.

•

• Battelle 

M.W. Wanlass, S.P. Ahrenkiel, R.K. Ahrenkiel, 

D.S. Albin, J.J. Carapella, A. Duda, J.F. Geisz, 

S. Kurtz, and T. Moriarty 

National Renewable Energy Laboratory 

R.J. Wehrer and B. Wernsman 

Bechtel Bettis, Inc. 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3-7, 

• 

Lattice-Mismatched Approaches 

for High-Performance, III-V 

Photovoltaic Energy Converters 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute 

Contract No. DE-AC36-99-GO10337 

2005 

February 2005 NREL/CP-520-37440 
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LATTICE-MISMATCHED APPROACHES FOR HIGH-PERFORMANCE, III-V �

PHOTOVOLTAIC ENERGY CONVERTERS �

M. W. Wanlass1

, S. P. Ahrenkiel1, R. K. Ahrenkiel1, D. S. Albin1, J. J. Carapella1, A. Duda1, J. F. Geisz1, Sarah Kurtz1, 

T. Moriarty1

, R. J. Wehrer2, and B. Wernsman2 

1) National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO, USA 

2) Bechtel Bettis, Inc., 814 Pittsburgh-McKeesport Boulevard, West Mifflin, PA, USA 

ABSTRACT 

We discuss lattice-mismatched (LMM) approaches 

utilizing compositionally step-graded layers and buffer 

layers that yield III-V photovoltaic devices with 

performance parameters equaling those of similar latticematched (LM) devices. Our progress in developing highperformance, LMM, InP-based GaInAs/InAsP materials 

and devices for thermophotovoltaic (TPV) energy 

conversion is highlighted. A novel, monolithic, multibandgap, tandem device for solar PV (SPV) conversion 

involving LMM materials is also presented along with 

promising preliminary performance results. 

INTRODUCTION 

The design options for III-V photovoltaic (PV) device 

structures based on LM heteroepitaxy are constrained due 

to the limited number of commercially available crystalline 

substrate materials. The design space is increased 

substantially by considering designs that involve LMM 

components. Traditionally, LMM approaches have been 

considered inferior due to the well-known associated 

problems such as high defect densities (dislocations and 

stacking faults), rough surface morphologies, epilayer 

cracking, and epiwafer bowing. At NREL, our goal is to 

make LMM heterostructures look and perform like LM 

ones (i.e., we strive to attain the highest Shockley-ReadHall lifetime, thinnest epistructure, and flattest epiwafer). In 

this paper, we discuss approaches utilizing 

compositionally step-graded layers and buffer layers that 

yield LMM PV devices with performance parameters 

equaling those of similar LM devices. Our progress in 

developing high-performance, LMM, InP-based 

GaInAs/InAsP materials and devices for 

thermophotovoltaic (TPV) energy conversion is discussed. 

Additionally, a novel, monolithic, multi-bandgap, tandem 

device for SPV conversion involving LMM materials is 

presented. 

All of the test devices and PV converter structures 

discussed in this paper were grown using atmosphericpressure metalorganic vapor-phase epitaxy (MOVPE) in 

home-built systems at NREL. Trimethylindium, 

triethylgallium, trimethylgallium, trimethylaluminum, arsine, 

and phosphine were used as the primary reactants, with 

hydrogen sulfide, hydrogen selenide, disilane, carbon 

tetrachloride, and diethylzinc used as the doping 

precursors. Growth was performed at temperatures 

ranging from 600 to 700°C in a purified hydrogen ambient. 

LMM, InP-BASED THERMOPHOTOVOLTAIC ENERGY 

CONVERTER MATERIALS & DEVICE DEVELOPMENT 

At NREL, we have been investigating low-bandgap, 

InP-based semiconductor alloys for TPV converter 

applications since 1991. The initial (LMM) device 

heterostructures utilized simple, mono-bandgap, shallowhomojunction devices and GaInAs compositionally graded 

regions to achieve the desired bandgaps (~ 0.5-0.6 eV) on 

InP substrates [1]. The converter structures were 

subsequently improved in 1993/1994 by employing two 

important design changes. Firstly, InAsP compositional 

step grades replaced GaInAs grades, as we found that the 

GaInAs material quality was improved. Secondly, GaInAs 

absorber layers were clad with InAsP layers to form 

double-heterostructure (DH) converters [2]. The InAsP 

cladding layers served to confine minority carriers to the 

GaInAs active layer, leading to a marked improvement in 

converter performance, both in improved carrier collection 

(i.e., increased quantum efficiency) and improved opencircuit voltage (i.e., reduced reverse-saturation current 

density). During the next ten years, several organizations 

became active in researching InP-based materials for 

TPV, and the work scope continued to expand to include 

the development of monolithic interconnected modules 

(MIMs) [3] and multi-bandgap, tandem converters [4, 5, 6, 

7]. 

The main perceived disadvantage of the InP-based 

approach for TPV converters is that low-bandgap, 

GaInAs/InAsP DH converters are LMM with respect to InP. 

As mentioned earlier, LMM is known to introduce a wide 

range of problems in epistructures. Additionally, thick 

epistructures are often employed due to compositionally 

graded regions that are typically included to mitigate some 

of the aforementioned problems. The good news, as 

discussed later in this paper, is that many of the perceived 

problems have been addressed and, largely, solved. The 

InP-based approach is very well suited to the TPV 

problem; some of the important advantages have been 

described in a previous paper [8]. 

In developing LMM, InP-based materials for TPV, we 

have used a combination of x-ray diffraction (XRD), 

transmission electron microscopy (TEM), and 

1 �
cathodoluminescence (CL) techniques to improve our 

understanding of the relaxation dynamics, and defect 

structure, in InAsP compositionally graded 

heterostructures and GaInAs/InAsP interfaces [9]. 

Additionally, we have employed resonant-cavity 

photoconductivity decay (RCPCD) [10] to characterize the 

minority-carrier lifetime as part of our 

qualification/optimization effort. 

The low bandgaps that are typically required for TPV 

converters (~ 0.5 - 0.7 eV) dictate that LMM epistructures 

must be considered when growing GaInAs/InAsP 

heterostructures on InP. Intermediate compositionally 

step-graded InAsP regions are included in the 

heterostructures to mitigate the deleterious effects of the 

LMM between the active GaInAs/InAsP DH and the InP 

substrate. Our studies have shown that all of the InAsP 

layers in a step grade, except the last one, which we refer 

to as the buffer layer, experience nearly complete 

relaxation. The high degree of relaxation in the step grade 

results in a flatter epiwafer. The InAsP buffer layer retains 

residual compressive strain, causing its in-plane lattice 

constant to be significantly smaller than the value for the 

fully relaxed condition. The reduction in lattice constant 

must be taken into account when growing a subsequent 

active layer of GaInAs in order to maintain a coherent 

GaInAs/InAsP interface. Otherwise, misfit dislocations will 

nucleate homogeneously at the interface, leading to a high 

interface recombination velocity and also, potentially, 

threading dislocations and stacking faults in the GaInAs 

active layer. Put simply, maintaining a coherent 

GaInAs/InAsP back interface is key to realizing a highquality DH. We are also investigating a “displacement 

layer”concept [11] that involves including a layer between 

the buffer layer and the DH. The displacement layer could 

take on different forms and functions, but its primary 

purpose is to improve the quality of the back interface in 

the DH. 

A significant amount of effort has been devoted to 

gaining an understanding of how the strain in the InAsP 

buffer layer varies with thickness. For typical thicknesses 

of 0.1 - 0.5 μm, we have observed that the buffer layer is 

strained significantly (~0.20-0.25%). We have also 

observed that the buffer layer strain is consistently higher 

than the equilibrium value according to theory [12]. The 

important point is that the buffer layer strain as a function 

of the buffer layer thickness is well characterized, and that 

it decreases in a monotonic fashion as the layer thickness 

increases. 

The data in Fig. 1 were obtained from a set of 

structures where the unintentionally doped GaInAs (ntype, ~ 5E14 cm-3) active layer composition was held 

constant (Eg = 0.52 eV, 2 μm thickness, a standard used 

for our lifetime measurements) while the number of 

compositional steps in the graded layer was varied 

(0.06 As mole fraction increment per step). 

Figure 1. XTEM and RCPCD low-injection minority-carrier lifetime data for undoped, 0.52-eV GaInAs/InAsP DHs grown 

on different InAsP buffer layers. The lifetime data are plotted as function of the differential lattice mismatch between the 

active layer and the buffer layer (calculated with respect to the InP substrate). 

2 �
Thus, the composition of the InAsP buffer layer was varied 

to put the GaInAs layer under different degrees of 

compression, and then tension. The region over which the 

active-layer/buffer-layer interface remains coherent (as 

determined empirically by XRD) is indicated. XTEM and 

RCPCD were used to correlate the quality of the back 

interface with the low-injection minority-carrier lifetime. 

Misfit and threading dislocations are readily seen for the 

cases outside of the coherent region, and the lifetimes are 

generally low. The high lifetimes correlate well with 

interface coherence. 

High lifetimes, and commensurately large estimated 

diffusion lengths, have been obtained in our laboratory for 

both n- and p-type LMM DHs. Unintentionally doped (~ 

5E14 cm-3), 0.5-eV (1.7% LMM) n-DHs have low-injection 

hole lifetimes as high as 4.4 μs, as measured by RCPCD. 

The minority-carrier diffusion length was estimated 

(conservatively) by assuming that the mobility is half the 

value obtained for majority carriers experiencing the same 

background carrier density, and then using Einstein’s 

relation, in conjunction with the relationship between the 

diffusion length, diffusivity, and lifetime, to calculate the 

diffusion length. Thus, for the n-DH above, the estimated 

hole diffusion length is 41 μm. A similar calculation was 

done for a 0.6-eV (1% LMM), p-DH (~ 1.5E16 cm-3, 

chosen so that recombination would be determined by 

radiative and SRH processes), which showed a 0.76 μs 

low-injection electron lifetime. In this case, the estimated 

electron diffusion length is 126 μm. The above results 

indicate that the LMM DHs are of exceptional quality ­

comparable to the best LM GaInAs/InP materials grown in 

our laboratory. 

High-performance levels have also been 

demonstrated recently for 0.52-eV GaInAs/InAsP TPV 

converters, which have a relatively large LMM with respect 

to the InP substrate (1.6%). Spectral quantum efficiency 

data for one of the better converters are given in Fig. 2 

below. 

1-868 Q2 SJ9C 

1 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

3.0E- 5.0E- 7.0E- 9.0E- 1.1E- 1.3E- 1.5E- 1.7E- 1.9E- 2.1E- 2.3E- 2.5E- 2.7E- 2.9EInternal Quantum Efficiency 

External Quantum Efficiency 

07 07 07 07 06 06 06 06 06 06 06 06 06 06 

Wavelength (m) 

Figure 2. Spectral quantum efficiency data for a highperformance 0.52-eV GaInAs/InAsP TPV converter. 

The data show near-unity internal quantum efficiency 

near the band edge, indicating that the material quality is 

excellent. It should be noted that the improved InAsP 

buffer-layer designs developed at NREL were used in 

these converter structures. The drop off in performance for 

wavelengths shorter than 1.7 μm can be attributed to 

absorption in the InAsP window layer. Window layers with 

increased transparency should lead to improved 

performance. Photocurrent density versus voltage data for 

these devices, measured at 25°C, have been analyzed, 

and a value of 1.07 for the ideality factor, and 39.6 μA/cm2 

for the reverse-saturation current density, were 

determined. The above results are very encouraging as 

they indicate low recombination in the space-charge 

region. Also, the reverse-saturation current density value 

is less than a factor of two higher than that measured for 

the best 0.52-eV, LM GaInAsSb/GaSb converter [13]. 

A NOVEL, ULTRA-THIN, MONOLITHIC, MULTIBANDGAP, TANDEM SPV ENERGY CONVERTER 

Realistic modeling studies of series-connected, multibandgap, tandem solar cells show that the ideal bandgap 

for the bottom subcell in a triple-junction device is ~1 eV. 

The above holds for a variety of potential tandem cell 

applications including those for space vehicle power and 

for terrestrial concentrators. Accordingly, a large effort has 

been devoted recently to the development of ~1-eV, LM 

GaInAsN subcells for monolithic integration in Ge-based 

tandem structures that utilize GaInP (~1.85 eV) and GaAs 

(~1.4 eV) as the upper subcells. Unfortunately, so far, the 

quality of GaInAsN has been too poor to realize improved 

tandem performance through its inclusion. Thus, the need 

for a high-performance, monolithically integrable, 1-eV 

subcell still remains. 

Here, we describe a novel approach for producing 

ultra-high-efficiency, monolithic, multi-bandgap, tandem 

(MMT) SPV converters that have improved performance, 

and other advantageous features, when compared with 

existing technologies. Accordingly, the objectives and key 

features of the approach are as follows: 

1) To achieve SPV power conversion efficiencies of 

35-40+% for a variety of typical applications, the approach 

provides designs for MMT SPV power converters with 

optimal, or near-optimal, bandgaps for both space and 

terrestrial uses. Particularly, our approach provides a 

means of successfully combining the well-developed, 

medium- and high-bandgap, LM semiconductors grown on 

GaAs (or Ge) substrates (e.g., GaAs, GaInAsP, AlGaAs, 

GaInP, and AlGaInP), with lower-bandgap (~1 eV), LMM 

GaInAs alloys, in a monolithic structure. To date, ultrahigh-efficiency MMT SPV converters have not been 

demonstrated due to the lack of a suitable, highperformance, optimum, low-bandgap subcell option; the 

present approach provides a variety of schemes for 

including such an option. 

2) To provide MMT SPV converter designs that can 

be grown epitaxially on GaAs or Ge substrates 

(monocrystalline or polycrystalline), which are thus 

compatible with current industrial techniques for producing 

SPV power generation systems. The design given here 

3 �
could also be grown on appropriate compliant or 

engineered substrates, if available. 

3) To provide a method to fabricate ultra-thin MMT 

SPV converters that have the advantages described 

below: 

a) For both terrestrial concentrator and space 

applications, ultra-thin devices provide the ultimate 

solution to thermal management, since the devices can be 

mounted on a robust, surrogate substrate (handle) of 

choice, and intimate contact with the heat sink can also be 

achieved. In contrast, devices processed on Ge or GaAs 

substrates have relatively poor thermal conductivity. 

b) For space applications, the highest specific 

power (W/kg) is possible with handle-mounted, ultra-thin 

devices. 

c) Ultra-thin MMT SPV converters can mounted 

on strong, flexible handles (e.g., metal foil or kapton), 

resulting in flexible devices, which may have specific 

advantages (e.g., PV system deployment in space). 

d) The robust nature of handle-mounted, ultrathin devices is also a key advantage; devices processed 

on Ge or GaAs substates are relatively fragile. 

e) Ultra-thin devices also open the possibility of 

reclaiming the scarce, expensive elements that make up 

the parent substrate (e.g., Ga in GaAs). 

Two key enabling concepts are used in our approach. 

The first is the use of inverted epistructures, which make 

possible the growth of the LM and LMM subcells in one 

growth run, on one side of the parent substrate, and in the 

proper optical sequence. With this scheme, the higherbandgap, LM subcells are grown first in the structure, thus 

possibly retaining their pristine (i.e., defect-free) quality. 

The second key feature is the use of transparent, 

compositionally step-graded layers, which allow the 

integration of LMM, infrared-responsive subcells (e.g., 

LMM GaInAs) into a monolithic scheme that includes LM 

higher-bandgap subcells. Inverted epistructures require 

handle mounting and parent substrate removal for efficient 

operation, thus, ultra-thin tandem device fabrication is a 

natural consequence of our approach. 

Our initial work on the above-mentioned tandem cell 

approach has been focused on a series-connected, threesubcell device. The approach can be generalized to cover 

a wide range of designs; in a recent patent application, we 

discuss potential tandem device structures with two to six 

subcells [14]. The present device is grown on a GaAs 

substrate. LM n-GaInP{ (1 μm) etch-stop and n-GaAs (0.4 

μm) contact layers are first grown. A LM DH top subcell, 

consisting of an n-AlInP (25 nm) front-surface confinement 

layer (FSCL), n-GaInP (0.1 μm, ordered) emitter layer, pGaInP (1.2 μm, ordered) base layer, and a p-GaInP (0.1 

μm, disordered) back-surface confinement layer (BSCL), 

is then deposited, followed by a p+

/n+ GaAs tunnel junction 

(24 nm total thickness). The LM middle subcell consists of 

an n-GaInP (0.1 μm) emitter layer, p-GaAs (2.5 μm) base 

layer, and a p-GaInP (50 nm, ordered) BSCL, followed by 

another p+

/n+ GaAs tunnel junction (24 nm total thickness). 

A transparent, compositionally step-graded GaInP layer is 

then grown to affect a change in lattice constant to match 

that of the low-bandgap (~1 eV) GaInAs alloy (Ga mole 

fraction of 0.75, LMM ~2.2%). The step grade contains 9 

compositional steps with a Ga mole fraction increment of 

0.03 per step, and a step thickness of 0.25 μm. The grade 

terminates with a Ga mole fraction of 0.24 in the GaInP 

buffer layer (1 μm) to compensate for residual 

compressive strain (a Ga mole fraction of 0.26 in the 

GaInP buffer layer would be LM to the 1-eV GaInAs if both 

layers were fully relaxed). The LMM, 1-eV, DH subcell (nGaInP FSCL (50 nm), n-GaInAs emitter layer (150 nm), pGaInAs base layer (2.9 μm), and p-GaInP BSCL (50 nm)) 

is then grown, along with a final p+

-GaInAs (0.1 μm) 

contact layer to facilitate processing. It should also be 

noted that a back-surface reflector (BSR) can be easily 

incorporated into the device at the top surface (which is 

actually the back of the device). The BSR can serve dual 

roles as both a sub-bandgap IR heat reflector for improved 

thermal management, and as a reflector that allows the 

LMM, 1-eV subcell to be grown half the usual thickness. 

The thinner LMM subcell takes less time to grow and 

should produce ~20 mV more output voltage due to the 

“narrow diode” effect (i.e., ~half the reverse-saturation 

current). 

A general device processing sequence for inverted 

tandem cell structures is now described. The first step 

involves preparation of the top surface of the epistructure, 

which is actually the back surface of the device, by 

applying a back electrical contact, and, optionally, a backsurface reflector (BSR). The structure is than inverted and 

affixed to a pre-metallized “handle” material (in this case, 

conductive epoxy is used on an Al-coated Si wafer), which 

can be chosen to have a variety of advantageous 

properties and functions (e.g., strength, flexibilty, low cost, 

good thermal conductivity, selected electrical properties, 

etc.). The resultant handle-mounted device should have 

enhanced mechanical properties, and access to the back 

electrical contact should be established. The third step 

involves removal of the parent substrate and associated 

etch-stop layer (accomplished here using selective wetchemical etchants). In the final step, the top surface of the 

device has an electrical contact applied (gridded, 

electroplated Au), and an antireflection coating (ARC) is 

also deposited. Additionally, the GaAs contact layer is 

selectively removed between the grids, and the active 

device layers are mesa isolated to define the device area 

and to eliminate electrical shunting. The ultra-thin tandem 

device is then complete and ready for operation. 

We have performed semi-realistic modeling 

calculations, based on a rigorous approach for seriesconnected tandem subcells [15], to serve as a guide for 

the choice of the bottom subcell bandgap, and to predict 

potential performance, under operating conditions relevant 

to ultra-high performance devices. We assume that the 

bottom subcell quantum efficiency is 0.95 (spectrally 

independent) in all of the calculations. Also, the top and 

middle subcells are fixed to be GaInP (1.87 eV) and GaAs 

(1.42 eV), respectively. Results for the one-sun AM0 

spectrum, at 25°C, show that the optimum bottom subcell 

bandgap is 1.02 eV, with a tandem efficiency of 33.1%. 

For terrestrial concentrator applications, we modeled for 

4 �
the low-AOD Direct spectrum at 250 suns concentration, to perform optimally under both space (AM0, one sun), 

25°C, and obtained an optimum bottom subcell bandgap and terrestrial concentrator (low-AOD Direct, hundreds of 

of 1.01 eV, with a tandem efficiency of 41.5%. The above suns), conditions are planned for near-term future work. 

results bode well for achieving the performance goals 

envisaged for ultra-high efficiency SPV converters. 

In a preliminary effort, we have been successful in 

growing, processing, and testing monolithic, seriesconnected, handle-mounted, ultra-thin 

GaInP/GaAs/GaInAs tandem solar cells. Performance 

data for the best device fabricated to date are included in 

this section of the paper. Quantum efficiency (QE) and 

reflectance (R) data are given in Fig. 3. The data generally 

show excellent carrier collection across a broad spectral 

range for all of the subcells. The R data, however, show 

that photocurrent gains are still possible at the far edges of 

the tandem response range. Improving the two-layer 

ZnS/MgF2 ARC will be a focus of future work. Interference 

effects are also observed in the QE data for the 1.02-eV 

bottom subcell, which occur because the subcell is 

optically thin with a BSR, causing it to behave like a FabryPerot cavity. The interference effects are also evident in 

the R data over the response range of the bottom subcell. 

It is important to note that the QE for the bottom subcell is 

excellent despite its 2.2% LMM with respect to the GaAs 

substrate. 

100


90


80


70


60


50


40


30


20


10


0


300 400 500 600 700 800 900 1000 1100 1200 1300


Wavelength (nm)


Figure 3. Composite spectral absolute external quantum 

efficiency (solid lines) and spectral reflectance (dotted line) 

data for an ultra-thin, handle-mounted 

GaInP/GaAs/GaInAs series-connected tandem solar cell. 

Current-voltage data for the most efficient 

GaInP/GaAs/GaInAs tandem solar cell fabricated to date 

are shown in Fig. 4. The tandem cell is 31.1% efficient 

under the one-sun Global spectrum at 25°C, which is only 

~1% absolute lower than the highest efficiency ever 

demonstrated for a solar cell under these conditions [16]. 

With continued development, we anticipate that these 

devices will be 34-35% efficient under the above 

conditions. Furthermore, versions of these devices tuned Quantum Efficiency or Refelectance (%)


Figure 4. Current-Voltage data for a high-efficiency, ultrathin, handle-mounted GaInP/GaAs/GaInAs seriesconnected tandem solar cell measured under the one-sun 

Global spectrum at 25°C. 

CONCLUSION 

The design space for epitaxial III-V PV device 

heterostructures is increased substantially by considering 

approaches that involve LMM components. Recent 

advances in the understanding of relaxation dynamics and 

heterointerface behavior in compositionally step-graded 

heterostructures with significant LMM (~2%) has resulted 

in the demonstration of LMM III-V materials and devices 

with properties equaling those of similar LM cases. 

Examples of highly successful applications of LMM 

approaches to both TPV and SPV devices have been 

discussed. LMM, InP-based TPV materials and devices 

have advanced considerably; GaInAs/InAsP DH devices 

with exceptional minority-carrier lifetimes and diffusion 

lengths have been demonstrated, along with 0.52-eV 

GaInAs/InAsP TPV converters with excellent performance 

characteristics. A novel, monolithic, ultra-thin 

GaInP/GaAs/GaInAs series-connected tandem solar cell, 

which uses a LMM, 1-eV bottom subcell, has been 

demonstrated with excellent performance. A conversion 

efficiency of 31.1% (one-sun Global spectrum, 25°C) has 

been achieved for the SPV tandem in preliminary work, 

5 �
which is the second highest efficiency reported so far for 

any PV device under such operating conditions. 

ACKNOWLEDGEMENTS 

The authors gratefully acknowledge the support 

of both Bechtel Bettis, Inc., and the U. S. Department of 

Energy (under Contract DE-AC36-99GO10337) for this 

work. We also thank Charlene Cramer and Michelle 

Young for assistance with the epitaxial growth and device 

processing, respectively, of the ultra-thin SPV tandem 

devices. 

REFERENCES 

[1] M. W. Wanlass et al., “GaxIn1-xAs 

Thermophotovoltaic Converters,” Sol. Energy Mater. Sol. 

Cells 41/42, 405 (1996). 

[2] M. W. Wanlass, “Low-Bandgap, DoubleHeterostructure InAsP/GaInAs Photovoltaic Converter,” U. 

S. Patent No. 6,300,557, 10/9/01. 

[3] D. Wilt et al., “Monolithic Interconnected Modules 

(MIMs) for Thermophotovoltaic Energy Conversion,” 

Semcond. Sci. Technol., vol. 18, pp. S209-S215, 2003. 

[4] R. J. Wehrer et al., “0.74/0.55-eV GaxIn1-

xAs/InAsyP1-y Monolithic, Tandem, MIM TPV Converters: 

Design, Growth, Processing and Performance,” Proc. 29th 

IEEE Photovoltaic Spec. Conf., 5/20-24/02, New Orleans, 

LA, ISBN: 0-7803-7472-X, pp. 884-887, 2002. 

[5] Rebecca Wehrer et al., “InGaAs SeriesConnected, Tandem, MIM TPV Converters,” Proc. 

WCPEC-3 (30th IEEE PVSC), Osaka, Japan, May 15, 

2003. 

[6] S. L. Murray et al., “InGaAs Thermophotovoltaic 

Tandem Cell with Multi-Junction Interconnects on a Single 

Chip,” Proc. 19th European Photovoltaic Solar Energy 

Conference and Exhibition, Paris, France, June 7-11, 

2004. 

[7] Mark W. Wanlass and Jeff Carapella, “LowBandgap, Monolithic, Multi-Bandgap Optoelectronic 

Devices,” NREL IR # 01-48, patent pending. 

[8] M. W. Wanlass et al., “Recent Advances in LowBandgap, InP-Based GaInAs/InAsP Materials and Devices 

for Thermophotovoltaic (TPV) Energy Conversion,” AIP 

Conf. Proc. 738, 427-435 (2004). 

[9] S. P. Ahrenkiel et al., J. Electronic Mat. 33, 185 

(2004). 

[10] R. K. Ahrenkiel and S. W. Johnston, “An 

advanced technique for measuring minority-carrier 

parameters and defect properties of semiconductors,” 

Materials Science and Engineering B102 (2003) 161-172. 

[11] Mark W. Wanlass and S. Phillip Ahrenkiel, 

"Method for Optimizing Heteroepitaxial, LatticeMismatched Systems," NREL IR # 02-01, patent pending. 

[12] J. W. Matthews, in Epitaxial Growth (Part B), 

edited by J. W. Matthews (Academic Press, New York, 

1975). 

[13] Private communication with Dr. Michael Dashiell. 

[14] M. W. Wanlass, “Approaches for Ultra-High 

Efficiency, Monolithic, Multi-Bandgap, Tandem Solar 

Photovoltaic Energy Converters,” NREL IR # 05-05, patent 

pending. 

[15] M. W. Wanlass and D. S. Albin, “A Rigorous 

Analysis of Series-Connected, Multi-Bandgap, Tandem 

Thermophotovoltaic (TPV) Energy Converters,” AIP Conf. 

Proc. 738, 462-470 (2004). 

[16] Martin A. Green et al., “Solar Cell Efficiency 

Tables (Version 24),” Prog. Photovolt: Res. Appl. 2004; 

12:365-372. 

6 �
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February 2005 

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3-7 January 2005 

4. TITLE AND SUBTITLE 

Lattice-Mismatched Approaches for High-Performance, III-V 

Photovoltaic Energy Converters 

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DE-AC36-99-GO10337 

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6. AUTHOR(S) 

M.W. Wanlass, S.P. Ahrenkiel, R.K. Ahrenkiel, D.S. Albin, J.J. 

Carapella, A. Duda, J.F. Geisz, S. Kurtz, T. Moriarty, R.J. Wehrer, 

and B. Wernsman 

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NREL/CP-520-37440 

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52005000 

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National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 

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14. ABSTRACT (Maximum 200 Words) 

We discuss lattice-mismatched (LMM) approaches using compositionally step-graded layers and buffer layers that yield 

III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our 

progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic 

(TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) 

conversion involving LMM materials is also presented, along with promising preliminary performance results. 

15. SUBJECT TERMS 

PV; lattice-mismatched (LMM); step-graded layers; buffer layers; lattice-matched (LM); devices; thermophotovoltaic 

(TPV); energy conversion; 

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