

 
	
 4.5.33

 	
 Richter et al., Solid State
 Commun., 39 (1981), 625; I.H. Campbell, P.M. Fauchet, Solid State Commun. 58
 (1986) 739; Y. Sasaki, C. Horie, Phys. Rev. B47 (1993) 3811.

 
	
  

 	
  

 
	
 4.5.34

 	
 G.Conibeer, M.Green,
 E-C.Cho, D. KÖnig,Y-H.Cho, T Fangsuwannarak, G. Scardera, E. Pink, Y Huang, T
 Puzzer, S. Huang, D. Song, C. Flynn, S. Park,
 X. Hao, D. Mansfield, “Silicon quantum dot nanostructures for tandem
 photovoltaic cells”, Thin Solid Films, 516 (July 2008) 6748.

 
	
  

 	
  

 
	
 4.5.35

 	
 B. Zhang, S.K. Shrestha, P.
 Aliberti, M.A. Green, GJ. Conibeer, Characterisation of size-controlled and
 red luminescent Ge nanocrystals in multilayered superlattice structure, Thin
 Solid Films 518 (2010) 5483-5487.

 
	
  

 	
  

 
	
 4.5.36

 	
 B. Zhang, S.K. Shrestha,
 M.A. Green, GJ. Conibeer, “Size controlled synthesis of Ge nanocrystals in
 SiO2 at temperatures below 400 degree C using magnetron
 sputtering”, Applied Physics Letters, 96, 26901 (2010).

 
	
  

 	
  

 
	
 4.5.37

 	
 Y.Sasaki, C. Horie, Phys.
 Rev. B47 (1993) 3811; Kartopu et al., J. Appl. Phys. 103 (2008) 113518.

 
	
  

 	
  

 
	
 4.5.38

 	
 Karmous et al., Phys. Rev.
 B 73 (2006) 075323.

 
	
  

 	
  

 
	
 4.5.39

 	
 B. Zhang, S. Shrestha ,
 M.A. Green , G. Conibeer, Surface states induced high p-type conductivity in
 nanostructured thin film composed of Ge nanocrystals in SiO2
 matrix, Applied Physics Letters, 97, 132109 (2010).

 
	
  

 	
  

 
	
 4.5.40

 	
 XJ.
 Hao, E-C. Cho, C. Flynn, YS. Shen, S.C. Park, G. Conibeer, M.A. Green, Solar Energy
 Materials & Solar Cells, “Synthesis and characterization of boron-doped
 Si quantum dots for all-Si quantum dot tandem solar cells”, 93 (2009)
 273-279.

 
	
  

 	
  

 
	
 4.5.41

 	
 I. Perez-Wurfl, A. Gentle,
 X. Hao, M.A. Green, G. Conibeer and D-H. Kim, Proceedings of 24th EPVSEC,
 Hamburg, Germany, (2009).

 
	
  

 	
  

 
	
 4.5.42

 	
 J.M.
 Shah, Y-L. Li, Th. Gessmann, E. F. Schubert., Journal of applied physics. 94 (2003) 2627.

 
	
  

 	
  

 
	
 4.5.43

 	
 L. Ma, D. Lin, G. Conibeer,
 I. Perez-Wurfl, “Introducing dopants by diffusion to improve the conductivity
 of silicon quantum dot materials in 3rd generation photovoltaic devices”,
 Physica Status Solidi, accepted Sept10

 
	
  

 	
  

 
	
 4.5.44

 	
 G. Cantele, E. Degoli, E.
 Luppi, R. Magri, D. Ninno, G. Iadonisi, S, Ossicini S, “First-principles
 study of n-and p-doped silicon nanoclusters”, Physical Review B, 72 (2005)
 113303;G.M. Dalpian, J.R. Chelikowsky, “Self-Purification in Semiconductor
 Nanocrystals”, Physical Review Letters, 96 (2006) 226802.

 
	
  

 	
  

 
	
 4.5.45

 	
 A.R. Stegner, R.N. Pereira,
 R. Lechner, K. Klein, H. Wiggers, M. Stutzmann, Brandt MS, “Doping efficiency
 in freestanding silicon nanocrystals from the gas phase: Phosphorus
 incorporation and defect-induced compensation”, Physical Review B, 80 (2009)
 165326.

 

98

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ANNUAL REPORT

	
  

 	
  

 
	
 4.5.46

 	
 A.L. Tchebotareva, MJ.A. de
 Dood, J.S. Biteen, H.A. Atwater, A. Polmana, “Quenching of Si nanocrystal
 photoluminescence by doping with gold or phosphorous”, Journal of
 Luminescence, 114 (2005) 137-144.

 
	
  

 	
  

 
	
 4.5.47

 	
 M. Fujii, A. Mimura, S.
 Hayashi, K. Yamamoto, C Urakawa, H. Ohta, Journal of Appl. Phys., 87 (2000)
 1855-1857.

 
	
  

 	
  

 
	
 4.5.48

 	
 D. KÖnig, S. Shrestha, J.
 Rudd, G. Conibeer, M. A. Green, “Doping of Si-Based Dielectrics for Providing
 Majorities to Si Quantum Dots: Acceptors in SiO2”, Proc. 24th
 European PVSEC, (Hamburg 2009).

 
	
  

 	
  

 
	
 4.5.49

 	
 G. Conibeer, M.A. Green, D.
 Konig, I. Perez-Wurfl, S. Huang, X. Hao,
 D. Di, L. Shi, S. Shrestha, B. Puthen-Veettil, Y. So, B. Zhang and Z.
 Wan, “Silicon quantum dot based solar cells: addressing the issues of doping,
 voltage and current transport”, Progress in Photovoltaics: Res. Appl. 18
 (2010) In press, Published online in Wiley Online Library (wileyonlinelibrary.com).
 DOI: 10.1002/pip.1045, accepted Sept 2010.

 
	
  

 	
  

 
	
 4.5.50

 	
 Santosh K. Shrestha,
 Pasquale Aliberti, Gavin J. Conibeer, Energy selective contacts for hot
 carrier solar cells, Solar Energy Materials and Solar Cells, 94 (Sept 2010)
 1546-1550.

 
	
  

 	
  

 
	
 4.5.51

 	
 P. WÜrfel, Sol. Energy
 Mats. and Sol. Cells. 46 (1997) 43.

 
	
  

 	
  

 
	
 4.5.52

 	
 R. Ross, AJ. Nozik, J. Appl
 Phys (1982) 53, 3318.

 
	
  

 	
  

 
	
 4.5.53

 	
 V.Y. Davydov, et al.,
 “Absorption and emission of hexagonal InN: Evidence of narrow fundamental
 band gap”, Physica Status Solidi (B), 229 (2002) p. r1-r3.

 
	
  

 	
  

 
	
 4.5.54

 	
 D. Fritsch, H. Schmidt, M.
 Grundmann, “Band dispersion relations of zinc-blende and wurtzite InN”,
 Physical Review B, 69 (2004) 165204.

 
	
  

 	
  

 
	
 4.5.55

 	
 YTakeda,T. Ito,T. Motohiro,
 D. KÖnig, S.K. Shrestha, G. Conibeer, “Hot carrier solar cells operating
 under practical conditions”, Journal of Applied Physics, 105 (2009)
 074905-10.

 
	
  

 	
  

 
	
 4.5.56

 	
 Wurfel, P., A.S. Brown,
 T.E. Humphrey, and M.A. Green, Particle conservation in the hot-carrier solar
 cell”, Progress in Photovoltaics, 13 (2005) 277-285.

 
	
  

 	
  

 
	
 4.5.57

 	
 P. Aliberti, Y Feng, Y Takeda,
 S.K. Shrestha, M.A. Green, GJ. Conibeer, “Investigation of theoretical
 efficiency limit of hot carrier solar cells with bulk InN absorber”, Journal
 of Applied Physics, 108 (2010) 094507-10.

 
	
  

 	
  

 
	
 4.5.58

 	
 G.
 Conibeer, C.W.Jiang, D. KÖnig, S.K. Shrestha, T. Walsh, M.A. Green, “Selective energy
 contacts for hot carrier solar cells”, Thin Solid Films, 216 (2008)
 6968-6973.

 
	
  

 	
  

 
	
 4.5.59

 	
 P.G. Klemens, Phys. Rev.
 148 (1966) 845.

 
	
  

 	
  

 
	
 4.5.60

 	
 G. Conibeer, R. Patterson,
 P. Aliberti, L. Huang, J-F Guillemoles, D. KÖnig, S. Shrestha, R. Clady, M.
 Tayebjee, T. Schmidt, M.A. Green “Hot Carrier solar cell Absorbers”, 24th
 European PVSEC, Hamburg (Sept. 2009).

 
	
  

 	
  

 
	
 4.5.61

 	
 G. Conibeer, NJ.
 Ekins-Daukes, J-F. Guillemoles, D. KÖnig, E-C. Cho, C-W. Jiang, S. Shrestha
 and M.A. Green, “Progress on Hot Carrier Cells”, Solar Energy Materials and
 Solar Cells, 2009, 93, 713.

 
	
  

 	
  

 
	
 4.5.62

 	
 D. KÖnig, K.
 Casalenuovo,YTakeda, G. Conibeer, J.F. Guillemoles, R. Patterson, L.M. Huang,
 M.A. Green, “Hot Carrier Solar Cells: Principles, Materials and Design”,
 Physica E, 42 (2010) 2862-2866.

 
	
  

 	
  

 
	
 4.5.63

 	
 H. Bilz, W. Kress, “Phonon
 dispersion relations in Insulators”, Springer-Verlag (1979).

 
	
  

 	
  

 
	
 4.5.64

 	
 R. Clady, MJ.Y Tayebjee, P.
 Aliberti, D. Konig, NJ. Ekins-Daukes, GJ. Conibeer, T.W. Schmidt, M.A. Green,
 “Interplay between Hot phonon effect and Intervalley scattering on the
 cooling rate of hot carriers in GaAs and InP”, Progress in Photovoltaics,
 accepted March 2011.

 
	
  

 	
  

 
	
 4.5.65

 	
 K.T
 Tsen, J.G. Kiang, D.K. Ferry, H. Lu, WJ. Schaff, H.W. Lin, S. Gwo, “Direct measurements of
 the lifetimes of longitudinal optical phonon modes and their dynamics in
 InN”, Applied Physics Letters, 90 (2007) 3.

 
	
  

 	
  

 
	
 4.5.66

 	
 YC.
 Wen, YC, C.Y Chen, C.H. Shen, S. Gwo, C.K. Sun, “Ultrafast carrier thermalization in InN”,
 Applied Physics Letters, 89 (2006) 3; M.D.Yang, et al., “Hot carrier
 photoluminescence in InN epilayers”, Applied Physics A - Materials Science
 & Processing, 90 (2008)
 123-127.

 
	
  

 	
  

 
	
 4.5.67

 	
 F. Chen, A.N. Cartwright,
 H. Lu, WJ. Schaff, “Ultrafast carrier dynamics in InN epilayers”, Journal of
 Crystal Growth, 269 (2004) 10-14.

 
	
  

 	
  

 
	
 4.5.68

 	
 R. Patterson, M. Kirkengen,
 B. Puthen Veettil, D Konig, M.A. Green, G. Conibeer, Sol. Ener. Mats. And
 Sol. Cells, 94 (2010) 1931-1935.

 
	
  

 	
  

 
	
 4.5.69

 	
 S. Huang, K. Minami, H.
 Sakaue, S. Shingubara, T Takahagi, “Effects of the surface pressure on the
 formation of Langmuir-Blodgett monolayer of nanoparticles”, Langmuir, 20
 (2004), 2274.

 
	
  

 	
  

 
	
 4.5.70

 	
 C. R. Kagan, C. B. Murray,
 M. Nirmal, M. G. Bawendi, Phys. Rev. Lett. “Electronic Energy Transfer in
 CdSe Quantum Dot Solids”, 76 (1996) 1517.

 
	
  

 	
  

 
	
 4.5.71

 	
 D. KÖnig, “Photovoltaic
 Device Physics at the Nanoscale”, Chapter 3 (pp. 73 - 146) in L. Tsakalakos
 (Ed.), “Nanotechnology for Photovoltaics”, Taylor & Francis, April 2010;
 ISBN-13:9781420076745

 
	
  

 	
  

 
	
 4.5.72

 	
 TK. Todorov, K.B. Reuter,
 D.B. Mitzi, Advanced Materials, 22 (21010) 1-4.

 
	
  

 	
  

 
	
 4.5.73

 	
 W.R. L. Lambrecht,
 “Structure and phonons of ZnGeN2”,
 Phys. Rev. B 72 (2005) 155202

 
	
  

 	
  

 
	
 4.5.74

 	
 T.R. Paudel, W.R.L.
 Lambrecht, Phys. Rev. B, 79 (2009) 245205.

 
	
  

 	
  

 
	
 4.5.75

 	
 S.C. Erwin, I. Zutic,
 Nature Materials, 3 (2004) 410-414.

 
	
  

 	
  

 
	
 4.5.76

 	
 Z.T
 Liu, J.Z. Zhu, N.K. Xu, X.L. Zheng, Jap. J. Appl. Phys., 36 (1997) 3625.

 
	
  

 	
  

 
	
 4.5.77

 	
 A. Shalav, B.S. Richards,
 M.A. Green, “Luminescent layers for enhanced silicon solar cell performance:
 Up-conversion”, Sol. Energy Mats. and Solar Cells, 91 (2007) 829-842.

 
	
  

 	
  

 
	
 4.5.78

 	
 C.M. Johnson, PJ. Reece,
 GJ. Conibeer, “Up-conversion luminescence enhancement in erbium-doped porous
 silicon photonic crystals for photovoltaics”, Photonics West Conference, San
 Francisco, Jan 2011.

 
	
  

 	
  

 
	
 4.5.79

 	
 S. Pillai, K. R. Catchpole,
 T Trupke, and M. A. Green, J. Appl. Phys., 101 (2007) 093105.

 
	
  

 	
  

 
	
 4.5.80

 	
 Z.
 Ouyang, S. Pillai, F. Beck, O. Kunz, S. Varlamov, K. R. Catchpole, P. Campbell, and M. A. Green,
 App. Phys. Lett, 96 (2010) 261109.

 
	
  

 	
  

 
	
 4.5.81

 	
 S. H. Lim, W. Mar, P.
 Matheu, D. Derkacs, and E. T. Yu, J. Appl. Phys., 101 (2007) 104309.

 
	
  

 	
  

 
	
 4.5.82

 	
 S.
 Pillai, K. R. Catchpole, T Trupke, G. Zhang, J. Zhao, and M. A. Green, Appl. Phys. Lett. 88 (2006)
 161102.

 
	
  

 	
  

 
	
 4.5.83

 	
 S. Pillai, I. Perez-Wurfl,
 G. J. Conibeer, and M. A. Green, physica status solidi (c) 8 (2011) 181.

 

99UNITED STATES

 

 

 

ARC PHOTOVOLTAICS CENTRE OF EXCELLENCE 2010/11 ANNUAL

REPORT, pp 61-99

ARC PHOTOVOLTAICSCENTRE OF EXCELLENCE ANNUAL REPORT,

2012, pp 169-224

 

 

 

Article referenced as support for the following
sections:

 

Page 8 & 9: Figure and Text on Tandem Solar Cells

4.5 Third Generation Strand – Advanced
Concepts

	
  

 
	
 University Staff:

 
	
 Prof.
 Gavin Conibeer (group leader)

 
	
 Dr Richard Corkish

 
	
 Prof. Martin Green

 
	
  

 
	
 Senior Lecturer:

 
	
 Dr Santosh Shrestha

 
	
  

 
	
 Senior Research Fellows:

 
	
 Dr Dirk KÖnig

 
	
  

 
	
 Lecturers:

 
	
 Dr Ivan Perez-Wurfl

 
	
  

 
	
 Research Fellows:

 
	
 Dr Shujuan Huang

 
	
 Dr Supriya Pillai (ASI
 Fellow)

 
	
 Dr Xiaojing “Jeana” Hao
 (ASI Fellow)

 
	
  

 
	
 Postdoctoral Fellows:

 
	
 Dr Pasquale Aliberti
 (until June 2012)

 
	
 Dr Robert Patterson (ASI
 Fellow)

 
	
 Dr Binesh Puthen-Veettil
 (ASI Fellow)

 
	
 Dr Xiaoming Wen (from Dec
 2012)

 
	
 Dawei Di (March to Sept
 2012)

 
	
 Craig
 Johnson (from Aug 2012) (ASI Fellow)

 
	
 Dr Siva Karaturi (from Feb
 2013)

 
	
  

 
	
 Professional officers:

 
	
 Dr Tom Puzzer (part time)

 
	
 Dr Patrick Campbell
 (shared with Thin Film)

 
	
 Mark Griffin (shared with
 Thin Film)

 
	
  

 
	
 Research Associates:

 
	
 Dr
 Didier Debuf (adjunct fellow)

 
	
 Yidan Huang

 
	
  

 
	
 Higher Degree Students:

 
	
 Andy Hsieh (until Nov
 2012)

 
	
 Dawei Di (until March
 2012)

 
	
 Craig
 Johnson (until Aug 2012)

 
	
 Sammy Lee (until Aug 2012)

 
	
 Haixiang Zhang

 
	
 Tian Zhang

 
	
 Yu “Jack” Feng

 
	
 Pengfei Zhang

 
	
 Sanghun Woo

 
	
 Yao Yao

 
	
 Neeti Gupta

 
	
 Chien-Jen
 “Terry” Yang

 
	
 Ibraheem Al Mansouri

 
	
 Yuanxun “Steven” Liao

 
	
 Suntrana Smyth

 

- 169 -

	
  

 
	
 Lingfeng Wu (from Mar
 2012)

 
	
 Xuguang Jia (from Mar
 2012)

 
	
 Ziyun Lin (from Mar 2012)

 
	
 Hongze Xia (from Mar 2012)

 
	
 Xi Dai (from Mar 2012)

 
	
 Shu Lin (from Aug 2012)

 
	
 Pei Wang (from Aug 2012)

 
	
 Simon Chung (from Aug
 2012)

 
	
  

 
	
 Casual Staff:

 
	
 James Rudd (part time)

 
	
 Adrian
 Shi (Aug to Dec 2012)

 
	
  

 
	
 Visiting Researchers:

 
	
 Dr Yukiko Kamikawa (AIST,
 Tsukuba, Japan, until Mar 2012)

 
	
  

 
	
 Undergraduate Students:

 
	
 4th year Thesis Projects:

 
	
 Dongchen Lan

 
	
 Guillaume Pauvert

 
	
 Aden Brown

 
	
 Li Shen

 
	
 Alexander Robinson

 

Abstract

There has been a significant increase in the work on hot
carrier cells in 2012 with the start of both the Australian Solar Institute (ASI) project and
US-Australia Solar Energy Collaboration (USASEC) projects. The Group IV nanostructure tandem cells project - the
“all-Si” tandem cell – has also seen an increase with the start of its own
ASI project. There has also been further work on up-conversion,
photo-electrolysis and on plasmonics.

There
has been increased understanding of the parameters required to grow good Si
nanostructure materials. Characterisation
of materials is leading to better understanding of the defects which narrow the
effective bandgap and hence limit VOC. Improvements in VOC
have been seen with lower temperature
measurements and in current with light trapping by various techniques.
Modelling using equivalent circuits is giving greater insight to device
performance.

Hot carrier cells efficiencies are being modelled with
greater input of real material properties and with integration of more complex structures.
Combined contact and absorber materials mdoels and those including both
multiple quantum well and phononic bad gap material have been developed. New
designs of contacts for rectification or for incorporation of energy selective
contacts directly in absorbers are being
developed. A wide range of potential materials for phononic modulated
structures have been identified and are being developed. Growth of
nanoparticle ordered arrays has progressed with
modelling of such superstructures also being further developed. Fully
integrated devices are now being designed and are expected to be
fabricated with collaborators in the near future.

The
up-conversion project now has good synthesis routes for Er containing phosphors
and for PbS downshifting quantum dots (QDs) used to sensitise Er to a wider
wavelength range. Progress on incorporating such materials with light trapping
structures is expected to give improvements in up-conversion for Si cells.

This
progress in all the main Third Generation project areas is improving
understanding and allowing optimisation of modelling, structures and devices.
Developments to come in the next year will see significant advancement in these
areas, with excellent prospects for good demonstration devices.

- 170 -

4.5.1 Third Generation Photovoltaics

The
“Third Generation” photovoltaic approach is to achieve high efficiency whilst
still using “thin film” second generation deposition methods. The concept is to
do this with only a small increase in areal costs and to use abundant and
non-toxic materials and hence reduce the cost per Watt peak [4.5.1]. Thus these
“third generation” technologies will be compatible with large scale
implementation of photovoltaics. The aim is to decrease costs to well below
US$0.50/W, towards US$0.20/W or better, by dramatically increasing efficiencies
but maintaining the economic and environmental
cost advantages of thin film deposition techniques (see Fig. 4.1.3 showing the
three PV generations) [4.5.1, 4.5.2]. To achieve such efficiency
improvements such devices aim to circumvent the Shockley-Queisser limit for
single band gap devices that limits efficiencies to the “Present limit”
indicated in Fig. 4.1.3 of either 31% or 41% (depending on concentration
ratio). This requires multiple energy threshold devices such as the tandem or
multi-colour solar cell. The Third Generation Strand is investigating several
approaches to achieve such multiple energy threshold device [4.5.1, 4.5.3].

The
two most important power loss mechanisms in single-band gap cells are the
inability to absorb photons with energy less than the band gap (1 in Fig.
4.5.1), and thermalisation of photon energy exceeding the band gap, (2 in Fig.
4.5.1). These two mechanisms alone amount to the loss of about half of the
incident solar energy in solar cell conversion to electricity. Multiple energy
threshold approaches can utilise some of this lost energy. Such approaches do
not in fact disprove the validity of the Shockley-Queisser limit, rather they
avoid it by the exploitation of more than one energy level for which the limit
does not apply. The limit which does apply is the thermodynamic limit shown in
Fig. 4.1.3, of 68.2% or 86.8% (again depending on concentration).

Figure 4.5.1: Loss processes in a standard solar cell: (1) non-absorption of below
band gap photons; (2) lattice
thermalisation loss; (3) and (4) junction and contact voltage losses; (5)
recombination loss.

In
the Third Generation Strand, we aim to introduce multiple energy levels by
fabricating a tandem cell based on silicon and its oxides, nitrides and
carbides using reduced dimension silicon nanostructures to engineer the band
gap of an upper cell material. We are aiming to collect photo-generated
carriers before they thermalise in the “hot carrier” solar cell. Also we are
investigating absorption of two below bandgap photons to produce an electron-hole
pair in the cell by up-conversion in a layer behind the Si cell using erbium
doped host materials. In order to optimise the requisite properties, all these
structures are likely to be thin hence maximising absorption of light in thin
structures through light trapping is very important. Hence we are also
investigating localised surface plasmon enhanced coupling of light into these
Third Generation devices.

4.5.2 Si Nanostructure Solar Cells

Researchers: Shujuan Huang, Ivan Perez-Wurfl, Dirk KÖnig, Tom Puzzer, Xiaojing Hao,
Dawei Di, Zhenyu Wan, Sammy Lee, Yidan Huang, Lingfeng Wu, Xuguang Jia, Ziyun
Lin, Tian Zhang, Terry Yang, Gavin Conibeer, Martin Green

- 171 -

4.5.2.1 The ‘‘all-Si’’ tandem cell

We
are developing a material based on Si (or other group IV) quantum dot (QD) or
quantum well (QW) nanostructures, from which we can engineer a wider band gap
material to be used in tandem photovoltaic cell element(s) positioned above a
thin film bulk Si cell, see Fig. 4.5.2.

Previously
we have demonstrated the ability to fabricate materials which exhibit a blue
shift in the effective band gap as the QD or QW size is reduced, using
photouminescence [4.5.4] and absorption [4.5.5] data [4.5.6]. A thin film
deposition of a self-organised QD nanostructure is achieved through a sputtered
multi-layer of alternating Si rich material and stoichiometric dielectric
[4.5.4]. On annealing the excess Si precipitates into small nanocrystals which
are limited in size by the layer thickness, thus giving reasonable size uniformity,
as first demonstrated by Zacharias [4.5.7]. Demonstration of doping of these
layers with both phosphorus and boron to create a rectifying p-n junction has
resulted in devices with a photovoltaic open circuit voltage of 490mV [4.5.6,
4.5.7, 4.5.8].

Figure 4.5.2: A tandem photovoltaic cell using quantum confined QDs or QWs to
engineer the band gap of the top cell and potentially also the lower cells.
Short wavelength light is absorbed in the top cell and longer wavelengths in
lower cells, thus boosting the overall
voltage generated and hence efficiency. Formation of Si (or Ge or Sn)
QDs through layered thin film deposition of Si rich material which crystallises
into uniform sized QDs on annealing.

A
cell based entirely of Si, or other group IV elements, and their dielectric
compounds with other abundant elements (i.e. silicon oxide, nitride or carbide)
fabricated with thin film techniques, is advantageous
in terms of potential for large scale manufacturability and in long term
availability of its constituents. Such thin film implementation implies
low temperature deposition without melt processing, it hence also involves
imperfect crystallisation with high defect densities. Hence devices must be thin to limit recombination due to their
short diffusion lengths, which in turn means they must have high
absorption coefficients.

For
photovoltaic applications, nanocrystal materials may allow the fabrication of
higher band gap solar cells that can be used as tandem cell elements on top of
normal Si cells [4.5.11, 4.5.12]. For an AM1.5 solar spectrum the optimal band
gap of the top cell required to maximize conversion efficiency is 1.7 to 1.8eV
for a 2-cell tandem with a Si bottom cell [4.5.13]. To date, considerable work
has been done on the growth and characterization of Si nanocrystals embedded in
oxide [4.5.7, 4.5.14] and nitride [4.5.15, 4.5.16] dielectric matrices.
However, little has been reported on the experimental properties of Si
nanocrystals embedded in SiC matrix [4,5,17]. These are of particular interest
for application in photovoltaic devices because of an expected significant
increase in carrier transport due to a decrease in the barrier height between
adjacent nanocrystals [4.5.18]. As a result, sufficient carrier mobility can be
obtained to satisfy device fabrication requirements.

- 172 -

4.5.2.2 Fabrication of Si QD nanostructures

Thin
film techniques are used for nanostructure fabrication. These include
sputtering and plasma enhanced chemical vapour deposition (PECVD). The
deposition is a variation of the multi-layer alternating ‘stoichiometric
dielectric / Si rich dielectric’ process, shown in Fig. 4.5.3, followed by an
anneal during which Si nanocrystals precipitate limited in size by the Si rich
layer thickness [4.5.12, 4.5.7]. The most successful and hence most commonly
used technique is sputtering, because of its large amount of control over
deposition material, deposition rate and abruptness of layers. A multi-target
remote plasma sputtering machine with two independent RF power supplies as well
as additional DC power supplies is used in this work.

Figure 4.5.3: Multilayer deposition of alternating Si rich dielectric and
stoichimetric dielectric in layers of a few nm. On annealing the Si
precipitates out to form small nanocrystals of a size determined by the layer
thickness. Nanocrystal or quantum dot size is therefore uniform.

RF magnetron sputtering is used to deposit alternating
layers of SiO2 and SRO of thicknesses down to 2nm. [SRO refers to Si
rich oxide, formed by co-sputtering Si and SiO2.] Deposition of
multi-layers, consisting
typically of 20 to 50 bi-layers, is followed by an anneal in N2 from
1050 to 11500C. During the anneal, the excess silicon in the SRO layer
precipitates to form Si nanocrystals between the stoichiometric oxide layers.

For Si QDs in SiO2 the precipitation occurs
according to the following:

Precipitation
of excess Si from Si rich dielectrics in SiNx and SiC follows a
similar crystallisation reaction as Si precipitates from the amorphous matrix.
The techniques have also been applied to growth of Sn and Ge QDs in SiNx
in SiO2. Ge quantum dots can be precipitated at substantially lower
temperature, as discussed below.

4.5.2.3 Silicon nanocrystal devices on quartz substrates

Researchers: Ivan Perez-Wurfl, Xiaojing Hao, Dawei Dai, Adrian Shi, Lingfeng Wu,
Xuguang Jia, Ziyun Lin, Tian Zhang, Terry Yang

As
previously reported in previous annual reports, devices have been fabricated
using the SiQD in SiO2
materials, with p-n junction formation using B or P of multilayers,
respectively [4.5.9, 4.5.10, 4.5.19]. The fabricated p-n diodes
consisted of sputtered alternating layers of SiO2 and SRO onto
quartz substrates with in-situ boron and phosphorus doping. The top B doped
bi-layers were selectively etched to create isolated p-type mesas and to access
the buried P doped bi-layers. Aluminium contacts were deposited by evaporation,
patterned and sintered to create ohmic contacts on both p and n-type layers.
The fabricated interdigitated solar cells have an effective area of up to
0.12cm2. The devices exhibit rectification and a photovoltaic
response with VOC up to 493 mV, but with as yet very small currents
and very bad fill factors. These are partly due to the very high

- 173 -

resistance
of the material and in particular to the relatively long lateral paths to
contacts at the back contact, necessitated by growth on an insulating quartz
substrate. The device structure with appropriate contacting is shown
schematically in Fig. 4.5.4. This also shows the current crowding effect which
results from the high lateral resistance.

Figure 4.5.4: Schematic diagram of the of the fabricated inter-digitated Si QD in
SiO2 devices, with B and P doping to create the p-n junctions and
illustrating the effect of current crowding due to high lateral resistance to
the back contact.

4.5.2.4 Equivalent circuit model of nanocrystal devices on
quartz substrates

In
order to better understand the limitation imposed by the device on the solar
cell performance, it is necessary to find a good equivalent circuit model
specific to our devices. The high resistivity of the base layer makes it
imperative to consider the two dimensional effects of current flow [4.5.20]. In
order to analyse the I-V characteristics
of these diodes we first generalised the model to include any number of diodes
in series. The series connection of diodes is used to explain the ideality
factors higher than two normally observed in our structures. We believe this is
a reasonable model as the ideality factor observed is almost independent of the
diode current. This type of behaviour has also been observed in multi quantum
well laser diodes [4.5.21], where it has been proven to arise from an
unintended series combination of diodes. Based on this series combination of
diodes, it is possible to obtain an expression relating temperature dependent I-V measurements to the band gap [4.5.9].
It is further possible to linearise the expression around an average measuring
temperature, Tavg, as
follows:

 (4.5.1)

where
k is Boltzmann’s constant, q is the electron charge, Tnom is the temperature at
which the saturation current, Ioi,
is defined, ni is the
diode ideality factor, and αi
is the saturation current temperature exponent. Notice that this equation
shows that the I-V characteristics
are related to a sum of band gaps.

As
the current flows from the base contact to the emitter, a linear voltage drop
along the base and under the diode isolation mesa causes an exponential change
in the diode current. This crowding of the current at the edge of the diode
mesa, depicted in Fig. 4.5.4, can be modelled adding a current dependence on
the series resistance. This series resistance, RS,
can be expressed as the sum of a current
independent, Rext and
a current dependent series resistance Rint
arising from current crowding:

 (4.5.2)

The
value of this resistance can be found by numerically solving the following
transcendental equation:

- 174 -

 (4.5.3)

Within
this mathematical framework it is then possible to extract the value of the
series resistance, remove its effect from the measured I-V characteristics to extract the actual
ideality of the diode current. We normally observe an ideality factor of 3.
Based on the proposed model of at least two diodes
in series, without any assumptions, it is only possible to establish that the
value of the band gap extracted from temperature dependent I-V measurements corresponds to a sum of
the band gaps (or activation energies) of these diodes.

An
interesting behaviour observed in these devices was the apparent lack of
correspondence between the dark and light I-V
characteristics. The series resistance extracted from the diode dark
I-V characteristic is too small
to explain the limited short circuit current as well as the low fill factor
measured under a simulated 1-sun condition. A more complete circuit model is
necessary to explain this discrepancy. We have proposed a model where the
observed behaviour is due to two distinct areas in the fabricated devices. The
photocurrent is produced only in a small area of the device, this area being
proportional to a fraction of the normalized diode area. In a Spice circuit
model this area is given a value smaller than 1, that we denote as fraction.
This will be a fitting parameter to reproduce the measured dark and illuminated
I-V characteristics. Figure 4.5.5
shows the circuit proposed.

Figure 4.5.5: Equivalent Spice circuit representation of the fabricated devices.

A
relatively large percentage of the device area is responsible for the measured
characteristics in the dark. The current in this area is caused by the
diffusion of minority carriers caused by the applied voltage (V1 in Fig. 4.5.5)
from the p or n side to the opposite region. This current is expected to be
large due to the low lifetime of the minority carriers (mostly recombination
current in the depletion region). The observed series resistance will be
proportional to the total series resistance, Rtot, and inversely proportional to the area, R2=Rtot/(1-fraction), where the diffusion current
occurs. The dark I-V behaviour
is modelled by the top branch of the circuit depicted in Fig. 4.5.5. D_QD1 and
D_Sch represent the series connection of diodes whose band gaps are extracted
using Eq. (4.5.1). The series resistance, R2, has the temperature and current
dependence detailed in Eqs. (4.5.2) and (4.5.3).

Only
a small part of the diode area may have a large enough lifetime to produce a
photocurrent. As this photocurrent flows only through this fractional area, the
series resistance is inversely proportional to
this fraction: R1=Rtot/fraction.
Since the photocurrent, Iph, flows through R1, the illuminated I-V characteristics are limited by
a larger resistance than that observed in the dark condition, as long as the fraction
of the diode area is smaller than one half. The simulations depicted in Fig.
4.5.6 show the reduction of ISC as the fraction, f, is varied from 99% to 1% of the total diode area.

- 175 -

Figure 4.5.6: Spice simulations of 1-sun I-V characteristics based on the circuit
depicted in Fig. 4.5.5. The fraction, f,
represents the normalized area of the diode where the photocurrent is produced.

With
the circuit model described, it will be possible to extract complementary
information from dark and illuminated I-V measurements.

In view of these simulations, it is clear that the
electrical characterisation of our devices needs to take into consideration previously overlooked
limitations. For example, great care should be taken when interpreting the
Quantum Efficiency extracted from a spectral response measurement as the
assumed condition of short circuit current may be incorrect even if the device
is externally short circuited (internally, the diode may be forward biased).
Moreover, as the current is proportional to the photon flux, and the flux is
generally different at each wavelength tested, the internal bias of the device
can be different at each point of the spectrum investigated.

4.5.2.5 Modelling for device optimisation

Researchers: Ivan Perez-Wurfl, Ziyun Lin, Tian Zhang

When
designing and implementing the Si-QD solar cell model the most important
performance limiting factor identified was the in-plane resistivity of the
material. To take this into consideration a 2-D model needed to be implemented.
This model takes into consideration the lateral flow of current in the solar
cell. By implementing this 2-D effect we have been able to recreate the
current-voltage (I-V)
characteristics of diodes made with Si-QD on insulating substrates. The model
takes into consideration the temperature dependence of the resistivity and
saturation current. The actual behaviour of the resistivity and saturation
current are modelled in terms of activation energies obtained from temperature dependent measurements. The activation energy
of the resistivity is related to defects in the material which act as
electronic traps. The activation energy of the saturation current is related to an effective electronic bandgap
that, in some cases, can be modified by the doping itself. In crystalline
semiconductors such as Si, doping does not affect the electronic bandgap unless
extreme levels of dopants are used (> 1x1019cm-3).
However, in disordered or amorphous semiconductors, doping the material will
always affect the electronic bandagap. For example, doping hydrogenated
amorphous silicon (a:Si:H) increases the defects in the material which in turn
act as traps. The end result is a highly resistive material despite the very
high doping levels used. Furthermore, the high doping itself adds such a high density of defects that the electronic
bandgap of the material is clearly reduced. To minimize the effect, undoped
materials are commonly used for the active PV layer of the device. We
have identified that the type of Si-QD films required for PV devices, has
indeed a similar behaviour as that of a-Si:H when it comes to doping. What is
more, the native defects in the Si-QD layers we produce are high enough to
affect the electronic bandgap even when no intentional doping is present.

- 176 -

The realisation that the effective electronic bandgap is
in fact the most important aspect limiting VOC has clarified an optimisation plan to follow.

Efforts
are now in place to establish optical methods to quantify the effective
electronic bandgap. Specifically, we are working on tailoring standard
characterisation methods, Photo Thermal Deflection Spectroscopy (PDS) and Photo
Luminescence (PL), to specifically study Si-QD materials. Both techniques offer
the advantage of allowing evaluation of defects. These defects should be
minimised to reap the benefits of the expected enhanced bandgap and, with the
aid of these techniques, could be
quantified without the need to fabricate devices. Once the quality of material
has been optimised according to minimised defects determined using these
optical methods, this material can then be used to fabricate devices.

4.5.2.6 Light trapping for increased current in devices

Researchers: Ivan Perez-Wurfl, Supriya Pillai, Lingfeng Wu, Ziyun Lin, Xuguang Jia

The photogenerated current in p-n unction Si
nanostructure devices has been increased by more than 100% compared to a cell
without any light trapping. The application of an Aluminium back-reflector gave a current enhancement of up to 46%.
Plasmonic silver nanoparticles gave an even more impressive result increasing
the current by up to 109%.

The
work concentrated around devices fabricated on quartz substrates in the
substrate configuration. This means that the devices were designed such that
illumination was applied from the top of the structure. The substrate is used
as a support for the device as well as the back-reflector as shown in Fig.
4.5.7.

Figure 4.5.7: Schematic of the structure used to optimise photocurrent using an Al
back-reflector.

The
amount of light absorbed by these structures is limited by the thinness of the
structure and the relatively low silicon content. Furthermore, light trapping
is not possible using a textured substrate as the material is deposited by
sputtering which is not as conformal as CVD or ALD, for example. Furthermore,
the thinness of the layer does not allow the typical chemical texturing used in
standard Si solar cells. The most straightforward way to increase light
absorption, and therefore short circuit current, is with the use of a metal
back-reflector. The efficiency of a back-reflector can be further enhanced (or
substituted) with the use of plasmonic effects that could increase the optical
absorption path by scattering incoming light.

A
sample was chosen such that the current was as high as possible for multiple
devices contained within the same insulating substrate. We will refer to this
sample as SM2A from here on. The devices showed a reasonable uniformity within
the sample to allow a statistically significant comparison between cells with
and without a back-reflector. The resistivity of the base had to be chosen to
be as low as possible to reduce its effect on the current without compromising
the quality of the diodes fabricated.

- 177 -

The
results of the photocurrent enhancement are summarised in Table 4.5.1. Notice
that the lowest enhancement is 31.3%. The average enhancement across the sample
was 40% with a maximum enhancement of 46.1%.

Table 4.5.1: Summary of
photocurrent enhancement achieved with an Aluminium back-reflector on sample SM2A.

	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 
	
  

 	
  

 	
 5mm2 device

 	
  

 	
 1mm2
 devices

 	
  

 
	
  

 	
  

 	 

 	
  

 	 

 	
  

 
	
  

 	
  

 	
 Device A

 	
  

 	
 Device B

 	
  

 	
 Device C

 	
  

 	
 Device D

 	
  

 
	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 
	
 ISC
 without Reflector (mA)

 	
  

 	
 18.7

 	
  

 	
  

 	
 4.1

 	
  

 	
  

 	
 4.6

 	
  

 	
  

 	
 4.6

 	
  

 	
  

 
	
 ISC with
 Reflector (mA)

 	
  

 	
 27.3

 	
  

 	
  

 	
 5.4

 	
  

 	
  

 	
 6.7

 	
  

 	
  

 	
 6.4

 	
  

 	
  

 
	
 % increase using Al (M2A)

 	
  

 	
 46.1

 	
 %

 	
  

 	
 31.3

 	
 %

 	
  

 	
 45.1

 	
 %

 	
  

 	
 37.5

 	
 %

 	
  

 

Figure 4.5.8: Typical dark and illuminated I-V characteristics
of a 1mm2 diode tested in sample SM2A. Three curves are shown:
Illuminated with Reflector (green), Illuminated with No Reflector (grey) and in
the Dark (blue).

Larger devices were also tested with the best result
shown on a 5mm2 device.

The largest device showed the highest percent
enhancement in photocurrent when tested with a back-reflector or with silver
nanoparticles. This particular enhancement is probably due to variations in the
device performance across the sample rather than having an actual dependence on
the area. It is worth pointing
out that the largest device is closer to the centre of the sample which
positively impacts the quality of the device. An interesting feature observed
in Fig. 4.5.8 is the larger VOC observed when tested without a back
reflector. This is probably due to heating of the sample while testing,
consequently, the temperature may not have been the same for all the
measurements. The actual temperature of the device is difficult to control due
to the thermally insulating substrate used. Details of the thermal management
issues will be discussed below.

Silver
nanoparticles have also been investigated. The enhancement obtained in the
photocurrent was significant. When tested
on a 5mm2 solar cell, the current improved more than two fold.
Specifically, the current was increased by 109% compared to that
obtained without any light trapping scheme.

The
silver nanoparticles were deposited on the top of the cell. Illumination was
then applied from the side of the substrate after removing the Al back
reflector as shown in the schematic in Fig. 4.5.9.

- 178 -

Figure 4.5.9: Schematic of the structure used to optimise photocurrent using silver
nanoparticles.

The
nanoparticles were obtained by evaporating 18nm of Silver followed by a 1 hour
anneal in a nitrogen purged oven at 200oC. Figure 4.5.10 shows the
comparison between the I-V characteristics
of the cell tested with an aluminium back-reflector or silver nanoparticles.

Figure 4.5.10: Illuminated I-V
characteristics of a 5mm2 diode showing an enhanced
photocurrent due to Silver nanoparticles (green curve) or an Al backreflector
(blue curves) compared to the same cell without any light trapping (red curve).

4.5.2.7 Devices on conductive substrate

We
have investigated three metals and one highly doped semiconductor thin film
suitable for use as conductive virtual substrates on a quartz supporting
substrate.

The
first all Si-QD devices demonstrated by our group were fabricated on quartz
substrates. The main reason for using quartz substrates is to demonstrate that
any PV response is in effect due to the Si-QD material and not the substrate
itself. The disadvantage of using these substrates is the current crowding that
occurs due to the high resistivity of the Si-QD layers. This current crowding
effect limits the maximum current that can be extracted from the solar cell.
Conductive substrates are investigated to mitigate this detrimental effect.

We
note that a thick substrate is not actually necessary. A thin, highly
conductive film is sufficient to reduce current crowding effects to a negligible
level. The conductive film of choice will be deposited on a quartz substrate to
ensure compatibility with our current process.

The high temperature needed to precipitate the Si-QD
from the silicon rich oxide layers, precludes the use of low melting point metals such as
Aluminium. To ensure compatibility with the unavoidable

- 179 -

high
temperature of our process, we chose three refractory metals with the highest
melting points, namely Tungsten (W),
Tantalum (Ta) and Molybdenum (Mo). We also investigated the use of silicon rich
silicon carbide (SRC) layers. SRC can be deposited with a low resistivity and
is perfectly compatible with the sputtering tool used for depositing SRO
materials.

4.5.2.7.1 Metals as back contacts

Researchers: Ziyun Lin, Ivan Perez-Wurfl, Xuguang Jia, Terry Yang

Sputtering
is the most common technique used to deposit refractory metals. The deposition
itself is straightforward but much optimisation is needed to ensure the stress
in the films is low enough to avoid the films delaminating from the substrate.
Early on in the optimisation process we identified severe difficulties in obtaining good quality films with W and Ta. We
concentrated our efforts then on improving the quality of Mo thin films.
It was found that a good indicator of the compatibility of the film with a high
temperature process was the minimisation of pinholes in the film. To accurately
quantify the density of pinholes we developed a technique based on the optical
transmission of Mo thin films. We have been able to use this characterisation
technique to quantify the pinhole density down a 1/100,000 total area of the
film. Fig. 4.5.11 shows the measured optical transmittance of two Mo films
deposited on quartz.

Figure 4.5.11: Optical transmission of two Mo films on quartz.

The
transmittance was measured using a Perkin Elmer Lambda 1050 spectrophotometer.
The measurement has been optimised by
ensuring a baseline calibration compatible with measurements of very low
transmittance. Figure 4.5.11 shows the nominally zero transmittance (10-6
%) of a 3mm thick Al slab. This “zero” level shows a clear differentiation with
a film transmitting only 0.001%. This optical characterisation method is fast
and accurate and allows us to pre-screen Mo films before using them as a substrate.

The
resistivity, surface roughness and the areal density of pinholes of the
optimised Mo layer is reported in Table 4.5.2.

Table 4.5.2: Characteristics of the best overall Mo film used as a substrate for a
Si-QD p-i-n structure.

	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 	
  

 
	
 Substrate 

 deposition 

 temperature (oC)

 	
  

 	
  

 	
 Film 

 thickness (nm)

 	
  

 	
  

 	
 Surface
 roughness (nm)

 	
  

 	
  

 	
 Sheet
 resistance 

 (W/square)

 	
  

 	
  

 	
 Area of pinholes 

 (% of total area)

 	
  

 
	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 	 

 
	
 420

 	
  

 	
  

 	
 350
 ± 50

 	
  

 	
  

 	
 0.84
 ± 0.2

 	
  

 	
  

 	
 2.35
 ± 0.8

 	
  

 	
  

 	
 0.0018

 	
  

 

- 180 -

A 300 nm Mo film similar to that of Table 4.5.2 but with
50X higher pinhole density was sputtered on quartz at room temperature. A bilayered
structure consisting of 25 bilayers (4nm SRO:4nmSiO2), nominally
un-doped with 45% Si by Volume in the SRO, was simultaneously deposited on a
quartz substrate and the Mo film. Both structures were also simultaneously
annealed at 1100oC for 1 hour in a nitrogen purged quartz tube
furnace. Visually, there was no apparent difference between the structure
deposited on Mo and that deposited on quartz after annealing. Comparing the PL
emission from the two samples also showed no difference as shown in Fig.
4.5.12.

Figure 4.5.12: a) Picture of the annealed 25 bilayer
structure on Mo b) Comparison of the PL emission from the multilayer on quartz
and Mo.

This
experiment proved that Mo is indeed compatible with the high temperature
process and has no obvious interaction with the multilayer structure.

The
same Mo layer was then used as a back contact for a standard full p-i-n
structure (70 bilayers total, bilayer = 4nm SRO/2nm SiO2, SRO = 66%
Si by volume). After the annealing process it was clear that Mo had interacted
with the p-i-n structure as can be seen in Fig. 4.5.13.

Figure 4.5.13: Picture of p-i-n structure after 11000C
anneal. The bottom left corner shows clearly that a reaction occured between
the p-i-n structure and the underlying Mo thin film.

Electrical
measurements showed that a dead short was created across the full p-i-n
structure precluding the measurement of VOC
or ISC. It is very likely that the formation of a Silicide caused
the shunt observed across the diode. The silicide formation is very
sensitive to the presence of SiO2 between Si and Mo. The thicker
oxide and lower Si content of the 25 bilayers may have been enough to avoid a
reaction with the underlying Mo thin film. In the case of the p-i-n structure,
the high concentration of Si in the SRO and the thin SiO2 barriers
promote the formation of a Silicide. Mo films
are still under investigation. Further optimisation of these films is under
intensive study. The use of a silicide is being considered to limit the
reaction of the metal and the p-i-n structure. Our findings in this critical
area will be reported in future reports.

- 181 -

4.5.2.7.2 Wide bandgap semiconductor as back contacts

Researchers: Dawei Di, Ivan Perez-Wurfl, Gavin Conibeer

A
second option investigated, geared at reducing the lateral resistivity, is the
use of a thin, highly doped, wide bangap material.

Silicon
rich carbide (SRC) layers have been investigated as the back contact for a
p-i-n structure. A significant advantage of using SRC rather than a metal
conductive substrate is the large amount of transmission
of SRC as compared to zero transmission for metals. This allows a wider range
of device testing including rear illumination and is more compatible
with a full tandem cell structure.

The
SRC layers are deposited directly on quartz. The films are doped with
phosphorous. After 1100oC annealing for 1 hour, the SRC layers show
a sheet resistance an order of magnitude smaller than that of bilayered SRO
layers of similar thickness and with the highest silicon content of interest
(66% Si by Volume).

Despite
its greater transmission than metals SRC as a conductive substrate nonetheless
does have an unavoidable absorption loss caused by the layer itself. The SRC
layer has to be as thin as possible to reduce
this loss. In the interest of investigating the effectiveness of SRC as a
conductive substrate, we choose a relatively thick layer that would be
easier to handle and characterise. With enough evidence to prove the
effectiveness of SRC we can then optimise the thickness without significantly
sacrificing its conductivity.

A
237 nm thick phosphorous doped SRC layer was deposited on quartz, annealed and characterised
prior to using it as a substrate. The measured sheet resistance of this film
was 5 k /square. The layer was then used as a substrate for a standard p-i-n
structure. The full stack was annealed once more, devices were fabricated and
characterised. It was possible to demonstrate a working device on this structure. Dark and light I-V characteristics of a 1mm2
diode is shown in Fig. 4.5.14.

Figure 4.5.14: I-V characteristics
of a standard p-i-n structure on and SRC substrate.

The
measured ISC and VOC are lower than those of a standard
p-i-n structure deposited directly on quartz. The reason for this lower
performance is mainly due to the absorption of light in the SRC layer which is
most probably not converted into current.

- 182 -

4.5.2.8 Increase in open circuit voltage

Researchers: Lingfeng Wu, Xuguang Jia, Ziyun Lin, Ivan Perez-Wurfl

The
original intention to show a path towards improving the open circuit voltage of
these devices beyond 493mV (the previous record voltage) was to vary the silicon
content in the SRO in order to exploit a suspected trend relating it to the
open circuit voltage. Our previous experience had shown that by reducing the
silicon content it is possible to increase the open circuit voltage sacrificing
some of the short circuit current due to
the inherent increase in the resistivity of the material. This suspected trend
had to be revised after developing the proper techniques to reproduce material
with the same silicon content on a regular basis. Contrary to expectations, the
change in silicon content of the SRO has very little effect on the open circuit
voltage.

An open circuit voltage of 503mV was measured on a 1mm2
solar cell at 250 K under an approximate 0.8 suns illumination condition. The demonstration of a voltage higher
than 500mV was one of the outcomes of this more structured line of work. The
reason for these non-standard testing conditions will be explained in detail in
the following paragraphs. It is important to point out that the demonstration
of a voltage larger than 500mV was measured on a device fabricated on an
oxidised Si wafer. Such devices can absorb less than half the number of photons
compared to a similar cell on a transparent quartz substrate. The I-V curve of this solar cell is shown in
Fig. 4.5.15.

Figure 4.5.15: Dark and illuminated I-V of cell with VOC = 503 mV,
under an approximate 0.8 suns, 250
K.

Having
developed the desired control on the composition and thickness, we deposited
p-i-n structures with variable silicon content (65.8, 63.7 and 60.9% Si by
Vol.) with nominally the same size of Si-QD. Using photoluminescence (PL) as a
metric to compare the similarity of QD in three different structures, we
confirmed that indeed the Si-QD were very similar in the three cases. The PL
spectra measured on these structures is shown in Fig. 4.5.16.

- 183 -

Figure 4.5.16: Measured PL on three p-i-n structures with
variable silicon content in the SRO (Si percent by Volume: 65.8%, yellow;
63.7%, pink; 60.9% blue).

As
can be seen in Fig. 4.5.16, the peak emission of the three structures is nearly
equal. The obvious differences are a small red shift of the PL emission spectra
for the 65.8% Si by Vol. sample and a slightly higher tail in the high energy
emission for the sample with the lowest Si content. The low variation in the silicon content was intentional
as these 5% variation represents a change in the lateral resistivity of
four to five orders of magnitude. The detailed relationship between Si content
and resistivity is currently under study.

The measured optical absorption of these three samples
confirms that indeed, the sample with a higher silicon content has the highest optical
absorption becoming slightly less pronounced as the silicon content is
decreased.

The fabricated devices expected to highlight the
relationship between VOC and Si content in the SRO, do not in fact show any particular trend. The
open circuit voltage measured ranges between 200mV and 300mV without any
apparent correlation to the silicon content. The lowest silicon content investigated
in fact did not produce any measurable devices, probably due to the extreme
high series resistance which in turn limits the device performance.

Having determined that the silicon content is not indeed
determining the open circuit of these devices, we set ourselves to the task of determining
the discrepancy of these observations with our previous experience showing an
actual dependence. We have concluded that the lack of accurate control in the
material composition in the early days of this research at UNSW, along with the
variation in the deposition rate, lead to unintended variations in the
deposited material which was interpreted as a trend
in VOC as a function of Si content in the SRO. We believe that there
may much more important factors affecting VOC besides the Si
content. Investigation of other options to increase VOC led us to
consider devices not fabricated on our standard quartz substrates.

Our
strongest proof that the substrates indeed affect the quality of the devices
built on them, come from an experiment performed by depositing single SRO
layers on quartz substrates. An apparent random
variation in obtaining a PL signal from only a handful of the samples prepared
was eventually correlated to an absorption peak observed in the IR
region of the spectrum, present only in the substrates that showed no PL
signal. This absorption peak (2.7mm) was later identified as an indicator of
OH groups. The higher concentration of OH groups caused out-diffusion of oxygen
from the substrate which oxidised the single layer deposited on them. The
vendor of the quartz substrates, however, does not differentiate between the
substrates with and without the OH related peak. In the same manner, there are
other impurities that could be infiltrating the Si-QD structures which could
potentially limit their electronic and PV quality.

- 184 -

With
this in mind we tested a p-i-n structure with an intended 64% Si by Volume in
the SRO layers. The structure was deposited on an oxidised Si wafer instead of
quartz. The oxide on the wafer is 97nm thick and ensures that there is no
possible electrical contact between the substrate and the Si-QD devices.

The
open circuit voltage measured in seven devices under 1sun illumination varied
between 420 mV and 440 mV. However, these devices are thermally isolated due to
the thick oxide present on the substrate. As the device is illuminated, it
heats up due to the light absorbed in the film however, due to the thermal
insulation caused by the substrate, a temperature gradient is bound to exist
with the highest temperature always being on the side of the device. An
advantage of measuring the I-V properties of these devices as a function of
temperature, is the possibility of determining the electronic bandgap of
the material used in the devices. The uncertainty in the actual temperature of
the device results in an uncertainty in the
determination of this bandgap. Fig. 4.5.17 presents a plot of VOC
vs. temperature measured on a 1mm2 Si-QD solar cell on a thermally
oxidised Si wafer.

Figure 4.5.17: VOC vs. Temperature measured on a
1mm2 Si-QD solar cell on a thermally oxidised Si wafer. The bandgap
extracted depends on the actual temperature of the device measured.

The
bandgap can be estimated based on the intercept on the y-axis of the linear fit
to the measured data. This intercept is
then the bandgap plus a correction factor in the order of 3kTaverage.
Based on this approach, we conclude that the effective electronic bangap
of the Si-QD layer is somewhere between 0.84 and 0.91 eV. These values are well
below the bandgap estimated from the PL peak emission reported in Fig. 4.5.16
(1.4 eV). The large discrepancy can be explained if the Si-QD material has a
very high density of defects bellow and above the conduction and valence band
edges respectively. We note that a lower electronic bandgap than anticipated
will make it difficult to obtain open circuit voltages as large as one would
expect from a material with a bandgap given by the peak of the PL emission.
Having identified this issue we can now concentrate on reducing the density of
electronic defects rather than tackling the resistivity or varying the diameter
of the Si-QD.

4.5.2.9 Other Si nanostructure materials studies

The use of matrices other than SiO2 can give
advantages in carrier transport. Or the use of NCs other than Si can give lower processing
temperatures and different possibilities for bandgap engineering.

- 185 -

4.5.2.9.1 Carrier tunnelling transport in Si QD superlattices

Transport
properties are expected to depend on the matrix in which the silicon quantum
dots are embedded. As shown in Fig. 4.5.18 different matrices produce different
transport barriers between the Si dot and the matrix, with tunnelling
probability heavily dependent on the height of this barrier. Si3N4
and SiC give lower barriers than SiO2 allowing larger dot spacing
for a given tunnelling current.

Figure 4.5.18: Bulk band alignments between silicon and its carbide, nitride and
oxide. Tunnelling probability between QDs separated by d depends exponentially on the square root
of the barrier height (ΔE1/2) multiplied by d. (e.g. [4.5.22] p244)

The results suggest that dots in a SiO2
matrix would have to be separated by no more than 1-2 nm of matrix, while they could be separated by more
than 4 nm of SiC. Fluctuations in spacing and size of the dots can be
investigated using similar calculations. It is also found that the calculated
Bloch mobilities do not depend strongly on variations in the dot spacing but do
depend strongly on dot size within the QD material [4.5.18]. Hence, transport
between dots can be significantly increased by using alternative matrices with
a lower barrier height, ∆E.
For the same tunnelling current the spacing of QDs can increase for oxide to
nitride to carbide matrix.

4.5.2.9.2 Hetero-Interlayers

Researchers: Dawei Di, Zhenyu Wan, Gavin Conibeer

Experiments
have been carried out in which the layer between the Si rich layers is a
different dielectric. Si3N4 (nominally stoichiometric)
has two advantages. The higher density of the nitride restricts diffusion of Si
to nucleating nanocrystals during annealing. Thus size control and a more
mono-disperse size distribution of the nanocrystals result. Secondly the lower
barrier height of the nitride, as compared to the oxide within the Si rich
layers, leads to a higher tunneling probability in the growth direction, whilst
good quantum confinement is maintained in the nanocrystals planes.

Initial
results on this approach show promising decreases in the vertical resistivity
of such Si QD nanostructures with SiNx interlayers [4.5.23]. Figure
4.5.19 shows I-V curves measured on vertical samples on a conducting Si wafer
substrate, for both SiNx or SiO2 interlayers, each doped
with either B or P. In each case the substrate used was of the same carrier
type as the nominal type of the doped layer
on top. The most striking feature of the data is the much lower resistivity of
the B doped nitride interlayer samples as compared to the B doped oxide
interlayer sample. Whilst for P doping the oxide and nitride interlayer samples
show almost no difference in resistivity.

Hence
there is some evidence for significantly reduced resistivity for nitride as
compared to oxide interlayers, for the B doped samples. But the similar
resistivities for the two interlayers for P doping is not as first expected and
requires further investigation. The explanation is related to the differing
effects on Si QD crystallisation of P and B.

- 186 -

Figure 4.5.19: Logarithmic I-V curves
for Si QD samples grown with either nitride or oxide interlayers, and
subsequently annealed in H2 forming gas at 3500C. (a) B doped
samples: the nitride interlayer shows a dramatically lower resistivity; and (b)
P doped samples: the resistivities of the nitride and oxide interlayer samples
are similar. Insets show schematic diagrams of QD sizes and spacings.

Figure
4.5.20 shows the XRD spectra for SiO2 and SiNx
interlayers between SRO layers. Figure 4.5.20(a)
shows that for the oxide interlayer (i.e. for the SiO2-SRO-SiO2
structure) the effect of P (B) increasing (decreasing) the Si QD size is
demonstrated in the narrowing (broadening) of the XRD peaks for all of the
{111}, {220} and {311} Si peaks. The fact that all these principal peaks are
narrowed (broadened) to the same extent for a given sample indicates that the
effect is due to an increase (decrease) in QD size allowing a narrower
(broader) range of angles that satisfy the Bragg condition. This is in contrast
to inhomogeneous strain or a disordered structure in the nanocrystals which, if
it were present, would distort lattice spacing differently for different plane
families. Hence these QD sizes can be estimated using the Scherer equation for
peak broadening to give 5.7nm and 14.8nm for B and P, respectively [4.5.23].
Thus indicating that P incorporation significantly increases the size of Si
QDs. But in Figure 4.5.20(b) for the nitride interlayer the QD sizes are much
closer at 3.3nm and 5.0nm for B and P, respectively. This suggests that the
nitride interlayers suppress the growth of larger (smaller) nanocrystals for P
(B) doping respectively.

Figure 4.5.20: XRD comparison of P and B doped multilayers
for both (a) SiO2 and (b) SiNx interlayers between SRO
layers, all on quartz substrate.

The
likely reason for this uniformity of QD sizes for the nitride interlayer with P
and B doping, as compared to the oxide interlayer, is that the presence of a
strong gradient in N concentration at the interface between the silicon rich
oxide and silicon nitride layers will tend to promote heterogeneous nucleation
whether or not B or P are present. This will tend to suppress the differing QD
sizes in P and B material caused by their differing nucleation behaviour, thus
reducing the difference in QD spacing and its differing effects on resistivity.

This
suggests a reason for the differing I-V behavior shown in Fig. 4.5.19 based on
the opposite effects on Si QD crystallisation of P and B. For the oxide
interlayer samples the QDs are expected to

- 187 -

be
smaller with B and larger with P incorporation. The large size of the QDs in P
doped material are such that they penetrate several layers and significantly
reduce the number of tunneling events required in the vertical current
transport direction. This will tend to reduce resistivity. But in the nitride
interlayer the much more similar QD sizes for P and B doping shown in Fig.
4.5.20, mean this effect will be significantly reduced. Hence for B doping in
Fig. 4.5.19 (a) the QD sizes are very similar
and the presence of the lower barrier height nitride interlayer dominates in
reducing resistivity. Whilst for the P doping of Fig. 4.5.19 (b) the
larger QDs in the oxide interlayer material reduce resistivity to approximately
the same extent as does the lower barrier height of the nitride in the nitride
interlayer material even though this latter has larger spacing between the
smaller QDs.

4.5.2.9.3 Alternative group IV materials for tandem cells

Researchers: Sammy Lee, Shujuan Huang, Zhenyu Wan, Gavin Conibeer

Ge
nanocrystals in SiO2, made by a similar phase separation from solid
solution process, are being investigated as an alternative material with a
lower precipitation temperature and wider potential range of effective band gap
than Si nanocrystals. The temperature range employed is typically around 6500C,
but with substrate heating during growth can be as low as 3500C [4.5.24]. These
materials demonstrate a weak p-type conductivity without intentional doping and
can be made more p-type by addition of Sb during growth, although the exact
mechanism of doping is still under investigation.

Other
analogues involving Sn nanocrystals in dielectric matrices have been
fabricated. These require even lower processing temperatures, around 250-3000C,
and can be annealed in situ. However, Sn nanocrystals
formed in SiO2 matrix tend to undergo significant oxidation
[4.5.25]. In a Si3N4 matrix there is no oxidation
but the tetragonal β form of Sn always forms, because the β-Sn
allotrope is more thermodynamically stable than the cubic α-Sn allotrope
[4.5.26]. But as β-Sn is also semi-metallic as compared to the
semiconducting α-Sn, this also means that no semiconducting optical or
electronic properties of these Sn nanocrystal materials have yet been measured.

Other
alternatives to group IV elements in dielectric matrices are alloys of group IV
elements. Several of these are attractive
for controlling the band gap in a group IV tandem cell as summarized in [4.5.27].
SiC alloys with excess Si have been investigated both as material with
precipitated Si and SiC nanocrystals and as amorphous unannealed alloys. Band
gap control over the range of about 1.7 to 2.2eV is possible for the Si
nanocrystals material by control of nanocrystal size [4.5.28]. Si:Ge alloys are
of course well known as a means of continually varying the band gap from 0.7 to
1.1eV. But whilst they are suitable in a
tandem cell for cells under a Si cell, they are not suitable for a tandem cell
element on top of a Si cell.

Alloying
of Sn with small percentages of Ge can stabilize the cubic α-Sn phase
[4.5.29]. The Ge rich Ge:Sn alloy can also be grown. These alloys have band
gaps from 0.2-0.7eV and are hence potentially useful for cells to go under a Si
cell in a tandem, but again are not suitable for cells with higher band gap
than Si. However, the Ge:Sn alloy can also be used to stabilize the α
phase in nanocrystals incorporated in a dielectric matrix. SiGeSn alloys are a
variation of Sn:Ge alloys. The addition of less than 1% of Si allows lattice
matching of the alloy with Ge substrate [4.5.30] with addition of more Si also
allowing a higher range of direct band gap of 0.8-1.4eV [4.5.31]. The advantage
of the ternary alloy/compound is that the extra degree of freedom allows
independent control of band gap and lattice parameter, which in turn allows for
better quality materials.

Ge:C
and Sn:C alloys and GeC and SnC compounds are interesting as tandem cell
materials [4.5.27]. Ge:C offers a potentially wide band gap range from 0.6 to
1.1eV and a fairly continuous range of composition. Several of the material
properties of GeC have been calculated [4.5.32] and GeC thin films can be
sputtered [4.5.33] with material approaching stoichiometric GeC for high
methane gas flows. In addition high sputter powers lead to a greater degree of
sp3 hybridization of carbon bonds thus leading to cubic diamond or
zinc-blende structures as opposed to the graphitic structures for sp2 hybridization
[4.5.34].

- 188 -

4.5.2.10 Transfer to Si QD growth using PECVD

Researchers: Tian Zhang, Ivan Perez-Wurfl

Collaboration with: Manuel
Mogano, University of Milan; Birger Berghoff, RWTH Aachen

Sputtering
is a high energy process in which many defects are incorporated in deposited
material, it is also relatively slow for
thin film deposition. A transfer of the technology to plasma assisted chemical vapour
deposition (PECVD) has the potential advantages of improving material quality
for Si QD nanostructures in SiO2 matrix and of reducing long term
material costs.

PECVD
has been used to date primarily for growth of Si QD nanostructures in SiNx
and in SiC [4.5.35, 4.5.36]. It has now been extended to a SiO2
matrix.

Figure 4.5.21: XRR measurements of (a) as deposited PECVD
multilayers of SRO/SiO2 on quartz substrate (b) after annealing at
1100oC.

Figure 4.5.21 shows the X-ray reflectivity (XRR)
measurements of multilayer SRO/SiO2 material as deposited by PECVD and after annealing at
1100oC. They clearly show a good quality multilayered structure as
deposited with reduced quality after annealing. This is very consistent with
the formation of Si nanocrystals breaking up the multi-layer arrangement.

Figure 4.5.22: Raman spectroscopy of
annealed multilayer PECVD sample (blue line) compared to c-Si wafer.

Figure
4.5.22 shows Raman data on the annealed multilayer nanostructure. The clear
asymmetric broadening of the peak at around
510cm-1 strongly indicates formation of Si nanocrystals.

- 189 -

4.5.2.11 Summary of Group IV nanostructures for tandem cell
elements

Modelling
of the parameters of Si nanostrucrue growth and measured performance has
indicated that the band gap of these materials is dominated by defects. This is
allowing a more systematic investigation of the material quality in order to
move towards higher quality material to give higher VOC devices.
Currents have been improved through light trapping using both rear reflection
and plasmonics. On measuring devise significant temperature increase is seen
because of the high resistivity, therefore
lower temperature measurements of Voc have seen an increase to over
500mV for devices that are in effect locally at room temperature.
Reduced resistivity has been seen in structures with nitride interlayers and
other group IV elements have been incorporated into Ge QW and Ge:C alloys.
Further improvement of device parameters is expected as greater understanding
of the underlying material properties is gained with the current robust
characterisation methodology.

4.5.3 Hot Carrier Cells

Researchers: Shujuan Huang, Santosh
Shreshtha, Dirk KÖnig, Yukiko Kamikawa, Robert Patterson, Pasquale Aliberti, Binesh Puthen Veettil,
Ivan Perez-Wurfl, Andy Hsieh, Yu Feng, James Rudd, Pengfei Zhang, Yao Yao,
Hongze Xia, Neeti Gupta, Yuanxun Liao, Suntrana Smyth, Xi Dai, Pei Wang, Simon
Chung, Martin Green, Gavin Conibeer

Hot carrier solar cells offer the possibility of very
high efficiencies (limiting efficiency above 65% for unconcentrated
illumination) but with a structure that could be conceptually simple compared
to other very high efficiency PV devices – such as multi-junction monolithic
tandem cells. For this reason, the approach lends itself to ‘thin film’ deposition techniques, with their
attendant low costs in materials and energy usage and facility to use abundant,
non-toxic elements.

An
ideal hot carrier cell would absorb a wide range of photon energies and extract
a large fraction of the energy to give very
high efficiencies by extracting ‘hot’ carriers before they thermalise to the
band edges. Hence an important property of a hot carrier cell is to slow
the rate of carrier cooling to allow hot carriers to be collected whilst they
are still at elevated energies (“hot”), and thus allowing higher voltages to be
achieved from the cell and hence higher efficiency. A hot carrier cell must
also only allow extraction of carriers from the device through contacts which
accept only a very narrow range of energies (energy selective contacts or
ESCs). This is necessary in order to prevent cold carriers in the contact from cooling the hot carriers, i.e.
the increase in entropy on carrier extraction is minimized [4.5.37]. The
limiting efficiency for the hot carrier cell is over 65% at 1 sun and 85% at
maximum concentration – very close to the limits for an infinite number of
energy levels [4.5.1, 4.5.38, 4.5.39]. Fig. 4.5.23 is a schematic band diagram
of a hot carrier cell illustrating these two requirements.

Figure 4.5.23: Band diagram of the hot carrier cell. The
device has four stringent requirements: a) To absorb a wide range of photon energies;
b) to slow the rate of photogenerated carrier cooling in the absorber; c) To
extract these ‘hot carriers’ over a narrow range of energies, such that excess
carrier energy is not lost to the cold contacts; d) to allow efficient
renormalisation of carrier energy via carrier-carrier scattering.

- 190 -

Modelling
of Hot Carrier efficiencies has progressed with implementation of real material
properties to give more realistic efficiencies for InN which include Auger
processes and more realistic contact structures.
Significant progress has been made on demonstrating resonance in double barrier
selective energy structures. Further work on triple barrier double Si QW
structures has been carried out for rectifying ESCs. This is complemented by
improvements in 2/3D modelling of transport in these ESC structures. For
absorbers, modelling of nanocrystals superlattice arrays has been applied to
real material systems. The growth of such systems in both III-V QD
superlattices with collaborators and with colloidal Langmuir-Blodgett
dispersion of Si nanocrsystals has produced structures which are now being
characterised for their modulation of phononic properties. Also measurement of
carrier cooling rates has been extended to other large phononic gap bulk
materials including InN, demonstrating the importance of material quality.
Design of structures for hot carrier cells which should be practical and
realisable has developed, with the device properties more carefully specified
and plans for fabricating such structures in real devices.

4.5.3.1 Modelling of hot carrier solar cell efficiencies
using optimised absorber and ESCs

Researchers: Yu Feng, Pasquale Aliberti, Hongse Xia, Gavin Conibeer

The
performance of a hot carrier solar cell device has been evaluated taking into
account the non-equilibrium distributions of electrons and holes as well as
different rates of Coulomb scattering between charge carriers. This includes
processes of electron-electron (e-e) scattering, hole-hole (h-h) scattering and
electron-hole (e-h) scattering. Their effects on the carrier renormalization
and the electron-hole energy transfer have been quantitatively examined under
the relaxation time approximation (RTA). Other processes relevant to an
operating device, including photo-generation, radiative recombination, carrier
extraction, carrier cooling (inelastic carrier scattering) and impact
ionisation/Auger recombination (AR-II), have also been incorporated with RTA.

The
1000-sun performance has been predicted for the cell configuration of an intrinsic
InN absorber (thickness: 50nm) with two symmetric ESCs, in accordance with
previous work. The maximum efficiencies have been computed with different
relaxation time parameters, following the A and B given in the following
equation.

 

where
τe eq, τheq, τehehs are the
relaxation time of the respective processes (i.e. e-e/h-h/e-h scattering), N is
the carrier concentration.

The
results are shown in Fig. 4.5.24. In the upper figure efficiency decreases with
A, suggesting insufficient rates of e-e and h-h elastic scattering could reduce
the efficiency. This is because the carriers
with energies within the transmission window of the ESCs will be depleted
unless the elastic scattering is quick enough to supply new carriers
with these energies. The curve at B = 108 is almost flat, for e-h
elastic scattering quick enough to prevent carrier depletion, noting that e-h
scattering allows both carriers to exchange their energies, providing
possibilities of supplying carriers with the energies of extraction. For the
same reason, with negligible e-e and h-h scattering (A > 10-16)
the efficiency tends toward a constant value instead of dropping to zero. The
lower figure in Fig. 4.5.24 shows that the efficiency decreases with the
relaxation time of e-h scattering. It is partly due to the carrier depletion
especially with insufficient e-e and h-h scattering, at A = 10-14.
With sufficient e-e and h-h scattering, for example the equilibrium limit in
which carriers are completely re-normalised, the same trend still exists. In
fact a long relaxation time of e-h scattering blocks the energy flow from
electrons to holes, allowing the holes to relax rapidly due to their fast
thermalisation events compared to electrons
hence leaving the electron population hot. This would limit the hole flux out
of the device since only holes with an elevated energy and within a
small energy window could be extracted. On the contrary, the efficiency goes up
with B if we replace the hole ESC with a normal contact, one that

- 191 -

is
transmissive for all holes, since less e-h scattering means less energy loss
from hot electrons. To optimize the cell performance, the selection of the hole
contact depends on the e-h scattering rate in the absorber. A normal
non-selective valence band contact is beneficial for slow scattering rates (B
> 1011) while the ESC
configuration adopted here has a greater advantage for rapid scattering rates
(B < 1011).

Figure 4.5.24: Maximum efficiency with different A and B.
The e-h isolation limit indicates no scattering between electrons and holes.
The equilibrium limit indicates ultrafast e-e scattering and h-h scattering.

The
energy-dependent distributions of electrons and holes are shown in Fig. 4.5.25,
corresponding to the Maximum Power Points
(MPPs) with different A and B values. From left to right the figures show the
trend of increasing carrier temperature with e-e and h-h scattering rates, as
with less carrier depletion a faster
extraction rate can occur reducing the retention time of carriers. From top to
bottom the figures demonstrate a larger deviation between electron distribution
and hole distribution with less e-h scattering. The carrier depletion
within the transmission window of ESCs is illustrated in the corresponding
insets. The resupply of depleted holes is more rapid than electrons because of
the proportionality between the scattering rate and the carrier effective mass.
With a small A, i.e. fast e-e/h-h scattering rates, the supply of holes is
sufficient for the carrier extraction, leading to a larger current density and
a higher overall efficiency. In this case the electron depletion could be even
more severe since the increase in current depletes electrons more quickly. This
is observed with B = 1010 and 1012. Apart from the region
of carrier depletion the distributions are close to Fermi functions, which may
be used for defining a “quasi-temperature” for each type of carrier.

- 192 -

Figure 4.5.25: Energy-dependent distributions of electrons
(black solid line) and holes (red dashed line) when operating at the Maximum
Power Points (MPPs) with different A and B: A = 10-14 (left), A = 10-19 (right); B = 1010
(upper), B = 1012 (middle), B = 1014 (lower). The
depleted region within the transmission window is enlarged in the
corresponding inset.

The
calculated current-voltage curves are shown in Fig. 4.5.26. With faster e-h
scattering (smaller B) the short- circuit
current density is increased while the open-circuit voltage is reduced. It is
difficult to extract holes at the elevated energy level required by ESCs
unless e-h scattering is quick enough to resupply highly energetic holes. On
the other hand the increase of voltage with slower e-h scattering comes from
the hotter electron distribution at the open-circuit condition, with less
energy lost to the thermalised holes. If a
HCSC is designed with the narrow high energy transmission window for holes, as
adopted in this work, the maximum efficiency is always higher for quicker e-h
scattering. The drawback of slow e-h scattering would be pronounced for insufficient
e-e and h-h scattering rates, since the significant carrier depletion would
inhibit the carrier extraction resulting in a colder carrier distribution.
Hence the quasi-temperatures of both electrons and holes are low at MPPs, if
e-h interaction is weak, while in the lower inset the contrast between them
increases. The carrier quasi-temperature insets also indicate a test of the
validity of commonly used assumptions: electrons and holes share the same
temperature only for fast e-h scattering (B < 1010) while holes
are completely thermalized when e-h scattering rates are slow (B > 1014).

- 193 -

For
real materials the choice of parameters A and B can be determined by computing
the realistic scattering rates between carriers. We have calculated the scattering
relaxation times for bulk cubic-InN, taking into account both the direct
Coulomb potential and the exchange potential. The results closely follow the relation shown in the equation
above for relaxation times, with A = 2.5X10-19 and B = 2.4X1011.

The
results show that the maximum efficiency varies significantly with the carrier
scattering properties due to energy-dependent carrier depletion and asymmetric
statistics of electrons and holes. With different hole contacts the dominant
loss mechanism switches: with an ESC at an elevated energy the hole extraction
is blocked until significant heat is transferred from hot electrons. Thus at
the hole side an ESC is more suitable for fast e-h scattering while a normal
contact is beneficial for slow e-h scattering rates. In addition, the RTA model
suggested here provides the possibility of analyzing any realistic HCSC device.
It also serves to optimize the transmission properties of both contacts. According to this work it heavily
depends on the carrier scattering properties in the absorber.

4.5.3.3 Energy Selective Contacts (ESCs)

Researchers: Binesh Puthen-Veettil,
Yu Feng, Andy Hsieh, Yuanxun Liao, Santosh Shrestha, Gavin Conibeer

One
practical implementation of the requirement for a narrow range of contact energies
is an energy selective contact (ESC) based on double barrier resonant
tunnelling. Tunnelling to the confined energy levels in a quantum dot layer
embedded between two dielectric barrier layers, can give a conductance sharply
peaked at the line up of the Fermi level on the ‘hot’ absorber side of the
contact with the QD confined energy level. Conductance both below this energy
and above it should be very significantly
lower. This is the basis of the current work on double barrier resonant
tunnelling ESCs.

4.5.3.3.1 Modelling optimised materials for Energy Selective
Contacts

Energy selective contacts (ESCs) for a HC solar cell can
be implemented using different materials and structures. The main property that needs to
be achieved is precise energy selectivity in carrier extraction. Different
models have been developed to identify possible materials for ESCs and to optimize their properties. Amongst the different
possible material configurations, group IV and group III-V double
barrier resonant tunnelling structures look to be the most promising.

Figure 4.5.26: Schematic diagram of a
double barrier structure. Silicon QDs are formed in a dielectric matrix that may not be the same as the
barrier material. The growth direction of the structure is along the z axis.

Modelling
of group IV materials has shown that the structure with the highest potential
to be used as ESC consists of double barrier structure (DBS) with quantum dots
(QDs) embedded in a dielectric matrix.

Modelling
of electrical properties for the structure shown in Fig. 4.5.26 has been
performed for Si QDs in Si based dielectric materials. The reasons for focusing
on Si in the first instance for group IV

- 194 -

materials
are abundance and well established growth techniques. Structures consisting of
Si QDs and the following dielectric
barriers have been modelled: SiO2, Si3N4 or
SiC. Results show that optimal energy selectivity properties are
obtained with higher lateral confinement within the middle layer of the
structure and higher vertical conductivity. Amongst the material taken into
account in the simulations, structures consisting of Si QDs in a SiO2
with SiC barriers have shown the best overall energy extraction properties.

ESCs
for a III-V based HC solar cell can be realized using a QW structure in a DBS,
probably requiring either MBE or MOVPE growth. Modelling results have shown
that, for a HC cell based on an InN absorber, the optimum configuration for
ESCs is constituted by an InN/InXGa1-XN DBS [4.5.40].
This structure allows modulation of the extraction energy and shape of the
transmission probability function
independently for the electrons and holes contacts. The value of extraction
energy can be modified by engineering the stoichiometry and the
thickness of the QW structure. In general the optimal extraction energy for
electrons and holes is different, due to the different values of effective
mass. Thus, the hole ESC QW should be physically thinner and have a higher
barrier than the electron ESC QW to achieve similar currents at reasonable
extraction energies, as illustrated in Fig. 4.5.27.

Figure 4.5.27: Schematic of a HC solar cell based on a InN
absorber and InN/InGaN ESCs.

The
transmission coefficient of a double-barrier semiconductor structure has been
numerically calculated using a tight-binding method. The phonon scattering
during the transmission has been incorporated with a Greens function method.
This constructs a framework for simulating a real resonant tunneling system
with ultra-thin layers. It will help in optimizing the material system for the
ESCs. Some sampling results for the DBS of AlAs/GaAs/AlAs are shown in Fig.
4.5.28.

Figure 4.5.28: The total transmission (y axis: 01) profile
as a function of the normally-penetrating electron energy (x axis: eV) through
the zinc-blende DBS AlAs(3 unit cells)/GaAs (3 unit cells)/AlAs (3 unit cells),
with different phonon scattering properties. Left: No phonon scattering
considered; Right: with phonon scattering and the initial phonon occupation
number = 0.

- 195 -

Here
we mainly aim to utilize the 1st energy peak. From the transmission
calculation of an AlAs/GaAs/AlAs double-barrier structure, this peak is visible
and reasonably significant if only normally incident electrons are considered.
(These constitute the majority of energy transmission in any case.) With the
consideration of phonon scattering, side-peaks appear which are not favorable
for energy-selective extraction. This
indicates a preference of choosing less polar materials. Besides, the addition
of arbitrary directions off normal may broaden the transmission peak with a
little loss on the selectivity. Such a Greens function model can treat
any type of atomic system in terms of the scattering problem. It is ready for
use as a tool to choose the optimal material system for the energy-selective
transmission.

4.5.3.3.2 Triple barrier resonant tunnelling structures for
carrier selection and rectification

Researchers: Gavin Conibeer, Andy Hsieh, Santsoh Shrestha

A
normal energy selective contact (ESC) using double barrier resonant tunnelling
is symmetric and does not rectify electrons from holes. An ESC which uses
triple barrier resonant tunnelling through two separately tuned quantum well or
quantum dot layers can give a much greater energy selectivity and also is a
filter to separate electrons and holes in opposite directions (i.e.
rectification). The mini-band line-ups of the confined levels need to be tuned
by the well thicknesses, effective mass of electrons and holes and barrier
heights such as illustrated schematically in Fig. 4.5.29. With appropriate choice of these for the left contact
there can be an alignment of the confined levels such as to give a
resonant channel in the conduction band for electrons but misalignment of
levels in the valence band such that there is no channel for holes. Similarly
for the right contact the valence band confined
levels are aligned to allow holes to pass but misaligned in the conduction band
so as to block electrons. For non-equal electron and hole effective
masses (as is the case for most materials) this condition will in general be
met [4.5.41, 4.5.42].

Figure 4.5.29: Triple barrier double well resonant
tunnelling energy selective contacts. Wells are of different width designed to
give allignment of 1st and 2nd confined levels in the conduction
band on the left and for 1st and
2nd confined levels in the valence band on the right. In general for
non-equal effective masses allignment of first and second confined
energy levels for electrons will not give the same condition for holes thus giving
electron/hole rectification and a preferred collection direction in the
external circuit.

The collection channels for electrons and holes would
ideally be aligned with the peak occupancies of energy of the hot electron and hot hole
populations respectively, which in turn will depend on the respective
temperatures of these populations. This condition will give the highest
efficiency, but as discussed in Section 4.5.3.1, for high electron-electron and
hole-hole carrier scattering rates, it is not very critical.

- 196 -

Ideally
there would be two of these triple barrier double QW/QD devices on either side
of the absorber, but as there is much less energy in the high effective mass
hole population a simpler double barrier or simple band contact on the hole
side is also possible, without much loss in collected energy, as investigated
in Section 4.5.3.1. Various materials combinations could be used for such a
design, but specifically we are looking at III-V triple barrier QWs based on
the InGaN/GaN system. Again this is addressed to some extent in Section
4.5.3.1.

The
exact thickness of QWs or size of QDs and barrier thicknesses and contact work
functions will have to be tuned for the
particular confined energy levels involved in each material. Use of more than two
QW or QD layers on either side is also possible. This will increase energy
selection by requiring even more careful alignment.

This
is the first application of this concept to energy selective contacts for hot
carrier, Hot Lattice or Thermoelectric cells, but the concept itself was first
suggested by [4.5.43] and has been used for energy filtering in quantum cascade
lasers for several years. It is also not the first application of the concept to solar cells as [4.5.44], at Imperial
College, have used the concept for a multi-QW solar cell design. But it
is the first application to Hot Carrier or Hot Lattice devices and its ability
to rectify is likely to be a very significant advantage.

4.5.3.3 Hot Carrier Absorbers: slowing of carrier cooling

Carrier
cooling in a semiconductor proceeds predominantly by carriers scattering their
energy with optical phonons. This builds up a non-equilibrium ‘hot’ population
of optical phonons which, if it remains
hot, will drive a reverse reaction to re-heat the carrier population, thus
slowing further carrier cooling. Therefore the critical factor is the
mechanism by which these optical phonons decay into acoustic phonons, or heat
in the lattice. The principal mechanism by which this can occur is the Klemens mechanism, in which the optical phonon
decays into two acoustic phonons of half its energy and of equal and
opposite momenta [4.5.45]. The build-up of emitted optical phonons is strongly
peaked at zone centre both for compound semiconductor due to the FrÖlich
interaction and for elemental semiconductors due to the deformation potential
interaction. The strong coupling of the FrÖlich interaction also means that
high energy optical phonons are constrained to near zone centre even if
parabolicty of the bands is no longer valid [4.5.46]. This zone centre optical
phonon population determines the dominant optical phonon decay mechanism is
this pure Klemens decay.

4.5.3.3.1 Modelling of electron-phonon interactions:

Researchers: Yu Feng, Hongze Xia, Santosh Shrestha, Robert Patterson, Gavin Conibeer

Another
important factor is the Frohlich interactions between hot electrons and
longitude optical phonon modes. The
importance of investigating the fundamental particle interactions occurring in
the device is demonstrated by the significant dependence between the
carrier energy relaxation time and the output efficiency. The underlying
mechanism of phonon bottleneck effect involves two parts, the first is the
block of Frohlich interaction between electrons and phonons; while the second
is the block of phonon decay from the
optical modes into acoustic modes. Also the build-up of non-equilibrium of
acoustic phonons may also contribute to reduced carrier energy relaxations. In
order to understand the mechanism of slow relaxation rates in different
types of materials the three interaction processes need to be simulated.

The
Frohlich interaction occurs in polar semiconductors and creates energy exchange
between electrons and longitudinal phonons. It affects the total energy and
state transition rates by an interaction
Hamiltonian. The Frohlich interaction Hamiltonian for superlattice structures
involves an overlap integral over one dimension (the direction
perpendicular to planes). The derivation of the Hamiltonian involves the
interaction between electrons and polarization fields induced by longitudinal
mode vibrations. By obtaining the divergence of the polarization field an
effective phonon envelope function is developed and incorporated in the overlap
integral.

- 197 -

4.5.3.3.2 Phonon decay mechanisms

Researchers: Gavin Conibeer, Santosh Shrestha, Robert Patterson, Yu Feng, Hongze
Xia, Suntrana Smyth

An
optical phonon will decay into multiple lower energy phonons. The processes
require the conservation of energy and momentum. However, additionally, the
principal decay path must be into two LA
phonons only, which are of equal energy and opposite momenta, via an
anharmonicity in the lattice, this was suggested by Klemens [4.5.45] and
has been shown to apply to a wide range of materials.

Wide Phononic Gaps in III-Vs and analogues

For
some compounds in which there is a large difference in masses of the
constituent elements, there exists a large gap in the phonon dispersion between
acoustic and optical phonon energies. If large enough this phonon band gap can
prevent Klemens’ decay of optical phonons, because no allowed states at half
the LO phonon energy exist. InN is an example of such a material with a very
large phonon gap. The prevention of the Klemens’ mechanism forces optical
phonon decay via the next most likely, Ridley mechanism, of emission of one TO
and one low energy LA phonon. Such a mechanism only has appreciable energy loss
(although still much less than Klemens’ decay) if there is a wide range of
optical phonon energies at zone centre. This is only the case for lower
symmetry structures such as hexagonal. For a high symmetry cubic structure, LO
and TO modes are close to degenerate at
zone centre and the Ridley mechanism is severely restricted or forbidden.
Unfortunately cubic InN is very difficult to fabricate precisely because
of the large difference in masses that give it its interesting phononic
dispersion.

Slowed
cooling has been observed in some III-V compounds in which there is a large
difference in atomic mass. This has been shown in slowed carrier cooling for
InN [4.5.47] and for slowed carrier cooling in InP compared to the small mass
ratio GaAs [4.5.48].

Analogues of InN with abundant elements, but also with
narrow Eg include II-IV-VI compounds such as ZnSnN2, IIIA
nitrides such as LaN and YN which should have large phonon gaps, and IVA
nitrides such as ZrN and
HfN which are readily available. Bi and Sb compounds should also have large
phonon gaps but are not abundant. Group IV compounds have large calculated gaps
and small Egs, as well as several other advantages [4.5.49]. Fig. 4.5.30 shows
calculations of phonon dispersions for group IV compounds which have large
phonon band gaps, sufficient to block Klemens’ decay.

Figure 4.5.30: Adiabatic bond charge calculations of phonon dispersions for group IV
compounds. Phonon gaps increase as the mass ratio increases with those for GeC
and SnC large enough to block Klemens’ decay.

Slowed carrier cooling in MQWs

Low
dimensional multiple quantum well (MQW) systems have also been shown to have
lower carrier cooling rates. Comparison of bulk and MQW materials has shown
significantly slower carrier cooling in the latter. Fig. 4.5.31 shows data for
bulk GaAs as compared to MQW GaAs/AlGaAs

- 198 -

materials as measured using time resolved transient
absorption by Rosenwaks [4.5.50], recalculated to show effective carrier temperature as a
function of carrier lifetime by Guillemoles [4.5.51]. It clearly shows that the
carriers stay hotter for significantly longer times in the MQW samples, particularly
at the higher injection levels by 11⁄2 orders of magnitude. This is due to an
enhanced ‘phonon bottleneck’ in the MQWs allowing the threshold intensity at
which a certain ratio of LO phonon re-absorption to emission is reached which
allows maintenance of a hot carrier population, to be reached at a much lower
illumination level. More recent work on strain balanced InGaAs/GaAsP MQWs by
Hirst [4.5.52] has also shown carrier temperatures significantly above ambient,
as measured by PL. Increase in In content to make the wells deeper and to
reduce the degree of confinement is seen to increase the effective carrier
temperatures.

The
mechanisms for the reduced carrier cooling rate in these MQW systems are not
yet clear. However there are three effects that are likely to contribute. The
first is that in bulk material photogenerated hot carriers are free to diffuse
deeper into the material and hence to reduce the hot carrier concentration at a
given depth. This will also decrease the density of LO phonons emitted by hot
carriers as they cool and make a phonon bottleneck more difficult to achieve at
a given illumination intensity. Whereas in a MQW there are physical barriers to
the diffusion of hot carriers generated in a well and hence a much greater
local concentration of carriers and therefore also of emitted optical phonons.
Thus the phonon bottleneck condition is achieved at lower intensity.

The
second effect is that for the materials systems which show this slowed cooling,
there is very little or no overlap between the optical phonon energies of the
well and barrier materials. For instance the optical phonon energy ranges for
the GaAs wells and AlGaAs barriers used in [4.5.50] at 210-285meV and
280-350meV, respectively, exhibit very little overlap in energy, with zero
overlap for the zone centre LO phonon energies of 285 and 350meV [4.5.53].
Consequently the predominantly zone centre LO phonons emitted by carriers
cooling in the wells will be reflected from the interfaces and will remain
confined in the wells, thus enhancing the phonon bottleneck at a given
illumination intensity.

Thirdly,
if there is a coherent spacing between the nano-wells (as there is for these
MQW or superlattice systems) a coherent Bragg reflection of phonon modes can be
established which blocks certain phonon
energies perpendicular to the wells, opening up one dimensional phononic band
gaps (analogous to photonic band gaps in modulated refractive index
structures. For specific ranges of nano-well and barrier thickness these
forbidden energies can be at just those energies required for phonon decay. This coherent Bragg reflection
should have an even stronger effect than the incoherent scattering of the
second mechanism above at preventing emission of phonons and phonon decay in
the direction perpendicular to the nano-wells.

It
is likely that all three of these effects will reduce carrier cooling rates.
None depend on electronic quantum confinement and hence should be exhibited in
wells that are not thin enough to be quantized but are still quite thin
(perhaps termed ‘nano-wells’). In fact it may well be that the effects are
enhanced in such nano-wells as compared to full QWs due to the former’s greater
density of states and in particular their greater ratio of density of
electronic to phonon states which will enhance the phonon bottleneck for
emitted phonons. The fact that the deeper and hence less confined wells in
[4.5.52] show higher carrier temperatures is tentative evidence to support the
hypothesis that nano-wells without quantum confinement are all that are
required. Whilst several other effects might well be present in these MQW
systems, further work on variation of nano-well and barrier width and
comparison between material systems, will distinguish which of these reduced
carrier diffusion, phonon confinement or phonon folding mechanisms might be
dominant.

- 199 -

Figure 4.5.31: Effective carrier temperature as a function
of carrier lifetime for bulk GaAs as compared to GaAs/AlGaAs MQWs: time
resolved transient absorption data for different injection levels, from
Rosenwaks [4.5.50], recalculated by Guillemoles [4.5.51].

4.5.3.3.3 Hot carrier cell absorber requisite properties

The
above discussion allows us to estimate the major properties required for a good
hot carrier absorber material. These are listed below in approximate order of
priority, although their relative importance may well change in light of future
research:

	
  

 	
  

 
	
 1.

 	
 Large
 phononic band gap (EO(min) - ELA) - to suppress Klemens’ decay, requires
 large mass difference between elements.

 
	
  

 	
  

 
	
 2.

 	
 Narrow
 optical phonon energy dispersion (ELO - EO(min)) – to minimise the loss by
 Ridley decay, requires a high symmetry.

 
	
  

 	
  

 
	
 3.

 	
 Small Eg < 1eV – to allow broad range of
 photon absorption.

 
	
  

 	
  

 
	
 4.

 	
 A
 small LO optical phonon energy (ELO) – to reduce the amount of energy lost
 per LO phonon emission.

 
	
  

 	
  

 
	
 5.

 	
 A
 small maximum acoustic phonon energy (ELA). This maximises (EO(min) - ELA)
 and is important if ELO is also small.

 
	
  

 	
  

 
	
 6.

 	
 Good
 renormalisation rates in the material, i.e. good e-e and h-h scattering. This
 condition is met in most semiconductors quite easily, with e-e scattering
 rates of less than 100fs. But it may be comprised in nanostructures.

 
	
  

 	
  

 
	
 7.

 	
 Good
 carrier transport in order to allow transport of hot carriers to the
 contacts.

 
	
  

 	
  

 
	
 8.

 	
 Ability
 to make good quality, ordered, low defect material.

 
	
  

 	
  

 
	
 9.

 	
 Earth
 abundant and readily processable materials.

 
	
  

 	
  

 
	
 10.

 	
 No,
 or low, toxicity of elements, compounds and processes.

 

4.5.3.3.4 Hot carrier absorber: choice of materials

InN
has most of these properties, except 4, 8 & 9, and is therefore a good
model material for a hot carrier cell
absorber. InN is investigated below in combination with its alloys with GaN.
However the abundance of In is very low, so it is difficult to see InN as a
long term material suitable for large scale implementation. Hence analogues of
InN, which retain its interesting property of a large phonon band gap,
are also investigated.

Analogues of InN

As
InN is a model material, but has the problems of abundance and bad material
quality, another approach is to use analogues of InN to attempt to emulate its
near ideal properties. These analogues can be II-IV-nitride compounds, large
mass anion III-Vs, group IV compounds/alloys or nanostructures.

- 200 -

Figure 4.5.32: Use of the periodic table to analyse possible
analogue compounds of InN based on atomic mass combination and
electro-negativity.

II-IV-Vs: ZnSnN; ZnPbN; HgSnN;
HgPbN

With
reference to Fig. 4.5.32, it can be seen that replacement of In on the III
sub-lattice with II-IV compounds is analogous and is now quite widely being
investigated in the Cu2ZnSnS4 analogue to CuInS2
[4.5.54].

ZnGeN
can be fabricated [4.5.55] and is most directly analogous with Si and GaAs.
However, its band gap is large at 1.9eV. It also has a small calculated
phononic band gap [4.5.56]. ZnSnN has a smaller electronic gap (1 eV) and
larger calculated phononic gap [4.5.56]. It is however difficult to fabricate, and also its phononic gap is not as
large as the acoustic phonon energy making it difficult to block Klemens
decay completely. HgSnN or HgPbN should both have smaller Eg and
larger phononic gaps. These materials have not yet been fabricated [4.5.57].

Large mass cation:

The
Bi and Sb compounds have large predicted phononic gaps and Bi is a relatively
abundant material, with only low toxicity [4.5.57]. BiB has the largest
phononic gap but AlBi, Bi2S5, Bi2O3 (bismuthine)
are also attractive. Similarly SbB has a large predicted phononic gap. That for
AlSb is the same size as the acoustic phonon energy and its band gap is 1.5eV,
making it marginal as an absorber material and similar to InP.

Group IIIA III-Vs

LaN
and YN both have large phononic gaps whilst that for ScN is too small. The
Lanthanides can also form III-Vs. ErN and other RE nitrides can be grown by
MBE. The phononic band gaps of the Er compounds are predicted to be large,
because of the heavy Er cation, but its
discrete energy levels make it not useful as an absorber, although the
combination of properties in a nanostructure could be advantageous.

Group IV alloy/compounds:

All
of the combinations Si/Sn, Ge/C or Sn/C look attractive with large gaps
predicted in 1D models. However being all group IVs they only form weak
compounds. Unfortunately SiC, whether 3C, 4H or 6H, has too narrow a phononic
gap. Nonetheless GeC does form a compound and is of significant interest
[4.5.58].

- 201 -

There
are also several other inherent advantages of group IV compounds/alloys all of
which are associated with the four valence electrons of the group IVs which
result in predominantly covalent bonding:

	
  

 	
  

 
	
 a)

 	
 The
 elements form completely covalently bonded crystals primarily in a diamond
 structure (tetragonal is also possible as in βSn). However for group IV
 compounds the decreasing electronegativity down the group results in
 partially ionic bonding. This is not strong in SiC and whilst it tends to
 give co-ordination numbers of 4, can nonetheless result in several allotropes
 of decreasing symmetry: 4c, 4h, 6h. However, as the difference in period
 increases for the as yet theoretical
 group IV compounds, so too does the difference in electronegativity and hence
 also the bond ionicity and the degree of order. For a hot carrier
 absorber this is ideal because it is just such a large difference in the
 period which is needed to give the large mass difference and hence large
 phononic gaps. All of GeC, SnSi, SnC (and the Pb compounds) have computed
 phononic gaps large enough to block Klemens decay, and should also tend to
 form ordered diamond structure compounds.

 
	
  

 	
  

 
	
 b)

 	
 Because
 of their covalent bonding, the group IV elements have relatively small
 electronic band gaps as compared to their more ionic III-V and much more
 ionic II-VI analogues in the same period: e.g. Sn 0.15eV, InSb 0.4eV, CdTe
 1.5eV. In fact to achieve approximately the same electronic band gap one must
 go down one period from group IV to III-V and down another period from III-V
 to II-VI: e.g. Si 1.1eV, GaAs 1.45eV, CdTe 1.5eV. This means that for group IV compounds there is greater scope for large
 mass difference compounds whilst still maintaining small electronic
 band gaps. A small band gap of course being important for broadband
 absorption in an absorber - property 3 in the desirable properties for hot
 carrier absorbers listed above.

 
	
  

 	
  

 
	
 c)

 	
 The smaller Eg would tend to be for the
 larger mass compounds of Pb or Sn. Which, to give large mass difference, would be compounded with
 Si or Ge. This trend towards the lower periods of group IV also means that
 the maximum optical phonon and maximum acoustic phonon energies will be
 smaller for a given mass ratio - the desirable properties 4 and 5.

 
	
  

 	
  

 
	
 d)

 	
 Furthermore,
 unlike most groups, the group IV elements remain abundant for the higher mass
 number elements – desirable property 9. Property 10 is also satisfied because
 the group IVs have low toxicity.

 

Nanostructures:

As
discussed in Section 4.5.3.3.9, the phonon dispersion of QD nanostructures can
be calculated in the same way as compounds.
Their phononic properties can be estimated from consideration of their combination
force constants. Hence it is possible to ‘engineer’ phononic properties in a
wider range of nanostructure combinations.
Of the materials discussed above the Group IVs lend themselves most readily
to formation of nanostructures instead of compounds due to their predominantly
covalent bonding, which allows variation in the coordination number. Therefore
the nanostructure approaches of Section
4.5.3.3.9 are consistent with a similar description as analogues of InN,
whether it be III-V QDs, colloidally dispersed QDs or for core shell
QDs.

4.5.3.3.5 Combined phononic / mnw absorber design

Researchers: Gavin Conibeer, Yu Feng, Hongze Xia, Robert Patterson, Suntrana Smyth

The
multiple nano-well structures have demonstrated reduced carrier cooling by enhancing
the phonon bottleneck, probably at a stage prior to optical phonon decay. Large
phononic band gap materials would seem to have strong potential to block
Klemens’ decay of optical phonons. A structure combining both structures should
give even greater reduction in carrier cooling rates as the mechanisms should not interfere directly with
each other and hence their effects should be additive. A combined
absorber for a hot carrier cell should look similar to Fig. 4.5.33.

This
has narrow nano-wells of phononic band gap material, not thin enough to give
quantum confinement, separated by thin barriers. The barriers would either have
an electronic band offset to block hot carrier diffusion or an optical phonon
energy offset with the nano-wells and probably a coherent nano-well structure,
or all three, depending which of the multiple nano-well (MNW)

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mechanisms
discussed above is dominant. This will enhance the phonon bottleneck either by
preventing hot carrier diffusion, by reflecting and confining optical phonons
or by blocking certain folded phonon modes, respectively, or all three. The
phononic nano-wells themselves will further enhance phonon bottleneck by
preventing Klemens’ decay of optical to acoustic phonons. This should maximise
the phonon bottleneck and minimise carrier cooling for a given illumination
intensity. A lattice matched pair of large phononic band gap nano-well and
barrier materials (InN/InGaN) is suggested in Fig. 4.5.33 (b), whereas in (c) a
wider range of possible InN analogue phononic band gap materials could be used
in a thin film structure with thin probably oxide barriers. The thin barriers
(a few nm) should facilitate tunnelling between wells and hence transport of
the carriers to the contacts.

Figure 4.5.33: Hot Carrier cell design combining Multiple
Nano-Well (MNW) with phononic band gap materials: (a) schematic and (b) band
structure for InN/InGaN epitaxial multiple nanowell with phononic band nano-wells; (c) band structure with
nano-wels of analogues of InN with large phononic band gap with thin
film barriers.

4.5.3.3.6 Alternative quantum well structure with integrated
energy selective contacts

Researchers: Dirk KÖnig, Binesh Puthen-Veettil

Use of quantum wells in the absorber region has the
advantage of a continuous density of states in the plane above the first confined energy level.
This allows continuous absorption of photon energies above this energy. With
barriers of different phonon energy such that they reflect optical phonons, phonons can be confined in the z direction and
increase phonon bottleneck the consequent decrease in carrier cooling
rate. If in addition the materials and barrier are chosen such that they give
two confined energy levels below the barrier a significant distance apart, then
these can select only a very few carrier energies to be transported through the
device, thus effectively playing the role of energy elective contacts. This
thus reduces the need for external energy selective contacts and allows the
contacts to be optimised for the appropriate Fermi level and work function
alignment to give electron collection and hole reflection at one contact and
hole collection and electron reflection at the other, thus giving the
rectification required.

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Figure 4.5.34: InAs/AlSb MQW on AlSb with GaP top electron
contact layer to give integrated hot carrier absorber and carrier energy
selected structure without need for external ESCs.

Figure
4.5.34 shows such a structure with InAs wells and AlSb barriers, designed to
give confined energy levels such that the 2nd
level aligns with the conduction band of heavily n-type GaP. The GaP and
AlSb contact materials act as hole and electron blocking layers respectively.
The valence band offset between InAs and AlSb is very small. This allows the
AlSb to collect thermalised holes from the valence band of the superlattice.
InAs and AlSb have a very small lattice mismatch (1.3%) so the whole structure
could be grown epitaxially on an AlSb substrate [ 4.5.59, 4.5.60].

Such
an integrated structure would give an ideal test bed for integration of
absorber MQW and internal energy selection of carriers which would not require
external ESCs and which gives rectification in the external circuit. The
specific materials and their thicknesses required would not allow optimisation
for slowing of hot carrier cooling, but would be able to be optimised for
appropriately spaced energy selection and collection.

4.5.3.3.7 Phononic dispersion modelling to predict phonon
lifetimes and decay rates in specific III-V or core-shell structures

Researchers: Hongze Xia, Yu Feng, Robert Patterson, Gavin Conibeer

InN/
InxGa1-xN multiple quantum-well superlattices (MQW-SL)
with wurtzite crystal structure are studied as the absorber of the hot carrier
solar cell. Such a structure will exploit both the significant
hot-phonon-bottleneck effects of the component materials and the known slowed
carrier cooling in MQWs. The former is due to the large contrast of the atomic
masses, and hence a large band-gap between high-lying and low-lying phonon modes.
The phonon band-gap effectively stops Klemens decay, i.e. one high-lying phonon
decaying into two low-lying phonons, and produce hot population on the polar
modes, feeding energy back to electrons. InN and InxGa1-xN
have very similar lattice structures, with
almost the same lattice constant. This benefits the solar energy conversion, in
terms of both the carrier transport and the reduction of recombination
sites. The 1-dimensional superlattice structure ensures a continuous electronic
energy spectrum and hence a broad-band absorption. The absorption is further
enhanced by the small electronic band-gap of InN.

The dispersion relations of phonon modes in
superlattices have been computed with a 1-dimensional atomic-plane model. Since the epitaxial
growth is intended to be along the high-symmetrical Γ - A direction for
both InN and InxGa1-xN layers, the periodicity of MQW-SL
and hence the zone-folding is along this
direction. For simplicity we assumed that the mode frequency of each
zone-folded mini-band only depends on the vertical component of the
wave-vector (i.e. along Γ - A). The reason for this assumption partly
comes from the high concentration of electron-emitted polar phonons around the
zone-center, where the dispersions are relatively flat compared to the mini-gaps.
In wurtzite

- 204 -

structure periodic atomic planes with alternating
elements align perpendicular to the Γ-A direction. If only considering the vertical component of
wave-vectors, all atoms in one atomic plane vibrate in phase and can be treated
as one uniform displacement. The phononic model adopted here treats a
1-dimensional chain with an equal length of the superlattice periodicity,
taking into account the plane-to-plane force constants. The plane-to-plane
force constants are calculated by adopting the conventional Keating potentials
for all bonds. A sample phononic dispersion relation is shown in Fig. 4.5.35.
The electronic structures, involving the wavefuntions and energies of all the
states, can be calculated by using the Kronig-Penney model for superlattices.
In equilibrium conditions space charges occur in the well layers (negative) and
in the barrier layers (positive) due to the difference of their electron affinities. The electron affinity
of InGaN is significantly lower than that of InN, giving a larger
off-set of the conduction band edge rather than that of the valence band edge.
A sample electronic dispersion relation is shown in Fig. 4.5.36.

Figure 4.5.35: The left figure indicates the phonon decay
paths and the sample dispersion for a SL structure with 3 layers of unit cells
inside each layer (barrier or well). The right figure shows the phonon
modulation function of a representative mode from each category of modes,
computed from the eigenfunctions of the lattice dynamic equation. The transverse
modes have similar modulation functions and hence only that part of the
longitude modes is shown.

Figure 4.5.36: The electronic dispersion relation and the
zone-center wave functions for a SL structure with 3 layers of unit cells
inside each layer (barrier or well). The wave vector is along the Gamma-A
crystal direction.

The calculation of the rate of polar interaction between
hot electrons (here emission by holes is not of concern) and polar phonons are based on the
Frohlich-type Hamiltonian and the 1-st order

- 205 -

perturbation theory. A hot reservoir of electrons at
1000K is assumed, with heat transferred to the cold reservoir of lattice modes
at 300K. The energy relaxation times referring to the polar phonon emission are illustrated in Fig. 4.5.37, for all the
MQW-SL configurations, i.e. for different well and barrier thicknesses and
different barrier materials. 18X18 combinations of the thicknesses are sampled
for representing all the possible structures from 2nm to 9nm. The number of
combinations comes from the fact that a
complete well/barrier layer should include an integer number of unit cells.
Figure 4.5.37 is generated by interpolating the relaxation time data of
the sampling combinations.

Figure 4.5.37: Energy relaxation times of the hot electron
system (1000K), in the reservoir of phonons
(300K), with superlattice structures of different well/barrier thicknesses and
InxGa1-xN compositions (Left: x=0 Right: x=0.2)

The
energy relaxation times of high-lying longitude optical phonons are
demonstrated in Fig. 4.5.38, for different combinations of well and barrier
thicknesses. For Indium mole fraction x=0 (left figure), the contrast of
relaxation times are larger than that for the case for x=0.2 (right figure);
this is reasonable for as the higher Indium content in the barrier layer would
make the structure closer to bulk InN. Both
figures show the same trend with different barrier/well thicknesses: the
relaxation time increases with thicker well layers and thinner barrier layers,
in spite of some irregular local variations. The regular variation
mainly results from the change of numbers of InN-like modes and InGaN-like
modes. Here a hot reservoir of high-lying modes at 1000K is assumed, with heat
transferred to the cold reservoir of low-lying lattice modes at 300K. The
non-equilibrium between the two systems is physical due to their significant
frequency separation.

Figure 4.5.38: Energy relaxation times of the high-lying LO
phonon system (1000K), in the reservoir of 1000K high-lying phonons and 300K
low-lying phonons, with superlattice structures of different well/barrier
thicknesses and InxGa1-xN compositions (Left: x=0 Right:
x=0.2)

To
explain the variation, we need to first examine the phonon dispersions of the
MQW-SL. Taking x=0 as an example, the left figure in Fig. 4.5.35 shows the
dispersions of InN/GaN SL structure, with

- 206 -

6
layers of nitrogen atoms inside each layer (the same for both the InN layer and
the GaN layer). From the number of atomic layers involved we could expect the
same number of high-lying optical modes which are InN-like and GaN-like
respectively. The GaN-like optical modes (blue color: solid line for LO, dash
line for TO) have much higher frequencies than the InN-like optical modes
(green color), for the bond force constants
of GaN is larger than for InN and the atomic mass of Ga is smaller than
In. There are also four branches (orange colour: two of LO and two of TO)
corresponding to the interfacial modes (IF), with frequencies between GaN-like
modes and InN-like modes. From the right figure
of Fig. (4), the vibrations can be seen to be almost completely confined in the
respective layers, for InN-like and GaN-like optical modes. For IF most
of the vibrational energy is within the 1st and 2nd nitrogen atoms from the interface, and hence has little overlap
with InN-like or GaN-like modes.

Considering
all three-phonon processes, only the Ridley channel and the Shrivistava-Barman
channel (decaying into a high- lying optical phonon and a low-lying optical
phonon) are allowed due to the energy conservation law. As in
wurtzite-structured SL, the difference between the acoustic branches and the low-lying optical branches becomes almost
indistinguishable with both having partial optical-like character. These
low-lying branches can be separated into two categories: GaN-confined modes (red lines) and GaN-InN mixed modes (grey lines).
In fact the InN-like low-lying modes sit within the allowed frequency
band of GaN, hence vibrations in GaN layers are excited too. Therefore no InN-confined modes exist in the low-lying branches.
Among all allowed three-phonon processes, GaN-like optical modes can
decay into both types of low- lying modes (See the solid arrow and the dashed
arrow in Fig. 4.5.35), while InN-like optical modes can only decay into the
mixed modes as they only overlap with the mixed modes. Besides, the energy gap
between GaN-like LO modes and GaN-like TO modes is relatively large; hence the
resulting low-lying phonons have relatively high energies, compared to the decayed
low-lying phonons from the InN-like modes. Since the low-lying branches with
high energies are generally flatter, leading to a larger joint density of
states of transition, the decay rates of GaN-like LO modes could be enhanced
further. Due to the two reasons explained above the GaN-like LO modes decay
faster than the InN-like LO modes. Therefore with a thicker well layer (or a thinner barrier layer), the
energy relaxation time of the high-lying LO phonon system becomes
longer, for it introduces more InN-like modes (or fewer InGaN-like modes).

According
to Fig. 4.5.38, the relaxation time of high-lying phonons could go up to 300ps,
significantly longer than the bulk value of around 1ps. And according to Fig.
4.5.37 the relaxation time of hot electrons
(if phonons are completely thermalized) can go up to more than 1ps, this is
also much longer than the bulk value. Combining these two, an optimized
MQW-SL structure should involve a thin barrier layer and a thick well layer.
The contrast in phonon energies between the well and the barrier also needs to
be large, indicating a small Indium content in the barrier. In addition, such a
structures could potentially prevent hot phonons diffusing out, making it even
more attractive than bulk materials.

4.5.3.3.8 Time resolved photoluminescence measurements of
bulk phononic band gap materials

The
potential efficiency boost, which can be achieved by hot carrier solar cells,
is directly related to the possibility of extracting high energy carriers from
the absorber layer before thermalisation, increasing
the voltage and hence the conversion efficiency. The poor conversion efficiency
of photons with energies above the band gap of the absorber is the main
loss mechanism in conventional single junction
solar cells. The investigation of thermalisation time constants of hot carriers
is a crucial step towards the engineering of hot carrier cells. The
efficiency of an InN based hot carrier solar cell has been calculated using the
theoretical model in Sections 4.5.3.3.7 and 4.5.3.1. It was found that the
limiting efficiency is strongly related to hot carriers relaxation velocity in
the absorber [4.5.61].

A
comparison of femtosecond time resolved photoluminescence (tr-PL) spectroscopy
between InP and GaAs was reported in the annual report two years ago and now
published in [4.5.48]. This showed a
distinctly longer carrier cooling time constant for the wide phononic gap InP
as compared to almost zero phononic gap of GaAs. It also showed further
evidence for excitation into higher side valleys for both GaAs and InP for
appropriate excitation wavelengths. Hot carrier cooling in InN has

- 207 -

been
investigated using tr-PL. The wide gap between optical and acoustic branches in
the InN phononic dispersion relation (wider than that for InP) prevents the
Klemens decay of optical phonon into acoustic phonons. This can lead to slower
carrier cooling due to the “Hot Phonon Effect” [4.5.62]. The decay of hot
carriers for different excitation wavelengths InN has been investigated.

Figure 4.5.39: Schematic representation of time resolved
photoluminescence arrangement.

Tr-PL
experiments have been performed on an InN sample grown on sapphire substrate,
obtained from collaborators in Taiwan, using the measurement configuration
shown in Fig. 4.5.39. In this technique a laser pulse acts as a switching gate
relating the photoluminescence signal to the time domain. The PL signal is
collected from the sample, after a femtosecond laser excitation, and focused in
a non linear crystal. The gate signal is generated from the same laser and is
focused on the same crystal after passing through an optical delay stage. The
signal is detected using a monochromator and a
PMT. Our system configuration provides 150 fs pulses with tunable wavelength
over a range of 256 nm (4.84 eV) to 2.6 μm (0.48 eV).

Figure 4.5.40: 3D representation of InN time resolved PL
data.

Figure
4.5.40 is a three dimensional representation of PL as a function of time for
all the probed wavelengths. It can be observed that the PL sharply rises when
the carriers are photo-excited by the laser pulse. The fast decay of the PL
shows the thermalisation of carriers towards respective band

- 208 -

edges.
The decay is faster for highly energetic carriers compared to carriers closer
to the bandgap. Thus the carrier population quickly degenerates towards the
band edges during the thermalisation process. In InN the thermalisation is most
probably due to interaction of highly energetic electrons and holes with LO
phonons.

To
investigate the rate of the carrier cooling process, the effective temperature
of the carrier population has been calculated fitting the high energy tail of
the PL spectrum for every single time during the cooling transient. The PL has
been fitted assuming that carriers form a Boltzmann-like distribution in a
femtosecond time scale using the following equation.

L(ε)
represents the measured PL intensity at energy ε, α(ε) is the
measured sample absorption coefficient, EG
is the InN energy gap, 0.7 eV in this case, and kB is the Boltzmann
constant. TC is the fitted parameter and represents the hot carrier
temperature. Figure 4.5.41 shows the tr-PL data for two sets of samples
(a) In N grown on Sapphire substrate and (b) InN grown by MBE at Saitama
University for zinc blende structure on sapphire and as wurtzite structure on
MgO substrate. For the data in (a) the
carrier temperature transient is seen to follow an exponential behaviour quite
well (blue line – fit). The data for the higher quality samples in (b)
show significantly slower carrier cooling rates than for (a).

Figure 4.5.41: Carrier relaxation curves for InN using
tr-PL: (a) Lower quality InN with soPhotoexcited carrier density is 1.5 x 1019
cm-3. The blue dashed curve is a single exponential fit. (b) Zinc
blende and wurtzite InN samples grown by MBE on MgO substrates by Shuei Yagi,
Saitama University (Cubic=crosses, Wurtzite structure = circles). This high
quality material shows much slower carrier cooling rates.

The
fitting of the calculated temperature data has been performed using a single
exponential.

Here
τTH represents the carriers thermalisation time constant,
whereas C and K are two constant parameters. The fitted value for τTH
is 7 ps. This long cooling constant can be attributed to the hot phonon effect
due to the long lifetime of the A1(LO) phonon [4.5.48,4.5.63]. The variation in
carrier cooling rates measured is attributed to the difficulty of growing good
quality InN on sapphire substrate, such that the carrier cooling velocity is
strictly related to the quality of the material. Other slow carrier cooling
constants have been reported for InN in the literature [4.5.64].

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4.5.3.3.9 Nanostructures for absorbers

Nanostructures offer the possibility of modification of
the phonon dispersion of a composite material. III-V compounds or indeed most of the cubic
and hexagonal compounds can be considered as very fine nanostructures
consisting of ‘quantum dots’ of only one atom (say In) in a matrix (say N) with
only one atom separating each ‘QD’ and arranged in two interpenetrating fcc
lattices. Modelling of the 1D phonon dispersion in this way gives a close
agreement with the phonon dispersion for zinc-blende InN extracted from real
measured data for wurtzite material.

Similar
‘phonon band gaps’ should appear in good quality nanostructure superlattices,
through coherent Bragg reflection of modes such that gaps in the superlattice
dispersion open up [4.5.65]. There is a close analogy with photonic structures in which modulation of
the refractive index in a
periodic system opens up gaps of disallowed photon
energies. Here modulation of the ease with which phonons are
transmitted (the acoustic impedance)
opens up gaps of disallowed phonon energies.

Force constant modelling of III-V QD materials by SK growth

3D
force constant modelling, using the reasonable assumption of simple harmonic
motion of atoms in a matrix around their rest or lowest energy position,
reveals such phononic gaps [4.5.66]. The model calculates longitudinal and
transverse modes and can be used to calculate dispersions in a variety of
symmetry directions and for different combinations of QD sub-lattice structure
and super-lattice structure.

III-V
Stranski-Krastinov grown QD arrays of InAs in InGaAs and InGaAlAs matrices are
fabricated at the University of Tokyo using
MBE. We are investigating these for evidence of phonon dispersion modulation
and potential slowed carrier cooling. In order to understand the expected
phonon dispersions these are being modelled using the 3D force constant
technique.

Lattice
matched and strain compensated material pairs that may produce large phonon
bandgaps are of interest. Previous
iterations in the design of these structures indicated the importance of
separating “light” and “heavy” atoms to different parts of the
nanostructure. Initially, the lightest atom in the system, As, was present in
both the QD and the matrix. This meant that the reduced mass of both regions
(proportional to the sum of the inverses of each atomic mass) was very similar
as the light element dominates in this case.

On this iteration structures with an In0.5Ga0.5-xAlxAs
matrix (with x=0.4) and InAs QDs were grown. Significant Al content
was introduced into these structures with the expectation that this light
element, segregated to the matrix material, might produce appreciable phonon
bandgaps. Some images derived from
characterisation of the structure are presented in Fig. 4.5.42. The
superlattice of QDs has a simple hexagonal structure. Extraordinary periodic
out-of-plane stacking is achievable and largely defect free structures can be
grown on the order of microns.

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Figure 4.5.42: Some images of the structure as grown and
modelled. (Above) the near-perfect stacking of the structure is shown. (Above right) in-plane InAs QD arrangement. (Right)
simple hexagonal superlattice structure showing stacking. The QDs are similar
to flattened disks with in plane dimensions
of 40-50 nm and heights of 7 nm.

Force
constant modelling of this structure predicts an appreciable phonon bandgap, as
shown in Fig. 4.5.43 and Fig. 4.5.44. This bandgap is due almost entirely to
the presence of the Al in the system. A small
bandgap is present due to the mass difference between In, Ga and As, but it is
less than a quarter of the size shown in Fig. 4.5.43 and Fig. 4.5.44.
Due to computational constraints the size of the QDs is quite small, only about
a nanometre in diameter. While actual sizes for these structures are too
computationally intensive to model without extreme effort, recent modelling
with gradually increasing size suggests that the dispersion relations scale
linearly with size. That is, once the discrete distances (bond lengths) are
small relative to the superlattice unit cell dimension, the dispersion should
look exactly the same when scaled such that the relative dimensions are
preserved. This will be investigated in greater detail in further work.

- 211 -

Figure 4.5.43: Dispersion curves for InAs QDs in a simple
hexagonal SL. The matrix is In0.5Ga0.5-xAlxAs
with (x = 0.4). There is a large phonon bandgap present in the system
due to the presence of Al. The gap seems very tolerant to Al location. (Top):
<100>, (Middle): <110>, (Bottom): <111> directions.

Figure 4.5.44: Densities of states (DOS) for the dispersion
curve immediately to the left. Note small changes in the DOS could be due to
pseudo-random locations of the Ga and Al particles or to differences in
crystalline direction. (Top): <100>, (Middle): <110>, (Bottom):
<111> directions respectively.

4.5.3.3.10 Fabrication and characterisation of highly ordered
nanoparticle arrays for hot carrier absorber

As demonstrated by the work on modelling phonon
modulation, periodic core-shell QD arrays offer a way to significantly change the phonon
modulation in a superlattice because the core and shell can be of materials of
very different force constant, directly leading to a strong phonon confinement.
Deposition methods are being investigated to fabricate such highly ordered QD
arrays.

The
Langmuir-Blodgett (LB) deposition technique is conventionally used for the
fabrication and characterisation of single
and multilayer films with precise control of thickness, molecular orientation and
packing density. In recent years, it has been applied to metallic nanoparticle
depositions for formation of highly ordered array.

We
have employed an LB system to fabricate gold (Au) and silicon (Si) nanoparticle
layers. A LB system has been set up at UNSW. We have designed the trough top
for nanoparticle deposition to optimise the dispersability. The system is shown
in Fig. 4.5.45, which consists of a trough, a pair of barriers, a surface
pressure sensor, a dipping unit for film transferring and an interface
controller.

- 212 -

Figure 4.5.45: A customised LB system
for nanoparticle deposition, installed in Room 1013 Chemical Engineering Building UNSW.

LB
deposition and formation of monolayer with Si nanoparticles:

Figure
4.5.46(a) is a schematic diagram showing the LB system, consisting of an LB
trough, a solid substrate, a dipping unit, a Wilhelmy plate and two barriers.
To make a Si NP array, a small amount of 0.5 mg/mL chloroform solution with
dodecane-capped Si NPs dissolved with diameters of 2.7 nm or 6.9 nm, was spread
onto the water surface drop wise over a time interval of 30 seconds. After the
complete evaporation of the solvent (chloroform), the Langmuir monolayer of Si
NPs on the water surface was dynamically
manipulated by compression with the barriers. During the compression, the
surface pressure (SP) was measured using the Wilhelmy plate, and the isotherm
curve (SP versus film area) was recorded. Fig. 4.5.46(b) shows the
isotherm curve for a 30 mL chloroform solution of 2.7-nm-diameter Si NPs with a
concentration of 0.5 mg/ml. Upon application of an appropriate level of
compression, suitable packing within the monolayer is achieved on the water
surface. Subsequently, the monolayer was transferred onto a hydrophilic
substrate (Si or Quartz wafer washed in piranha solution) through
vertical-dipping controlled by the dipping unit. From the AFM image (Fig.
4.5.46(c)), a film is observed. The cross-sectional view clearly shows the
thickness of the film is 2.7 nm, equal to the size of Si NPs, which indicates
a monolayer.

Figure 4.5.46: (a) a schematic of the LB system; (b)
measured isotherm curve for Si NPs of 2.7 nm and the chosen depositing surface
pressure (SP); (c) AFM image and cross-sectional profile of the fabricated 2.7
monolayer on a Si wafer.

- 213 -

Amorphous and short-range ordered close-packed arrays:

Unlike
other semi-conductor NPs such as CdSe or metallic NPs, due to their irregular
shapes and lighter weight, Si NPs are not mobile enough for self-assembly to
happen in a short timescale; this is due to the weaker attractive capillary
forces between the particles. Therefore, the reported results always show a
random array of Si NPs or aggregated clusters. In our work, we found that by
increasing the stabilizing time for monolayer formation on the water surface or
slowing down the moving rate of the barriers before performing the vertical
dipping, we can achieve decently uniform ordered close-packed arrays as shown
in Fig. 4.5.47 (d).

Figure 4.5.47: TEM images of (a) amorphous (b)
short-range-ordered (c) more uniform short-range-ordered close packed arrays
deposited at a SP of 50 mN/m with different LB processes (d) an enlarged area
in (c); Process 1 is to prolong the stabilizing time of the monolayer on the
water surface, 2 is to slow down the moving rate of the barriers.

In
summary, we have established a process for deposition of nanoparticle
monolayers on a solid substrate, with reasonably close packed ordered arrays of
nanoparticles. Future work will concentrate on improving periodicity of the
monolayer for both HC absorbers and energy selective contacts.

4.5.3.4 Implementation of nanostructures for hot carrier
cells

In
order to use a nanoparticle array as a hot carrier absorber, and hence utilise
slowed carrier cooling due to modified phonon dispersion, then a way to
fabricate a complete structure with ESCs must be established. A structure in
which nanoparticles are arranged in a uniform 3D array should give the required
phononic band gap. The degree to which this needs to be ordered is still under
investigation. Certainly a large difference between masses is beneficial
between the nanoparticles and the matrix

- 214 -

and
this can be best achieved in an array of ‘core shell QDs’ in which the core and
shell have very different acoustic impedance in order to promote coherent
reflection and hence confinement of phonons [4.5.65]. The matrix in which such
an array is embedded needs to allow transport of carriers to contacts and also
electron-electron and preferably hole-hole scattering to renormalise carrier
energies. Such a structure should also have a narrow electronic band gap so as
to absorb a wide range of photon energies.
This combination of properties is challenging but not mutually exclusive
because phononic properties are largely independent of electronic
properties and such a structure would not need
to have electronic confinement of carrier energies just vibronic modulation of
phonon energies. Energy selective contacts are also required for such a
structure. These would most ideally be arranged
at the top and bottom of the absorber. However, in a realistic device a
selective hole contact might not be required as the hole population only
contains a small fraction of the hot carrier energy and hence thermalisation of
holes is less important, whereas a structure with only one contact (presumably
the top contact) as an ESC would be much easier to fabricate. These ESCs would
be QD or QW double barrier structures. (Addition of extra layers to give double
QD/QW triple barrier structures (or even triple QD/QW quadruple barriers) would
give greater resonance and enebale the contact to be rectifying – important if it
is to collect carriers of only one. Such a QD array / double barrier QD ESC
structure is shown schematically in Fig. 4.5.48.

Figure 4.5.48: Conceptual integrated hot carrier device,
incorporating double (or triple) barrier QD resonant tunnelling contacts and
absorber probably based on either a bulk or nanostructure phononic band gap
material and/or MQW nanostructure, for slowed carrier cooling.

4.5.3.5 Summary of hot carrier solar cell research

There has been significant development in most areas of
hot carrier solar cell work. The modelling of efficiencies now includes combinations of
absorber and energy selective contacts with reasonable values chosen using real
material parameters based on III-nitrides. This has been carried out both for
bulk absorber InN and for multiple quantum well materials again based on
nitrides. Such devices are capable of being fabricated and then tested against
these predicted results and work with collaborators is moving towards growing
such devices. In addition the theory of the slowed carrier cooling behaviour in
MQWs is being investigated with plans to test the competing mechanisms again
using material from a range of collaborators. Measurement of InN with
time-resolved PL has indicated some evidence for slowed carrier cooling,
further corroborating the importance of a large phonon band gap to block
optical phonon decay, but also highlights the importance of material quality. Progress on the Langmuir-Blodgett approach to
ordered nanoparticle arrays has seen development of highly ordered arrays of
nanoparticles. The potential application of nanostructures to fully integrated devices
has been investigated conceptually, with various designs considered. Similarly
the possibility of absorber materials which are analogous to InN is also being
investigated. These many aspects of Hot carrier cells will see further
development and consolidation in 2013 with working devices planned for early
2014.

- 215 -

4.5.4 Up-Conversion

Researchers: Sanghun Woo, Craig Johnson, Supriya Pillai, Shujuan Huang, Richard
Corkish, Gavin Conibeer

Collaboration with: Peter Reece (Physics, UNSW), John Stride (Chemistry, UNSW)

Up-conversion in novel silicon-based materials

Up-conversion
(UC) in erbium-doped phosphor compounds (particularly NaYF4:Er) has
been shown to be a promising means of enhancing the sub-band-gap spectral
response of conventional Si solar cells without modification of the electrical
properties of the cell [4.5.67]. In this scheme, a layer containing the
phosphor is applied to the rear of a high-efficiency bifacial cell. After
absorbing two long-wavelength (1500nm)
photons - which are transmitted by the cell - the excited Er ions can relax by
emitting a photon with an energy greater than the Si band gap, thereby
increasing the current that can be extracted from the cell.

Synthesis of Er doped NaYF4
phospors for UC

Fabrication of in-house phosphors allows control over
the growth process and the ability to control Er doping levels and the possibility to include
co-dopants.

NaY(1-x)F4:Erx
nanocrystals were synthesized by thermal decomposition of metal
trifluoroacetate (TFA) precursors [4.5.68, 4.5.69]. After the reaction at 800C
for two hours, oleic acid (OA) and 1-octadecene (1-ODE) were added to the
reaction and slowly heated for 1 hour at 2800C. At this stage, NaYF4:Er
NCs formed in the alpha phase. To convert NCs from alpha to the beta phase, the
temperature was slowly increased to 3200C for 3 hours. The resulting solution
was treated to remove unreacted precursors, excess OA and 1-ODE. NaYF4:Er
NC films were fabricated by spin casting followed by nucleophilic ligand
exchange. NC films were coated on the glass by multiple spin castings.

Fig.
4.5.49(A) shows HRTEM images of hexagonal beta phase NaYF4:Er NCs.
The dominant and most distinguishable lattice (100) of the hexagonal beta phase
NaYF4 NCs had a fringe distance of 0.54 nm. The histogram in Fig.
4.5.49 (B)indicates that particles had sizes ranging from 25 to 35 nm with a
narrow size distribution peaking around 30 nm.

Figure 4.5.49: Electron microscope
data for NaYF4: 10% Er. (A) low and high resolution TEM. Both scale bars correspond to 50nm. (B) Histogram
of diameters of Er dope NCs.

- 216 -

The absorption spectrum of UC NCs is shown in Fig.
4.5.50 and corresponds to all energy levels up to 3.1 eV of NaYF4:Er NCs. Fig.
4.5.50(B) shows five red-shifted peaks compared to the absorbance which were resulted from radiative relaxations
from 2H11/2, 4S3/2, 4F9/2,
4I9/2 and 4I11/2. As shown by UC luminescence
images in insets of Fig. 4.5.50(B) increased Er concentrations in UC NCs
accelerated multi-phonon relaxation processes between 4S3/2
and 4F9/2, which emphasizes the importance of
optimization between slowing non-radiative decay rates and increasing
absorption sites in NCs.

Figure 4.5.50: Absorption spectra for NaYF^4:Er NCs (A).
(B) detil 300-1000nm, with insets showing the luminescence.

The
SEM images of spin cast films before and after the soaking process in a MPA
solution are shown in Fig. 4.5.51(A) and (B) respectively. Without the post
soaking process, NCs were coated separately from neighbouring crystals, and
packed closely. By treating with a 3-MPA solution, NCs were agglomerated and linked to each other resulting
in a bulk-like feature. The cross-linked NC film made with 3-MPA brings
about easy handling, light trapping in NC layers, and a reduced film thickness.

Figure 4.5.51: (A) SEM images of morphology of upconverting
layer (A) before and (B) after soaking in a MPA solution. (C) Up-convesrion
spectroscopy showing the intensities of emitted photons in terms of laser power densities and Er doping concentration.
(D) Film thickness dependent upconversion under three different
excitation powers.

Figure 4.5.51 (C) shows the total intensity of
upconverted photons from energy levels above 4I11/2, which are all able to contribute to the
photoconversion process in c-Si solar cells, depending on the excitation power
between 30 μW and 215 mW and the Er concentration between 2 and 100% substituting Y sites of the NaYF4 host
structure. To minimize the effect of the total number of Er ions

- 217 -

in
UC samples, all samples were prepared thicker than the required thickness for
saturated upconversion, as described in the thickness dependent analysis below.
Considering the result of Er concentration effects, 15% Er doped UC layers
always gave the best upconversion regardless of the excitation power. This
means that increasing Er doping concentration did not affect the number of photons that could meet with each trivalent Er
ion. The ratio between the Er doping concentration and the photon flux
was not a predominant factor because the number of photons that could be
absorbed by each Er ion does not change when the films were thick enough. Thus,
increased or decreased Er content under fixed laser density did not have the
same effect as changing the laser power at the fixed Er concentrations. By
increasing Er densities in UC NCs, the number of neighbouring ions is increased, and the distance between Er ions is
reduced. Both can boost non-radiative cross relaxation processes as well
as energy transfer upconversion mechanisms. From the results, it can be concluded
that 15% Er doped NaYF4 can maximize the rate of the energy transfer
upconversion minus cross relaxation decay. This optimisation is practical
because it is independent of the solar concentrations.

Using the brightest UC NCs with a 15% doping concentration
of Er, the film thickness was optimized under various excitation fluxes, as illustrated in Fig. 4.5.51 (D).
From the excitation coefficient information,
the UC films had to be thicker than 20 μm, so that all photons within
energies of 4I13/2 could be absorbed by the UC
phosphor. However, the required film thicknesses to perform the saturated
emission were only about 2.5 μm under a 1 mW laser, 3 μm under a 5 mW
laser and 5 μm under a 10 mW laser. This is because upper UC NC layers
re-absorbed upconverted photons emitted from the lower layers. Also, the stable
bulk-like layers may have a higher absorption coefficient than simply packed
NCs, resulting in relatively thinner thicknesses required for the saturated UC
efficiency.

Light trapping in slowed light modes to enhance UC efficiency

While
phosphors have demonstrated high-efficiency UC behaviour, their use presents
particular challenges with regard to fabrication and cost. Last year we
presented data on use of Er-doped porous Si (PSi:Er) as an alternative UC
material. PSi is unique in that its porosity - and hence the material
refractive index - can be varied as a function of depth, allowing for the
elaboration of high-quality monolithic Si optical structures such as
distributed Bragg reflectors (DBRs).

A
cross-sectional electron micrograph of such a structure is shown in Fig.
4.5.52. Its porous substructure also allows for deep infusion of dopant atoms
via techniques such as electroplating.

Figure 4.5.52: Cross-sectional electron micrograph of a
30-bilayer porous silicon distributed Bragg reflector.

Figure
4.5.53 shows the calculated field intensity profile for these DBRs in the
spectral region between 1500 and 1600nm. The results have been normalized to
the incident field intensity and each

- 218 -

plot
is independently colour-scaled to show maximum detail. They clearly demonstrate
that considerable field enhancement is possible throughout the depth of such a
multilayer structure, with peak relative intensities of more than 11 for the
30-bilayer structure and more than 18 for the 40-bilayer structure. However, it is also clear that the regions of
enhancement are increasingly narrowly-concentrated as the number of bilayers
increases. This is due to the additional interference fringes in the
reflectivity characteristic of the DBR that arise with additional layers, as is
apparent from Fig. 4.5.53. Calculations for more than 40 layers show that
enhancements of up to 80 are possible for a large number of bilayers though the
peak narrows to less than 1nm wide for 100 bilayers.

Figure 4.5.53: Intensity-valued mode profile in DBRs with
varying numbers of high/low-porosity bilayers
for a leading band edge fixed at 1550nm. All intensities are normalized to the
incident field intensity and each plot is independently color-scaled to
show maximum detail. The air and substrate interfaces with the DBR are
indicated by the dashed black (top) and red (bottom) lines, respectively.

Sensitisation of Er phosphors to a wider wavelength range

Er has an absorption window at the I13/2
level from 1480nm to 1580nm. This narrow range means that it is inefficient at absorbing below Si band
gap photons. Sensitizing to the 1100-1500nm range would significantly enhance
the absorption and hence have a dramatic effect on the up-conversion
efficiency, because of its non-linear dependence.

PbS
nanoparticles have a wide absorption range below 1100nm. They can also be tuned
in size to emit at 1500nm, with absorbed photons being downshifted to 1500nm at
some quantum efficiency less than 100%.

Such PbS NCs have been synthesized based on the
procedure developed by Cademartiri et al. [4.5.70] by decomposition of PbCl2 with
sulphur solution. PbS QDs and Er-doped NaYF4 NC films were

- 219 -

fabricated
by the simple spin casting method followed by nucleophilic cross-linking of
NCs.

The
sizes, the size distribution and the fringe distance of resulting UC NCs were
determined using HRTEM and glancing incident x-ray diffraction (GIXRD) was
carried out to investigate the crystalline structures of PbS QDs.

Figure 4.5.54: Absorbance and emission data from UV-Vis
spectroscopy. (a) for PBs quantm dots showing the desired slight mismatch
between absorbance and emission such that re-absorption is unlikely. (b)
absorbnce for Er doped NCs and emission from PbS QDs, showing the good match
between PbS emission to pump Er absorption at the.

Figure 4.5.54 shows absorbance and emission data for PbS
QDs and Er doped NaYF4 NCs. The range of absorbance for PbS QDs is
close to ideal for absorbance of light below the Si bandgap but stopping short of the emission. This emission, tuned
to 1500nm, is ideal for pumping the I13/2 level at 1520nm in Er,
without being re-absorbed by the PbS. Thus the sensitization of Er to a
significantly wider wavelength range should be very valuable in increasing the
flux of photons which can be absorbed and up-converted by the Er doped
phosphor.

Conclusion

Up-conversion
in Er doped phosphors shows very promising results. Fabrication of NaYF4 NCs allows greater control over doping levels and
geometries for efficient up-conversion. Light trapping in DBR structures
based on P-Si should enable greater absorbance of 1500nm light through the
greater density of photonic states. Progress towards sensitisation of Er to a
wider wavelength range using down shifting PbS quantum dots tuned to emit at
1500nm has been made. Combination of all these elements in an up-conversion structure is expected to significantly
boost UC efficiencies for Si cells.

4.5.5 Concluding Remarks for the Third Generation Section

Work
has proceeded significantly in all the areas of Third Generation research, with
improved fabrication and characterisation of materials and complexity of
modelling which together give an overall better understanding and optimisation
of devices.

Group
IV based nanostructure materials have seen significant improvement in
understanding of the parameters required for growth and the influence of these
on material and device properties. The modelling of equivalent circuits for the
devices is giving greater understanding and now allowing prediction of growth
conditions to give improvement. The realisation that the effective bandgap and hence the VOC in these materials is
dominated by defects has led to a greater effort on characterising and
reducing defects. Light trapping through back reflectors and the use of
plasmonics has led to more than double the current in nanostructure devices.
Further optimisation in this area is very likely. The use of conductive
substrates to reduce the lateral resistivity has made promising progress, with
SiC and Mo substrates showing some improvements. Optimisation of these will
continue to give better lateral transport and should lead to better photovoltaic
properties. Higher VOCs over 500mV

- 220 -

have
been measured at lower temperatures. The justification for this is that due to
the high resistivities, devices heat up considerably and cooling is valid to
bring them back to room temperature. A temperature of 50degress below ambient
is calculated to achieve this condition and gives
VOC of 503mV, although significantly high VOCs occur at
even lower temperatures as would be expected. Work on other related
materials has seen lower resistivities for Si3N4
interlayers in Si QDs in SiO2; high quality Ge QW growth; initial
data for growth of Ge:C materials with the aim of engineering band gaps through alloying and the potential for transfer
of growth processes to PECVD with it lower levels of damage and thermal
budget.

Hot
carrier cells have seen a great deal of advance in the last year. Modeling of
overall devices now includes a much larger range of real material properties in
a more integrated structure. In addition modelling of combined multiple quantum
well an phononic bandgap structures is leading to new designs for combined structures which offer promise for slowed carrier
cooling. Modelling of energy selective contacts has progressed again
with real material properties and integration into full conceptual devices.
Such devices are entirely feasible and plans to grow these with collaborators
are underway. New designs for rectifying ESCs or incorporation of ESCs within
the absorbed material have been suggested and these also can be incorporated
into real structures. Increased understanding of the mechanisms involved in
both phononic band gap materials and in multiple quantum well structures is
leading to new experiments to determine the exact relationship between reduced
carrier cooling mechanisms. The results of these will lead to the optimum
design requirements for combined structures which should allow significantly
slower carrier cooling rates. Deposition of nanoparticle arrays has made progress with very ordered arrays
now possible using Langmuir-Bodgett deposition. These arrays have been modelled
to give significant phonon bandgaps and characterisation of these is now
underway. New materials suitable for some of these phononic and nanostructure
properties are being identified and growth and characterisation of these
materials is planned in-house and with a range of collaborators.

Up-conversion
has seen a further significant development with synthesis of NaYF4 doped with
Er now allowing greater flexibility in doping and geometry of devices. Light
trapping work is continuing to increase the
number of photonic modes available for absorption in Er. Sensitisation of Er to
a wider wavelength range is being enabled using PbS QDs which are now being
synthesised and characterised and which are showing close to ideal
properties for such sensitisation. Combination of all these approaches is
expected to give significantly improved up-conversion.

The
development of all the 3rd generation projects now allows much
greater understanding of the materials and devices. Work in 2013 will see
consolidation of this into improved devices. Several new areas of funding will
contribute to this and also allow development of new project areas.

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