**Development of Optically Enhanced Back Reflectors and Improved
Deposition Processes for Amorphous Silicon-Based Photovoltaic
Technologies**

ANNUAL TECHNICAL STATUS REPORT

####### June 15 2002 - September 15 2003

Scott J. Jones (Principal Investigator), Greg Demaggio, Masat Izu

Tongyu Liu, David Tsu, Jeff Steele and Rey Capangpangan

**Energy Conversion Devices, Inc.**

NREL Technical Monitor

Bolko von Roedern

###  

Prepared under Subcontract No. ZDJ-2-30630-22

**Preface**

This Annual Subcontract Report covers the work performed by Energy
Conversion Devices, Inc. for the period June 15, 2002 to September 15,
2003 under DOE/NREL Subcontract No. ZDJ-2-30630-22. The following
personnel participated in the research program.

R. Capangpangan, G. Demaggio, M. Izu, S. Jones, T. Liu, S.R. Ovshinsky,
J. Steel and D. Tsu.

We would like to specially thank Professors Hellmut Fritzsche and Joe
Doehler for the many discussions and input he has made in advancing this
program.

**Executive Summary**

**Objectives**

In this program, we plan to improve the module efficiencies through
development of a new optically enhanced Al/ZnO back reflector and
improved i-layer deposition process. In the case of the back reflector
development, a multi-layered thin film structure consisting of films
with contrasting indices of refraction placed between the Al and ZnO
layers of the back reflector will be developed to improve the
reflectivity of the back reflector structure. The ultimate goal is to
achieve the high currents and cell efficiencies typically obtained with
the Ag/ZnO back reflector with a new optically enhanced back reflector
that can be used in the solar module products. Ag/ZnO back reflectors
are presently used only in R&D applications due to long term
instabilities related to electromigration of Ag. For the multi-layered
structure, focus will be on preparing the layers using sputtering
techniques so that this technology might be quickly applied to ECD's
present back reflector fabrication process that uses sputtering
techniques.

In the case of the i-layer, focus will be on preparing microcrystalline
silicon based intrinsic layers for low cost, high stable efficiency
solar cells through the use of high intensity (decomposition rate)
plasmas. In these studies, the effects of such deposition conditions as
ion bombardment, substrate temperature and etchant gases on the grain
size and film transparency will be studied and correlated with cell
performance.

Achievement of the goals of this program and application of these
advancements to ECD's joint venture company's production lines would
lead to an immediate improvement in module efficiencies. These advances
along with ECD's participation in the NREL a-Si teams with other
development programs will contribute to the ultimate goal of achieving
stable efficiencies of 15% using a low-cost, scalable, manufacturable
techniques and inexpensive substrates.

**Approach**

To improve the back reflector performance, a new multi-layer Al/(low
index layer)/(high index layer)/(low index layer)/ZnO structure is being
developed in this program. Fabrication of the back reflector layers is
initially being done using the dc sputtering technique presently used in
the production plant. As a first step to development of the back
reflector structure, a number of materials are being developed. In
particular, deposition conditions for a transparent conductor with an
index of refraction of 1.6 or less is being devised. We are first
testing different ZnO alloys as a low index material with ZnOMgF and
ZnOSi alloys already developed. Also the conditions used to make the
high index materials (n\>3.0) are being determined with doped a-Si and
a-SiGe as the first two candidates.

Having developed the necessary materials, a number of Al/(low index
layer)/(high index layer)/(low index layer)/ZnO structures are being
made to optimize the reflectivity of the stack through variation of the
layer thicknesses. Low bandgap a-SiGe nip semiconductor stacks will be
deposited on the back reflector structures followed by top TCO/Al
contacts to fabricate small area solar cells to test device performance.
With these cells, we will be looking for improvements in short circuit
currents and cell efficiencies with the use of these new types of back
reflector structures. Besides testing for reflectivity, we will also
test the use of different layer texturing schemes to obtain light
scattering properties which enhance the probability of photon absorption
and increase the short circuit currents.

Microwave techniques are being used to prepare microcrystalline Si nip
cells to achieve high performance at i-layer deposition rates near 15
Å/s. This is a continuation of previous studies in which 7.0% stable
efficiencies were obtained for single-junction cells using these high
deposition rates. The i-layers are presently being made in a single
chamber system while the doped layers are prepared by the standard 13.56
MHz PECVD technique in a second multi-chamber system. Deposition
conditions are being varied to optimize small area cell performance.

**Results/Status**

The following has thus far been achieved during this program:

1)  We have refurbished a sputtering machine to complete the back
    reflector depositions,

2)  We have developed conditions to make transparent, conductive ZnO
    layers with index of refraction values near 1.8,

3)  We have developed conditions to make transparent, conductive ZnOSi
    and ZnOMgF alloys with index of refraction values between 1.6 and
    1.7,

4)  We have tested the reflectivity of Al/ZnO/ ZnOSi/doped Si/ ZnOSi/ZnO
    back reflector structures and found it to be lower than that for
    Al/MgF/Si/MgF structures in the 600-1000nm range (the range of
    interest for back reflectors). The reflectivity for Al/MgF/Si/MgF is
    similar to that for Ag/ZnO in the 600-1000 nm range. We are
    presently determining the reason for the low reflectivity for
    Al/ZnO/ ZnOSi/doped Si/ ZnOSi/ZnO structure. Because of the low
    reflectivity, improvement in the cell performance has yet to be
    obtained.

5)  We have found that replacing doped a-Si with doped a-SiGe in the
    multi-layer structure does lead to a significant improvement in the
    reflectivity of the back reflector.

6)  We have tested the use different SiH~4~+SiF~4~ total gas flows
    during microcrystalline Si i-layer depositions to increase the
    degree of etching during growth but no improvement in cell
    performance was obtained.

**Publications**

Listed below are articles in which these works were published during the
entire program.

S.J. Jones, G. Demaggio, J. Doehler, T. Liu and D. Tsu*,* "Development
of Optically Enhanced Back Reflectors for Amorphous Silicon-Based
Photovoltaic Technologies. ", Program and Proceedings: NCPV and Solar
Program Review Meeting 2003, March 24-26, Denver, CO, 823.

**Table of** **Contents**

Title Page 1

Preface 2

Executive Summary 3

Table of Contents 5

List of Figures 6

List of Tables 7

Introduction 8

Experimental 10

Results 11

Back Reflector Studies 11

Back Reflector Deposition Equipment Upgrade 11

######  Single layer material studies 12

ZnO alloys 13

Doped SiO~x~ 17

Al/Multi-layer/ZnO results 17

Solar Cells 18

Discussion and Future Work 20

Microcrystalline Si studies 21

**List of Figures**

Figure 1. ECD/United Solar's triple-junction solar cell structure. 8

Figure 2. Cell structure using the new type of back reflector. 9

Figure 3. Test structures used for optimizing back reflectors. 10

## Figure 4. Depiction of dc sputtering system. 12 

Figure 5. New back reflector structure. 12

Figure 6. Effect of index of refraction of layers on reflectivity. 13

Figure 7. Quantum efficiency data for Al/ZnO and Al/ZnO/Si/ZnO
structures with room temperature Al. 14

Figure 8. Index of refraction for ZnO made at different temperatures. 14

Figure 9. Index of refraction for ZnOSi with different Si contents. 15

Figure 10. Index of refraction for ZnOMgF with different MgF contents.
16

Figure 11. Resistivity vs. index of refraction for ZnO alloys. 16

Figure 12. Reflectance spectra for various structures. 18

Figure 13. New deposition chamber for microcrystalline Si deposition. 22

**List of Tables**

Table I. Properties for cells with Al/ZnO and Al/ZnO/Si/ZnO back
reflectors. 14

Table II. a-SiGe cell performance for devices made with different back
reflectors. 19

Table III. Data for μc-Si cells made by the Gas Jet technique. 21

**Introduction**

Over the past 15 years \[6-8\], ECD has pioneered and continued the
development of several key a-Si PV technologies. An a-Si/a-SiGe/a-SiGe
triple junction, spectrum splitting cell structure has been developed to
achieve high efficiencies for a-Si based solar modules. This structure
is depicted in Figure 1. With this design, ECD and United Solar have set
several light-to-electricity efficiency records including United Solar's
world record 10.5% stable triple junction a-Si alloy solar panel with
over 1 ft^2^ area and a 13% stable active area efficiency for a 0.25
cm^2^ cell. To market this technology, a roll-to-roll manufacturing
process was developed through ECD's machine division. This inexpensive
process uses a 14", ½ mile long steel web as a substrate material on
which the light absorbing layers is deposited. ECD has developed the
process through several generations of manufacturing machines with the
out come of a production process which offers significant
economy-of-scale advantages and results in dramatic cost savings as the
volume of production increases. This technology is presently being used
in United Solar's 25 MW production line in Auburn Hills, MI.

![](media/image1.wmf){width="2.8506944444444446in"
height="2.1243055555555554in"}

**Figure 1. ECD/United Solar's triple-junction solar cell structure.**

While several advances have been made on the R&D level to increase small
area cell and module efficiencies, the stable efficiencies presently
obtained for the large area modules made in Uni-Solar's production
machine are between 8.0 and 8.5%, roughly 20% lower than the 10.5% value
obtained for the 1 sq. ft. module in R&D. The deficiencies in the module
performance in production are in part related to the following
alterations to the fabrication process from that used in R&D:

1)  Use of deposition rates of 3 Å/s as compared with the 1 Å/s rate
    used in R&D -- in order to use gas utilization rates which make the
    process economically feasible, conditions must be used that increase
    the i-layer deposition rates to roughly 3 Å/s. This leads to poorer
    i-layer and cell performance.

2)  Use of Al/ZnO back reflectors rather than the Ag/ZnO used in R&D --
    because of poor long term yield issues, the Ag/ZnO layer is not
    presently used in production, instead a sturdier Al/ZnO structure is
    used. Because of the poorer reflectance properties of Al as compared
    with Ag, roughly a 10% drop in current and cell efficiency is
    observed with the use of the Al/ZnO back reflector.

In terms of addressing these issues in order to improve the module
efficiencies obtained in production, several research and development
programs have been undertaken. To improve the performance of cells made
using i-layer deposition rates of 3-10 Å/s, both United Solar and ECD,
through the Thin Film Partnership Program, have carried out studies of
i-layers made at high rates using the standard 13.56MHz PECVD technique
and VHF PECVD techniques using frequencies between 70 and 100
MHz\[9,10\]. While some progress has been made, the efficiencies for the
cells made at the higher deposition rates remain lower than those made
using the 1 Å/s conditions.

In contrast to the high deposition rate issue, little work has recently
been done on improving the back reflector used in the manufacturing of
the solar modules. Considering the potential 10% increase in module
performance if an environmentally stable back reflector that has similar
performance capabilities to Ag/ZnO is developed, it is important that
some research efforts be undertaken in this area. Some attempts have
been made to using barrier layers to improve the stability of the Ag/ZnO
back reflectors\[4\]. While some short term improvement of the cell
yields have been observed, the long term air exposure issue has yet to
be solved without substantial decreases in cell efficiency.

As part of this NREL supported program, we plan to improve the currents
of the modules through development of a new optically enhanced Al/ZnO
based back reflector. In the case of the back reflector development, a
multi-layered thin film structure consisting of films with contrasting
indices of refraction (n) placed between the Al and ZnO layers of the
back reflector will be developed (See Figure 2). The addition of the
multi-layer structure will improve the reflectance properties of the
back reflector. In this program, these new types of back reflectors will
be tested in amorphous silicon based single junction and multi-junction
devices. The differences in n of the different layers of the
multi-layered back reflector and electrical conduction through the
multi-layered structure will be optimized to obtain the highest
reflection values, highest currents and best cell performance. The
ultimate goal is to achieve the high currents and cell efficiencies
typically obtained with the Ag/ZnO back reflector with an
Al/(multi-layered structure)/ZnO back reflector that can be used in the
solar module products. For the multi-layered structure, focus will be on
preparing the layers using sputtering techniques so that this technology
might be quickly applied to ECD's present back reflector fabrication
process that uses sputtering techniques.

![](media/image2.wmf){width="3.5131944444444443in"
height="1.7631944444444445in"}

**Figure 2. Cell structure using the new type of back reflector.**

##### **Experimental**

For the back reflector development, the layers were primarily made using
the dc sputtering technique. The system used to prepare the layers
contains four different sputtering cathodes to deposit four different
types of materials in one system pumpdown. A movable substrate and rail
system are used to move the substrate past the targets at a fixed speed
during film deposition to simulate the roll-to-roll deposition process.
Presently, back reflector layers are deposited on six 2" x 2" substrates
during each run, however substrates as large as 1' x 1' can be used with
the existing hardware. During materials development, films were
deposited on 7059 glass and/or quartz substrates for various optical and
electrical measurements to determine such properties as reflectivity,
conductivity, index of refraction, absorption and film thicknesses. To
fabricate the Si layer, a P doped Si target (200ppm) was used while a
ZnAl alloy target with 5% Al was used to fabricate the ZnO alloys.

For multi-layer back reflector fabrication, multi-layer stacks were
deposited on 5 mil stainless steel substrate typically used in
Uni-solar's manufacturing facility. To initially characterize the back
reflector performance, nip amorphous silicon germanium semiconductor
structures were deposited on the back reflectors followed by top
contacts to create 0.25 cm^2^ solar cells (depicted in Figure 3). The
nip structures were fabricated using a research scale, multi-chamber
load locked deposition system and the standard 13.56 MHz Plasma Enhanced
Chemical Vapor Deposition (PECVD) technique. Both the ITO and Al layers
are prepared using standard evaporation techniques. For promising back
reflector conditions, a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells
will be made.

**[a-SiGe:H Cells]{.underline}**

![](media/image3.wmf)

**Figure 3. Test structures used for optimizing back reflectors.**

To characterize the solar cells, standard IV and spectral response
(quantum efficiency) measurements are made. Since our goal is to use the
back reflectors to enhance the currents for the a-SiGe:H middle and
bottom cells in the triple-junction cells, the AM1.5 light for the IV
measurements of the a-SiGe:H cells is filtered using 630nm cutoff
filters to simulate the absorption due to a top a-Si:H cell. To complete
light soaking studies, the cells are subjected to 600-1000 hrs. of one
sun light with the cell temperature fixed at 50ºC. The i-layer
thicknesses are determined using standard capacitance techniques.

For the microcrystalline Si growth studies, single-junction nip
microcrystalline Si solar cells are being fabricated to characterize the
microcrystalline Si quality. The Si materials have been prepared using
microwave techniques to achieve high decomposition and growth rates.
Since our goal is to create cells for red-light absorption, cells are
again characterized using unfiltered AM1.5 light and AM1.5 light
filtered using 630nm cutoff filters to complete IV measurements. Light
soaking of the cells are also completed using the method used for the
a-SiGe:H cells for the back reflector characterization.

**Results**

***Back Reflector Studies***

In devising the new back reflector structure, one should consider two
properties the present Ag/ZnO back reflectors provide. First is the high
reflectivity of the Ag metal and second is the texture surface at the
ZnO top surface which provides light scattering properties to enhance
the probability of light collection and higher solar cell currents. The
textured surface is typically obtained by using high deposition
temperatures during metal growth and/or thick layers (metal and/or ZnO).
With the new Al/ML/ZnO structure, the addition of the multi-layered
structure should lead to enhanced reflectivity. The exact source for
light scattering (surface texturing or particle scattering) in this new
back reflector structure has yet to be determined.

*Back Reflector Deposition Equipment Upgrade*

Prior to beginning the back reflector studies, the equipment used to
make the back reflectors required significant upgades. The system has
been used in the past for fabricating back reflector layers on 14"
stainless steel webs, however it has not been in operation for three
years and several key fixtures need to be repaired. These included:

1)  Repair of substrate heating system,

2)  Repair of diffusion pumping system,

3)  Repair of sputtering power supply,

4)  Installation of additional gas lines including those for oxygen
    incorporation for reactive sputtering,

5)  Replacement of web drive system with rail system for 14" x 14"
    substrate holder.

In particular, construction of the rail system was imperative and
required a significant amount of time. The existing web drive system was
obsolete and in need of significant repair. Also the drive system was
built to move web in only one direction and because of this would not
have been desirable for this program. All of the parts for this new rail
system were fabricated in ECD's model shop.

A depiction of the system after completion of the repairs is shown in
Figure 4. With this system, one is able to deposit with four different
targets in one system pumpdown. Between two targets is a gas manifold
that allows for uniform Argon or reactive gas flows (such as Oxygen)
across two of the targets. The substrate rail system allows for movement
of one 1' x 1' substrate or six 2" x 2" substrates across the deposition
areas to simulate the web movement presently used in the production
machines built by ECD.

## Figure 4. Depiction of dc sputtering system.

###### Single layer material studies

The multi-layer structure to be optimized in this program is depicted in
Figure 5. The multi-layer consists of a high index conductive material
(n =3.2 or greater) sandwiched between two low index materials (n\<2.0).
Both of the materials need to be somewhat transparent and conductive.
Early results from modeling of the reflectivity suggest that more focus
should be spent on developing the low index material than increasing the
index of the high index materials. Figure 6 shows the dependence of
absorption of the structure as a function of the low n and high n
values. This absorption value is roughly inversely proportional to the
reflection from the back reflector structure. From this data, it was
determined that lowering n for the two low n layers would be more
effective in increasing the reflection form the back reflector than
raising the index of refraction for the high n material. Thus we have
spent the initial portion of this program developing the low index,
transparent conductive oxide. As figure 3 shows, we have chosen to
initially use doped a-Si as the high index material because of minimal
light absorption in the red portion of the solar spectrum, a reasonably
high conductivity when doped, and an index of refraction of 3.2.

**Figure 5. New back reflector structure.**

![](media/image4.wmf){width="4.75in" height="2.765972222222222in"}

**Figure 6. Effect of index of refraction of layers on reflectivity.**

*ZnO alloys*

As a first attempt to prepare the desired low index materials, a number
of, ZnO films were deposited under a variety of dc reactive sputtering
deposition conditions. The conditions were varied to observe if the
index of refraction could be significantly altered without the addition
of other elements to the ZnO alloy. The deposition conditions varied
included chamber pressure, substrate temperature, applied power and
oxygen flow. Of the conditions varied, oxygen flow and substrate
temperature had the greatest effect. With a fixed moderate substrate
temperature of 150°C, at low oxygen flows, the films were Zn-rich and
metallic in nature, as one would expect, and became low absorbing,
TCO-like when a certain oxygen threshold was reached. Throughout this
range of oxygen flow, we were not able to find conditions where the
index of refraction was less than 1.8 and reasonable transparency and
conductivity values were obtained.

Using conditions which gave an n=1.8 value, an Al/ZnO(1.8)/Si/ZnO(1.8)
stack was made and compared with an Al/ZnO back reflector stack. For
these initial studies, specular Al layers with little texture were used.
To characterize the back reflectors, a-SiGe:H nip stacks were deposited
onto the back reflectors and ITO/Al top contacts subsequently deposited
onto the semiconductor stack to create small area (0.05 cm^2^) solar
cells. In Figure 7, quantum efficiency spectra for devices made using
the two different types of back reflectors are displayed. Enhanced
quantum efficiency in the red portion of the spectrum for the cell with
the Al/ZnO/Si/ZnO back reflector can be seen from the figure. This
increased collection was confirmed with IV measurements (see Table I)
where increases in J~sc~ values of 0.5-1.0 mA/cm^2^ have been observed
for the Al/ZnO/Si/ZnO cells. However, the currents are still low
partially due to the specular surface of the Al layer. To obtain the
desired large increases in J~sc~, the index of reflection of the low n
material must further be reduced.

![](media/image5.wmf)

**Figure 7. Quantum efficiency data for Al/ZnO and Al/ZnO/Si/ZnO**

**structures with room temperature Al.**

Table I. Properties for cells with Al/ZnO and Al/ZnO/Si/ZnO back
reflectors.

+--------------------+------------+-----------+--------+--------------+
| Type of Back       | Jsc        | Voc       | FF     | Pmax         |
| reflector          |            |           |        |              |
|                    | (mA/cm2)   | \(V\)     |        | (mA/cm2)     |
+--------------------+------------+-----------+--------+--------------+
| Al/ZnO             | 15.1       | 0.683     | 0.395  | 4.07         |
+--------------------+------------+-----------+--------+--------------+
| Al/ZnO/Si/ZnO      | 15.8       | 0.687     | 0.416  | 4.52         |
+--------------------+------------+-----------+--------+--------------+

Lowering the index of refraction below n=1.8, the effect of substrate
temperature on the ZnO film growth was also studied. Figure 8 displays
the change in n with light wavelength for films deposited at 25°C and
250°C. The back reflector is only effective in the region between

![](media/image6.png){width="5.375196850393701in"
height="3.4035433070866143in"}

**Figure 8. Index of refraction for ZnO made at different
temperatures.**

roughly 550nm and 1000nm, as depicted in the figure. Below 550nm, top
and middle cells absorb most of the light while the a-SiGe:H bottom cell
typically does not contain enough Ge to absorb light beyond 1000nm. One
can see from the figure that as the substrate temperature is increased
from 25 to 250°C, n is decreased from roughly 1.90 to 1.73. The
conductivities of the films deposited at 250°C were actually higher than
those deposited at the lower substrate temperature (resistivity of 230
ohms/cm^2^ as compared with 5x10^4^ ohms/cm^2^). With further
alterations in other deposition parameters, we have been able to obtain
ZnO films with average index of refractions in the region of interest of
1.65 however the films did not have the desired transparency.

To obtain TCOs with n\<1.7, we studied the addition of other elements to
the ZnO alloy. Figure 9 displays optical data for ZnOSi alloys.
Specifically, the index of refraction is plotted as a function of
wavelength for three different ZnOSi films with different Si contents.
Si was added to the process by placing Si wafers on the Zn target. For
this material's use as a component layer of the back reflector
structure, the wavelength region of interest is roughly between 550 and
1000 nm, as is identified in the figure. In this wavelength region, one
can see a favorable decrease in the index of refraction with increasing
Si content. We have also explored a second alloy, ZnOMgF. Figure 10
displays variation of the index of refraction as a function of
wavelength for three ZnOMgF films with different concentrations of
MgF~2~ placed on the Zn target. The red curve has the largest amount of
MgF~2~ and one can see that in the region of interest for its use as a
back reflector material, the average n is roughly around 1.64.

![](media/image7.png){width="5.4992125984251965in"
height="3.501968503937008in"}

**Figure 9. Index of refraction for ZnOSi with different Si contents.**

Wavelength (nm)

0 250 500 750 1000 1250 1500 1750 2000 2250 2500

![](media/image8.png){width="6.372916666666667in"
height="2.3194444444444446in"}

**Figure 10. Index of refraction for ZnOMgF with different MgF
contents.**

In terms of the conductivity of these materials, Figure 11 displays the
trends in resistivity with index of refraction changes with alloying of
ZnO with Al, Si and MgF~2~. For both alloys with Si and MgF~2~, under
certain deposition conditions, we have been able to prepare materials
with high conductivity (ρ~o~\<10^-3^ Ωcm), high transparency (96-97%,
see numbers on figure) and n

![](media/image9.wmf){width="6.4992125984251965in"
height="4.14251968503937in"}

**Figure 11. Resistivity vs. index of refraction for ZnO alloys.**

between 1.6 and 1.7 through alloying ZnO with Si and MgF~2~. As n is
decreased from 1.88 to 1.65 with the addition of more Si or MgF~2~, a
slight rise in resistivity is observed as Si is increased while
virtually no change in resistivity as more of MgF~2~ is added to the
alloy. Increasing the amount of Si and MgF~2~ further such that n\<1.6,
the resistivity increases to values of 15 x 10^-3^ Ωcm or greater (in
the case of ZnOMgF, the resistivity values are greater than 20 x 10^-3^
Ωcm). Doping of the materials to increase the conductivity will likely
be required for these n\<1.6 materials to achieve the desired
performance. When Al is added to the ZnOSi alloy, no improvement in the
conductivity was observed and when the amounts of Al became excessive,
AlO~x~ formation dominated the structure leading to high resistivity
values and fixed indices of refraction near 1.7, the value typically
obtained for Al~2~O~3~.

*Doped SiO~x~*

As an alternative to the ZnO alloys, we have completed a small number of
experiments (less than 10) to fabricate conductive doped SiO~x~ films.
The layers were made by dc sputtering of a Si target with W pieces
placed on top of the target in an Ar + O~2~ atmosphere. Unfortunately,
we have yet to prepare the correct W doping level to obtain a
transparent conductive layer. Because of the small number of
experiments, obtaining the desired film characteristics may be a matter
of further optimization of deposition conditions or it could be related
to an inability to dope the sputtered films due large defect densities.
Preparing these layers by PECVD techniques might lead to less defective,
dopable materials. Adding a PECVD process to the present dc sputtering
back reflector fabrication method would make the process significantly
more complex but could be done. In the coming months, we will decide
whether to further pursuit of doped SiO~x~ is worth our team's efforts.

*Al/Multi-layer/ZnO results*

Prior to testing the back reflectors with nip solar cell stacks, a
number of Al/multi-layer/ZnO structures have been fabricated for
reflectivity measurements. Since there was no nip stack on top of the
back reflector and the ZnO interfaced with atmosphere (n=1), these
measurements were used not to determine the thicknesses of the various
layers comprising the back reflectors. They were used to determine if
the multi-layer structure does lead to a significant increase in
reflection in the red region of the light spectrum. To eliminate the
possibility of different degrees of texturing affecting the reflectance,
the Al layers were deposited at room temperature leading to a surface
that is close to specular. Figure 12 compares reflectivity data for four
different structures, 1) a single Al layer made in the same dc
sputtering system used to make the ZnO alloys, 2)
Al/ZnO/ZnOSi/Si/ZnOSi/ZnO, 3) Al/ZnO/ZnOSi/SiGe/ZnOSi/ZnO, and 4)
Al/MgF~2~/Si/MgF~2~. For structures 2-4, the layer thicknesses were
varied to obtain the highest reflection in the region of interest
(600-100nm) where light is absorbed by the bottom and middle component
cells of the triple-junction a-Si/a-SiGe/a-SiGe structure. The
Al//MgF~2~/Si/MgF~2~ structure was made in an earlier program with the
MgF~2~ layers made using evaporation techniques. It was demonstrated
that this stack has similar reflectivity properties to specular Ag/ZnO
in the 600-100nm region. In this region, one can see that on average the
reflectance for the Al/ZnO/ZnOSi/Si/ZnOSi/ZnO structure is slightly
lower that that for the bare Al. This was unexpected since modeling
suggested that the structure would lead to improved total reflectance.
The lower values for the Al/ZnO/ZnOSi/Si/ZnOSi/ZnO structure could be
related to some source of absorption that has yet to be determined or a
textured surface (interface) which would lead to a scattering source not
taken into account by our modeling. In an attempt to increase the
reflectance, we have just recently fabricated a structure with an a-SiGe
layer as the high index layer of the multi-layer structure. This will
lead to an increase in n of the high index layer and thus an increase in
∆n between the high index and low index (ZnOSi) layers. As would be
expected, the reflectance increased with the replacement of Si with SiGe
and the new structure has reflectivity properties significantly higher
than for the Al layer. While the reflectance is higher, use of a-SiGe
leads to higher red light absorption and thus may not be desirable for
this application. We are also now in the process of re-checking the
optical properties of the sputtered Si.

![](media/image10.wmf){width="5.5625in" height="3.8854166666666665in"}

**Figure 12. Reflectance spectra for various structures.**

*Solar Cells*

A number of a-SiGe solar cells were fabricated with the back reflector
structures to demonstrate the back reflector performance. To optimize
the performance, the various layer thicknesses were altered to obtain
the high solar cell currents and efficiencies. In particular, focus was
on cell performance in the red portion of the light spectrum where the
back reflector quality is of importance. The amount of Ge in the a-SiGe
layer for these cells is higher than what is typically used for the
bottom cells of the triple-junction structure in order to more closely
monitor the back reflector performance. The Al/ZnO/ZnOSi/Si/ZnOSi/ZnO
back reflector structure is the only structure yet to be optimized using
the nip solar cells. There is some question as to whether the Al should
be textured for current enhancement as is typically done with the Al/ZnO
back reflectors or if the Al should be specular to obtain the most
benefit from the addition of the ML structure. Thus we have fabricated
cells with both textured (high substrate temperature) and specular (low
substrate temperature) surfaces for the Al layer.

Table II compares data for cells with Al/ML/ZnO back reflectors with
those for cells with standard Al/ZnO and Ag/ZnO back reflectors. All of
the back reflectors were made in the same deposition system except for
the Al/ZnO made in United Solar's production machine (labeled as from
production machine). The amorphous semiconductor and ITO depositions for
each of the back reflectors were made using the same R&D deposition
systems using nominally the same deposition parameters. Comparing the
cells with Al/ML/ZnO having textured Al to those with specular Al, the
cell current and overall performance is significantly higher for the
cells with the textured Al surface. This demonstrates the improvement
one is able to obtain with the textured surface. Comparing the
performance of the Al/ML/ZnO back reflector having the textured Al with
Al/ZnO structures made using both the R&D and Production machines, the
overall performance is quite similar. There are some differences in the
V~oc~, J~sc~, FF and R~s~ values, but the P~max~ values are all in the
2.5-2.7 mW/cm^2^ range. The 2-3 Ohms cm^2^ difference in the series
resistance for the Al/ML/ZnO and the Al/ZnO may suggest a slight problem
with the Al/ML/ZnO back reflector, however the difference may also
reflect slight variations in the Ge contents in the i-layers. The
performance of all of the Al based back reflectors are still
significantly poorer than that for the Ag/ZnO back reflectors, as is
shown in the table.

**Table II.**

**a-SiGe cell performance for devices made with different back
reflectors.**

**Cell properties obtained using AM1.5 light filtered with 630nm cutoff
filter.**

(ML structure is ZnO/ZnOSi/Si/ZnOSi/ZnO)

+---------------------------+-----+--------+-----+----------+--------+
| Type of Back Reflector    | V~  | J~sc~  | FF  | R~s~     | P~max~ |
|                           | oc~ |        |     |          |        |
|                           |     | (mA/   |     | (Ohms    | (mW/   |
|                           | \(  | cm^2^) |     | cm^2^)   | cm^2^) |
|                           | V\) |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ML/ZnO from R&D BR     | 0.  | 9.47   | 0.  | 14.5     | 2.59   |
| machine                   | 519 |        | 527 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ML/ZnO from R&D BR     | 0.  | 9.05   | 0.  | 15       | 2.57   |
| machine                   | 538 |        | 527 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ML/ZnO from R&D BR     | 0.  | 7.03   | 0.  | 18.9     | 1.93   |
| machine                   | 538 |        | 511 |          |        |
|                           |     |        |     |          |        |
| (Specular Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ZnO from R&D BR        | 0.  | 9.50   | 0.  | 12.1     | 2.69   |
| machine                   | 520 |        | 545 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ZnO from R&D BR        | 0.  | 9.29   | 0.  | 12.2     | 2.54   |
| machine                   | 509 |        | 538 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ZnO from Production    | 0.  | 9.63   | 0.  | 20.9     | 2.49   |
| machine                   | 541 |        | 477 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Al/ZnO from Production    | 0.  | 9.66   | 0.  | 18       | 2.47   |
| machine                   | 520 |        | 492 |          |        |
|                           |     |        |     |          |        |
| (Textured Al)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Ag/ZnO from R&D BR        | 0.  | 11.4   | 0.  | 13.6     | 3.53   |
| machine                   | 573 |        | 541 |          |        |
|                           |     |        |     |          |        |
| (Textured Ag)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+
| Ag/ZnO from R&D BR        | 0.  | 11.4   | 0.  | 18.4     | 3.26   |
| machine                   | 556 |        | 515 |          |        |
|                           |     |        |     |          |        |
| (Textured Ag)             |     |        |     |          |        |
+---------------------------+-----+--------+-----+----------+--------+

*Discussion and Future Work*

The fact that the addition of the multi-layer stack to the Al/ZnO
structure has yet to lead to increased back reflector performance may be
attributed to two reasons. First the reflectance data suggests that the
addition of the ZnOSi/Si/ZnOSi structure has yet to lead to an increase
in reflection which is surprising since simulations that use optical
data for single layers of these films to simulate the reflectivity from
the multi-layer structure suggest that we should exceed the values for
the Al/ZnO structure. Possibly there is some source of absorption or
light scattering (textured surface?) that we have not taken into
account. The fact that we can obtain increased reflectivity by
substituting the Si layer with an a-SiGe layer suggests that there are
avenues to achieve the desired reflectivity. An improvement in the back
reflector performance with the addition of the ML stack may also be
related to the fact that the comparisons made thus far between Al/ZnO
and Al/ML/ZnO have been with those having textured Al surfaces.
Deposition of the ML structure on the textured Al surface may minimize
the affect of ML structure where the reflectivity is dependent on the
thicknesses of the layers comprising the ML.

Considering these issues, there are few tests we will complete during
the next year:

1)  Fabricate cells with Al/ML/ZnO back reflectors having doped a-SiGe
    layers replacing the doped Si layer. We have shown that the
    reflectivity will increase with the use of a-SiGe and thus may lead
    to improved back reflector performance. However increased absorption
    by the a-SiGe layer may lead to similar or even lower currents and
    solar cell performance. We will test this with a variation of Ge
    contents.

2)  Fabricate Al/ZnO stacks with a specular Al surface with the same R&D
    system used in these studies to compare cell performance with that
    for Al/ML/ZnO. It is important that we set this baseline so that any
    improvement in solar cell performance with the addition of the ML
    stack can be observed. Since texturing the Al surface could minimize
    the effect of the ML stack, it is important that we do this with a
    specular Al surface by using low substrate temperatures during Al
    growth. The Al/ZnO back reflector made with specular Al whose data
    is shown in Figure 7 was made under unoptimized conditions and the
    cells made with different semiconductor processing conditions and
    thus can not be used for comparison with the optimized Al/ML/ZnO.

3)  Fabricate Al/ML/ZnO structures using ZnOMgF rather than ZnOSi as the
    low index layer. We have shown that indices of refraction near 1.6,
    high transparency and conductivity values can be obtained for the
    ZnOMgF alloys. The deposition process is more difficult than for the
    ZnOSi alloys, but there performance could be better.

4)  Fabricate Al/ML/ZnO structures with a specular Al surface and thick
    ZnO top layers. If a textured surface is required for light
    scattering, there is some question as to whether this should be the
    Al surface that is textured or the top ZnO surface should be the
    textured surface. Texturing the Al surface could minimize the effect
    of the ML while texturing the top ZnO will scatter the light in
    various angles and again possibly minimizing the effect of the ML
    structure. We plan to test both scenarios.

5)  Explore the use of different light scattering sources such as layers
    having low density regions or voids. Finding alternative light
    scattering source to layer texturing might be required. We plan on
    testing a number of different materials in a attempt to achieve the
    light scattering from a layer that is still transparent, highly
    conductive and when added does not disturb the enhanced reflectivity
    of the ML structure.

6)  Continue development of materials with n=1.6 or less that are
    transparent and conductive.

In addition to the Al/ML/ZnO studies, we plan on pursuing the
development of potential diffusion barriers for Al and Cu. In
particular, we plan on testing the use of high density a-SiC materials
which previously have been used to successfully limit Cu diffusion.

***Microcrystalline Si studies***

Improving the performance of the back reflectors used for the solar
panels fabricated with our machines is presently one of the major goals
of ECD. Also other members of the team are developing the
microcrystalline Si cells while little work is being done on the back
reflector issue. Thus in this program we are focusing on the back
reflector development and are spending less time on the microcrystalline
Si development. During this reporting period, we have fabricated some
microcrystalline Si single-junction cells in an attempt to improve the
performance of the cells fabricated at high i-layer deposition rates
(15-20 Å/s) over the efficiencies obtained in previous studies. As in
these earlier studies, the i-layers were made in a single chamber system
using microwave techniques. The doped layers were deposited in a
separate system using the standard 13.56MHz technique and deposition
rates near 1 Å/s. During transport between the deposition systems, air
exposure of the samples was minimized through the use of a mobile
evacuated chamber. The nip structures were deposited on Ag/ZnO coated
stainless steel substrates and ITO/Al were deposited on top of the nip
structures to complete the solar cell structures.

Table III compares data from our previous studies (8/01) with the
studies completed in this program. Prior to 8/01, we were able to obtain
red light efficiencies of 2.8-2.9 mW/cm^2^ using a technique in which a
Gas Jet was subjected to high intensity microwaves. Using the same
system, we have altered the SiH~4~/SiF~4~ ratio in the deposition
process in an attempt to improve the FF of the single-junction devices.
The idea was to increase the degree of etching by increasing the amount
of SiF~4~ used while maintaining a deposition rate of around 15 Å/s by
increasing the amount of SiH~4~. As can be seen from the data in the
table, on average the cells with high SiH4 + SiF4 total flows have
slightly poorer performance.

**Table III.**

**Data for μc-Si cells made by the Gas Jet technique.**

**IV data taken using AM1.5 light filtered with 630nm cutoff filter (red
light).**

+--------+--------+--------+---------+--------+---------+-----------+
| Report | S      | V~oc~  | J~sc~   | FF     | R~s~    | P~max~    |
|        | iF~4~+ |        |         |        |         |           |
| Date   | SiH~4~ | \(V\)  | (mA     |        | (oh     | (         |
|        | flow   |        | /cm^2^) |        | mcm^2^) | mW/cm^2^) |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/01   | Low    | 0.483  | 10.32   | 0.570  | 8.1     | 2.84      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/01   | Low    | 0.475  | 10.92   | 0.559  | 7.8     | 2.90      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/01   | Low    | 0.452  | 11.47   | 0.528  | 7.8     | 2.74      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/01   | Low    | 0.458  | 11.70   | 0.514  | 8.4     | 2.75      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/03   | High   | 0.461  | 10.40   | 0.571  | 8.0     | 2.74      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/03   | High   | 0.436  | 10.55   | 0.574  | 7.7     | 2.64      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/03   | High   | 0.460  | 10.63   | 0.544  | 8.4     | 2.66      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/03   | High   | 0.425  | 11.26   | 0.524  | 8.7     | 2.51      |
+--------+--------+--------+---------+--------+---------+-----------+
| 8/03   | High   | 0.418  | 11.50   | 0.516  | 8.7     | 2.48      |
+--------+--------+--------+---------+--------+---------+-----------+

There is some concern that the results we obtain from the single chamber
system are restricted to a certain level of performance because of the
exposure of the doped/intrinsic interfaces to atmosphere. Thus, we are
presently adding a new process chamber (see Figure 13) to our load lock
system which will be dedicated to microwave high growth rate processes
in the milli-torr pressure regime. This load lock system presently
consists of three other chambers dedicated to n, i or p-layer PECVD
depositions that are separated by gate valves. Thus in this system, we
will be able to prepare microcrystalline Si nip structures using high
i-layer deposition rates without breaking vacuum.

![](media/image11.jpeg){width="3.377083333333333in"
height="4.502777777777778in"}

**Figure 13. New deposition chamber for microcrystalline Si
deposition.**