September 1999 • NREL/SR-520-26795

S.J. Jones, T. Liu, D. Tsu, and M. Izu

Energy Conversion Devices, Inc.

Troy, Michigan

Use of Very-High-Frequency

Plasmas to Prepare a-Si:HBased Triple-Junction Solar

Cells at High Deposition Rates

Annual Technical Status Report

11 March 1998—11 March 1999

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

NREL is a U.S. Department of Energy Laboratory

Operated by Midwest Research Institute • Battelle • Bechtel

Contract No. DE-AC36-98-GO10337
September 1999 • NREL/SR-520-26795

Use of Very-High-Frequency

Plasmas to Prepare a-Si:HBased Triple-Junction Solar

Cells at High Deposition Rates

Annual Technical Status Report

11 March 1998—11 March 1999

S.J. Jones, T. Liu, D. Tsu, and M. Izu

Energy Conversion Devices, Inc.

Troy, Michigan

NREL Technical Monitor: K. Zweibel

Prepared under Subcontract No. ZAK-8-17619-18

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

NREL is a U.S. Department of Energy Laboratory

Operated by Midwest Research Institute • Battelle • Bechtel

Contract No. DE-AC36-98-GO10337
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1

Preface

This Annual Subcontract Report covers the work performed by Energy Conversion Devices, Inc. for the

period March 11, 1998 to March 11, 1999 under DOE/NREL Subcontract No. ZAK-8-17619-18. The

following personnel participated in the research program.

M. Izu, S.J. Jones, T. Liu, A. Myatt , S.R. Ovshinsky, D.V. Tsu, B. Viers.

Collaborations with the Colorado School of Mines, University of Toledo, and United Solar Systems

Corp. are gratefully acknowledged. We would like to specially thank Professor Xunming Deng for the

many discussions and input he has made in advancing this program he previously led.
2

Executive Summary

Objectives

The main objective of this program is to develop a high rate thin film deposition method in order to

increase the throughput of ECD’s roll-to-roll solar module production design and in doing so reduce the

cost of the solar modules. In particular, a very high frequency (70 MHz) plasma enhanced chemical

vapor deposition (PECVD) process is being developed for the fabrication of intrinsic layers for high

efficiency amorphous silicon-based triple-junction solar cells at high deposition rates. Intrinsic layers

consisting of either amorphous silicon or amorphous silicon germanium alloy materials are being

developed. The program goal is to prepare these materials at rates of 10 Å/s or higher while maintaining

the cell efficiencies at the high values presently obtained for devices made using the standard 13.56 MHz

frequency and low deposition rates (near 1 Å/s). Upon completion of a successful program, this high rate

process will be added to ECD’s roll-to-roll solar cell production design to reduce solar module cost.

Approach

In order to test the feasibility of using the VHF method for high rate intrinsic layer preparation, the

deposition conditions used to prepare small area (0.25 cm2

) single-junction amorphous silicon (a-Si:H)

and silicon germanium alloy (a-SiGe:H) cells by the very high frequency (VHF) technique are being

optimized to obtain the highest cell efficiencies. These component cells will then be combined to create

high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells. The deposition conditions for these

multi-junction cells will also be optimized to further increase the device performance. In particular for

the triple-junction cells, the conditions used to prepare p/n tunnel junctions will be optimized to minimize

absorption and series resistance and maximize carrier collection and cell performance. Besides achieving

high small area cell performance, large area cathode hardware designs which will allow for the uniform

deposition of i-layers over a large area using the VHF technique and high deposition rates will be tested

using a single chamber vacuum system. This large area hardware will be required for the future

incorporation of the technique into an ECD built roll-to-roll solar cell production line.

Results/Status

The following has thus far been achieved during this year:

1) 8.0% stable efficiencies have been achieved for a-Si:H single-junction cells whose i-layers

are prepared at rates near 10 Å/s using the VHF technique. This performance compares with 8.2%

stabilized efficiencies for a-Si:H cells made using the same deposition equipment, the standard 13.56

MHz technique and deposition rates near 1 Å/s. While we have yet to achieve the 8.4% program goal for

the a-Si:H cells, we hope that our recent improvement of the n-layer conditions as well as further

optimization of other deposition conditions will lead to the achievement of the goal in the coming

months. This achievement may also come from the general improvement of system baseline efficiencies

for all cells by optimization of chamber heating and pre-deposition treatments.

2) In terms of the a-SiGe:H cells, a number of studies of devices with properties appropriate for

middle junction cells have been made. The stabilized properties for these cells prepared at i-layer rates

near 10 Å/s are again similar to a-SiGe:H cells made using the same deposition hardware and the low rate

13.56 MHz method. We are looking to improve these efficiencies further again through optimization of

doped layer conditions and the general improvement of the baseline efficiencies for the semiconductor

deposition system.
3

3) Deposition conditions have been identified for which uniform i-layer deposits can be obtained

over our 4” x 4” substrate area at the high 10 Å/s rate. As the deposition rate is increased, the ability to

obtain uniform deposits over a significant area becomes more difficult. It was found that by using

moderately low chamber pressures, one can maintain uniform thin film deposits at the 10 Å/s rate while

other deposition parameters are varied in order to optimize the cell performance.

4) 10.5% efficiencies have been obtained for a triple-junction cells whose i-layers are prepared at

the 10 Å/s rate. These efficiencies degrade by 10-13% with long term light exposure. This amount of

degradation is similar to that obtained for triple-junction cells prepared at the standard low i-layer

deposition rates (1 Å/s). The achievement of higher efficiencies in the coming year is likely with further

optimization of the triple-junction structure. In terms of improving the efficiencies for the triple-junction

cells, focus has thus far been on optimizing the top and middle component cells. We plan to put a major

effort on the optimization of both the bottom cell and tunnel junction performance in the coming months.

Publications

Listed below are some articles in which this work, or portions of this work, were published.

“Preparation of a-Si:H and a-SiGe:H i-layers for nip solar cells at high deposition rates using a very high

frequency technique.” S.J. Jones, X. Deng, T. Liu, and M. Izu, Amorphous and Microcrystalline Silicon

Technology – 1998, edited by R. Scropp, H. Branz, M. Hack, I. Shimizu and S. Wagner, Vol 507, 113.

“Preparation of a-Si:H and a-SiGe:H nip cells at high rates using a 70 MHz VHF PECVD technique.”,

S.J. Jones, T. Liu and M. Izu, NCPV Photovoltaics Program Review; Proceedings of the 15th

Conference, Denver, CO, 1998; AIP Conference Proceedings, Vol 462, 303.
4

Table of Contents

Preface...........................................................................................................................................................1

Executive Summary ......................................................................................................................................2

Table of Contents..........................................................................................................................................4

List of Figures ...............................................................................................................................................5

List of Tables ................................................................................................................................................6

Introduction...................................................................................................................................................7

Experimental .................................................................................................................................................9

First Year Results........................................................................................................................................10

Component Cells ..................................................................................................................................10

a-Si:H Cells ....................................................................................................................................11

a-SiGe:H Cells ...............................................................................................................................12

Variation in Cell Properties with Different Deposition Conditions........................................12

Present Status of a-SiGe:H Cells.............................................................................................14

Triple-Junction Cells ............................................................................................................................15

Deposition Uniformity Across Substrate Platform........................................................................15

Present Status of a-Si:H/a-SiGe:H/a-SiGe:H Triple-Junction Cells .............................................23

Other Studies ........................................................................................................................................24

Small –Angle X-ray Scattering Measurements of a-Si:H/a-SiGe:H multi-layered structures.......24

Conclusions and Future Plans.....................................................................................................................26

References...................................................................................................................................................29
5

List of Figures

Figure 1. Structures used for optimizing component cells............................................................................9

Figure 2. Cell efficiencies for VHF a-Si:H devices with Jsc = 10 ± 0.5 mA/cm2......................................10

Figure 3. IV plot for a-Si:H cell whose i-layer was prepared at 10 Å/s using VHF technique. .................11

Figure 4. IV curve for a-SiGe:H cell made using the VHF technique........................................................15

Figure 5. IV curve for a-SiGe:H cell made using the VHF technique.......................................................15

Figure 6. Non-uniform deposition Rate distributions across the substrate platform. .................................17

Figure 7. Non-uniform Jsc distributions across the substrate platform. .....................................................18

Figure 8. Variation of Voc across the substrate platform. ..........................................................................19

Figure 9. Variations in the deposition rate profiles as power and chamber pressure are varied. ...............20

Figure 10. Changes in the deposition rate profiles as power is varied at low-pressure conditions. ...........21

Figure 11. Changes in the deposition rate profiles as active gas flow is varied. ........................................22

Figure 12. Deposition rate distribution across the substrate platform at low-pressure conditions.............22

Figure 13. Voc distribution across the substrate platform at low pressure conditions...............................23

Figure 14. IV curve for triple-junction cell.................................................................................................24

Figure 15. IV curve for triple-junction cell.................................................................................................24

Figure 16. Quantum efficiency plot for triple-junction cell made using VHF technique...........................24

Figure 17. Samples structures analyzed using small angle x-ray scattering. ..............................................25

Figure 18. Small angle x-ray scattering results for a-Si:H, a-SiGe:H and multi-layered structures...........26
6

List of Tables

Table 1. Data for a-Si:H nip cells whose i-layers were made using 13.56 or 70MHz frequencies. ...........11

Table 2. Data for a-Si:H cells made using old and new n-layer deposition conditions..............................11

Table 3. Data for a-SiGe:H cells made with and without Ge grading of the i-layer...................................12

Table 4. Effect of substrate bias on the properties of single-junction VHF a-SiGe:H cells.......................13

Table 5. Effect of substrate bias on the properties of single-junction VHF a-Si:H cells. ..........................13

Table 6. a-SiGe:H cells made with different amounts of helium and hydrogen dilution. ..........................14

Table 7. Data for VHF a-SiGe:H cells prior to light soaking. ...................................................................14

Table 8. Data for a-SiGe:H cells after light soaking for 600 hrs. under white light conditions. ................15

Table 9. Deposition times for triple-junction cell fabrication using different layer deposition rates.........27
7

Introduction

Throughout the past 20 years, Energy Conversion Devices, Inc (ECD) and it’s commercialization

subsidiaries and joint ventures have developed amorphous silicon (a-Si:H) materials for thin film

photovoltaic (PV) devices. In particular, ECD has used the 13.56 MHz rf Plasma Enhanced Chemical

Vapor Deposition (PECVD) process to deposit the amorphous and microcrystalline layers that comprise

its proprietary triple-cell, spectrum splitting thin-film solar cell (Guha et al. 1994). Several energy

conversion efficiency world records have been established for small-area devices (Yang et al. 1997) and

for 4 sq. ft. modules (Izu et al. 1993) using this technology. To achieve these high stable efficiencies, the

13.56 MHz PECVD processes have been optimized at rather low deposition rates of 1-3 Å/s. Operating

this process at higher deposition rates leads to unacceptably low stable solar cell efficiencies due to

higher electronic defect densities in the a-Si:H alloys, and to a greater susceptibility to light-induced

material quality degradation that results from changes in the film growth kinetics and microstructure at

the higher deposition rates.

Increasing the deposition rate at which high performance solar cells can be manufactured would have a

significant impact on manufacturing cost, since both the capital depreciation and labor costs would be

reduced by higher product throughput. ECD has experimented with a number of novel a-Si:H alloy

deposition technologies over the past 10 years, and the company has had considerable success in

developing and commercializing a high deposition rate, low pressure microwave PECVD process to

make a-Si:H photoreceptors. The process has also been used to prepare a-Si:H alloys for solar cells at

rates up to 100 Å/s (Doehler et al. 1988). Although initial device performance was good, the light

stability of these devices was poor, with efficiencies degrading by more than 20% after 1000 hrs of light

soaking under air mass 1.5 (AM1.5) illumination (where as state-of-the-art PECVD devices degrade by

10-15%).

Prior to the beginning of this program, we have tested the feasibility of using a 70 MHz, Very High

Frequency (VHF) PECVD process to prepare semiconductor layers for high efficiency cells at high

deposition rates. Earlier studies (Chatham et al. 1989, Shah et al. 1992) have shown that by using the

VHF process, the deposition rate of a-Si:H films could be increased to 10-15 Å/s without an observable

deterioration in the film and cell properties. We have used this VHF PECVD process to deposit highly

conductive, low optical absorbing microcrystalline p-layers which have led to improved solar cell

efficiencies (Deng et al 1997). Expanding this work, we prepared i-layers for a-Si:H single-junction cells

by the VHF PECVD technique (Deng et al 1997). In these initial studies, a-Si:H based nip cells whose ilayers were prepared at 6-10 Å/s have shown 10% initial active area efficiencies and degrade by only

15% when subjected to the extended 1000 hrs of light soaking .

In this program, we are expanding on these initial encouraging results by conducting extensive

optimization studies of the stable efficiencies of single-junction a-Si:H and a-SiGe:H nip cells as well as

a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells prepared by the VHF PECVD technique at high

deposition rates. The principal goal of this program is to demonstrate the feasibility of using the

alternative VHF PECVD deposition technique to manufacture high stable efficiency, triple-junction solar

cells at deposition rates of 10 Å/s. Accomplishing the program’s principal goal will permit this new

technology to be incorporated into ECD’s future solar module roll-to-roll production lines, and the

existing 5MW line at United Solar, ECD’s joint venture company in Troy, MI. Applying this high

deposition rate technique on a large scale to prepare high stable efficiency triple-junction cells and

modules will lead to significant cost savings due to:
8

1) Reduced material costs -

The gas utilization for the VHF PECVD process has already been shown to be much higher than

for the 13.56 MHz PECVD technique. Thus the amount of process gas used to make a module will be

less.

2) Higher throughput and reduced hardware costs -

The increased deposition rates will reduce the time required to fabricate each module in the

existing manufacturing lines. In the designs for new lines, the amount of a-Si:H deposition hardware,

which is the most costly hardware in the lines, can be reduced. Thus incorporation of this new deposition

technique will lead to a reduction in the labor and capital depreciation costs per module.

3) Improved module efficienciesBecause of the potential for making improved a-SiGe:H material and cells, implementation of the

technique may potentially lead to higher module efficiencies. Any such improvement in the efficiencies

will significantly effect the cost of the a-Si:H PV generated energy since the cost is inversely

proportional to the efficiency.

This program will provide the basis for incorporation of this high deposition rate technique in ECD’s

present and future large area, roll-to-roll solar module manufacturing lines (Izu et al. 1984) to increase

throughput and reduce module cost. These cost reductions will contribute to DOE’s and NREL’s longterm goal of fabricating inexpensive thin film modules with 15% stable efficiencies. With ECD’s

experience in a-Si:H PV technology in both R&D and manufacturing phases as well as the great potential

for the VHF PECVD technique, the probability of a successful program is high.

In the first stage of this program, the deposition conditions used to prepare a-Si:H and a-SiGe:H singlejunction cells are being systematically varied to optimize the cell properties. The focus of these studies

is on optimizing the cell properties while maintaining i-layer deposition rates around 10 Å/s. The

information obtained from these studies is being used to prepare high efficiency triple-junction cells with

the high i-layer growth rates. In the later stages of the program, large-scale hardware designs will be

developed which will allow for the incorporation of this high rate technique into a production line

environment.

The exact program goals are:

1) Prepare a-Si:H single-junction nip small area devices with stable 8.4% AM1.5 efficiencies

using i-layer deposition rates near 10 Å/s,

2) Prepare a-SiGe:H single-junction nip small area devices with stable 2.8% efficiencies when

measured using 630 cutoff filtered AM1.5 light using i-layer deposition rates near 10 Å/s,

3) Prepare a-Si:H/a-SiGe:H/a-SiGe:H triple-junction small area devices with stable 9.4%

AM1.5 efficiencies using i-layer deposition rates near 10 Å/s,

4) Develop cathode hardware for large area thin film depositions using the VHF technique for

future implementation in production.

The following sections outline the progress made in the first year of the program and our future plans.
9

Experimental

The solar cells used in these studies were composed of nip structures with stainless steel substrates.

These nip structures were fabricated using a research scale, multi-chamber load locked deposition system.

Figure 1 depicts the structures typically used in optimizing the a-Si:H and a-SiGe:H component cells. To

fabricate the a-Si:H and a-SiGe:H i-layers, a fixed VHF frequency of 70 MHz was used while

maintaining a deposition rate near 10 Å/s. At the time being, our focus is to prepare only the i-layers at

the high rates since they are the thickest layers in the nip structure. Thus, both the doped layers and the aSi:H buffer layers, grown between the VHF deposited a-SiGe:H i-layers and the doped layers, were

prepared using the conventional PECVD process in which a 13.56 MHz frequency is used. To improve

the cell properties, several buffer layer and i-layer deposition conditions were altered including the

substrate temperature, the hydrogen dilution and active gas flows, and the applied power.

 a-Si:H Cells a-SiGe:H Cells

 

ITO

p-layer (13.56 MHz )

i-layer (VHF)

n-layer (13.56 MHz)

stainless steel substrate

Al contacts

 

ITO

p-layer (13.56 MHz)

i-layer (VHF)

n-layer (13.56 MHz)

Ag/ZnO back reflector

Al contacts

stainless steel substrate

a-Si:H

Buffer

Layers

(13.56

MHz)

Figure 1. Structures used for optimizing component cells.

After fabrication of the nip structure, the devices were completed by depositing Indium Tin Oxide (ITO)

conductive layers and then Al collection grids. Both the ITO and Al layers were prepared using standard

evaporation techniques. In some cases, Ag/ZnO back reflectors were deposited on the stainless steel

substrates prior to nip fabrication in order to enhance the collected current and improve the overall

efficiency. The Ag/ZnO back reflectors were prepared in a roll-to-roll manufacturing machine using a

DC sputtering technique.

To characterize the cells, standard IV and spectral response (quantum efficiency) measurements were

made. For the a-Si:H and the triple-junction cells, standard white AM1.5 light was used to obtain the IV

data. Since our goal is to use the a-SiGe:H cells as middle and bottom cells for triple-junction cells, the

AM1.5 light for the IV measurements of the a-SiGe:H cells was filtered to simulate the absorption due to

a top a-Si:H cell. For a-SiGe:H cells made without Ag/ZnO back reflectors, a 530 nm cutoff filter was

used while 610 or 630 nm filters were used for cells with back reflectors. To complete light soaking

studies, the cells were subjected to 600-1000 hrs. of one sun light with the cell temperature fixed at 50oC.

The i-layer thicknesses were determined using standard capacitance techniques.
10

First Year Results

Component Cells

a-Si:H Cells

We have earlier reported results for a-Si:H cells whose i-layers were prepared using the VHF technique

at 10 Å/s (Deng et al 1997). The general conclusion from these studies was that by using the VHF

technique and careful selection of deposition conditions, the initial and stable cell efficiencies could be

made to remain relatively constant with varying i-layer deposition rate up to 10 Å/s. Figure 2

demonstrates this result where the initial and stable efficiencies for a number of cells prepared under a

variety of conditions are plotted as a function of deposition rate. All of these cells had Jsc near 10

mA/cm2

, i-layers which were roughly 2300 Å thick and had no current-enhancing Ag/ZnO back

reflectors. The stable efficiencies were obtained by light soaking the cells for 1000 hrs. Below a

deposition rate of 10 Å/s, the initial and stable cell efficiencies are relatively insensitive to the deposition

rate with average values of 6.6 and 5.5%, respectively. The small amount of scatter (±5%) is related to

experimental measurement procedures and variations in a few deposition parameters. Beyond 15 Å/s, the

efficiencies are lower.

Figure 2. Cell efficiencies for VHF a-Si:H devices with Jsc = 10 ± 0.5 mA/cm2.

To truly demonstrate the advantage of using the VHF technique to prepare a-Si:H i-layers at high rates,

we compare in Table I data for cells whose i-layers were prepared using the same deposition hardware by

either the standard 13.56 MHz frequency or the VHF 70 MHz frequency at different deposition rates. At

low deposition rates of ≤1 Å/s, the cells prepared using the 13.56 MHz frequency had efficiencies

between 6.5 and 6.6 % which degraded by about 15% after 1000 hrs. of light soaking. These efficiencies

and the amounts of degradation are similar to what were found for the cells prepared at 10 Å/s by the

VHF method. Thus, we can indeed use the VHF technique to prepare cells at a high rate of 10 Å/s with

similar quality to those prepared at 13.56 MHz. Comparing cells made at even higher rates, the cells

prepared with the 13.56 MHz frequency have significantly lower efficiencies than the cells produced at

the low rates while the efficiencies for the cells made by the VHF method are not as low and are more

stable. It is clear that for high deposition rates, the VHF is superior to the standard 13.56 MHz method

for a-Si:H preparation.

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

0 5 10 15 20 25 30 35

Deposition Rate (A/s)

Pmax (mW/cm2)

P int

P1000
11

Table 1. Data for a-Si:H nip cells whose i-layers were made using 13.56 or 70MHz frequencies.

Plasma Frequency

(MHz)

Deposition Rate

(Å/s)

Initial Pmax

(mW/cm2)

Light Degradation

(%)

13.56 0.61 6.65 14.3

70 9.9 6.51 11.8

13.56 16.0 5.30 36.2

70 24.8 6.04 21.9

For a-Si:H cells with Ag/ZnO back reflectors, average initial cell efficiencies of 10.3% have been

achieved for cells with i-layer thicknesses of 3000 Å. An IV curve for such a cell is shown in Figure 3.

After 1000 hrs. of light soaking in white light, these cell efficiencies degrade by 20%, typical degradation

percentages for cells of a similar i-layer thickness prepared by the standard rf PECVD technique at ilayer deposition rates of 1 Å/s. Presently, our best single-junction a-Si:H cell whose i-layer was prepared

using the VHF process at 10 Å/s has a stable efficiency of 8.0% (Voc=0.936 V, Jsc=14.3 mA/cm2

,

FF=0.599).

In order to improve the a-Si:H cell performance to achieve our 8.4% goal, we have altered our deposition

parameters in order to improve the stabilized efficiencies. We have recently completed an optimization

study of the conditions used to make the n-layers. Table 2 compares a-Si:H cells made with similar ilayer and p-layer conditions but different n-layer preparation conditions. These cells were made without

the benefit of a current enhancing Ag/ZnO back reflector. Using the new n-layer conditions, we have

increased the short circuit current significantly due to a less absorbing n-layer. a-Si:H cells with Ag/ZnO

back reflectors will be made using the new n-layer recipe in the coming months. Also, these new nlayers will be incorporated into the recipe used for making the triple-junction cells in the second year of

the program.

-18

-14

-10

-6

-2

2

6

-0.2 0 0.15 0.3 0.45 0.6 0.75 0.9 1.05

Voltage (V)

Current (mA/cm2)

Jsc=15.6 mA/cm2,Voc=0.954 V

FF=0.689, Rs=5.2 ohm cm2

Pmax=10.3 mW/cm2

 Figure 3. IV plot for a-Si:H cell whose i-layer was prepared at 10 Å/s using VHF technique.

Table 2. Data for a-Si:H cells made using old and new n-layer deposition conditions.

n-layer

Conditions

i-layer

dep.rate

Voc

(V)

Jsc

(mA/cm2)

FF Rs

(Ohmcm2)

Pmax

(mW/cm2)

Old

Standard

Conditions

9.0 0.951 10.62 0.689 6.1 6.96

New

Optimized

Conditions

10.3 0.953 11.29 0.685 6.1 7.35
12

a-SiGe:H Cells

Variation in cell properties with different deposition conditions

To optimize the efficiencies for the a-SiGe:H cells, a number of common PECVD parameters were varied

including the substrate temperature, the gas flows, the amount of hydrogen dilution, and the chamber

pressure. As is commonly found for a-SiGe:H materials and cells (Mackenzie et al 1985, Guha et al.

1981), particularly for those made at high rates, use of high substrate temperatures and large amounts of

hydrogen dilution are important to obtain good film quality and high cell efficiencies.

A variable that significantly effects the a-SiGe:H cell properties is the Ge grading across the i-layer

thickness. Table 3 compares IV data taken using AM1.5 white light for cells made with and without

grading of the Ge content through the i-layer. These a-SiGe:H cells were made without a back reflector

using deposition rates of 10 Å/s. As has been previously observed by a number of researchers using low

rate techniques (Guha et al. 1989), we were able to obtain significantly higher currents as well as

improved FF by grading the amount of Ge across the i-layer thickness.

Table 3. Data for a-SiGe:H cells made with and without Ge grading of the i-layer.

Grad.

Profile

Jsc

(mA/cm2)

Voc

(V)

FF Rs

(Ωcm

2

)

Pmax

(mW/cm2)

No 16.5 0.759 0.573 7.2 7.15

Yes 18.1 0.739 0.595 6.3 7.96

Specifically for high growth rate process, improvements in the quality of a-SiGe:H materials with

increased ion bombardment have previously been reported. Also several studies of the plasma kinetics

have shown that as the frequency of the plasma increases, the average energy of the ions which bombard

the growing surface decreases while their number increases (Howling et al. 1992, Heintze et al. 1993,

Oda et al. 1991).

Since a moderate amount of bombardment has previously been found to be beneficial in increasing the

surface mobilities of adatoms during the growth of a variety of thin film materials (Yehoda et al. 1988),

our group has studied the effect of bombardment conditions on the growth of our high rate i-layers. We

have varied the bombardment conditions at the growing surface by systematically applying a d.c. bias to

the substrate and adding He dilution to the plasma. Table 4 summarizes data for a-SiGe:H cells prepared

with various substrate biases. These cells were made without the benefit of the back reflectors and the

IV measurements were made using 530 nm filtered AM1.5 light to simulate absorption due to a top aSi:H cell. With increasing positive electrical bias, the deposition rate of the a-SiGe:H i-layer was found

to drop significantly from nearly 10 Å/s at a bias of 0V to less than 5.8 Å/s at 50V. With this drop in

deposition rate, there was also a significant decrease in Voc from 0.75V to 0.64V. Increasing the positive

bias further to 100V led to a much smaller change in deposition rate and no change in Voc.

With increasing negative bias, there is also a decrease in the deposition rate for the a-SiGe:H i-layers,

however the decrease is not as great as in the case of positive bias. Between 0 and –100V bias, there is

only a small (less than 7%) decrease in deposition rate and very little change in the cell properties.

Beyond –100V, the deposition rate decreases more rapidly and a significant drop in Voc is apparent.
13

For the a-Si:H cells, many similar trends are noted with varying bias (see Table 5). With increasing

positive bias, there is a significant decrease in the i-layer deposition rate as was observed for the aSiGe:H cells. Also, there is the same decrease in Voc observed for the a- SiGe:H cells with increasing

positive bias. Thus, the decrease in Voc for the a-SiGe:H alloys is not, for at least the most part, due to a

decrease in Ge content. The changes in Voc for both types of cells are likely due to decreases in the

hydrogen content and/or changes in the film microstructure. With increasing negative bias, virtually no

change in the Voc is observed however a smaller decrease in deposition rate is seen.

Table 4. Effect of substrate bias on the properties of single-junction VHF a-SiGe:H cells.

Substrate

Bias

(V)

Deposition

Rate

(Å/s)

i-layer

thickness

(Å)

Jsc

(mA/cm2)

Voc

(V)

FF Rs

(Ωcm

2

)

Pmax

(mW/cm2)

0 9.5 2000 8.15 0.750 0.571 12.1 3.49

50 5.8 1220 7.62 0.638 0.561 15.9 2.73

100 4.4 920 6.74 0.641 0.632 11.9 2.73

-30 10.5 2210 8.15 0.742 0.562 10.3 3.40

-50 9.3 1950 8.18 0.749 0.559 11.1 3.43

-100 8.9 1860 7.74 0.745 0.580 11.1 3.46

-150 8.5 1780 7.33 0.736 0.531 14.5 2.87

-200 6.8 1430 7.40 0.730 0.555 15.4 3.00

Table 5. Effect of substrate bias on the properties of single-junction VHF a-Si:H cells.

Substrate

Bias

(V)

Deposition

Rate

(Å/s)

i-layer

thickness

(Å)

Voc

(V)

Jsc

(mA/cm2)

FF Rs

(Ωcm

2

)

Pmax

(mW/cm2)

0 9.9 3840 0.963 15.68 0.659 6 9.95

100 4.5 3830 0.882 15.66 0.557 6.3 7.69

100 4.8 2960 0.882 15.24 0.613 5.7 8.24

-100 9.8 3750 0.954 15.45 0.675 4.9 9.95

-200 7.6 3200 0.966 15.17 0.664 5.3 9.73

-300 7.0 3380 0.956 15.05 0.658 5.3 9.47

Since the focus of this program is on maintaining a high deposition rate, we found no benefit to using an

applied negative or positive substrate bias. This method of altering the ion bombardment, in particular

the ion energy, of the film surface does not lead to higher efficiencies.

Table 6 displays the cell data for the devices made with different ratios of He/H2 dilution. The cells were

made using Al/ZnO back reflectors and the IV data was obtained using 630nm filtered AM1.5 light.

Also these cells were made without grading of the buffer layer and thus have lower efficiencies than what

is typically obtained (see last column in table for cell results for graded cell made at the time when this

He dilution study was made). One can see from the data that increasing the He flow lead to lower FF,

lower Jsc and poorer cell efficiencies. The deteriorations in these properties are unlikely to be a result of

the small increase in deposition rate with increased He flow. Thus, at least in the case of the 70MHz rf

plasma, use of He dilution leads to lower cell efficiencies and poorer a-SiGe:H i-layer quality. None of

the attempts to improve the quality of the a-SiGe:H materials through increased ion bombardment has led

to improved cell properties.
14

Table 6. a-SiGe:H cells made with different amounts of helium and hydrogen dilution.

He

(sccm)

H2

(sccm)

i-layer

thickness

(Å)

Deposition

Rate

(Å/s)

Jsc

(mA/cm2)

Voc

(V)

FF Rs

(Ωcm

2

)

Pmax

(mW/cm2)

0 42 1790 11.9 6.41 0.640 0.523 15.3 2.15

5 42 1820 12.1 5.78 0.646 0.501 19 1.87

10 42 1910 12.7 5.40 0.641 0.491 19.6 1.70

20 42 1870 12.5 5.15 0.646 0.482 23.7 1.61

10 32 1900 12.7 5.52 0.652 0.505 18.1 1.81

20 22 1940 13.0 5.57 0.640 0.490 19.4 1.75

0 graded 1700 11.0 6.93 0.645 0.594 13.7 2.66

Present status of a-SiGe:H cells

In Table 7, we compare data for a-SiGe:H cells prepared using the VHF technique and the 10 Å/s rate

with those made using the standard 13.56 MHz and a 1 Å/s rate. These cells were prepared using the

same deposition hardware and during the same time period. To judge the potential use of the VHF

technique as a method to prepare middle and/or bottom cells for triple-junction structures, we compare in

the table IV data obtained using 530 nm filtered AM1.5 light. Again, the 530 nm filter eliminates light

typically absorbed by the a-Si:H top cell. Comparing cells with similar Jsc values and i-layer thickness, aSiGe:H cells made by the VHF technique have similar initial efficiencies to those prepared in the same

deposition system using the 13.56 MHz frequency.

The high rate VHF produced a-SiGe:H cells and low rate 13.56MHz produced cells also have similar

efficiencies after light soaking as can be seen from the data in Table 8. Cells with similar Jsc values have

similar stable efficiencies. Some interesting results are the properties for the cell made by the VHF

method at a rate of 2 Å/s. The stable efficiencies and FF are actually higher for devices made under these

conditions than those made by the 13.56 MHz method at low rates. We have recently improved the

efficiencies for the VHF prepared cells and are now achieving higher efficiencies for the a-SiGe:H

devices. IV plots for these improved cells prior to light soaking are shown in Figures 4 and 5. Again

these data were obtained using AM1.5 light filtered with the 530nm low-bandpass filter. These cells are

presently being light soaked.

Table 7. Data for VHF a-SiGe:H cells prior to light soaking.

Freq.

(MHz)

Dep.

Rate

(Å/s)

i-layer

thickness

(Å)

Jsc

(mA/cm2)

Voc

(V)

FF Rs

(Ωcm

2

)

Initial

Pmax

(mW/cm2)

13.56 0.95 2400 8.50 0.751 0.552 12.3 3.51

13.56 0.92 2100 7.93 0.752 0.575 11.8 3.42

13.56 6.1 2260 7.43 0.711 0.550 14.4 2.91

70 9.5 2000 8.07 0.760 0.579 10.7 3.55

70 10.2 2150 8.15 0.750 0.571 12.1 3.49
15

Table 8. Data for a-SiGe:H cells after light soaking for 600 hrs. under white light conditions.

Freq.

(MHz)

Dep.Rate

(Å/s)

Jsc

(mA/cm2)

Voc

(V)

FF Pmax

(mW/cm2)

13.56 0.5 6.93 0.703 0.517 2.52

13.56 0.5 7.02 0.704 0.510 2.52

13.56 0.79 6.97 0.703 0.518 2.54

70 2.0 7.06 0.699 0.543 2.68

70 10.0 7.08 0.675 0.527 2.52

70 9.3 6.92 0.686 0.521 2.47

 

-11

-9

-7

-5

-3

-1

1

3

-0.15 0 0.15 0.3 0.45 0.6 0.75

Voltage (V)

Current (mA/cm2)

Jsc=9.03 mA/cm2,Voc=0.716 V

FF=0.602, Rs=9.6 ohm cm2

Pmax=3.89 mW/cm2

 

-9

-6

-3

0

3

-0.15 0 0.15 0.3 0.45 0.6 0.75

Voltage (V)

Current (mA/cm2)

Jsc=7.85 mA/cm2,Voc=0.754 V

FF=0.633, Rs=9.3 ohm cm2

Pmax=3.71 mW/cm2

Figure 4. IV curve for a-SiGe:H cell made using Figure 5. IV curve for a-SiGe:H cell made using

 the VHF technique. the VHF technique.

Triple-Junction Cells

Our initial attempts to prepare high efficiency triple-junction cells using i-layer deposition rates near 10

Å/s and the VHF technique were hindered by non-uniform depositions across the substrate platform.

Below we discuss this problem and later discuss the properties for the cells we have prepared after this

uniformity problem was solved.

Deposition uniformity across substrate platform

Our early attempts to improve the efficiencies through systematic variations in the deposition parameters

led to difficulties in correctly current matching the cells due to non-uniform deposits. Figures 6 displays

the severity of this uniformity problem. In the figure, the i-layer deposition rate across the substrate

platform area is plotted for two single-junction component cells, the top a-Si:H and the bottom a-SiGe:H

cells. The i-layer thicknesses were determined using standard capacitance techniques. In both cases,

thicker i-layers are observed near the gas inlet side of the substrate platform and the thinnest layers near

the pump port side. A smaller decrease in thickness is seen from the center to the other sides of the

platform. Later experiments have shown that these variations in thickness are not entirely due to gas

depletion effects.
16

These variations in deposition rate obviously lead to variations in the Jsc and FF, which depend strongly

on the i-layer thickness, and thus variations in the cell efficiencies. Figure 7 displays a typical variation

in Jsc values for an a-SiGe:H cell with the currents obtained using AM1.5 light filtered with a 630 nm low

pass filter. These large changes in current lead to similar variations in the Jsc for the triple-junction cells

(See Figure 7) and the production of low efficiency triple-junction cells across a large region of the

substrate platform due to improper component cell current matching.

Figure 8 displays the trends in Voc across the platform for the top and bottom cells. The figure shows that

while Voc is relatively uniform for the top a-Si:H cell, the Voc varies significantly for the a-SiGe:H cells

due to variations in the Ge content in the i-layers. This variation in Voc is also seen for the triple-junction

structures.

Using the same deposition system that was used to obtain the profiles shown in Figures 6-8, one can

obtain uniform deposits (within ±5%) using either the 13.56 MHz or the 70 MHz frequency by lowering

the deposition rate to 1-3 Å/s. Thus, the poor uniformity is almost exclusively dictated by the high

deposition rates, not the very high frequencies. To further increase the efficiencies of the triple-junction

devices, we felt it was necessary to determine a region of deposition parameter space in which fairly

uniform deposits could be obtained when high rates were used. Thus during this reporting period, we

studied the effect that a number of deposition parameters had on this deposition uniformity profile. This

study will prove useful to determine the conditions, if any, at which uniform deposits and high rates

might be obtainable with the existing large area cathode design which is a scaled-up version of our R&D

reactor.

We have found that the changes in deposition uniformity for the a-SiGe:H i-layers follow similar trends

to the changes for the a-Si:H i-layer deposits when disilane gas is used as the source gas. Thus for

simplicity, we will discuss in the following sections only changes in the top cell i-layer deposits made

with disilane and varying deposition parameters, with the implication that similar trends were noted for

the a-SiGe:H i-layers. In later discussions, we will also mention differences between top cells made

using silane and disilane.
17

Figure 6. Non-uniform deposition Rate distributions across the substrate platform.

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Deposition Rate Across Substrate Platform

a-Si:H Top Cell

10.8

10.7

10.0

 8.7

 7.5

 6.9

11.9

11.4

10.4

8.2

7.9

7.7

11.9

11.2

10.3

 8.2

11.3

10.3

 8.0

 7.7

 7.5

11.1

 9.1

 8.1

 7.8

 7.6

 7.2

11.6

11.1

10.0

 8.9

 7.7

 7.6

 6.7

10.7

 9.7

 8.7

 7.3

 7.0

 10.0

 9.8

 8.1

 6.9

 6.8

 6.5

Line where thickness 

profiles were measured

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Deposition Rate Across Substrate Platform

a-SiGe:H Bottom Cell

 8.2

 8.2

 6.8

 6.6

 6.2

 5.4

 9.0

10.2

 9.9

 9.9

 9.0

 8.1

 6.6

 9.8

10.5

10.3

 9.0

 8.3

 6.7

 7.9

 7.6

 7.0

 6.1

 11.3

 10.5

 8.2

 7.7

 7.9

 7.7

 9.6

11.0

10.3

 7.7

 8.5

 7.3

 6.5

 8.8

10.2

10.3

10.0

 7.2

 7.1

 7.6

 8.8

 9.0

 

 6.0

 6.0

 5.6

 5.1

Line where thickness 

profiles were measured
18

Figure 7. Non-uniform Jsc distributions across the substrate platform.

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Jsc Across Substrate Platform

a-SiGe:H Bottom Cell

7.57

7.33

6.47

6.37

6.06

5.48

7.82

7.64

7.43

7.37

6.47

6.64

6.23

7.76

7.47

7.37

6.73

6.58

6.30

6.64

6.55

6.40

5.97

7.53

7.29

6.77

6.55

6.37

6.08

7.76

7.49

7.28

6.56

6.23

7.79

7.53

7.47

7.16

6.33

5.90

 7.66

 7.62

 7.31

 

 6.36

 6.01

 5.50

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Jsc Across Substrate Platform

Triple Cell

6.55

6.58

6.49

6.22

6.05

5.73

6.87

6.60

6.58

6.37

6.27

6.11

6.96

6.71

6.70

6.45

6.39

6.10

6.78

6.58

6.45

6.26

6.18

6.81

6.77

6.71

6.27

6.04

5.97

6.89

6.84

6.71

6.10

6.02

6.01

6.91

6.79

6.66

6.68

6.31

6.03

5.87

 6.78

 6.68

 6.62

 6.68

 6.41

 6.31

 6.13

6.83 7.07
19

Figure 8. Variation of Voc across the substrate platform.

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Voc (mV) Across Substrate Platform

a-Si:H Top Cell

 954

 953

 953

 944

954

955

944

945

944

 949

 955 948

 955

 944

 946

 945

 950

 945

 943

 947

 949

 952

954

952

949

949

944

948

950

 952

 952

 954

 951

 946

 948

 954

 952

 

 

 

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Voc (mV) Across Substrate Platform

a-SiGe:H Bottom Cell

 590

 607

 661

 671

 678

 691

525

536

563

590

639

652

678

 508

 522

 553

 636

 647

 677

 647

 658

 669

 681

 549

 586

 649

 657

 665

 679

518

527

588

657

684

 548

 548

 570

 597

 676

 689

 581

 581

 596

 

 681

 687

 697
20

In Figure 9, the deposition rate across the center of the substrate platform (see note in Figure 6) is plotted

as a function of the distance from the gas inlet side of the platform for a number of a-Si:H i-layers

prepared with disilane. These i-layers were prepared using different applied powers and chamber

pressures with all other deposition conditions nominally fixed. In each figure, the average deposition rate

for each deposit is listed in the legend. Figure 9a displays profiles for cells made using moderate

chamber pressures and various powers. As the applied power is decreased under moderate chamber

pressure conditions, the uniformity across the platform improves slightly. But even at low power levels

where the average deposition rate dips below our targeted 10 Å/s, there is still over a factor of two

difference in the thinnest and thickest deposits in the 4” x 4” area.

Figure 9b displays the variation of the profile with alterations to the chamber pressure while the applied

power is fixed at the moderate value. Increasing the pressure from low to high values leads to a drop in

the deposition rate and some improvement in the deposition uniformity. However even at the highest

Thickness Profiles for a-Si:H i-layers Prepared 

Using Different Applied RF Powers

0

10

20

30

40

50

0 2 4 6 8 10

Distance Across Substrate Platform (cm)

Dep. Rate (A/s)

Low Power (8.0 A/s)

Moderate Power (13.1 A/s)

High Power (21.3 A/s)

a)

b)

Figure 9. Variations in the deposition rate profiles as power and chamber pressure are varied.

Deposition Rate of a-Si:H i-layers Prepared 

Using Different Chamber Pressures

0

5

10

15

20

25

30

0 2 4 6 8 10

Distance Across Substrate Platform (cm)

Dep. Rate (A/s)

Low Pressure (18.2 A/s)

Moderate Pressure (13.1 A/s)

High Pressure (4.6 A/s)

Very Low Pressure (11.2 A/s)
21

pressure, a significant drop in the deposition rate is observed near the gas inlet side of the platform

remains (also the deposition rate is too low under these conditions). This decrease in the deposition rate

with increasing pressure is of interest since in the same pressure regime using the same deposition

reactor, the deposition rate increases with increasing pressure, as one might expect, when a 13.56 MHz

signal and lower deposition rates are used. Decreasing the pressure from low to very low conditions leads

to a drop in the deposition rate. Also at the very low pressure regime, the uniformity is significantly

better while high deposition rates are maintained. Thus, in raising the pressure from very low to low

values, the uniformity becomes poor as the deposition rate increases, as one might expect. Near this low

pressure regime, the plasma chemistry must drastically change causing the sudden downward trend in

deposition rate. It is not clear whether this change is related directly to the VHF technique or to the Si

radical rich plasmas required for the high deposition rates.

Shown in Figure 10 are profiles for a-Si:H i-layers made in the very low pressure regime using different

power levels. At each of the power levels, the deposit is fairly uniform demonstrating that the use of

very low pressures is a key to make uniform, high rate a-Si:H deposits when disilane is used as the Si

source gas. In the moderate-low pressure regime, we have also attempted to improve the deposition

uniformity through systematic variation of the active gas flows, the diluent gas flow, and the total gas

flow. While some improvement in the deposition uniformity were observed when these parameters were

varied (See Figure 11, for example), the improvements were either small or led to deposition rates below

the desired 10 Å/s. To obtain the desired uniformity and deposition rate, very low pressures are required.

Figure 12 displays the uniform deposition rate over the entire substrate area when the new optimized

conditions are used. As was mentioned previously, the a-SiGe:H deposits behave similarly to the a-Si:H

deposits made with disilane in that very low pressures were required to obtain uniform deposits. Figure

13 demonstrates that the Voc, and thus the Ge content, is also uniform across the substrate area when

these new deposition conditions are used.

Figure 10. Changes in the deposition rate profiles as power

 is varied at very low pressure conditions.

Deposition Rates for a-Si:H i-layers Prepared 

Using Different Applied Powers

0

5

10

15

20

25

0 2 4 6 8 10

Distance Across Substrate Platform (cm)

Dep. Rate (A/s)

Low P ower (9.8 A/s)

Moderate P ower (11.4 A/s)

High P ower (16.4 A/s)
22

Figure 11. Changes in the deposition rate profiles as active gas flow is varied

 at moderately low pressure conditions.

 Figure 12. Deposition rate distribution across the substrate platform at low pressure conditions.

When SiH4 was used to make the a-Si:H i-layers, the 10 Å/s deposition rate could not be obtained using

the very low pressures due to the lower decomposition rate. Through systematic variation of the silane

flow, the applied power and the chamber pressure, we are presently optimizing the conditions to obtain

the desired uniformity. However with the work completed thus far, it is clear that the region of parameter

space in which this uniformity is obtained is much smaller than the area in which uniform a-Si:H deposits

made with disilane are obtained. Thus there is a benefit to using disilane for this high rate deposition

technique in terms of a wider range of parameter space to optimize the thin film growth parameters while

maintaining uniform substrate coverage.

Deposition Rate for a-Si:H i-layers Prepared 

Using Different Active Gas Flows

0

5

10

15

20

25

0 2 4 6 810

Distance Across Substrate Platform (cm)

Dep. Rate (A/s) 

Low Active Gas Flow (8.6 A/s)

Moderate Active Gas Flow (12.0 A/s)

High Active Gas Flow (13.1 A/s)

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Deposition Rate Across Substrate Platform

a-Si:H Top Cell

11.6

11.7

11.7

11.9

12.1

12.0

11.7

11.9

11.8

11.7

11.1

11.3

11.7

11.9

11.8

11.6

 11.2

11.1

11.2

11.7

11.7

11.4

11.6

 11.9

 11.1

 11.0

 10.8

11.8

11.8

11.5

11.3

11.2

11.9

11.9

11.7

11.5

11.5

11.4

 11.8

 12.1

 12.2

 12.1

 12.0

 12.1

 12.0

 11.6 11.4 11.6
23

 Figure 13. Voc distribution across the substrate platform at low pressure conditions.

As an additional comment to these studies of the deposition uniformity, we specifically focused on

varying the deposition parameters without making any geometry changes to the deposition hardware. It

is quite possible that several changes in the hardware configuration could lead to a widening of the

deposition parameter space in which uniform deposits are obtained. In particular, adjustment of the

cathode separation and/or the gas flow geometry could be beneficial.

Present status of a-Si:H/a-SiGe:H/ a-SiGe:H triple-junction cells

Incorporating the new deposition conditions that gave more uniform film deposits led to the achievement

of initial triple-cell active area efficiencies of 10.4 -10.5%. Representative IV curves for such devices are

shown in Figures 14 and 15 while a representative quantum efficiency curve for a VHF produced cell is

shown in Figure 16. The current matching for the cells have yet to be fully optimized, thus further

improvement in the efficiencies should come with proper matching. While these cells have yet to be

sufficiently light soaked, earlier VHF cells with 9.5-10% initial efficiencies were light soaked for 600

hrs. under white light conditions and the efficiencies were found to degrade by 10-13%. Considering the

similarity of the i-layer thicknesses for these cells and those of the 9.5-10% cells, it is likely that 10.4-

10.5% cells will also degrade by 10-13% after extended light soaking periods. Further optimization of

the deposition conditions for these triple-junction devices should lead to higher stable efficiencies.

In terms of obtaining higher efficiencies, we need to obtain higher currents, in particular for the bottom

cell. In this first year of the program, focus for the a-SiGe:H cells has been on the middle cells. In the

second year of the program, focus will be on improving the bottom cell properties as well as the quality

of the doped layers.

Gas Inlet

Gas Outlet (Pump Out Port)

4”

4”

Voc (mV) Across Substrate Platform

a-SiGe:H Bottom Cell

 659

 653

 625

 622

656

643

626

 660

 647

 655

 642

 630

 625

 624

 645

 640 636

632

 627

 657

 646

 638

 628 

 629

 628

 627

 661
24

 

-8

-6

-4

-2

0

2

-0.15 0.35 0.85 1.35 1.85 2.35

Voltage (V)

Current (mA/cm2)

Jsc=6.40 mA/cm2, Voc=2.30 V

FF=0.716, Rs=29.6 ohm cm2

Pmax=10.5 mW/cm2

 

-8

-6

-4

-2

0

2

-0.15 0.35 0.85 1.35 1.85 2.35

Voltage (V)

Current (mA/cm2)

Jsc=6.81 mA/cm2, Voc=2.31 V

FF=0.662, Rs=31.9 ohm cm2

Pmax=10.4 mW/cm2

 Figure 14. IV curve for triple-junction cell. Figure 15. IV curve for triple-junction cell.

Figure 16. Quantum efficiency plot for triple-junction cell made using VHF technique.

Top Cell Current =6.9 mA/cm2

, Middle Cell Current =7.1 mA/cm2,

Bottom Cell Current =7.8 mA/cm2

.

Other Studies

Small –Angle X-ray Scattering Measurements of a-Si:H/a-SiGe:H multi-layered

structures

The work completed at the Colorado School of Mines has shown that with the addition of Ge to the aSi:H lattice, a strong signal in the Small-Angle X-ray Scattering (SAXS) data appears which is associated

with the presence of highly oriented, elongated voids which are likely related to a “columnar like”

microstructure (Jones et al. 1993). To obtain this information, films that are at least 1 micron thick are

required to obtain sufficient SAXS signals for accurate data. As we know, these thicknesses are at least 5

times greater than those typically used for i-layers in high efficiency cell structures. Because of thickness

dependent effects such as the degree of coarsening of the film microstructure, there has always been a

question as to whether the microstructure described by the SAXS data for the thick films truly reflects

the microstructure of 1000-2000 Å thick films used in the devices.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

350 450 550 650 750 850 950

Wavelength (nm)

Quantum Efficiency

Top

Middle

Bottom

Total
25

In order to address this issue, we have completed an experiment with Don Williamson at the School of

Mines involving the use of a-Si:H/a-SiGe:H multi-layered structures. Specifically, we fabricated three

thin film structures depicted in Figure 17 for SAXS measurements. In each of the structures, the a-Si:H

films were made under similar conditions which lead to high quality i-layer fabrication: deposition rates

near 1 Å/s, low powers, moderate substrate temperatures, significant hydrogen dilution, 13.56 MHz

frequency. The a-SiGe:H layers were made under similar conditions except for the substitution of some

of the SiH4 source gas with GeH4. These conditions led to Ge contents near 35 at. %. In using these

conditions, we expected the a-Si:H materials to have few voids and none of the highly oriented voids

while the a-SiGe:H films should have a significant amount of the elongated voids. For the multi-layered

film shown in Figure 17C, because the a-Si:H layers are homogenous and should have a less course,

smoother structure, insertion of the 500 Å thick a-Si:H layers between the 2000 Å a-SiGe:H layers should

inhibit any large scale coarsening affect for the a-SiGe:H material that is usually observed for the thicker

films and thus the bulk of the SAXS from this sample should represent the scattering and microstructure

of 2000 Å thick a-SiGe:H layers.

Shaded

 1 micron thick 1 micron thick 2000 Å a-SiGe:H ,

a-Si:H a-SiGe:H Unshaded

 500 Å a-Si:H

 Fe free Al substrates

 A B C

Figure 17. Samples structures analyzed using small angle x-ray scattering.

Figure 18 displays the SAXS from the three samples shown above. As expected, the SAXS from the aSi:H films is small relative to the large scattering from the a-SiGe:H sample. The scattering from the

multi-layered sample depicted in Figure 17C is very similar to the scattering for the a-SiGe:H singlelayer. The extra scattering for the multi-layered film at the low q values is probably associated with the

thin 500 Å thick a-Si:H layers (see the scattering for the a-Si:H single-layer film in this region of q in

Figure 18). The similarity of the SAXS for the a-SiGe:H single-layered film and the multi-layered film

suggests that the SAXS data obtained from the measurements of the micron thick films does represent the

microstructure of 1000-2000 Å thick films that are used in the solar cells. Other information concerning

the SAXS measurements will be reported by Don Williamson.
26

Figure 18. Small angle x-ray scattering results for a-Si:H, a-SiGe:H and multi-layered structures.

Conclusions and Future Plans

In the first year of the program, we have made significant progress in advancing the VHF, high rate

technology. We have demonstrated that 8.0% stable efficiencies can be achieved for a-Si:H cells whose

i-layers are prepared at rates near 10 Å/s using the VHF technique. While we have yet to achieve to

8.4% program goal for the a-Si:H cells, we hope that our recent improvement of the n-layer conditions as

well as further optimization of other deposition conditions will lead to the achievement of the goal in the

coming months. This achievement may also come from the general improvement of system baseline

efficiencies for all cells by optimization of chamber heating and pre-deposition treatments. Presently,

there is not a great difference in the performance of a-Si:H cells made using the VHF technique and ilayer deposition rates near 10 Å/s and that for cells made using the standard 13.56 MHz technique and

rates near 1 Å/s in the same deposition system.

In terms of the a-SiGe:H cells, we have completed a number of studies of devices with properties

appropriate for middle junction cells. That is, cells without Ag/ZnO back reflectors having Voc values

near 0.75V and Jsc values near 8.0 mA/cm2

 when measured using AM1.5 light filtered using a 530 nm

low band pass filter. The stabilized properties for these cells prepared at i-layer rates near 10 Å/s are

again similar to a-SiGe:H cells made using the same deposition hardware and the low rate 13.56 MHz

method. We are looking to improve these efficiencies further again through optimization of doped layer

conditions and the general improvement of the baseline efficiencies for the semiconductor deposition

system. Optimization of the bottom cell conditions will be an issue we will focus much of our attention in

the coming year.
27

Establishing an initial 10.5% for a triple-junction cell whose i-layers are prepared at the high rates sets

the baseline for our future studies. Further optimization of the triple-junction structure should lead to the

achievement of higher efficiencies in the coming year. The result that the triple-junction cell degradation

(10-13%) with prolonged light soaking is similar to that regularly obtained for cells prepared at low ilayer deposition rates (1 Å/s) is important in light of the fact that the use of high rate methods to prepare

i-layers typically lead to less stable materials and cells (Doehler et al 1988). In terms of improving the

efficiencies for the triple-junction cells, focus has thus far been on optimizing the top and middle

component cells. The conditions used to make the bottom cells for the triple-junction cells with the

10.5% efficiencies were similar to those used for the middle cell except for the use of higher GeH4 flows

for i-layer preparation while the doped layers were made using conditions optimal for cells made using

low i-layer growth rates. In the coming months, work will done on optimizing the bottom cell or the p/n

tunnel junctions. Improvement of the bottom cell performance is of particular importance. In order to

achieve the high stable efficiencies, obtaining high red-light currents for the bottom cell is critical in

order to get the proper current matching between the component cells. We believe that improvement of

the bottom cell will come through use of increased hydrogen dilution of the plasma gases, known to be

beneficial for a-SiGe:H cell performance. Since increased hydrogen dilution typically leads to lower

deposition rates, there will be need to alter other conditions while the dilution is increased in order to

maintain the 10 Å/s i-layer growth rates. A great deal of time during the next year will be spent on

improving both the bottom cell and tunnel junction performance.

In order to fully understand the benefit of the high rate i-layer growth, the times required to fabricate

triple-junction cells with our R&D system using different layer growth rates are compared in Table 9.

Each time is the sum of times required to complete each layer deposition and do not include other

processing times (baking times, cooling times, chamber pump down time, etc.). By increasing the i-layer

deposition rates to 10 Å/s, we are able to decrease the total deposition time by nearly 70%, a significant

improvement. The payoff from increasing the rates of the other layers is not nearly as great, but of

course is beneficial. For the data in the table, we have taken a conservative approach of increasing the

deposition rates for the buffer and doped layers to a high rate of 6 Å/s. Since we have no experience in

depositing these layers at rate above 5 Å/s, we thought this conservative approach was appropriate.

Increasing the buffer layer deposition rate to 6 Å/s leads to nearly a 15 min decrease in the total

deposition time while the increase in the n-layer and p-layer deposition rates both decrease the total time

by 5 and 5.8 min, respectively. Thus besides the i-layer growth rates, increasing the buffer layer growth

rate has the strongest effect on the deposition time of the triple-junction semiconductor structures.

Because of this, we plan on exploring the use of VHF to increase not only the i-layer growth rate but also

the buffer layer deposition rate in the coming months. Considering the high quality of the a-Si:H cells

made with i-layer growth rates of 10 Å/s, one should be able to increase the rate of the buffer layer

without much of a change in the material quality.

Table 9. Deposition times for triple-junction cell fabrication using different layer deposition rates.

i-layer

deposition rates

(Å/s)

buffer layer deposition

rates (Å/s)

n-layer deposition

rates (Å/s)

p-layer deposition

rates (Å/s)

Total deposition

time (min.)

1 1 1 1 123.5

10 1 1 1 38.5

10 6 1 1 23.9

10 6 6 1 18.9

10 6 6 6 13.1
28

In terms of application the VHF, high rate deposition technique in the production of large area panels,

our studies demonstrate that obtaining uniform deposits will be a key issue. Proper selection of the largearea deposition hardware and the hardware geometry will be critical for obtaining the required deposition

uniformity in manufacturing. Without good thin film uniformity, obtaining the current matching needed

for high performance modules will not be possible. This does not seem to be an insurmountable problem

since some research on cathode hardware for the application of the VHF technology has already been

done with some reasonable solutions (Sansonnens et al. 1998). One fact that should be pointed is that we

have obtained uniform deposits using the VHF technique and low deposition rates (2 Å/s). Thus for

substrate sizes we are using, the non-uniformities we have outlined are primarily due to the high

deposition rates we are using, not specific to the use of high frequencies. In the coming months, we plan

to test new large area hardware designs in a single chamber for the application of high rate methods, such

as the VHF technique, into ECD’s module production line.

In summary, we plan to focus on the following issues in the next year,

1) Optimization of the bottom cell performance with focus on using increased hydrogen dilution

while maintaining the 10 Å/s i-layer deposition rates to obtain higher currents,

2) Optimization of the doped layers in the triple-junction structure to obtain higher efficiencies,

3) Improvement of our system baseline efficiencies through careful assessment of the procedures

used in preparing the devices,

4) Develop a recipe for high rate buffer layer depositions to decrease the processing time for the

triple-junction cells,

5) Development of large area cathode hardware for the high rate VHF process.

We believe these are the next important issues that need to be addressed to access the feasibility of using

the VHF technique for high rate deposition of high efficiency a-Si:H based triple-junction cells.
29

References

Chatam H; and P.K. Bhat, Mat. Res. Soc. Proc. 149 (1989) 447.

Deng, X; S.J. Jones, T. Liu, M. Izu and S.R. Ovshinsky, Mat. Res. Soc. Symp. Proc. 467 (1997) 795.

Deng, X; S.J. Jones, T. Liu, M. Izu and S.R. Ovshinsky, Proc. 26th IEEE PV Spec. Conf. (1997) 591.

Doehler, J; S. Hudgens, S.R. Ovshinsky, B. Dotter, L.R. Peedin, J.M. Krisko and A. Krisko, United

States Patents #4,883,686 and 5,093,149 (1988).

Guha, S; K.L. Narasimhan, S.M. Pietruszko, J. Appl. Phys. 52 (1981) 859.

Guha, S.; J. Yang, A. Pawlikiewicz, T. Glatfelter, R. Ross and S.R. Ovshinsky, Appl. Phys. Lett. 54(23)

 (1989) 2330.

Guha,S.; J. Yang, A. Banerjee, T. Glatfelter, K. Hoffman, S.R. Ovshinsky, M. Izu, H. Ovshinsky and X.

 Deng, Mat. Res. Soc. Symp. Proc. 336 (1994) 645.

Heintze, M.; R. Zedlitz and G.H. Bauer, Mat. Res. Soc. Symp. Proc. 297 (1993) 49.

Howling, A.A.; J.L. Dorier, Ch. Hollenstein, U. Kroll, and F. Finger, J. Vac Sci. Tech A 10 (1992) 1080.

Izu, M; and S.R. Ovshinsky, Thin Film Solids 119 (1984) 55.

Izu, M; X. Deng, A. Krisko, K. Whelan, R. Young, H.C. Ovshinsky, K.L. Narasimhan and S.R.

 Ovshinsky, Proc. 23rd IEEE PV Spec. Conf. (1993) 919.

Jones, S.J.; Y. Chen, D. L. Williamson, R. Zedlitz and G. Bauer, Appl. Phys. Lett. 62 (1993) 3267.

Mackenzie, K.D.; J.R. Eggert, D.J. Leopold, Y.-M. Li, S. Lin, and W. Paul, Phys. Rev. B 31 (1985) 2198.

Oda S.; and M. Yasukawa, J. Non-Cryst. Solids 137&138 (1991) 677.

Sansonnens, L.; A.A. Howling, Ch. Hollenstein, Mat. Res. Soc. Proc. 507 (1998) 541.

Shah, A; J. Dutta, N. Wyrsch, K. Prasad, H. Curtins, F. Finger, A. Howling, and Ch. Hollenstein, Mat.

Res. Soc. Proc. 258 (1992) 15.

Yang, J.; A. Banerjee and S. Guha, Mat. Res. Soc. Symp. Proc. Spring 467 (1997) 693.

Yehoda, J.E.; B. Yang, K. Vedam and R. Messier, J. Vac. Sci. & Tech. A6 (1988) 1631.
REPORT DOCUMENTATION PAGE Form Approved

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

September 1999

3. REPORT TYPE AND DATES COVERED

Annual Technical Status Report, 11 March 1998-11 March 1999

4. TITLE AND SUBTITLE

Use of Very-High-Frequency Plasmas to Prepare a-Si:H-Based Triple-Junction Solar Cells at 

High Deposition Rates; Annual Technical Status Report, 11 March 1998-11 March 1999

6. AUTHOR(S)

S.J. Jones, T. Liu, D. Tsu, and M. Izu

5. FUNDING NUMBERS

C: ZAK-8-17619-18

TA: PV905004

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Energy Conversion Devices, Inc.

1675 West Maple Rd.

Troy, MI 48084

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REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

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SR-520-26795

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13. ABSTRACT (Maximum 200 words)

This report describes work performed by Energy Conversion Devices, Inc. (ECD) during this phase of this subcontract. ECD

researchers have made significant progress in advancing the very high frequency (VHF), high-rate technology. They demonstrated

that 8.0% stable efficiencies can be achieved for a-Si:H cells whose i-layers are prepared at rates near 10 Å/s using the VHF

technique. Presently, there is not a great difference in the performance of a-Si:H cells made using the VHF technique and i-layer

deposition rates near 10 Å/s and that for cells made using the standard 13.56 MHz technique and rates near 1 Å/s in the same

deposition system. In terms of the a-SiGe:H cells, researchers have completed a number of studies of devices with properties

appropriate for middle-junction cells—that is, cells without Ag/ZnO back-reflectors having Voc values near 0.75V and Jsc values near

8.0 mA/cm2

 when measured using AM1.5 light filtered using a 530-nm, low-band-pass filter. The stabilized properties for these cells

prepared at i-layer rates near 10 Å/s are again similar to a-SiGe:H cells made using the same deposition hardware and the low-rate

13.56 MHz method. Establishing an initial 10.5% for a triple-junction cell whose i-layers are prepared at the high rates sets the

baseline for ECD’s future studies. The triple-junction cell degradation (10%-13%) with prolonged light soaking is similar to that

regularly obtained for cells prepared at low i-layer deposition rates (1 Å/s). This is important because the use of high-rate methods to

prepare i-layers typically leads to less-stable materials and cells. Increasing the buffer-layer deposition rate to 6 Å/s leads to nearly a

15-min decrease in the total deposition time, whereas the increase in the n-layer and p-layer deposition rates both decrease the total

time by 5 and 5.8 min, respectively. Thus, besides the i-layer growth rates, increasing the buffer layer growth rate has the strongest

effect on the deposition time of the triple-junction semiconductor structures.

 14. SUBJECT TERMS 15. NUMBER OF PAGES

photovoltaics ; amorphous silicon ; triple-junction solar cells ; very high frequency plasmas ;

high deposition rates 16. PRICE CODE

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