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Innovation for Our Energy Future 

High-Efficiency Amorphous 

Silicon and Nanocrystalline 

Silicon-Based Solar Cells and 

Modules 

Final Technical Progress Report 

30 January 2006 – 29 January 2008 

S. Guha and J. Yang 

United Solar Ovonic LLC 

Troy, Michigan 

Subcontract Report 

NREL/SR-520-43191 

May 2008 

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Contract No. DE-AC36-99-GO10337

Subcontract Report 

NREL/SR-520-43191 

May 2008 

High-Efficiency Amorphous 

Silicon and Nanocrystalline 

Silicon-Based Solar Cells and 

Modules 

Final Technical Progress Report 

30 January 2006 – 29 January 2008 

S. Guha and J. Yang 

United Solar Ovonic LLC 

Troy, Michigan 

NREL Technical Monitor: Bolko von Roedern 

Prepared under Subcontract No. ZXL-6-44205-14 
This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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Table of Contents 

Preface.......................................................................................................................v

Executive Summary ............................................................................................... vi

Objectives.....................................................................................................................vi

Approaches...................................................................................................................vi

Status/Accomplishments.............................................................................................vii

Publications...............................................................................................................x

1. Fundamental research on mixed-phase silicon solar cells and nc-Si:H 

solar cells...................................................................................................................1

1.1. Introduction....................................................................................................... 1

1.2. Experimental details.......................................................................................... 2

1.3. Results and discussion ...................................................................................... 2

1.3.1. Microscopic local current flow in mixed-phase Si:H solar cells.......................................................2

1.3.2. Light-induced change of local current flow in mixed-phase Si:H solar cells....................................8

1.3.3. Doping effect on the material structure in mixed-phase Si:H solar cells..........................................9

2. Correlation of hydrogen dilution profiling with material structure and 

solar cell performance .......................................................................................... 16

2. 1. Introduction..................................................................................................... 16

2. 2. Experimental details........................................................................................ 17

2. 3. Results and discussion .................................................................................... 17

2.3.1. Material structures...........................................................................................................................17

2.3.2. Solar cell performance ....................................................................................................................20

2. 4. Summary ......................................................................................................... 21

3. Optimization of Ag/ZnO back reflector..................................................... 26

3. 1. Introduction..................................................................................................... 26

3. 2. Experimental details........................................................................................ 27

3. 3. Results and discussion .................................................................................... 27

3.3.1. Structural analysis...........................................................................................................................27

3.3.2. a-SiGe:H solar cells on improved Ag/ZnO back reflectors.............................................................32

3.3.3. Calculation of optical enhancement in a-SiGe:H cells on Ag/ZnO back reflector..........................37

3. 4. Summary ......................................................................................................... 39

4. High efficiency a-Si:H/a-SiGe:H/nc-Si:H and a-Si:H/nc-Si:H/nc-Si:H 

triple-junction solar cells...................................................................................... 42

4. 1. Introduction..................................................................................................... 42

4. 2. Experimental details........................................................................................ 42

4. 3. Results and discussion .................................................................................... 42

4.3.1. a-Si:H top cell .................................................................................................................................42

4.3.2. a-SiGe:H middle cell.......................................................................................................................44

4.3.3. nc-Si:H bottom cell .........................................................................................................................45

4.3.4. Optimization of a-Si:H/a-SiGe:H/nc-Si:H Triple-junction design..................................................56

4.3.5. High efficiency triple-junction cells................................................................................................58

4. 4. Summary ......................................................................................................... 59

5. High rate deposition of a-Si:H and a-SiGe:H solar cells using modified 

very high frequency glow discharge.................................................................... 62

iii
5. 1. Introduction..................................................................................................... 62

5. 2. a-Si:H and a-SiGe:H single-junction solar cells made with MVHF at high 

deposition rates............................................................................................................ 62

5. 3. a-Si:H/a-SiGe:H double-junction solar cells made with MVHF at high rates69

5. 4. Summary ......................................................................................................... 69

6. Large-area a-Si:H/a-SiGe:H/a-SiGe:H triple-junction and a-Si:H/nc-Si:H 

double-junction solar cells.................................................................................... 71

6. 1. Introduction..................................................................................................... 71

6. 2. Experimental details........................................................................................ 71

6. 3. a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells under manufacturing 

constraints.................................................................................................................... 72

6. 4. Large-area a-Si:H/nc-Si:H double-junction modules ..................................... 75

6. 5. Summary ......................................................................................................... 77

References:............................................................................................................. 77

iv
Preface 

This final report covers the work performed by United Solar Ovonic LLC for the period 

from January 30, 2006 to January 29, 2008, under the support of the National Renewable Energy 

Laboratory Thin Film Partnership Program Subcontract No. ZXL-6-44205-14. The original 

subcontract was for a three year period starting from January 30, 2006 to January 29, 2009. Due 

to realignment with the Solar America Initiative Program, the Thin Film Partnership Program 

was terminated on January 29, 2008. Therefore, this final report summarizes the work 

performed in the first two phases. 

The following personnel participated in this research program: 

A. Banerjee, E. Chen, S. Guha (Principal Investigator), B. Hang, M. Hopson, A. Mohsin, 

J. Noch, J. M. Owens, T. Palmer, L. Sivec, D. Wolf, X. Xu, B. Yan, J. Yang (CoPrincipal Investigator), K. Younan, and G. Yue. 

Collaboration with the Colorado School of Mines, University of Oregon, Syracuse 

University, and the National Renewable Energy Laboratory is acknowledged. 

v
Executive Summary 

Objectives 

 United Solar Ovonic LLC (United Solar) has successfully used its spectrum splitting aSi:H/a-SiGe:H/a-SiGe:H triple-junction structure in their manufacturing plants. A 

manufacturing capacity of 118 MW was achieved in 2007 with plants in Auburn Hills and 

Greenville, Michigan. The global photovoltaic market has expanded dramatically over the last 

few years. United Solar has set up a very aggressive expansion plan to achieve grid parity. In 

order to achieve this challenging goal, we need to improve our solar panel efficiency, improve 

the manufacturing throughput, and reduce the manufacturing cost. In the Thin Film Partnership 

Program (TFPP), we have identified three areas of research: 

i) Optimize the a-Si:H and a-SiGe:H deposition parameters under the current 

manufacturing constraints for improving the solar module efficiency and 

manufacturing throughput, and reducing the manufacturing cost. 

ii) Explore new deposition methods for a-Si:H and a-SiGe:H materials to improve the aSi:H/a-SiGe:H/a-SiGe:H triple-junction cell efficiency at high deposition rates. 

iii) Explore new materials and new cell structures for higher efficiency at high deposition 

rates. 

The first area represents a short-term goal with the objective of improving the current 

manufacturing module efficiency and throughput without modification of the production 

machines. Therefore, certain constraints have to be considered. For example, the deposition rate 

of each layer has to be adjusted according to the web speed and chamber length. Under this 

premise, any improvement in the cell efficiency will improve the current manufacturing module 

efficiency and throughput. The second area represents an intermediate-term goal with the 

objective of considerably improving cell and module efficiency with a higher throughput but 

with only limited modifications of the manufacturing machines. The third area represents a 

long-term goal with the objective of significantly improving the module efficiency with high 

throughput by using new designs for the manufacturing machines in order to deposit the new 

materials. 

In order to achieve the goals in each area, the research in material properties and device 

designs are vary important, especially on the material structure and defect characterization. 

Under the TFPP team, we have made a significant amount of fundamental studies and gained 

remarkable knowledge in material deposition, material characterization, and device optimization. 

Although the TFPP was terminated due to the realignment with the Solar America Initiative 

(SAI) Project, the achievements in this project have built a solid foundation, which will help 

achieving the goals and milestones in the SAI project. 

Approaches 

 For the first area, we carried out the research using a two-step approach. We first 

optimized the a-Si:H and a-SiGe:H component cells as well as the a-Si:H/a-SiGe:H/a-SiGe:H 

triple-junction cells using one of our large-area batch RF glow discharge machines. The avi
Si:H/a-SiGe:H/a-SiGe:H triple-junction cells were deposited on Al/ZnO back reflectors made in 

the manufacturing machines. When experimental deposition parameters are improved, the new 

recipe would be transferred to the production line. 

 For the second area, we studied high rate a-Si:H and a-SiGe:H solar cells made with 

MVHF. Although the MVHF a-Si:H top cell has shown a significant advantage over the RF 

deposited a-Si:H cells at high rates, many challenges still remain, especially in the quality of 

high rate deposited a-SiGe:H middle and bottom cells. Under this program, we explored more 

deposition parameters and researched new deposition regimes. 

For the third area, we continued investigating issues related to nc-Si:H material 

properties, cell efficiency, and deposition rate. The major target is to demonstrate the feasibility 

of using nc-Si:H in multi-junction structures and to achieve higher cell and module efficiencies 

than those obtained using the conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. 

We have identified that the nanocrystalline evolution along the growth direction is one of the 

major challenges for high efficiency nc-Si:H solar cells. Therefore, we developed a hydrogen 

dilution profiling technique for controlling the nanocrystalline evolution during nc-Si:H 

deposition. We have further optimized hydrogen dilution for high rate nc-Si:H deposition. At 

the same time, we modified the MVHF deposition chamber and made it suitable for highpressure deposition. In addition, we optimized the a-Si:H top cell and a-SiGe:H middle cell for 

high efficiency a-Si:H/a-SiGe:H/nc-Si:H triple-junction structures. 

Another major issue for using nc-Si:H solar cells in a production line is the thickness of 

the absorber layer and its uniformity. Due to the decrease of electromagnetic wavelength with 

the increase of VHF frequency, the electric field distribution over a large-area cathode becomes 

non-uniform. A special design for the large-area VHF cathodes is required to achieve uniform 

deposition. We have worked on the large-area VHF cathode design for the nc-Si:H deposition. 

Parallel to optimizing the solar cell efficiency, we have carried out significant 

fundamental studies on material structures and device designs. Fundamental research was done 

through collaboration with NREL, University of Oregon, Colorado School of Mines, and 

Syracuse University. We investigated a-Si:H, a-SiGe:H, and nc-Si:H material properties and the 

mechanism of light-induced degradation in these materials and solar cells. Most results have 

been reported by our collaborators. In addition, we have worked on the improvement of the light 

trapping scheme in a-SiGe:H and nc-Si:H solar cells by optimizing the texture of Ag/ZnO back 

reflectors. The correlation of Ag and ZnO structure with solar cell performance has been studied 

and provides a guideline for making good back reflectors. 

Status/Accomplishments 

Fundamental research in mixed-phase silicon solar cells and nc-Si:H solar cells

1. We have studied the material structures of mixed-phase silicon materials with TEM, 

AFM, and C-AFM. We correlated the materials structure with the light induced 

degradation of solar cells. The aggregation of nanograins forms micrometer sized 

clusters observed from the top view by AFM, and cone structures from the side view by 

X-TEM. The nanocrystalline aggregation supports the previously proposed two-diode 

vii
model for the light induced degradation in mixed-phase solar cells. In addition, the 

reduction of current density in the crystalline phase during light soaking provided a direct 

support of the explanation. 

2. We studied the doping effect on the structure changes in the mixed-phase materials. We 

found that first the incorporation of P in mixed-phase silicon materials resulted in a 

higher hydrogen dilution ratio to reach the amorphous/nanocrystalline transition and 

second the nanocrystalline aggregation disappeared when P was incorporated into the 

mixed-phase materials. 

3. For the collaboration with NREL, we have studied material structures of nc-Si:H solar 

cells with various hydrogen dilution profiles. The TEM and AFM analyses showed that 

with an optimized hydrogen profiling, we can control the growth of nc-Si:H with no 

incubation layer, and uniform distribution of crystallinity along the growth direction. We 

correlated the material structure properties to the nc-Si:H solar cell performance. 

Optimization of solar cell performance at the amorphous/nanocrystalline transition regime

1. We have optimized a-Si:H solar cells at the amorphous/nanocrystalline transition regime. 

We found that when a-Si:H solar cells were made under a hydrogen dilution condition 

close to the transition regime, the cell performance was improved. In order to keep the 

material structure close to the transition regime throughout the intrinsic layer, hydrogen 

dilution profiling was proven to be a useful method. 

2. The same approach was also used to optimize the a-SiGe:H component cells for high 

efficiency triple-junction solar cells. 

Optimization of optical enhancement by back reflectors

1. We have developed an analysis method of optical enhancement in a-SiGe:H and nc-Si:H 

solar cells. We found that the optical enhancement with optimized Ag/ZnO back 

reflectors was as high as 20-30 folds at long wavelength. Although optical enhancement 

is already very large, it is still not as large as the theoretical value of 4n

2

 (n is the optical 

index of the semiconductor layers) and more improvement is expected by optimizing the 

Ag and ZnO layers. 

2. We have studied the correlation of Ag/ZnO structures and solar cell performance. We 

found that a flat Ag layer reduced the plasmon absorption but it also reduced the 

scattering at the Ag/ZnO interface. For back reflectors with thinner ZnO, a textured Ag 

is better than flat Ag layer. However, when the ZnO layer is thick enough, a flat Ag layer 

is better than a textured Ag layer. 

High efficiency a-Si:H, a-SiGe:H, and nc-Si:H multi-junction solar cells

1. We studied the optimization of multi-junction solar cell designs. The current mismatch 

for high efficiency multi-junction solar cells was systematically studied. We found that a 

current mismatch with nc-Si:H bottom cell limited current yields improved initial and 

stable efficiencies for a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cells. 

viii
2. We have achieved active-area (0.25 cm2

) initial and stable efficiencies of 15.4% and 

13.0%, respectively, for an a-Si:H/a-SiGe:H/nc-Si:H triple-junction cell, where the top 

and middle cells were made using RF at a low rate ~1 Å/s, and the nc-Si:H bottom cell 

using MVHF at a high rate ~5-8 Å/s. 

3. We have achieved active-area (0.25 cm2) initial and stable efficiencies of 14.1% and 

13.3%, respectively, for an a-Si:H/nc-Si:H/nc-Si:H triple-junction cell, where the top cell 

was made using RF at a low rate ~1 Å/s, and the nc-Si:H middle and bottom cells using 

MVHF at a high rate ~5-8 Å/s. 

High rate deposition of a-Si:H, a-SiGe:H, and a-Si:H/a-SiGe:H double-junction solar cells

1. We have investigated deposition parameters for a-Si:H and a-SiGe:H deposition with 

MVHF at high rates. We found that the performance and stability of a-Si:H singlejunction cells deposited with MVHF do not depend on the deposition rate in the range of 

1-15 Å/s. 

2. We have achieved active-area (0.25 cm2) initial and stable efficiencies of 9.2% and 8.5%, 

respectively, for nc-Si:H single-junction cells made with MVHF at a high rate ~5-8 Å/s. 

3. We have made a-Si:H/a-SiGe:H double-junction solar cells on Ag/ZnO back reflectors 

using MVHF. The a-Si:H and a-SiGe:H intrinsic layers were deposited in 4 minutes and 

4 minutes, respectively. An initial active-area efficiency of 11.7% has been achieved. 

Large area a-Si:H/a-SiGe:H/a-SiGe:H triple-junction and a-Si:H/nc-Si:H double-junction solar 

cells

1. We have achieved an aperture-area (416 cm2) stable efficiency of 8.6% on an a-Si:H/aSiGe:H/a-SiGe:H triple-junction solar cell deposited under the manufacturing constraints 

on an Al/ZnO back reflector made in the manufacturing line. The cell was encapsulated 

using a procedure similar to the manufacturing process. 

2. We have achieved an aperture-area (460 cm2

) initial efficiency of 11.7% on an a-Si:H/ncSi:H double-junction solar cell deposited on Ag/ZnO back reflectors with RF glow 

discharge at a high rate. The nc-Si:H intrinsic layer in the bottom cell was deposited 

within 50 minutes. 

ix
Publications 

1. G. Yue, B. Yan, G. Ganguly, J. Yang, S. Guha, and C. Teplin, “Material Structure and 

Metastability of Hydrogenated Nanocrystalline Silicon Solar Cells,” Appl. Phys. Lett. 88, 

263507 (2006). 

2. G. Yue, B. Yan, G. Ganguly, J. Yang, and S. Guha, “Metastability of Hydrogenated 

Nanocrystalline Silicon Solar Cells,” Mater. Res. Soc. Symp. Proc. 910, 29 (2006). 

(Invited). 

3. B. Yan, J. Yang, and S. Guha, “Temperature Dependence of Dark Current-Voltage 

Characteristics of Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar 

Cells,” Mater. Res. Soc. Symp. Proc. 910, 713 (2006). 

4. B. Yan, C.-S. Jiang, H. R. Moutinho, M. M. Al-Jassim, J. Yang, and S. Guha, “Local 

Current Flow in Mixed-Phase Silicon Solar Cells and Correlation to Light-Induced OpenCircuit Voltage Enhancement,” Mater. Res. Soc. Symp. Proc. 910, 647 (2006). 

5. P. G. Hugger, S. Datta, P.T. Erslev, G. Yue, G. Ganguly, B. Yan, J. Yang, S. Guha, and 

J.D. Cohen, “Electronic Characterization and Light-Induced Degradation in nc-Si:H Solar 

Cells,” Mater. Res. Soc. Symp. Proc. 910, 21 (2006). 

6. D. C. Bobela, T. Su, P. C. Taylor, A. Madan, and G. Ganguly, “The Concentration of 

(SiH2)n Sites in Low and High Defect Density a-Si:H,” Mater. Res. Soc. Symp. Proc. 

910, 251 (2006). 

7. N. H. Nickel, M. Weizman, I. Sieber, and B. Yan, “Influence of Compositional and 

Structural Changes on Hydrogen Bonding in Silicon and Silicon-Germanium Alloys,” J. 

Non-Cryst. Solids 352, 1037 (2006). 

8. D. C. Bobela, T. Su, P. C. Taylor, and G. Ganguly, “Microscopic Properties of Silicon 

Dihydride Bonding in a-Si:H,” J. Non-Cryst. Solids 352, 1041 (2006). 

9. M. Weizman, N. H. Nickel, I. Sieber, and B. Yan, “Successive Segregation in LaserCrystallized Poly-SiGe Thin Films,” J. Non-Cryst. Solids 352, 1259 (2006). 

10. S. Guha and J. Yang, “Progress in Amorphous and Nanocrystalline Silicon Solar Cells,” 

J. of Non-Crystalline Solids 352, 1917 (2006). 

11. B. Yan, G. Yue, J.M. Owens, J. Yang, and S. Guha, “Over 15% Efficient Hydrogenated 

Amorphous Silicon Based Triple-Junction Solar Cells Incorporating Nanocrystalline 

Silicon”, Conf. Record of the 2006 IEEE 4th World Conf. on Photovoltaic Energy 

Conversion, (Hawaii, USA, May 7-12, 2006), p. 1477. 

12. C.-S. Jiang, H. R. Moutinho, M. M. Al-Jassim, L. L. Kazmerski, B. Yan, J. M. Owens, J. 

Yang, and S. Guha, “Distribution of Local Open-Circuit Voltage on Amorphous and 

Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells,” Conf. Record of the 2006 

IEEE 4th World Conf. on Photovoltaic Energy Conversion, (Hawaii, USA, May 7-12, 

2006), p. 1552. 

13. G. Yue, B. Yan, G. Ganguly, J. Yang, S. Guha, C. Teplin, and D. L. Williamson, 

“Performance Improvement of Hydrogenated Nanocrystalline Silicon Solar Cells by 

x
Hydrogen Dilution Profiling,” Conf. Record of the 2006 IEEE 4th World Conf. on 

Photovoltaic Energy Conversion, (Hawaii, USA, May 7-12, 2006), p. 1588. 

14. G. Ganguly, G. Yue, B. Yan, J. Yang, and S. Guha, “Fabrication of Large Area 

Amorphous Silicon/Nanocrystalline Silicon Double Junction Solar Cells,” Conf. Record 

of the 2006 IEEE 4th World Conf. on Photovoltaic Energy Conversion, (Hawaii, USA, 

May 7-12, 2006), p. 1712. 

15. G. Yue, B. Yan, G. Ganguly, J. Yang, and S. Guha, “Metastability in Hydrogenated 

Nanocrystalline Silicon Solar Cells,” J. Mater. Soc. 22, 1128 (2007). 

16. B. Yan, C.-S. Jiang, C. W. Teplin, H. R. Moutinho, M. M. Al-Jassim, J. Yang, and S. 

Guha “Local Current Flow in Amorphous and Nanocrystalline Mixed-Phase Silicon Solar 

Cells”, J. Appl. Phys. 101, 033711 (2007). 

17. C.-S. Jiang, B. Yan, H. R. Moutinho, M. M. Al-Jassim1, J. Yang, and S. Guha, “Light 

Soaking and Thermal Annealing Effects on the Micro-Electrical Properties of 

Amorphous and Nanocrystalline Mixed-Phase Silicon Solar Cells,” Mater. Res. Soc. 

Symp. Proc. 989, 15 (2007). 

18. B. Yan, G. Yue, and S. Guha, “Status of nc-Si:H Solar Cells at United Solar and 

Roadmap for Manufacturing a-Si:H and nc-Si:H Based Solar Panels,” Mater. Res. Soc. 

Symp. Proc. 989, 335 (2007). (Invited). 

19. G. Yue, B. Yan, J. Yang, and S. Guha, “High Rate Deposition of Amorphous Silicon 

Based Solar Cells using Modified Very High Frequency Glow Discharge” Mater. Res. 

Soc. Symp. Proc. 989, 359 (2007). 

20. G. Yue, B. Yan, C. W. Teplin, J. Yang, and S. Guha, “Optimization and Characterization 

of i/p Buffer Layer in Hydrogenated Nanocrystalline Silicon Solar Cells”, J. Non-Crystal 

Solids, (2008), in press. 

21. C.-S. Jiang, B. Yan, Y. Yan, C.W. Teplin, R. Reedy, H.R. Moutinho, M.M. Al-Jassim, J. 

Yang, and S. Guha, “Effect of P Incorporation on Aggregation of Nanocrystallites in 

Amorphous and Nanocrystalline Mixed-Phase Silicon Thin Films”, J. Non-Crystal 

Solids, (2008), in press. 

22. A.H. Mahan, Y. Xu, H.M. Branz, and B. Yan, “Amorphous Silicon Germanium n-i-p 

Solar Cells Deposited by HWCVD Using a Tantalum Filament Operated at 1800°C”, J. 

Non-Crystal Solids, (2008), in press. 

23. P. G. Hugger, D. Cohen, G. Yue, B. Yan, J. Yang, and S. Guha, “Effects of LightInduced Degradation and Hydrogen Profiling on the Electronic Properties of 

Nanocrystalline Silicon and Their Influence on Solar Cell Performance”, J. Non-Crystal 

Solids, (2008), in press. 

24. C.-S. Jiang, B. Yan, Y. Yan, C.W. Teplin, R. Reedy, H.R. Moutinho, M.M. Al-Jassim, 

and J. Yang, “P-Induced nanocrystallite dispersion in amorphous-nanocrystalline mixedphase Si:H thin films”, J. Appl. Phys. (2008), in press. 

xi
1. Fundamental research on mixed-phase silicon solar cells and nc-Si:H 

solar cells 

1.1. Introduction 

 Hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) thin films 

have attracted a great deal of interest in photovoltaic devices and large-area thin-film electronics. 

One of the main fabrication techniques is radio frequency (RF) or very high frequency (VHF) 

glow discharge, which decomposes SiH4 or Si2H6 molecules in the plasma. Hydrogen dilution is 

a critical parameter for controlling the material structure. By varying the hydrogen dilution in 

the process gases, one can obtain conventional a-Si:H, a-Si:H with intermediate orders (or socalled protocrystalline Si:H), mixed-phase Si:H, and nc-Si:H with high crystallinity [1]. Three 

models have been proposed to explain the role of hydrogen during the film growth: etching 

amorphous phase by atomic hydrogen, H-mediated chemical annealing, and enhanced surface 

diffusion of Si radicals [2]. Many recent studies have analyzed nanocrystalline formation using 

in-situ characterizations, such as real time spectroscopic ellipsometry [3,4] and infrared 

attenuated total reflection [5]. Most results support the surface diffusion model, which suggests 

that the diffusion of Si radicals is increased on the growth surface with the presence of additional 

hydrogen on the growth surface [2]. The material structural properties and their correlations to 

the deposition conditions have also been widely studied using many ex-situ characterization 

techniques such as cross-sectional transmission electron microscopy (X-TEM) [6,7] atomic force 

microscopy (AFM) and conductive AFM (C-AFM) [8,9], and scanning Kelvin probe microscopy 

(SKPM) [10]. The general picture revealed from these characterizations is that under a constant 

hydrogen dilution over a given threshold, nanocrystalline nucleation appears after an amorphous 

incubation layer. We also found that cone-shaped Si nanocrystallites grow with the film 

thickness. When the hydrogen dilution is just above the threshold, individual and isolated 

nanocrystalline cones form in a large fraction of amorphous matrix. This kind of material is 

normally called mixed-phase material. When the hydrogen dilution is high enough, the 

nanocrystalline cones coalesce and fully nc-Si:H materials are obtained. Raman, X-TEM, and 

X-Ray diffraction measurements show that the grain size is in a few nm to tens of nm range, 

while the sizes of nanocrystalline cones can be as large as micrometer range. Therefore, a 

structural model consisting of nanometer-size grains in micrometer-size cones separated by 

amorphous tissue has been proposed [11]. This model explains most of the materials properties 

including the increase of crystalline volume fraction with film thickness. 

Among the three types of material structures, the mixed-phase material contains very rich 

physics for fundamental study and device optimization. Although the mixed-phase material is 

very difficult to use in real solar cell application, the optimized a-Si:H and nc-Si:H solar cells are 

made close to the mixed-phase regime [12,13]. In general, the mixed-phase solar cells show an 

open-circuit voltage (Voc) ranging from 0.5 to 1.0 V, which is between the Voc values of typical 

a-Si:H and nc-Si:H cells. One interesting phenomenon for the mixed-phase solar cells is that the 

Voc increases after light soaking [14,15], opposite to commonly observed for a-Si:H and nc-Si:H 

solar cells where the Voc decreases after light soaking due to the Staebler-Wronski effect [16]. 

The original explanation for the light-induced Voc increase in the mixed-phase solar cell was 

based on light-induced structural changes from the crystalline phase to amorphous phase [14, 

15]. Subsequently, a complementary model of two parallel-connected diodes (two-diode model) 

was proposed [17]. There are two key points made in the two-diode model. First, the 

1
amorphous phase and nanocrystalline phase can be considered as two separate diodes with 

significantly different characteristics. Second, the current versus voltage (I-V) characteristics of 

nc-Si:H diode in the mixed-phase cells degrade by light soaking. The degradation of nc-Si:H 

causes a decrease in the electric current, when a forward voltage larger than the Voc of the ncSi:H cell is applied, resulting in an increase in the Voc of the mixed-phase solar cells [17]. 

However, the size of the nanocrystallites observed by X-ray diffraction and Raman spectroscopy 

is very small, ranging from a few nm to 30 nm. It is difficult to believe that such small grains 

can form complete diodes through the entire thickness of the intrinsic layer. 

Under this program, we used atomic force microscopy (AFM), conductive atomic force 

microscopy (C-AFM), Kelvin probe microscopy (SKPM) to measure the surface structure, the 

local current flow [8], and local Voc distribution [9] in mixed-phase Si solar cells, respectively. 

We also studied the structural properties along the growth direction and explored the possible 

growth mechanisms using X-TEM. 

1.2. Experimental details 

The mixed-phase Si:H materials and solar cells were made on a 4×4 cm2 stainless steel 

substrate using a multi-chamber PECVD system used for high efficiency solar cell fabrication. 

We controlled the deposition parameters in the regime of amorphous to nanocrystalline transition 

to obtain various material structures on one substrate, such as the center of the deposition 

showing an amorphous structure, edges with a mixed-phase signature, and the corners with a 

substantially nanocrystalline volume fraction as previously reported [14,15]. 

 Secondary-ion mass spectrometry (SIMS) was used to measure the chemical composition 

in the films. Raman spectroscopy was used to measure the crystallinity of the films. A laser spot 

with a diameter of 1-2 μm and a wavelength of 532 nm was irradiated on the samples, and 

Raman scattering from the sample was measured by a monochrometer. AFM, C-AFM and 

SKPM were used to measure the morphology, microscopic current distribution, and local work 

function distributions, respectively. X-TEM was used to investigate the structures inside the 

films. The X-TEM samples were prepared by the focused ion beam (FIB) technique. A 50-nmthick Pt layer was deposited on the sample surface and an 80-nm-thick slice of the sample was 

cut off using the FIB. All of these characterizations were carried out at NREL. 

1.3. Results and discussion 

1.3.1. Microscopic local current flow in mixed-phase Si:H solar cells

In the mixed-phase regime, the cell performance is very sensitive to the deposition 

condition. A very small change in the plasma condition can change the cell performance 

dramatically. Even on a 4 cm × 4 cm substrate, the distribution of cell performance varying from 

a-Si:H like to nc-Si:H like can be obtained. Therefore, by carefully selecting the deposition 

parameters, we can obtain solar cells on one substrate having different characteristics at different 

locations. Figure 1 shows the cell locations on a 4 cm × 4 cm substrate. Figure 2 gives an 

example of Voc distribution of the sixteen 0.25-cm2

 cells on the substrate. It is clear that the cells 

at the center show amorphous-like characteristics with Voc around 1.0 V, mixed-phase-like 

2
characteristic with Voc between 0.7 V and 0.8 V on the edges, and nanocrystalline-like 

characteristic at the corners with Voc between 0.5 V and 0.6 V. 

 To further confirm the differences in material structures, Raman spectra were acquired at 

the areas marked A, B, and C in Fig. 1, where the amorphous region (area A) is between cells 22 

and 23, mixed-phase (area B) between cells 11 and 12, and substantially nanocrystalline (area C) 

between cells 51 and 11. The Raman spectra clearly show that in area A, the material has a 

complete amorphous structure without detectable crystalline peak; in area B, a small peak at 

~520 cm-1 indicates a mixed-phase nature with the crystalline peak fraction of ~ 10%; and in area 

C, a large peak at ~520 cm-1 indicates a substantially crystalline volume fraction of over 50%. 

11 12 13 14

21 22 23 24

31 32 33 34

41 42 43 44

51 52 53 54 55

61 62 63 64 65

71 72 73 74 75

81 82 83 84 85

92 93 94

A

B

C

Figure 1. The solar cell distribution on a 4 cm × 4 cm substrate, where the small circles are ITO 

dots with an area of 0.05 cm2

 and the large ones of 0.25 cm2. The squares, labeled A, B, and C, 

are the three areas where the Raman spectra, C-AFM image, and AFM morphology image were 

taken. 

3
1 2 3 4

S1

S2

S3

S4

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Voc (V)

X

Y 

Figure 2. The Voc distribution of the sixteen 0.25-cm2 cells on the 4 cm × 4 cm substrate. 

 C-AFM images along with AFM morphology images were also taken in areas A, B, and 

C on each sample. Figure 3 shows (a) the C-AFM image under a forward 1 V bias at the 

amorphous area (area A) of a mixed-phase silicon sample with 500-nm thick intrinsic layer and 

10-nm thick fully amorphous buffer layer, and (b) the corresponding AFM morphology image. 

Although there are some current fluctuations, the local current does not change from location to 

location. However, in Fig. 4 (a), the C-AFM image at the mixed-phase area (area B) on the same 

sample shows two locations having very high current. Note that in Fig. 4 (a), the current scale is 

1.0 nA/unit (compared to 30 pA/unit in Fig 3 (a)), which results in a flat current profile for the 

rest of area. The same data are shown in Fig. 4 (b) with a current scale of 30 pA/unit as used in 

Fig. 3 (a), where the background current mapping is similar to that in Fig. 3 (a). However, the 

two high-current spikes extend beyond the range of the graph. The most important observation 

is that the high current and low current regions are clearly distinguished and the transition edge is 

very sharp. Figure 4 (c) shows the AFM morphology of the same area as in Fig. 4 (a) and (b), 

where three large hill-like structures (marked with arrows) correspond to the two locations with 

high current spikes. Figure 5 (a) shows a C-AFM image for the substantially nanocrystalline 

region (area C) in the same sample. Numerous high-current spikes appear everywhere. 

Although the current-axis is 2.0 nA/unit (a factor of two larger than in image Fig. 4 (a)), the top 

parts of many spikes are still cut away. As shown in Fig. 5 (b), the morphology at the 

nanocrystalline area has many large hill-like structures. The size of these hill-like structures is 

around 500 nm in diameter. 

4
30 pA

0

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

0

100 nm

(a)

(b)

Figure 3. (a) The C-AFM image under 1.0 V forward bias in area A on an n-i-p sample with a 

500-nm thick intrinsic layer and a 10-nm thick fully a-Si:H i/p buffer layer and (b) the surface 

morphology image of the same area as the upper picture. 

5
1 nA

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

(a)

0

30 pA

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

(b)

100 nm

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

(c)

0

0

Figure 4. The upper (a) and middle (b) pictures are the C-AFM images under 1.0 V of forward 

bias in area B on the same sample as in Fig. 3, where current in the upper picture is in 1.0 

nA/unit, while the middle one is in 30 pA/unit. The lower picture (c) is the morphology image in 

area B, where the three large hill-like structures as indicated by the arrows are clusters of 

nanocrystallites. 

6
2 nA

0

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

(a)

(b)

100 nm

0

0

0.5

1.0

1.5 μm

0.5

1.0

1.5

Figure 5. (a) the C-AFM image under 1.0 V of forward bias in area C on the same sample as in 

Fig. 5-6, where the current is in 2.0 nA/unit. (b) the morphology image in area C, where many 

large hill-like structures appear. 

 The mixed-phase material can be considered as nanocrystallites embedded in an 

amorphous matrix. The sizes of the nanocrystallites vary from a few nm up to 30 nm as 

measured by XRD and TEM. It could be even smaller for the mixed-phase materials with low 

volume fraction of nanocrystallites. Two difficulties arise in explaining some of the 

experimental results with such small crystallites. First, it is hard to imagine a two-phase model 

with the nanocrystalline phase forming a diode with such small nanocrystallites [17]. Second, 

the Voc of mixed-phase cells drops dramatically for crystalline volume fractions much smaller 

than the percolation threshold. In fact, the Voc starts to drop with increasing hydrogen dilution at 

a level that the material does not even show any nanocrystalline signature by XRD or Raman 

measurements. The C-AFM results allow us to address these two issues. By comparing the CAFM and AFM morphology images, we find that the small nanocrystallites aggregate to form 

clusters with a size of ~500 nm in diameter. The significant difference of the forward current in 

the amorphous regions and the areas of nanocrystallite aggregations provides the evidence that 

nanocrystalline regions (aggregation or cluster of nanocrystallites) can be considered as diodes 

having nc-Si:H diode characteristics, while the surrounding amorphous regions act as an a-Si:H 

diode. The bandgap of the nanocrystalline areas should be around 1.1 eV (close to the crystalline 

silicon value); while in the amorphous areas, it is around 1.7-1.8 eV (similar to the a-Si:H value). 

With these bandgaps, the nc-Si:H diode should have a much higher forward current density than 

the a-Si:H diode under 1.0 V of forward bias. In addition, the aggregation of the nanocrystallites 

7
results in a very non-uniform nanocrystallite distribution in a micro-scale. In the area of 

nanocrystallite aggregations, the high forward dark current leads to a dramatic drop in Voc, even 

though the average nanocrystalline volume is much lower than the percolation threshold. 

1.3.2. Light-induced change of local current flow in mixed-phase Si:H solar cells

In order to investigate the light-soaking and thermal annealing effects on the current flow 

through the nanocrystalline phase, we applied a fixed bias voltage of 0.75 V to the sample during 

the C-AFM measurements. The value was chosen to be between the Voc of a-Si:H and nc-Si:H 

solar cells. As shown above, the nanocrystalline phase is not uniformly distributed in the 

amorphous matrix, but aggregates to form clusters. Each aggregate is substantially 

nanocrystalline and contains many small grains in the nanometer scale. The local current on the 

nanocrystalline aggregates is much larger than on the surrounding amorphous phase as shown in 

Figs. 6(a)-(f), where the high current regions are nanocrystalline aggregates. It should be pointed 

out that for multiple scans over the same area, the current image changes, as illustrated in Figs. 

6(a)-(f). This probably results from the surface modification by the AFM tip. In the first scan, 

the current flow is very non-uniform in each individual aggregate. The non-uniformity was 

reduced in the second scan. We suspect that the improvement in the current uniformity could be 

caused by a mechanical modification of the p layer when the AFM tip contacted the sample 

surface, because the current image does not significantly depend on the bias voltage 

200 pA

0 1

2

3 4 μm

200 pA

0 1

2

3 4 μm

200 pA

0 1

2

3 4 μm

200 pA

0 1

2

3 4 μm

0 4 μm 0 4 μm 0 4 μm

(b) (d) (f)

(a) As grown (c) Light-soaked (e) Annealed

Figure 6. Examples of 2-D and 3-D C-AFM current images measured on an a-Si:H and nc-Si:H 

mixed-phase solar cell in the initial state (a) and (b), light-soaked-state (c) and (d), and thermal 

annealed state (e) and (f). 

8
Ele

ctric

Current (p

A)

Area of aggregates (×10-1 μm2)

Vb = 0.75V

0

50

100

150

200

0.0 0.5 1.0 1.5 2.0

250 Initial

Light-soaked

Thermal-annealed

Figure 7. Electric current as a function of the aggregation area in the initial, light-soaked, and 

annealed states. Straight lines are linear fitting of the data and represent the current densities. 

We took a large number of C-AFM images at different areas of the samples in the different 

states for statistical comparisons. Although the current values showed a large scattering, we 

found that a larger aggregate exhibits a higher average current value over the aggregate area. 

Figure 7 plots the average current value versus aggregate area. Although the data are very 

scattered, the average current values with the same size of aggregates at the initial and annealed 

states are similar and significantly larger than that at the light-soaked state. We further linearly 

fitted the current data versus aggregate area, and got the current densities of (9.4±0.7)×10-1, 

(2.8±0.3)×10-1, and (10±1)×10-1 (nA/μm

2

) for the initial, light-soaked, and annealed states, 

respectively. The similar current density on aggregates with different sizes reveals that the 

current flow in individual aggregates is limited by the size of the aggregates, but not by the tipsample contact. This means that the current route spreads out in the nanocrystalline aggregates, 

and is not confined in small areas under the tip. The large scattering in the current data reflects 

structure fluctuations in the aggregates. Each aggregate contains a large number of small grains. 

The grain boundaries and amorphous component between the nanometer-size grains significantly 

affected the electron transport and resulted in the current density fluctuations. 

 The current density measurements by C-AFM above and other micro-electrical 

measurements reveal that the amorphous and nanocrystalline phases in the mixed-phase solar 

cells can be distinguished, and thus support the model that the Voc of the mixed-phase solar cells 

can be considered from the two parallel-connected diodes. The light-induced decrease in average 

current density in the nanocrystalline aggregates further supports this model. 

1.3.3. Doping effect on the material structure in mixed-phase Si:H solar cells

 As presented above, we have measured the current flow on completed n-i-p solar cells 

using conductive atomic force microscopy (C-AFM) and observed high conduction paths 

through the nanocrystalline aggregations. We further investigated light-soaking and thermal9
annealing effects on the current flow through the nanocrystalline aggregations and found that the 

average forward current density over the nanocrystalline areas was reduced by light soaking and 

recovered to the initial values after thermal annealing. 

 In this section, we report our recent studies of phosphorus (P) doping effects on the 

microstructures and micro-electrical properties of the mixed-phase Si:H material. We found that 

adding PH3 into the process gas moved the amorphous-to-nanocrystalline transition to a higher 

hydrogen dilution ratio required for achieving the similar Raman crystallinity. The 

nanocrystalline aggregation observed in the intrinsic Si:H materials disappeared in the mixedphase films with high P doping, where isolated grains of nanometer sizes were distributed 

throughout the amorphous matrix. In addition, we found a dramatic increase in film conductivity 

in the heavily doped mixed-phase material having a small crystal volume fraction of 5%–10% 

estimated from Raman measurements. 

 Si:H films were deposited onto 4×4 cm2

 stainless-steel substrates using a multiplechamber plasma-enhanced chemical vapor deposition (PECVD) system. The intrinsic and Pdoped films were deposited in the i and n chambers, respectively, for n-i-p solar cells. We 

adjusted the hydrogen-dilution ratio to obtain different structural properties and adjusted Pdoping concentrations to vary film conductivities. We controlled the hydrogen dilution ratio so 

that the films were deposited in the amorphous/nanocrystalline transition regime. We were able 

to obtain various material structures from a single deposition on the same substrate: amorphous 

(from the center of the substrate), mixed-phase (along the edges of the substrate), and a high 

nanocrystalline volume fraction (near the four corners), as previously reported [8,9,14,15]. We 

deposited many samples with different hydrogen-dilution ratios and different P-doping levels 

and found that adding PH3 in the gas phase significantly increased the hydrogen-dilution 

threshold required for reaching the amorphous/nanocrystalline transition. Here, we present three 

typical samples: one mixed-phase Si:H intrinsic film, one weakly P-doped mixed-phase Si:H 

film deposited in the n-chamber without intentional PH3 flows, and one heavily P-doped film 

with a PH3-to-Si2H6 ratio of 3.3% and an increased hydrogen dilution by a factor of 2.3. 

 We used the AFM and C-AFM measurements to measure the surface morphology and 

local current flow. We also used cross-sectional transmission electron microscopy (X-TEM) to 

characterize the material structure long the growth direction. Raman measurements were carried 

out in the mixed-phase regions of the sample to identify regions of similar crystallinity for each 

sample. Secondary-ion mass spectrometry (SIMS) measurements were made to obtain P 

concentrations. 

 Although many samples have been characterized, the results from the three different 

types of samples of intrinsic, weakly P-doped, and heavily P-doped layers are presented in this 

report. Raman measurements showed that the crystallinity is similar for the three samples. An 

estimation from de-convolution of the Raman spectra showed that the crystal volume fraction 

was ~5%–10%. The SIMS analyses showed that the P contents are 3×1018 at/cm3 in the weakly 

doped sample, and 2×1021 at/cm3

 in the heavily doped sample. 

10
200 nm

SS substrate

Si:H

Pt layer

2 nm

(a)

200 nm

SS substrate

Si:H

Pt layer

2 nm

(a)

200 nm SS substrate

Si:H

Pt layer

2 nm

(b)

Figure 8. X-TEM images taken on (a) intrinsic and (b) heavily doped mixed-phase samples. 

High resolution images taken on the nc-Si:H aggregation in (a) and a grain embedded in the aSi:H matrix in (b) are shown in the inset of the images. 

 Figure 8 shows X-TEM images of the intrinsic and heavily doped mixed-phase Si:H 

films, where the cone-shaped structures in Fig. 8(a) are nanocrystalline aggregations containing a 

large amount of nanometer-sized grains, as shown in the inset of the figure. This observation is 

consistent with our previously observed nanocrystalline aggregation in mixed-phase solar cells 

[8,9]. The low-level doping from the residual P in the deposition chamber did not affect the 

material structure, namely, the aggregation of nanometer-sized grains were still observed. The 

most important observation was the disappearance of the cone-shaped nanocrystalline 

aggregations in the heavily doped samples, as shown in Fig. 8(b), even though the Raman spectra 

obtained from this sample showed similar crystallinity to the other two samples. The highresolution X-TEM image in the inset of Fig. 8(b) displays isolated nanometer-sized grains 

embedded in the amorphous matrix. Most likely, the nanocrystallites are randomly distributed in 

the a-Si:H matrix. 

11
1 μm

(a) (b)

(c)

20 nm

(d)

Figure 9. (a) An AFM and (b) corresponding C-AFM images taken under Vb= +0.75 V on the 

intrinsic mixed-phase sample. (c) shows a C-AFM image under Vb= -0.75 V on the same sample 

but different location, and (d) is a high-resolution image of (c).

 We next investigate the local conduction path in the mixed-phase films with varying Pdoping concentration. Figure 9 displays the following images from the intrinsic sample: (a) 

AFM topography, (b) C-AFM image with a Vb= +0.75 V electrical bias applied to the tip, (c) CAFM image with a Vb= -0.75 V bias, and (d) negatively biased C-AFM image under high 

resolution. Figures 9 (a) and (b) were taken simultaneously in the exact same area, where the 

white spots in (a) are the aggregations of nanocrystallites and the corresponding white spots in 

(b) are the areas with high current through the tip. The clear correspondence between the 

nanocrystalline aggregations and the high current areas reveals that the transport paths are 

through the nanocrystalline aggregations [8–10,17]. The current on individual aggregations is 

relatively uniform, and the value depends on the size of the aggregations, exhibiting larger 

current values on larger-size aggregations, as shown in Fig. 9 (b). Under negative biases, the 

current value does not depend on the size of the aggregations and is non-uniform in individual 

aggregations, as seen in the higher-resolution image of Fig. 9 (d). The current pattern in Fig. 9 

(d) resulted from the convulsion of pyramidal AFM tip and spherical nc-Si:H aggregation, 

indicating the tip/film contact is the current-limiting factor when a negative bias voltage is 

applied to the tip. The averaged current values are Ī≅37 pA and Ī≅4 pA in Figs. 9 (b) and (c), 

showing a significant asymmetry under the positive and negative biases. By analyzing the 

current images and the asymmetry, we conclude that there is a contact diode between the AFM 

tip and the sample surface, and that any contact barrier that may exist between the film and the 

substrate is not a dominant factor for the current flow. For positive biases on the tip, the contact 

12
1 μm

(a) (b)

(c) (d)

1 μm

Figure 10. (a) An AFM and (b) corresponding C-AFM images taken on the weakly doped 

mixed-phase sample under Vb= +0.75 V. (c) and (d) show an AFM and the C-AFM images 

under Vb= -0.75 V. 

diode of tip/film is forward biased, and hence, the current is limited by the intrinsic resistance of 

the film; however, for negative biases, it is limited by the tip/film contact barrier. 

 The AFM and C-AFM images on the mixed-phase region in the weakly P-doped sample 

are shown in Fig. 10. Similar features to the intrinsic mixed-phased samples are observed, 

indicating that a slight P-doping of ~3×1018at/cm3

 does not affect the structure of the mixedphase Si:H film. The averaged current values in Figs. 10 (b) and (d) are Ī≅170 pA and Ī≅90 pA 

under Vb= +0.75 and Vb= -0.75 V, respectively, indicating that the small amount of P doping 

indeed increased the conductivity of the film. 

 Figure 11 shows the same measurements on the heavily P-doped mixed-phase Si:H 

samples, where both the positive and negative current images show no nanocrystalline 

aggregations formed in this sample. The averaged current values in Figs. 11 (b) and (d) are 

Ī≅130 nA and Ī≅110 nA under Vb= +0.75 V and Vb= -0.75 V, respectively. We also made the CAFM measurement in the a-Si:H region of the heavily doped sample. The images exhibit 

uniform current distribution, and the averaged current values are Ī≅2 nA and Ī≅70 pA under Vb= 

+0.75 V and Vb= -0.75 V, respectively, showing that the conductivity increased dramatically by 

almost two orders of magnitude by the small crystal volume fraction of 5%–10%. 

13
1 μm

(a) (b)

(c) (d)

1 μm

Figure 11. (a) AFM and (b) corresponding C-AFM images taken on the heavily doped mixedphase sample under Vb= +0.75 V. (c) and (d) show the AFM and C-AFM images under Vb= -

0.75 V. 

 Based on the atomic hydrogen enhanced surface diffusion model [2], the cone-shaped 

structure formation can be explained as follows. Once a nanograin is formed, the surface of the 

grain may have a higher sticking coefficient or a lower surface energy for Si radicals such as 

SiH3. This leads to a continuing growth of the nc-Si:H cone, and the growth rate of the nc-Si:H 

cone is higher than the surrounding a-Si:H area. Alternatively, the surface mobility on the 

nanocrystalline surface could be smaller than on the amorphous surface. Unbalanced surface 

diffusion rates could lead to a net flux of Si radicals laterally diffused into the nc-Si:H cone areas, 

resulting in an increased growth rate on the cone surfaces. This unbalanced diffusion model may 

explain X-TEM data of most nanocrystallites observed in the cone-shaped structure. 

 If the formation of nanocrystalline aggregations is caused by preferential growth on the 

crystalline surface, the nanocrystallite dispersion in the heavily P-doped mixed-phase Si:H 

materials could be explained as follows. P atoms or radicals have the tendency to reside on the 

surface of nanocrystallites. As discussed in the literature [18], phosphine can be adsorbed on the 

crystalline Si surface without being dissociated to form a stable layer with pentavalent P bridging 

adjoining Si atoms. The P-radical coverage on the grain surfaces may reduce the possibility of 

new nucleation because of the five-coordination of P. The P-radical coverage on the grain 

surfaces may also reduce the diffusion of Si radicals to grain surface areas. In addition, the Pdoping levels of ~2×1021 at/cm3

 might exceed the solubility of P in Si; therefore, the extra P tends 

to segregate on the grain boundaries and the surface of nanocrystallites. These mechanisms 

could prevent new nucleation on the existing grain surface. Therefore, new nucleation must 

14
occur in the area without existing grains, where the formation energy of nucleation should be 

higher than the nanocrystallite surface in the intrinsic materials. In this case, a higher hydrogen 

dilution for nanocrystalline formation is needed and the nanocrystallites should dispersively 

distribute in the amorphous matrix. In the case of the weakly P-doped mixed-phase sample, the 

doping level of 3×1018 at/cm3

 is well below the solubility of P in Si. The small amount of P is 

probably not sufficient to effectively prevent the nucleation on the existing grain surfaces to form 

nanocrystalline aggregations. Regarding the dramatic increase in current by the 5%–10% crystal 

volume fraction measured by Raman spectra, our X-TEM images show randomly distributed 

nanocrystllites embedded in the a-Si:H matrix; therefore, the possibility can be excluded of wellaligned nanocrystallite arrays along the film’s vertical direction to form micro-filaments. The 

second possibility is the nonuniform distribution of P atoms. However, our high-resolution TEM 

and energy-dispersion spectra did not find any considerable P clusters, indicating that P does not 

aggregate within the scale of the measurement resolution. A highly speculative hypothesis is that 

P atoms may form a percolation of the narrow atomic chains in the regions of grain boundaries, 

leading to the high conduction paths. The existence of a small amount of nanograins provides 

the grain boundaries for P segregations to form P atomic chains. The last possibility is that the 

real crystal volume fraction is higher than detected by the Raman technique, because Raman 

scattering only detects grains larger than ~3 nm. 

 In summary, the compositional, structural, and electrical properties of mixed-phase Si:H 

thin films were investigated using SIMS, Raman spectroscopy, X-TEM, AFM, and C-AFM. We 

found that Si:H nanocrystallites in intrinsic and weakly P-doped materials aggregate to coneshaped structures. The current under a positive bias on the tip in the nanocrystalline 

aggregations is significantly larger than that on the surrounding amorphous areas. A heavy P 

doping requires an increased hydrogen dilution to reach the amorphous/nanocrystalline 

transition. No nanocrystalline aggregations were observed in the heavily P-doped mixed-phase 

Si:H materials, where the nanometer-sized grains are dispersed in the amorphous matrix. With a 

5%–10% Raman-detectable crystal volume fraction in the heavily doped film, the conductivity 

showed a dramatic increase by almost two orders in the current amplitude. The formation of the 

cone-shaped aggregation is explained by the atomic hydrogen enhanced surface diffusion model. 

Based on this model, the P-induced dispersion of Si nanocrystallites is explained by the coverage 

of P-related radicals on the existing nanocrystalline surface during the deposition and the P 

segregation in grain boundaries, which prevent new nucleation on the surface of existing 

nanocrystallites. 

15
2. Correlation of hydrogen dilution profiling with material structure and 

solar cell performance 

2. 1. Introduction 

Hydrogen dilution is a key parameter for controlling the transition from a-Si:H to ncSi:H. The structure analyses showed that nc-Si:H films deposited with a constant hydrogen 

dilution normally exhibit an initial amorphous incubation layer, followed by nucleation and 

growth of cone-like nanocrystalline clusters. As the growth of the nanocrystalline conestructures, the crystalline volume fraction and average grain-size increase with the film thickness 

[6,19-21], a phenomenon called nanocrystalline evolution. It was also found that nc-Si:H solar 

cells made with a hydrogen dilution close to the amorphous/nanocrystalline transition showed a 

better performance than those with a higher crystalline volume fraction [13]. The nc-Si:H 

materials with a high crystalline volume fraction normally have a high density of micro-voids 

and cracks, which results in ambient degradation caused by impurity diffusion [22]. It is obvious 

that with a constant hydrogen dilution, it is very difficult to maintain the material structure close 

to the amorphous/nanocrystalline transition regime throughout the sample thickness. If the 

hydrogen dilution is low, there will be a thick amorphous incubation layer; on the other hand, if 

the hydrogen dilution is high, the top layer of the material will have a high crystalline volume 

fraction with a high porosity. In order to suppress the nanocrystalline evolution phenomenon, we 

have developed a hydrogen dilution profiling method, where the hydrogen dilution ratio is 

dynamically reduced during the deposition, to control the material structure along the growth 

direction [23]. We also showed that this method was an effective way to increase solar cell 

efficiency. Since then, many groups have used the hydrogen dilution techniques and improved 

their nc-Si:H solar cell efficiency [24-26]. Recent materials characterizations with Raman 

spectroscopy indeed showed that the crystalline volume fraction was controlled. In some cases, 

the crystalline volume fraction was suppressed with increasing film thickness [27]. Drive-level 

capacitance profile (DLCP) measurements demonstrated that defect density was reduced in the 

hydrogen dilution profiled samples. In addition, the defect density distribution along film 

thickness was flatter in the samples made with hydrogen dilution profiling than with constant 

hydrogen dilution [28]. Therefore, it is conclusive that proper hydrogen dilution is an effective 

method to increase nc-Si:H solar cell efficiency. However, one can use various hydrogen 

dilution profiles such as different average dilution ratios, different functional forms of dynamical 

flow rates of the active gas and hydrogen, and different ranges of the dilution. Furthermore, one 

optimized hydrogen dilution profile obtained under a given condition does not necessarily apply 

to other conditions. Therefore, a systematic study of material structure and solar cell 

performance as a function of hydrogen dilution profile should improve our understanding of the 

nc-Si:H growth mechanism and materials properties and provide additional information for 

optimizing nc-Si:H solar cell performance. 

Under this program, we have carried out a systematic study of material structures and 

solar cell performance for nc-Si:H solar cells made with various hydrogen dilution profiles. The 

material structure characterizations were carried out at NREL. 

16
2. 2. Experimental details 

nc-Si:H n-i-p solar cells were made on bare stainless steel (SS) and Ag/ZnO back 

reflector coated SS substrates. The doped layers as well as n/i and i/p buffer layers were 

deposited using RF glow discharge under the same conditions for all samples. The nc-Si:H 

intrinsic layer was made using modified very high frequency (MVHF) glow discharge at high 

rates with various hydrogen dilution profiles. Before nc-Si:H deposition, an optimized seeding 

layer was made with a very high hydrogen dilution to reduce the amorphous incubation layer. 

The solar cell performance was characterized using current density versus voltage (J-V) 

measurements under an AM1.5 solar simulator at 25°C and quantum efficiency (QE) 

measurements under the short-circuit condition for short-circuit current density (Jsc) calibration. 

The material structure properties were characterized using Raman spectroscopy, cross-sectional 

transmission electron microscopy (X-TEM), atomic force microscopy (AFM), and conductive 

AFM (C-AFM) at NREL. 

2. 3. Results and discussion

2.3.1. Material structures

First, we made four nc-Si:H solar cells on bare SS to compare the effect of hydrogen 

dilution profiling on solar cell performance, electronic structure, and microstructure. During the 

intrinsic layer growth, the hydrogen flow rate was kept constant and the same value was used for 

all the samples, whereas the Si2H6 flow rate varied as different functions of time as illustrated in 

Fig. 12. The identification numbers of the four samples are listed in Table I. The total amount 

of Si2H6 flow during the intrinsic layer deposition was the same for all the samples, and therefore 

the average hydrogen dilution ratio was the same. Because hydrogen dilution is normally 

defined as the ratio of H2/SiH4 or H2/Si2H6, the hydrogen dilution functions are actually the 

inverse functions of the curves in Fig. 12. Compared to the constant hydrogen dilution, the three 

ramping curves result in a high initial hydrogen dilution and it continually decreases with time. 

Among the three ramping curves, the t2 flow rate curve has the most rapid change in the dilution 

ratio, corresponding to the highest hydrogen dilution at the beginning of the deposition (the 

region close to the substrate) and the lowest hydrogen dilution at the end of the deposition (close 

to the top surface). The linear curve gives moderate ramping and the t1/2 curve produces the 

slowest ramping. 

17
0 10 20 30 40 50

Time (min)

Si2H6 Flow 

60

Square

Square root

Linear

Constant

Figure 12. Si2H6 flow rate as functions of time during the nc-Si:H intrinsic layer depositions of 

the four samples on SS substrates. 

The main purpose of the hydrogen dilution profiling is to control the nanocrystalline 

evolution and to obtain uniform crystalline volume fraction along the growth direction. 

Normally, an increase of crystalline volume fraction with thickness was observed when constant 

hydrogen dilution was used [6,19-21]. Based on the X-TEM observation, a cone growth model 

for the nanocrystalline evolution has been proposed [1,4]. In this model, under constant 

hydrogen dilution, higher than a threshold value, nanocrystallite nucleation appears after an 

amorphous incubation layer and then nanocrystalline cones grow with the thickness of the film 

until the neighboring cones meet each other. Therefore, in the initial growth of nc-Si:H, the 

crystalline volume fraction increases with the film thickness. In addition, new nucleation 

formations during the deposition could lead to extra contributions for the increase of crystalline 

volume fraction. The concept of hydrogen dilution profiling is to have a very high hydrogen 

dilution to produce large amounts of nucleation sites in the beginning of deposition and reduce 

the hydrogen dilution ratio to control the cone growth and reduce the probability of new 

nucleation sites. Ideally, an optimized hydrogen dilution profiling would result in uniform 

crystalline distribution along the film thickness. However, an over-profiled hydrogen dilution 

might cause an inversed nanocrystalline evolution, which means that the bottom layer of the film 

has a higher crystalline volume fraction than the top layer of the film. 

To check whether hydrogen dilution profiling controls the material structures, Raman 

measurements were made on four samples. Figure 13 shows the Raman spectra of the four ncSi:H samples excited by (a) a green laser with a wavelength of 532 nm and by (b) a red laser 

with a wavelength of 633 nm. Normally, the Si Raman TO mode in nc-Si:H can be deconvoluted into three components: a broad amorphous peak at ~480 cm-1, an intermediate peak 

18
at ~510 cm-1, and a crystalline peak at ~520 cm-1. The intermediate peak could be assigned to 

the scattering from distorted Si-Si bonds in the grain boundaries or in the grains with a size 

smaller than 3 nm. The crystalline volume fraction (fc) is calculated from the ratio of the 

component areas obtained by de-convolution of Raman spectra, 

520 510 480

520 510

III

II f c ++ γ

+ = , (1) 

Where γ is the ratio of Raman emission cross sections of crystalline and amorphous components 

[29-31]. Because the estimation of γ is complicated and no agreeable value in the literature is 

given, we simply use γ=1 for the calculation of Raman crystallinity. Although this may 

introduce errors in the absolute crystalline volume fraction values, nevertheless, it can give a 

reasonable comparison of crystal volume fraction between samples. 

 Table I lists the results of Raman spectrum de-convolutions and the crystalline volume 

fraction for the four samples with the excitation lasers used. Normally, one believes that the 

green laser probes the material structure in the top surface region and the red laser measures the 

material structure in the bulk of the film. It is surprising that the sample with a constant 

hydrogen dilution shows very similar crystalline volume fractions measured by the green and the 

red lasers, indicating uniform crystallinity distribution along the film growth direction. Two 

possible explanations can be proposed for this observation. First, there is indeed no 

nanocrystalline evolution because of the well-optimized seeding layer. Second, the measurement 

with the green laser was influenced by the a-Si:H i/p buffer layer. In order to distinguish the two 

mechanisms, additional measurements using different techniques are needed. The sample made 

with the t1/2 Si2H6 ramping also shows a reasonably uniform crystalline volume fraction. The 

samples made with linear and t2 Si2H6 ramping show inversed nanocrystalline evolution, where 

the crystalline volume fraction measured with the green laser is much smaller than with the red 

laser. In particular, the sample with the t2 ramping shows a very large amorphous peak as shown 

in the top plot in Fig. 13. 

Table I. Raman spectrum de-composition parameters and crystalline volume fractions of nc-Si:H 

samples made with different hydrogen dilution profiles, where a, i, and c represent the 

amorphous, intermediate, and crystalline peaks. 

Area Peak (cm-1 Run ) 

No. 

λex.

(nm) a i c a i c 

fc

(%) 

Si2H6

profiling 

15117 532 21.8 5.0 7.9 479.9 508.5 519.0 37.2 

632.8 11.1 4.8 7.1 482.5 513.1 520.5 51.8 

Linear 

15121 532 43.0 14.4 5.6 470.8 499.5 519.0 31.7 

632.8 15.1 4.9 6.5 482.5 514.0 521.4 43.0 

t

2 

15123 532 12.7 5.3 8.3 481.4 510.0 519.0 51.7 

632.8 8.5 4.8 6.2 483.5 514.9 521.4 56.3 

t

1/2 

15125 532 9.1 4.9 7.9 487.4 513.0 520.5 58.4 

632.8 7.3 5.1 6.5 487.2 514.9 521.4 61.4 

Constant 

19
 In order to confirm the Raman results, the samples were characterized using X-TEM at 

NREL. The X-TEM samples were prepared by the focused ion beam (FIB) technique. A 50-nm 

thick Pt layer was deposited on the sample surface and an 80-nm-thick slice of the sample was 

cut off using the FIB. Figure 14 shows the medium resolution X-TEM pictures of the four 

samples. It appears that all of the samples show some elongated structures perpendicular to the 

substrates. Based on the analyses in the literature [1,11], these structures are assigned to large 

nanocrystalline clusters called large grains. If this assignment is correct, the following 

observations are made: 

1. There is no incubation layer in the n/i interface region. The 70-nm thick smooth region 

near the substrate is the n layer and the a-Si:H buffer layer. 

2. The samples made with the constant hydrogen dilution and the t1/2 Si2H6 ramping showed 

uniform nanocrystalline distributions throughout the film thickness, consistent with the 

Raman measurement. 

3. By carefully evaluating the images, one can see that the sample made with the constant 

hydrogen dilution (15125) shows a certain degree of extra elongated structures in the top 

half of the sample than the bottom half of the sample. The sample made with the t1/2

Si2H6 ramping (15123) shows an opposite trend, which means that the crystalline volume 

fraction slightly decreases with the film thickness, as we expected. 

4. The inversed nanocrystalline evolution becomes clear for the samples with the linear and 

t

2

 ramping (15121, 15123). In particular, the sample made with the t2 ramping has a 200-

300-nm thick layer in the top region with no indication of nanocrystallite inclusions. 

High resolution images reveal that the top layer is indeed amorphous silicon. 

 To summarize the Raman and X-TEM measurements, we find that the sample made with 

constant hydrogen dilution shows no obvious nanocrystalline evolution, which might result from 

the optimized seeding layer. The hydrogen dilution profiles result in an inversed nanocrystalline 

evolution, where the crystalline volume fraction decreases with film thickness. The t2 of Si2H6

profile leads to an amorphous layer in the top region of the film. 

Figure 15 shows (upper picture) the AFM image on the top surfaces and (lower picture) 

C-AFM image of the same areas on the nc-Si:H solar cell made with constant hydrogen dilution. 

The C-AFM image was taken with 1-V positive electrical bias. From the topographic image, one 

can see that many hill-like structures are distributed on the surface. We believe that they are 

clusters of nanocrystallites. The C-AFM image shows many high current spikes on the surface, 

which are normally observed in nc-Si:H solar cells. Figure 16 shows similar images taken on the 

surface of the sample made with t2

 ramping of Si2H6. The AFM topography is similar to the one 

made with constant hydrogen dilution. The current spikes in the C-AFM image of the sample 

with t2

 Si2H6 profile are much smaller than those on the sample with the constant hydrogen 

dilution. Our previous results showed that a thick a-Si:H p/i buffer layer effectively reduces the 

shunt current [8]. The extra amorphous layer caused by the hydrogen dilution profile acts as an 

a-Si:H buffer layer. 

2.3.2. Solar cell performance

 We used hydrogen dilution profiles similar to those listed in Table I for nc-Si:H singlejunction solar cells on Ag/ZnO coated SS substrates. Table II lists the cell performance data of 

the cells with different profiles of Si2H6 flow rate. The first four cells were made with the same 

20
average hydrogen dilution ratio. It appears that the cell with a constant hydrogen dilution shows 

the lowest efficiency due to a lower Voc and poorer FF, which could be due to a high crystallinity 

near the i/p interface. The cell made with a Si2H6 flow rate following the t2

 function also shows 

a poor performance. The FF is lower due to a high series resistance caused by the amorphous 

component near the i/p interface as shown by the Raman and X-TEM results. The linear and t1/2

profiles of Si2H6 flow rate improved the cell performance remarkably; in particular the cell with 

the t1/2 Si2H6 ramping showed an efficiency of 8.7%. We further optimized the average 

hydrogen dilution with the t1/2 profile and improved the efficiency to 9.1% as listed in the last 

row of Table II. 

2. 4. Summary 

 We have systematically studied the structural properties of nc-Si:H materials made with 

various hydrogen dilution profiles using Raman spectroscopy, X-TEM, AFM, and C-AFM. The 

solar cell performance correlates well with the material structures. Based on the experimental 

results, we make the following conclusions: 

1. The nc-Si:H materials do not have an incubation layer, indicating that the seeding layer is 

well optimized and works as per design. 

2. With the optimized seeding layer, no significant nanocrystalline evolution is observed 

even though a constant hydrogen dilution was used. 

3. Hydrogen dilution profiling can control the microstructure. The best nc-Si:H singlejunction solar cell was made using a hydrogen dilution profile, consisting of a constant 

hydrogen flow rate and an increasing Si2H6 flow rate following a t1/2 function. 

 If the hydrogen dilution ratio decreases too steeply, an inversed nanocrystalline evolution 

can occur. In general, a slightly inversed nanocrystalline evolution should benefit the cell 

performance. However, too thick of an amorphous component in the i/p interface region causes 

additional series resistance and results in poorer cell performance. 

Table II. J-V characteristics of nc-Si:H single junction solar cells made with different Si2H6 flow 

rate profiles in the intrinsic layers. The first four cells have the same average dilution ratio and 

the last one has a reduced hydrogen dilution ratio. 

Run 

No. 

Voc

(V) 

FF Q (>610nm) 

(mA/cm2

) 

Pmax

(mW/cm2

) 

Si2H6

ramping 

15589 0.558 0.505 25.68(14.24) 7.24 Linear 

15590 0.539 0.611 26.32(14.60) 8.67 t

0.5

15592 0.572 0.385 25.33(14.08) 5.56 t

2

15593 0.528 0.388 25.43(13.79) 5.21 Constant 

15591 0.567 0.631 25.50(13.87) 9.12 t

0.5

21
-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

300 350 400 450 500 550 600

Wave number (cm-1)

Raman Intensity (a.u.) a

RF15121

RF15117

RF15123

RF15125

(a)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

300 350 400 450 500 550 600

Wave number (cm-1)

Raman Intensity (a. u.) a

RF15121

RF15117

RF15123

RF15125

(b)

Figure 13. Raman spectra of four nc-Si:H solar cells made with different hydrogen 

dilution profiles. The measurements were made with (a) a green laser of 532-nm 

wavelength and (b) a red laser with 632.8-nm wavelength. 

22
Figure 14. X-TEM images of the four samples made with different Si2H6 ramping, where the upper 

left picture is from the sample made with Si2H6 ramped with linear function, the upper right Si2H6

ramped with t2 function, the lower left Si2H6 ramped with t1/2 function, and the lower right a 

constant Si2H6 flow rate. 

23
Figure 15. The upper picture shows an AFM morphology image and the 

lower one is the corresponding C-AFM image on a nc-Si:H solar cell made 

with a constant hydrogen dilution (sample 15125). 

24
Figure 16. The upper picture shows an AFM morphology image and the 

lower one is the corresponding C-AFM image on a nc-Si:H solar cell made 

with a t2

 ramping of Si2H6 (sample 15121). 

25
3. Optimization of Ag/ZnO back reflector 

3. 1. Introduction 

High efficiency thin film solar cells need not only high quality active semiconductor 

layers, but also inactive contact layers such as top transparent contact and bottom contact. At 

United Solar, high efficiency a-Si:H and a-SiGe:H alloys, and nc-Si:H solar cells are deposited 

on Al/ZnO or Ag/ZnO back reflector coated stainless steel substrates. The function of the metal 

coating is to reflect the light reaching the substrate, and the ZnO enhances the scattering of light 

through increased surface texture. United Solar’s back reflectors have been developed since the 

early 90s [32]. We have successfully implemented the optimized back reflectors in several rollto-roll manufacturing lines. In recent years, nc-Si:H based solar cells have gradually dominated 

the research and development of high efficiency silicon based thin film solar cells because of 

their higher current density and better stability than the a-Si:H and a-SiGe:H cells. The multijunction solar cells with a-Si:H in the top cell and nc-Si:H bottom cell promise higher 

efficiencies than the conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. We have 

achieved an initial and stable active-area efficiency of 15.1% and 13.3%, respectively, using the 

a-Si:H/a-SiGe:H/nc-Si:H and a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells [33, 34]. These 

results have surpassed the records achieved using the conventional a-Si:H/a-SiGe:H/a-SiGe:H 

triple-junction structure [35]. The high efficiency was partially due to an improved back 

reflector with more effective light trapping [36]. 

 For thin film solar cells, when light reaches the substrates, it is either reflected back to the 

cell or absorbed by the substrate. In order to reduce the absorption in the substrate and enhance 

the reflection, metal layers such as Al and Ag are used as a back reflector, and the metal surface 

is textured for enhanced light scattering. In addition, a dielectric layer such as ZnO is used for 

additional texturing and more efficient light scattering. Furthermore, it has been reported that a 

flat metal layer and a textured ZnO layer are the best combination because the flat metal surface 

can reduce the plasmon absorption at the metal/dielectric interface [37]. Although a significant 

amount of work has been done on the optimization of the light trapping effect [32, 36], a reliable 

calculation method of the optical enhancement, defined as the number of paths that light can 

travel in the solar cells, has not been established. An early work by Yablonovitch showed that 

the maximum optical enhancement is 4n

2 [38], where n is the optical index of the thin film 

semiconductor layer. The 4n

2

 factor is called Yablonovitch enhancement for stochastic light 

trapping. In silicon, this number is about 50. 

 Under this program, we have made a systematic study of the Ag/ZnO back reflector 

deposition and characterization. We deposited nc-Si:H and a-SiGe:H single-junction solar cells 

on various back reflectors and correlated the cell performance to the back reflector properties. 

The QE curves of nc-Si:H and a-SiGe:H single-junction cells on various substrates were 

compared. We proposed an analysis method for estimating the optical enhancement by Ag/ZnO 

back reflector substrates. 

26
3. 2. Experimental details 

Ag/ZnO back reflectors were deposited in a batch machine with sputtering under 

different conditions. Two types of Ag layers were used: a textured Ag layer similar to our 

conventional Ag/ZnO and a flat, specular Ag layer reduce the parasitical loss at the Ag/ZnO 

interface. Different ZnO thicknesses were used to increase the texture. The material structures 

were measured at NREL using X-ray diffraction (XRD) and atomic force microscopy (AFM). 

The optical properties were characterized by total reflection measurements. 

We deposited a-SiGe:H and nc-Si:H single-junction solar cells using a multi-chamber 

glow discharge system on different back reflectors. Current-density versus voltage (J-V) 

characteristics were measured under an AM1.5 solar simulator and QE curves were measured 

under the short circuit condition. 

3. 3. Results and discussion 

3.3.1. Structural analysis

 Table III list the Ag/ZnO back reflector samples characterized at NREL using XRD and 

AFM. Figure 17 shows the XRD spectra of the baseline sample R1938. From the plot in a wide 

range (Fig. 17 (a)), eight peaks were observed with the identifications marked in the plot. 

Table III. Ag/ZnO structural properties. 

Sample ID ZnO thickness ZnO Ts Ag state 

(μm) 

5R1938 0.8 High Textured 

5R2040 0.6 High Textured 

5R2141 1.8 High Textured 

R8272 0.5 Low Textured 

R8273 1.1 Low Textured 

R8283 1.5 Low Textured 

R8286 2.0 Low Textured 

R8322 3.0 Low Textured 

R8289 0.5 Low Flat 

R8290 1.0 Low Flat 

R8306 1.5 Low Flat 

R8314 2.0 Low Flat 

R8316 3.0 Low Flat 

R8568 0.5 High Flat 

R8578 2.0 High Flat 

R8567 0.5 High Textured 

R8483 2.0 High Textured 

R8562 2.0 High Textured 

R8342 2.0 High Textured 

27
0

5000

10000

15000

20000

25000

25 35 45 55 65 75 85

2θ (Degree)

XRD Density (CPS)

5BR1938

ZnO: (002)Ag: (110)

SS

SS

ZnO: (103)

SS

ZnO: (004)

Ag: (113)

0

5000

10000

15000

20000

25000

32 33 34 35 36 37 38 39 40

2 (Degree)

XRD Density (CPS)

5BR1938

ZnO: (002)

Ag: (110)

Figure 17. XRD spectra of BR1938 (a) in a wide range and (b) in a narrow range. 

28
 

500 nm

Si

Ag

ZnO

Pt cap layer

Figure 18. X-TEM image of a Ag/ZnO back reflector deposited on c-Si substrate. 

 It appears that the ZnO layer has a strong (002) preferential orientation and Ag layer with 

(111) preferential orientation (Fig.17 (b)). We have estimated the grain size from the full width 

at the half maximum of the XRD peaks and found that the calculated grain sizes are in the range 

of 17-25 nm without a clear trend in film thickness. The Ag/ZnO structures were also 

characterized with X-TEM analysis. Figure 18 shows an X-TEM of Ag/ZnO layers deposited on 

c-Si. The image shows that the ZnO grain sizes are much larger than estimated from XRD. 

There are two possible explanations of the different grain sizes measured by XRD and X-TEM. 

First, it could be that the XRD peak width is not determined by the grain size but something else 

such as the stress in the films. Second, the large grains appeared in the X-TEM images contain 

many small grains as measured by XRD. From the X-TEM image, one can see that the ZnO 

crystals have a columnar structure with the vertical grain size similar to the film thickness and 

the lateral size ranging up to 0.3 μm. The lateral grain size determines the texture of the ZnO 

surface as seeing by the Pt cap layer. 

The Ag/ZnO back reflectors were also measured using AFM for surface roughness 

analysis. Figure 19 plots the AFM images of two samples. The top and bottom images were 

taken from R2040 and R2141. The two samples were deposited under the same conditions 

except the ZnO layer thickness is 0.6 μm in R2040 and 1.8 μm in R2141. From the AFM 

images, one can see that both the vertical roughness and the lateral sizes of the features are much 

larger in the thicker sample than the thinner one. Figure 20 plots the RMS roughness versus the 

ZnO thickness. For the series of samples with flat Ag, the RMS roughness linearly increases 

with ZnO thickness from 0.5 to 2 μm and then levels off. The RMS roughness of the samples on 

textured Ag shows an initial roughness from the textured Ag layer, increases in the range up to 

2.0 μm, and levels off for samples with even thicker ZnO layer. The samples with ZnO 

29
Figure 19. AFM images of Ag/ZnO back reflectors. The top image was taken from R2040 

and the bottom one from R2141. The sample properties are listed in Table III. 

30
0

10

20

30

40

50

60

70

80

0123

ZnO Thickness (μm)

RMS (nm)

4

Texture Ag, Low Ts ZnO

Flat Ag, Low Ts ZnO

Texture Ag, High Ts ZnO

Figure 20. RMS roughness as a function of ZnO layer thickness in Ag/ZnO back reflectors. 

deposited at a high temperature show more texture than the low temperature ones. If the RMS 

roughness is a critical parameter for light trapping in solar cells, a 2-μm-thick thick ZnO is 

probably the optimal thickness for the back reflectors. 

The RMS roughness only shows the vertical changes of the surface morphology. 

However, the lateral scale of the textured features is also important for light trapping in solar 

cells. There are two methods for the lateral size representation. The first is the autocovariance 

function, which is calculated by the following equation, 

= [ Δ+Δ+ yyxxhyxhyxc ),(),(),( ] , (2) 

where h(x,y) is the surface height at position (x,y) and 〈 〉 means an average over the entire 

surface for Δx and Δy. The mathematical definition of the correlation length is the distance over 

which the correlation function along a given direction decreases by a factor of 1/e=0.37. The 

important parameter is the minimum correlation length, which is the minimum value of the 

correlation length in all the directions. The minimum correlation length represents the lateral 

feature size on a rough surface. Figure 21 plots the minimum correlation length as a function of 

ZnO layer thickness. It shows that the lateral size increases with the ZnO thickness. The initial 

lateral size is smaller on the samples with a flat Ag layer than with a textured Ag layer because 

the textured Ag also contributes to the feature size. The lateral size continually increases with 

the ZnO layer thickness without the tendency of saturation as shown in the RMS plots in Fig. 20. 

31
0

50

100

150

200

250

300

350

400

0123

ZnO thickness (μm)

Correlation length (nm)

4

Texture Ag

Flat Ag

Figure 21. Minimum correlation length of Ag/ZnO back reflector versus the thickness of the 

ZnO layer. 

To summarize the observations, we found that both the vertical and lateral sizes of microfeatures on the Ag/ZnO back reflectors increases with the ZnO thickness. The vertical roughness 

saturates when the ZnO thickness is over 2 μm, but the lateral size does not show the saturation 

trend. It may imply that the slope of the micro-features decreases with the ZnO thickness. The 

X-TEM image shows that the ZnO grain size is much larger than estimated from XRD spectrum, 

indicating the XRD peak is not determined by the grain size, but something else such as the 

stress in the film; or the large grains contain many nanometer sized grains similar as observed in 

nc-Si:H films. 

3.3.2. a-SiGe:H solar cells on improved Ag/ZnO back reflectors

 In this project, we first optimized a-SiGe:H bottom cells on SS substrates. Table IV lists 

the J-V characteristics of a-SiGe:H bottom cells on SS substrates. Normally we characterize aSiGe:H bottom cells using both the AM1.5 and red lights. For a-SiGe:H bottom cells on SS, a 

530-nm long pass filter is used. A benchmark of Pmax under the red light illumination is 4 

mW/cm2. Although the cell performance has not reached the best results achieved previously 

[39], the cells showed very good performances. We also listed the FF measured under weak blue 

and red lights from a white light source through narrow band pass filters. The blue light provides 

information for the region near the i/p interface, while the red light measures the quality of the 

32
Table IV. Typical J-V characteristics of a-SiGe:H bottom cells on stainless steel substrates, 

where the data were measured under AM1.5 illumination and with a 530-nm long-pass filter. 

The Jsc data were calculated from convoluting the QE curves with the AM1.5 spectrum. The FF 

values under the blue and red lights were measured under narrow band pass filters. 

Sample FF 

# 

Light 

source 

Pmax

(mW/cm2

) 

Jsc

(mA/cm2

) 

Voc

(V) AM1.5 Blue Red 

16332 >530 nm 4.42 10.81 0.662 0.618 

AM1.5 7.08 16.71 0.679 0.624 0.694 0.635 

16337 >530 nm 4.46 11.11 0.662 0.607 

AM1.5 6.98 16.88 0.677 0.611 0.699 0.640 

16388 >530 nm 4.45 11.06 0.665 0.601 

AM1.5 6.88 16.75 0.680 0.604 0.682 0.624 

16339 >530 nm 4.40 10.48 0.660 0.636 

AM1.5 6.53 16.12 0.675 0.600 0.701 0.665 

16366 >530 nm 4.52 10.90 0.666 0.622 

AM1.5 7.23 16.90 0.682 0.627 0.692 0.640 

intrinsic layer. It appears that the red FF is much lower than the blue FF, indicating that the 

material quality in the intrinsic layer needs to be improved further. In addition, a proper Ge 

profiling may also improve the cell performance further. 

 We used the recipe of a-SiGe:H bottom cells on SS to start optimizing cells on Ag/ZnO 

back reflectors. Table V lists the J-V characteristics of a-SiGe:H bottom cells made with the 

same recipe as Sample 16332. The first two samples in Table V serve as baseline samples. The 

Ag/ZnO back reflector from a roll-to-roll machine was used for Sample 16387. Then, we 

compared the cell performance for the cells on Ag/ZnO back reflectors with flat and textured Ag 

layers. First, a pair of cells (16382 and 16380) on the Ag/ZnO back reflectors with a thinner 

ZnO showed that the textured Ag layer produces a much higher Jsc than the flat Ag layer, 

especially for the long wavelength region. However, when a thick ZnO layer was used, the 

Ag/ZnO with the flat Ag layer produced a similar or slightly higher Jsc than the textured Ag 

layer. 

 In order to investigate the optical loss mechanisms of a-SiGe:H bottom cells on various 

Ag/ZnO back reflectors, we measured the total reflection of the substrates using an integrating 

sphere. Figure 22 (a) shows the total reflection spectra of various substrates. First, the SS has a 

relatively flat reflection spectrum with a reflectivity ranging from 0.5-0.6, which slightly 

increases with the increase of wavelength. The Ag/ZnO back reflectors show a large reflection 

minimum in the short wavelength region around 350-400 nm, which has been identified as the 

plasmon absorption at the Ag/ZnO interface. It appears that the flat Ag results in a higher 

reflection than the textured Ag layer, presumably caused by the reduced plasmon absorption. 

The enhanced reflection by the flat Ag is observed in both cases with thinner and thicker ZnO 

layers. In addition to the loss by plasmon absorption, the absorption in the ZnO layer also plays 

a significant role, especially in the long wavelength region. The lower reflectivity of the 

Ag/ZnO back reflectors with thicker ZnO could be mainly caused by the extra absorption of 

ZnO. Figure 22 (b) plots the average reflections in the short (300-650 nm), long (650-1000 nm) 

33
and entire (300-1000 nm) wavelength regions. On average, the thinner ZnO layer results in 

higher reflection in the whole wavelength region. 

 The light trapping effect not only depends on the total reflection from the back reflector, 

but also on the scattering at the semiconductor/dielectric interface as well as the dielectric/metal 

interface. From Table V, one can see that the cells on the Ag/ZnO back reflectors with a thinner 

ZnO layer showed lower Jsc, especially for the one with a flat Ag layer. It is clear that for the 

Ag/ZnO back reflector with a thin ZnO layer, the texture of the Ag layer is very important 

because it provides the effect of scattering. Figure 23 (a) compares the quantum efficiency and 

reflection curves of the two cells on Ag/ZnO back reflectors with the thinner ZnO layer. The 

long wavelength reflection is much higher from the cell with a flat Ag layer than from the one 

with a textured Ag layer. Correspondingly, the long wavelength quantum efficiency is lower in 

the cell with a flat Ag layer than that with a textured Ag layer. 

Table V. J-V data of a-SiGe:H bottom cells made with the same recipe as in Sample 16332 but 

on flat stainless steel and different Ag/ZnO back reflectors. The long wavelength performance 

was measured with a 530-nm long pass filter for the cells on stainless steel substrates and with a 

630-nm long pass filter for the cells on Ag/ZnO back reflectors. 

Run # Light Jsc 

(mA/cm2) 

Voc

(V) 

FF Pmax

(mW/cm2) 

Substrate Comment 

16332 AM1.5 16.71 0.679 0.624 7.08 

>530 nm 10.81 0.662 0.618 4.42 

SS Flat 

16366 AM1.5 16.91 0.682 0.627 7.23 

>530 nm 10.90 0.666 0.627 4.52 

SS Flat 

16387 AM1.5 22.81 0.680 0.599 9.29 

>630 nm 10.58 0.646 0.601 4.11 

Ag/ZnO 

5BR2040 

Textured Ag 

0.6 μm ZnO 

16382 AM1.5 21.01 0.662 0.585 8.14 

>630 nm 9.01 0.624 0.585 3.29 

Ag/ZnO 

R8568 

Flat Ag 

0.5 μm ZnO 

16380 AM1.5 22.61 0.672 0.562 8.54 

>630 nm 10.35 0.638 0.569 3.76 

Ag/ZnO 

R8567 

Textured Ag 

0.5 μm ZnO 

16402 AM1.5 23.78 0.689 0.610 9.99 

>630 nm 11.24 0.661 0.619 4.60 

Ag/ZnO 

R8578 

Flat Ag 

2.0 μm ZnO 

16413 AM1.5 23.53 0.686 0.617 9.96 

>630 nm 10.94 0.655 0.613 4.39 

Ag/ZnO 

R8578 

Flat Ag 

2.0 μm ZnO 

16357 AM1.5 23.33 0.699 0.619 10.09 

>630 nm 10.89 0.667 0.619 4.50 

Ag/ZnO 

R8483 

Textured Ag 

2.0 μm ZnO 

16364 AM1.5 23.55 0.696 0.597` 9.79 

>630 nm 11.01 0.665 0.602 4.41 

Ag/ZnO 

R8562 

Textured Ag 

2.0 μm ZnO 

 

34
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

Reflection

Texture Ag/Thin ZnO

Flat Ag/Thin ZnO

Texture Ag/Thick ZnO

Flat Ag/Thick ZnO

Stainless Steel

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Texture Ag

Thin ZnO

 Flat Ag 

Thin ZnO

Texture Ag

Thick ZnO

Flat Ag

Thick ZnO

Structure of Back Reflector

Average Reflection

Total

Short

Long (b)

Figure 22. (a) Total reflection spectra of stainless steel and various Ag/ZnO back reflectors; (b) 

the average reflection for various spectrum regions. 

35
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE

QE-Textured Ag/500 nm ZnO

R-Textured Ag/500 nm ZnO

QE-Flat Ag/500 nm ZnO

R-Flat Ag/500 nm ZnO

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE

QE-Textured Ag/2000 nm ZnO

R-Textured Ag/2000 nm ZnO

QE-Flat Ag/2000 nm ZnO

R-Flat Ag/2000 nm ZnO

(b)

Figure 23. (a) A comparison of QE and R curves of a-SiGe:H solar cells on Ag/ZnO with 

textured Ag and flat Ag layers and with a 500-nm thick ZnO; (b) the same comparison except the 

ZnO layer is 2000-nm thick. 

36
 The comparison of the cells on Ag/ZnO with thicker ZnO layers reveals a different 

picture. From Table V, one can see that the flat Ag layer produces a Jsc similar to (or slightly 

higher than) the cells on Ag/ZnO back reflector with the textured Ag layer; even though the 

scattering from the back reflector with the textured Ag layer is as high as indicated by the 

suppressed interference fringes in Fig. 22 (a). Figure 23 (b) shows the comparisons of the 

quantum efficiency and reflection curves of a-SiGe:H cells on Ag/ZnO back reflectors with 

either flat Ag or textured Ag layer, where the ZnO thickness is 2.0 μm. The QE curves are very 

similar except for clear interference fringes in the sample with flat Ag, indicating lower 

scattering with the flat Ag layer. The reflection curves still show that the cell with the flat Ag 

layer has high reflection in the long wavelength region. For the samples with the thick ZnO, the 

flat Ag has a lower absorption loss in the metal/dielectric interface, but the scattering is also 

lower. On the other hand, the textured Ag layer provides higher scattering, but the absorption is 

higher than the samples with a flat Ag layer. 

 To summarize the quantum efficiency and reflection results, we make the following 

observations. First, for Ag/ZnO back reflectors with a thinner ZnO, the textured Ag is definitely 

a necessary component for providing sufficient light scattering. Second, for the Ag/ZnO back 

reflectors with a thicker ZnO layer, the quantum efficiency is similar for the samples with flat Ag 

and textured Ag layers. The flat Ag layer reduces the plasmon absorption in the metal/dielectric 

interface, but does not provide as high light scattering as obtained from the textured Ag layer. 

Third, a thicker ZnO also improves the Voc slightly, which could be due to the reduction of the 

density of sharp features on the ZnO surface as measured by AFM microscopic images [36]. 

Overall, the long wavelength (>630 nm) Pmax has been improved from 3.8 mW/cm2 to 4.6 

mW/cm2 when the Ag/ZnO is changed from flat Ag and thin ZnO to flat Ag and thick ZnO 

layers. The trend is clear that a flat Ag layer with a thicker ZnO having sufficient texture is the 

most desirable structure for high efficiency solar cells. 

3.3.3. Calculation of optical enhancement in a-SiGe:H cells on Ag/ZnO back reflector

 In order to obtain the optical enhancement factor, we have to establish a baseline for 

reference. Thin film solar cells on flat stainless steel substrates should be good baselines. 

Therefore, the optical enhancement obtained in this way is the effective increase of the 

absorption length for reaching the quantum efficiency ratio of solar cells deposited on back 

reflectors over stainless steel substrates at different wavelengths. The procedure includes a 

comparison of the quantum efficiency curves of a-SiGe:H single-junction solar cells made with 

the same recipe on bare stainless steel substrates and Ag/ZnO back reflectors. A mathematical 

calculation procedure is proposed with certain assumptions. 

 First, we calculate the quantum efficiency as a function of the absorption coefficient and 

cell thickness. Assume N0(λ) of photons with wavelength λ get into the solar cell. The 

absorption coefficient for this wavelength is α. When we assume the generation rate is 100%, 

the density of electron/hole pairs generated at the position of x is, 

= NxN −αλλ x)exp()(),( 0 . (3) 

The amount of electron/hole pairs (photons absorbed) generated between x and x+dx is, 

37
),( −= ααλ )exp( dxxNxdN

L 0

0 . (4) 

For the cells on stainless steel substrates, we assume that the light reaches the substrates and 

reflects back with a reflection coefficient R. We also assume that the reflected light will come 

out of the cell when it reaches the top surface, which means there are only two paths (one can 

consider more paths with a high order polynomial approach). In this case, we integrate eq.(4) 

over the intrinsic layer thickness L for the direct incident beam and the reflection beam and 

obtain, 

∫ ∫ = +− − −−

0

0 0 )( )exp( ))(exp()exp(

L

NN ααλ RNdxx L ααα dxxL . (5) 

λ [ α −+−−= α ]

[ ]2

)))(exp(1))(exp(1()( NN 0 LRL . (6) 

We assume that there is no recombination loss, which means every photon-generated 

electron/hole pair is collected and contributes to the quantum efficiency (QSS) measurements. In 

this case, the QSS is the ratio of absorbed photons to total incident photons. Then, we obtain, 

=−+−−+− 0)1()exp()1()exp( α α QLRLR SS , (7) 

From the positive solution of this quadratic equation, we obtain, 

⎪⎭

⎪

⎬

⎫

⎪⎩

⎪

⎨

⎧ −+−+− −= R

RQRR L SS

2

)1()1(4)1( ln

2

α . (8) 

 In the case of solar cells on a textured back reflector, we assume the reflection coefficient 

at the surface of BR is one hundred percent, and the reflection at the top surface is also one 

hundred percent (total reflection). Although these assumptions are not realistic, the losses at the 

interface (by absorption in the interface or escaping out at the top surface) are considered as the 

reduction of the effective paths of the trapped light. Therefore, the estimated optical 

enhancement is the lowest limitation. With these assumptions, we define an effective enhanced 

absorption length kL, where k is the optical enhancement factor. Based on this definition, we 

have, 

)( [ )exp(1)exp(

0

0 0 = NN ααλ Ndxx −−=− αkL ∫ ]

kL

Q kL)exp(1

. (9) 

Defining the quantum efficiency QBR of the cell on BR as N(λ)/N0, then we have, 

= − −α , (10) BR

and, 

38
L

− Q )1ln( k BR

α −= , (11) 

where αL is obtained from eq.(6) with the QSS data. As pointed out, the problem is oversimplified by the assumptions. In reality, there are always some optical losses at the 

metal/dielectric interface and top surface as well as in the doped layers. In addition, electrical 

loss by recombination in the intrinsic layers is also a factor for the accuracy of the estimation. 

Therefore, the estimation by this method gives the lowest limit of the optical enhancement as 

pointed above. In order to reduce the recombination losses, QSS and QBR data under a reverse 

bias should be used. The comparison to the calculation based on the Qss and QBR data under the 

short-circuit condition will provide the errors caused by the recombination process in the 

intrinsic layer. 

 We have done some calculations of the optical enhancement factor k for a-SiGe:H singlejunction solar cells. Figure 24 (a) shows a comparison of QE curves of a-SiGe:H single-junction 

solar cells made with the same recipe on a flat stainless steel substrate and a Ag/ZnO back 

reflector. The ratio of the two quantum efficiency curves is also plotted on the same figure. It is 

clear that the optical enhancement is mainly in the long wavelength region, because in the short 

wavelength region, the absorption coefficients are large enough to absorb the light within one 

path through the thickness of the intrinsic layer. Figure 24 (b) shows the calculated optical 

enhancement factor k using the procedure presented above, where two reflectivity R values at the 

semiconductor/stainless-steel were used. Normally, we use R=0.2 for the SS/a-Si:H interface. 

With this value, one can see that the maximum optical enhancement factor is about 25. The 

estimated optical enhancement factor strongly depends on the R value. If R=0.5 is used, the 

maximum optical enhancement factor can be as high as 30. In addition, the wavelength 

dependence of R is also a consideration for further improvement in the calculation accuracy. 

Figure 25 shows a similar calculation for a nc-Si:H single-junction solar cells. The optical 

enhancement was calculated using R=0.2. The result shows that the optical enhance for this ncSi:H is not as high as that shown in Fig. 24, which is partially due to the flatter back reflector as 

indicated by the interference fringes. 

3. 4. Summary 

 We have systematically studied the material structures of Ag/ZnO using XRD, AFM, and 

X-TEM at NREL. The microscopic characterizations provided an important guideline for the 

optimization of back reflectors. We correlated the material structure of back reflector to the 

optical properties. Then, we optimized the Ag/ZnO back reflectors to have a high light trapping 

effect. We found that for Ag/ZnO back reflectors with a thinner ZnO layer, a textured Ag layer 

results in a higher short-circuit current density even if the plasmon absorption at the 

metal/dielectric interface is high for the textured Ag layer. The textured Ag provides high light 

scattering, which is the dominant factor for the back reflector with a thinner ZnO layer. For 

thick ZnO, the flat and textured Ag layers showed a similar short circuit current density because 

with the increase of the ZnO thickness, the scattering by the textured ZnO is increased. At the 

same time, the absorption at the metal/dielectric interface is lower. In order to increase the short 

circuit current density further, we shall increase the texture at the ZnO/semiconductor interface. 

39
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE

0

5

10

15

20

25

30

QBR/QSS

Q (16364)-Ag/ZnO

Q (16332)-SS

Q(BR)/Q(SS)

(a)

0

5

10

15

20

25

30

35

300 400 500 600 700 800 900 1000

Wavelength (nm)

k 

R=0.5

R=0.2

(b)

Figure 24. (a) Comparison of QE curves of a-SiGe:H single-junction cells deposited with the 

same recipe on a flat stainless steel substrate and a Ag/ZnO back reflector substrate; (b) the 

optical enhancement calculated using the procedure described in the text. 

40
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE

0

2

4

6

8

10

12

14

16

18

20

QBR/QSS

Q (15122)-Ag/ZnO

Q (15123)-SS

Q(BR)/Q(SS)

0

5

10

15

20

25

30

300 400 500 600 700 800 900 1000

Wavelength (nm)

n 

Figure 25. The upper plot shows the comparison of QE curves of nc-Si:H single-junction cells 

deposited with the same recipe but on a flat stainless steel substrate and a textured Ag/ZnO back 

reflector substrate. The lower plot shows the optical enhancement calculated using the procedure 

described in the previous section, where the reflection coefficient of R=0.2 was used in the 

calculation. 

41
4. High efficiency a-Si:H/a-SiGe:H/nc-Si:H and a-Si:H/nc-Si:H/nc-Si:H 

triple-junction solar cells 

4. 1. Introduction 

We have achieved 15.1% [40] initial active-area efficiency and a stable active-area 

efficiency of 13.0% [35] using a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cells. In order to 

show that using nc-Si:H as the intrinsic layer in the bottom cell of multi-junction solar cells will 

improve the solar module efficiency, one has to demonstrate that the small area solar cells with 

nc-Si:H bottom cell have a high stable cell efficiency than the conventional a-Si:H/a-SiGe:H/aSiGe:H triple-junction structure. We have reported an initial active-area efficiency of 15.1% 

using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure [41], where the nc-Si:H bottom cell 

was made using MVHF at a high rate and the a-Si:H top and a-SiGe:H middle cells were made 

using RF glow discharge at low rates. In this project, we continued to optimize all the elements 

for the triple-junction structure for the highest achievable efficiency. The work covers the 

optimization of the component cells and Ag/ZnO back reflectors. Combining the optimized 

components, we have attained a new record initial active-area efficiency of 15.4%. 

4. 2. Experimental details 

We first optimized the component cells. The a-Si:H top cells were deposited with RF 

glow discharge at a low rate of ~1 Å/s on bare stainless steel substrates. The intrinsic layer and 

the doped layers were optimized under the guideline of producing a Jsc around 9 to 9.5 mA/cm2

for current matching in a-Si:H/a-SiGe:H/nc-Si:H triple-junction structures. The a-SiGe:H 

middle cells were deposited in the same RF system on Ag/ZnO back reflectors. Optimizations 

were carried out mainly for the intrinsic layer. Various parameters were optimized such as 

bandgap profiles, pressure, and hydrogen dilution. The nc-Si:H bottom cells were deposited 

using MVHF at a high rate of ~5 Å/s on Ag/ZnO back reflector coated stainless steel substrates. 

Major optimization efforts were focused on the hydrogen dilution profiling as discussed in the 

previous section. High efficiency a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cells were made 

by combining the optimized component cells and taking care of the tunnel junctions. The solar 

cells were measured under an AM1.5 solar simulator with quantum efficiency (QE) calibration 

for the Jsc. The a-SiGe:H middle cells were also measured under the AM1.5 solar simulator with 

a 610-nm long-pass filter for simulating the situation in the triple-junction structure. 

4. 3. Results and discussion 

4.3.1. a-Si:H top cell

 We first increased the hydrogen dilution to reach the amorphous to nanocrystalline 

transition regime. As previously reported, the geometry of the cathode design causes a nonuniform distribution of crystallinity over the substrate area [14, 15]. It was observed that in the 

amorphous to nanocrystalline transition regime, the nanocrystalline formation appears at the 

corners or edge first, while the center area exhibits more amorphous component. We used this 

phenomenon to monitor the phase transition. Table VI lists two sets of a-Si:H top cells made in 

the transition region. The first three samples were made with a constant hydrogen dilution ratio. 

42
Table VI. J-V characteristics of a-Si:H top cells made with flat hydrogen dilution and profiled 

hydrogen dilution. 

Sample No Position Eff (%) Jsc

(mA/cm2

) 

Voc

(V) 

FF Comments 

Center 6.75 9.34 1.036 0.697 

Middle 6.57 9.55 1.030 0.667 

16341 

Edge 4.12 9.04 0.786 0.580 

Constant H dilution 

Baseline 

Center 6.28 7.97 1.043 0.757 

Middle 6.19 7.98 1.042 0.745 

16345 

Edge 5.61 8.14 0.995 0.693 

Constant H dilution 

with thin i-layer 

Center 5.94 7.77 1.049 0.729 

Middle 5.94 7.79 1.047 0.729 

16346 

Edge 5.40 7.46 1.036 0.698 

Constant H dilution 

with thin i-layer and 

low Ts 

Center 7.30 9.92 1.034 0.712 

Middle 7.12 9.92 1.031 0.692 

16140 

Edge 5.48 9.68 0.880 0.644 

Profiled H dilution 

Baseline 

Center 6.25 8.34 1.027 0.730 

Middle 6.31 8.48 1.030 0.723 

16343 

Edge 5.53 8.36 0.964 0.686 

Profiled H dilution with 

thin i-layer 

Center 6.26 7.92 1.055 0.748 

Middle 5.90 7.60 1.048 0.740 

16344 

Edge 4.2 7.32 0.916 0.626 

Profiled H dilution with 

thin i-layer and low Ts 

 Sample 16341 is a baseline cell, which showed a Voc of 1.036 V in the center, indicating 

good a-Si:H properties. However, at the edge, the Voc drops to 0.786 V, resulting from the 

mixed-phase formation. Sample 16345 was made under the same condition as Sample 16341 

except for a thinner intrinsic layer. The reduced intrinsic layer thicknesses resulted in an Voc

increase to 1.043 V at the center and 0.995 at the edge, as expected from typical observations. 

This also confirms the nanocrystalline evolution with the thickness, especially at the edge. At 

the same time, the FF is also improved. Sample 16346 was made under the same condition as 

Sample 16345 except the substrate temperature (Ts) was reduced by 25 °C. As expected, the 

lower substrate temperature results in an even higher Voc but smaller Jsc. 

 The lower three samples in Table VI were made with a hydrogen dilution profiling, 

where the hydrogen flow rate was linearly decreased with time. The average hydrogen dilution 

of this set was about 10% lower than the first set. When the same average dilution ratio was 

used, the cells made with hydrogen dilution profiling became mixed-phased solar cells, 

indicating that a very high hydrogen dilution at the initial stage of deposition could produce a ncSi:H seed layer for subsequent deposition. Therefore, the average hydrogen dilution has to be 

reduced when a hydrogen dilution profiling is used for a-Si:H deposition. Sample 16140 was a 

baseline cell made with hydrogen dilution profiling, which performs better than Sample 16341 

made with constant hydrogen dilution. Reducing the thickness of the intrinsic layer by simply 

reducing the deposition time with the same ratio of hydrogen profiling did not increase the Voc as 

observed in the samples with the constant hydrogen dilution ratio. An explanation could be that 

43
0.7

0.8

0.9

1

1.1

1980 1985 1990 1995 2000 2005 2010

Year

Voc (V)

Microcrystalline p layer

High H2 dilution

Near the transition

Figure 26. United Solar’s historical improvements in open-circuit voltage in a-Si:H solar cells 

when different technologies were invented and used. 

the initial hydrogen dilution was too high in the profiled sample and the initial growth has more 

effect for the thinner samples than for the thicker samples. By reducing the substrate 

temperature, the Voc is improved dramatically, and an initial Voc of 1.055 V was achieved, which 

is the highest value achieved in our laboratory. Figure 26 shows the historical evolution of Voc

when different processing techniques were used. Normally, it is believed that the Voc is limited 

by the properties of the intrinsic layer and the quality of interfaces, especially the i/p interface. 

The upper limit of Voc is the built-in potential, which is around 1.1-1.2 V. Although we may still 

have room for further improvement, the new record has approached the built-in potential. 

4.3.2. a-SiGe:H middle cell

United Solar has extensive experience in the optimization of a-Si:H and a-SiGe:H 

component cells for a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures [36,40]. Compared to 

the component cells used in the conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure, 

the a-Si:H top and a-SiGe:H middle cells used in the a-Si:H/a-SiGe:H/nc-Si:H structure should 

have higher current to match the nc-Si:H bottom cell. Currently, our nc-Si:H single-junction 

cells on Ag/ZnO show Jsc around 27-28 mA/cm2

. Optimizing a-SiGe:H middle cells to have a 

high current is more difficult than a-Si:H top cells. In general, the defect density is higher in aSiGe:H than in a-Si:H. In the specific design of an a-SiGe:H middle cell in the a-Si:H/a44
SiGe:H/nc-Si:H triple-junction, the relatively higher Jsc makes the cell design more difficult 

because we have to increase the Jsc with minimum loss in the Voc and FF. Table VII lists the J-V 

characteristics of a few a-SiGe:H middle cells during different stages of the optimization. We 

started from a baseline cell (16109) with Pmax ~4 mW/cm2

 and Jsc ~8.9 mA/cm2 under an AM1.5 

solar simulator with a 610-nm long-pass filter. We have optimized the GeH4 and Si2H6 flow 

rates (cell 16112) and profiles and improved the long wavelength Jsc to ~9.5 mA/cm2

, but lost 10 

mV in Voc. We further changed the deposition pressure. Although we gained long wavelength 

current, we lost another ~10 mV in Voc. By optimizing the hydrogen dilution (16130), we 

increased the Jsc, Voc, and FF at the same time. Furthermore, in using the optimized Ag/ZnO 

back reflector as described in the last quarterly report, we increased the long wavelength Jsc to 

~10.9 mA/cm2

 and the long wavelength Pmax to ~5 mW/cm2, which are good numbers for aSiGe:H middle cells. 

4.3.3. nc-Si:H bottom cell

The key contribution for high efficiency a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar 

cells is a high performance nc-Si:H bottom cell. We have put significant effort into the 

optimization of nc-Si:H single-junction solar cells. The major improvement is the quality of the 

nc-Si:H material. The defect density has been reduced remarkably as measured by David 

Cohen’s group [28]. In addition, careful control of the microstructure along the growth direction 

[23] is important as discussed in the previous section. We also used the optimized Ag/ZnO back 

reflectors developed in the last quarter. 

The most important technique of improving nc-Si:H solar cell efficiency is to control the 

nanocrystalline evolution along the growth direction [23]. In section 2, we showed the material 

structure as a function of hydrogen dilution profiling. In order to achieve high efficiency, we 

need not only the optimized nc-Si:H material, but also an optimized the device structure in ncSi:H solar cells, mainly the interface control. 

We previously reported [42] that as the intrinsic layer thickness increases, Voc drops more 

significantly in nc-Si:H cells than in a-Si:H cells. We attributed this to the fact that the 

crystalline volume fraction increases with increasing thickness. We also found that the nc-Si:H 

cell performance strongly depends on the i/p buffer layer [42]. Without a proper buffer layer, the 

nc-Si:H cell shows a poorer FF and a smaller Voc, which, in turn, leads to low conversion 

efficiency. Recently, we found that the metastable behavior after prolonged light soaking in ncSi:H solar cells is different from a-Si:H cells in many cases. Although this is mainly caused by 

the difference in the intrinsic material properties, the interface also has certain potential influence 

on the metastability. In this project, we carried out a systematic study of the effects of the 

intrinsic and i/p buffer layer thicknesses on nc-Si:H cell performance and stability. To avoid the 

microstructure evolution with thickness as mentioned before, the hydrogen dilution profiling 

technique [23] has been used so that the intrinsic layers in all the cells maintain similar 

crystalline volume fraction along the growth direction. The results show that both the cell 

performance and the stability are related to the intrinsic and i/p buffer layer thicknesses. 

nc-Si:H n-i-p single-junction solar cells were deposited using a multi-chamber glow 

discharge system with RF chambers for the doped layers and a MVHF chamber for the intrinsic 

layers at a deposition rate of ~3-5 Å/s. The same hydrogen dilution profile was used for all the 

45
Table VII. Typical J-V characteristics of a-SiGe:H middle cells on Ag/ZnO back reflector for aSi:H/a-SiGe:H/nc-Si:H triple-junction structures. 

Sample 

No. 

Light Jsc 

(mA/cm2) 

Voc

(V) 

FF Pmax

(mW/cm2

)

Comment 

16109 AM1.5 19.94 0.775 0.597 9.23 

>610 nm 8.85 0.743 0.613 4.03 

Previous baseline

16112 AM1.5 20.52 0.761 0.608 9.45 

>610 nm 9.46 0.731 0.598 4.14 

Optimization of 

GeH4 flow rate 

16116 AM1.5 20.84 0.753 0.606 9.51 

>610 nm 9.92 0.724 0.605 4.35 

Optimization of 

pressure 

16130 AM1.5 21.21 0.760 0.632 10.19 

>610 nm 10.18 0.730 0.637 4.73 

Optimization of 

H2 dilution 

16138 AM1.5 21.82 0.766 0.635 10.61 

>610 nm 10.61 0.731 0.650 5.04 

Optimization of 

Ag/ZnO BR 

16141 AM1.5 22.22 0.769 0.610 10.53 

>610 10.88 0.741 0.624 5.03 

Optimization of 

Ag/ZnO BR 

16148 AM1.5 22.07 0.763 0.627 10.56 

>610 nm 10.92 0.735 0.629 5.05 

Optimization of 

Ag/ZnO BR 

intrinsic layer depositions. The same n/i buffer and seeding layers were used for all the samples. 

The samples with different intrinsic layer thicknesses were made by controlling the intrinsic 

layer deposition time. The a-Si:H i/p buffers were made with RF glow discharge at a deposition 

rate of ~1 Å/s. For comparison, high rate a-Si:H cells with different intrinsic layer thicknesses 

were also made with MVHF at a deposition rate of ~6 Å/s. Both nc-Si:H and a-Si:H cells were 

deposited on Ag/ZnO coated stainless steel substrates. The material structure of the intrinsic 

layer was directly measured on the cells with no i/p buffer layer using Raman spectroscopy with 

a green light (532 nm) excitation. Light soaking was carried out under 100 mW/cm2

 of white 

light at 50 o

C under the open circuit condition for 1000 hours. Dark J-V characteristics of the 

cells were measured in a vacuum chamber using a programmable multi-meter at a controlled 

temperature. 

Table VIII lists the J-V characteristics of nine nc-Si:H single-junction solar cells with 

different intrinsic and i/p buffer layer thicknesses. The same hydrogen dilution profile was used 

for all the nc-Si:H intrinsic layer depositions. The different intrinsic layer thicknesses were 

obtained by controlling deposition time as listed in column two. For each intrinsic layer 

thickness, three different thicknesses of a-Si:H i/p buffers with the deposition times of 0, 3, and 7 

minutes (corresponding to thicknesses of 0, 180, and 420 Å) were added to study the effect of 

buffer layer thickness. From Table VIII, the following observations are made. For the effect of 

the intrinsic layer thickness, we found that the FF decreases and the Jsc increases with increasing 

intrinsic layer thickness. The Voc does not change much with intrinsic layer thickness for the 

cells with zero and 3-minute i/p buffer layers. However, for the cells with a 7-minute i/p buffer 

layer, the Voc decreases noticeably with an increase in the intrinsic layer thickness. Also, the 

conversion efficiency increases with the intrinsic layer thickness. For the effect of the i/p buffer 

layer thickness, three points can be made. First, the Voc increases with increasing i/p buffer layer 

thickness. Second, the FF does not improve by adding the i/p buffer layer. For the thin cells, the 

FF is even reduced. Third, the i/p buffer layer has a greater effect on the thin cells than the thick 

46
ones. For example, when the i/p buffer layer thickness was increased from 0 to 7 minutes, the 

Voc increased from 0.463 to 0.532 V for the cells with a 7-minute intrinsic layer, which is 

significantly larger than the increase from 0.455 to 0.483 V for the cells with a 30-minute 

intrinsic layer. This phenomenon is even more pronounced in the FF. For the cells with a 7-

minute intrinsic layer, the FF is reduced from 0.714 to 0.677 when the i/p buffer layer is 

increased from 0 to 7 minutes. However, for the cells with a 30-minute intrinsic layer, the FF is 

not sensitive to the i/p buffer layer thickness. 

As mentioned previously, to control the effect of microstructure evolution on the cell 

performance, we have used the hydrogen dilution profiling technique in the intrinsic layer 

deposition to keep the crystalline volume fraction constant in the material along the growth 

direction. To confirm this, Raman spectroscopy measurements were made on the cells with 

different intrinsic layer thicknesses by using the green laser (532 nm). In order to avoid the 

potential influence of the a-Si:H i/p buffer on measurement results, the cells with no i/p buffer 

layer (samples 14098, 14099, 14100) were selected for the Raman measurements. Figure 27 (a) 

shows the Raman spectra for the three samples. One can clearly see that the three cells have 

very similar crystalline volume fractions. For a detailed analysis, we deconvoluted the Raman 

spectra into three components of amorphous TO (~ 480 cm-1), intermediate (~ 500 cm-1), and 

crystalline (~520 cm-1) modes using three Gaussian functions as shown in Fig. 10 (b). The 

results are summarized in Table IX. The crystalline volume fraction listed in the table is simply 

defined as the ratio of the areas of the crystalline and the intermediate modes to the total. The 

crystalline volume fractions for the cells with 7, 15, and 30 minutes of intrinsic layers are 46.6%, 

56.8%, and 60.9%, respectively. We noted that the volume fraction in the thicker cell is still 

slightly higher than in the thinner one. 

 Dark J-V measurement is a useful tool in studying the recombination mechanism in aSi:H based solar cells. An ideal solar cell follows the typical diode characteristics of 

J=J0[exp(qV/nkT)-1], where J0 is the reverse saturated current density, q the unit charge, T the 

measurement temperature, k the Boltzmann constant, and n the diode quality factor. By studying 

the dark J-V characteristics and examining the deviation of experimental results from the diode 

Table VIII: J-V characteristics of nine nc-Si:H single-junction solar cells on Ag/ZnO substrate 

with different thicknesses of the intrinsic and i/p buffer layers. 

Sample 

No. 

Time (i) 

(min) 

Time (i/p) 

(min) 

Voc

(V) 

FF Jsc 

(mA/cm2) 

Pmax

(mW/cm2

) 

14098 7 0 0.463 0.714 14.54 4.81 

14099 15 0 0.455 0.641 18.71 5.46 

14100 30 0 0.455 0.586 22.49 6.00 

14101 7 3 0.481 0.700 14.18 4.77 

14111 15 3 0.473 0.642 18.78 5.70 

14112 30 3 0.473 0.590 22.87 6.38 

14113 7 7 0.532 0.677 16.00 5.76 

14114 15 7 0.491 0.622 19.73 6.03 

14106 30 7 0.483 0.590 22.82 6.50 

47
300 350 400 450 500 550 600

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

(a)

Intensity (a.u.)

Raman Shift (cm-1)

 14098-7 min

 14099-15 min

 14100-30 min

(b)

Intensity (a.u.)

Raman Shift (cm-1)

 14098-7 min

 a-Si

 intermediate

 c-Si

 Fitting

Figure 27. (a) Comparison of Raman spectra for the samples with different intrinsic thicknesses 

and with no i/p buffer, and (b) an example of a Raman spectrum deconvolution. 

48
Table IX. Deconvolution results of the Raman spectra, where s, p, w, and f denote the area, peak 

position, width, and percentage of each component, respectively. R represents the ratio of the 

areas of the c-Si and intermediate peaks to the total areas of the three peaks. 

Sample amorphous intermediate crystalline 

No. sa pa wa fa(%) si pi wi fi(%) sc pc wc fc(%) 

R 

(%) 

14098 18.1 481 86 53.4 6.2 505 29 18.3 9.6 520 11 28.3 46.6

14099 12.7 479 70 43.2 6.9 507 29 23.5 9.8 520 13 33.3 56.8

14100 10.8 478 69 39.1 6.8 507 29 24.6 10.0 520 13 36.3 60.9

characteristics, one can examine the shunt current and the recombination mechanism of the 

carriers in solar cells. In this study, we used the dark J-V measurement to study the nine nc-Si:H 

solar cells. The results are plotted in Fig. 28. One can see that the dark current density reduces 

with increased i/p buffer layer thickness. The cells with a thinner intrinsic layer show a larger 

reduction in the dark current density. Moreover, we found that the cells with no i/p buffer layer 

have a high shunt current density. When the i/p buffer layer thickness is increased, the shunt 

current density is continually reduced. 

For the cells with a 7-minute i/p buffer, the shunt current density becomes minimized and 

the dark J-V curves basically follow the diode characteristics as shown in Fig. 28 (d). By fitting 

the data of these three samples with the diode characteristics, we obtained J0 and n, as listed in 

Table X. One can see that the diode quality factors are very similar, which means that the carrier 

recombination mechanism is the same for the samples. The saturated current is increased by 

increasing the thickness of the intrinsic layer, indicating a greater recombination rate in the 

thicker samples. Previously, we found that the dark J-V characteristics of a-Si:H solar cells are 

largely independent of the intrinsic layer thickness. This phenomenon could be explained with a 

weak dependence of thickness on the recombination width, which is smaller than the intrinsic 

layer thickness. However, in the nc-Si:H solar cells, the dark J-V characteristics, especially the 

J0, strongly depend on the intrinsic layer thickness. A thicker intrinsic layer results in a larger J0, 

which could indicate that the recombination width is the same as the intrinsic layer thickness. A 

time-of-flight measurement found that the hole mobility in nc-Si:H is over one hundred times 

larger than in a-Si:H [43]. Correspondingly, the hole diffusion length in nc-Si:H solar cells is 

much larger than in a-Si:H solar cells and the improved hole diffusion length leads to a 

recombination width similar to the intrinsic layer thickness. 

Table X. The dark J-V fitting results of three nc-Si:H cells with different intrinsic layer 

thicknesses. The i/p buffer layer is the same for the samples (7 minutes). J0 denotes the reverse 

saturated current, and n the diode quality factor. 

Sample 

No. 

i layer 

deposition time 

J0

(A/cm2

) 

n 

14113 7 min 3×10-8 1.75 

14114 15 min 6×10-8 1.67 

14106 30 min 2×10-7 1.70 

49
1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage (V)

Current Density (A/cm2

)

no i/p

3 min i/p

7 min i/p

I layer = 7 min

(a)

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage (V)

Current Density (A/cm2

)

no i/p

3 min i/p

7 min i/p

I layer = 15 min

(b)

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage (V)

Current Density (A/cm2

)

no i/p

3 min i/p

7 min i/p

I layer=30 min

(c)

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage (V)

Current Density (A/cm2

)

i=7min

i=15 min

i=30 min

Fitting

Fitting

Fitting

7 min i/p buffer

(d)

Figure 28. Dark J-V characteristics of the cells with (a) a 7-minute intrinsic layer but with different 

i/p buffer thicknesses; (b) a 15-minute intrinsic layer but with different i/p buffer layer thicknesses; 

(c) 30-minute intrinsic layer but with different i/p buffer thicknesses; (d) different intrinsic layer 

thicknesses but with the same i/p buffer layer thickness of 7 minutes. 

50
 We now discuss the main points obtained from the experimental results. First, we know 

that the FF usually decreases with an increase in the intrinsic layer thickness because of the 

reduction of built-in electric field and increased recombination rate. The intrinsic layer thickness 

dependence of FF in the nine samples follows this common trend. It is also understandable that 

the Jsc increases with increased intrinsic layer thickness since the light absorption is increased. 

Here, we need to point out that this is true for the cells with a certain thickness range. Beyond 

this range, other factors such as microstructure evolution, carrier collection length, or collapse of 

the built-in electric field, might play dominant roles. Compared to the FF and Jsc, the effect of 

intrinsic layer thickness on the Voc is more complicated. Previously, we found that the Voc

decreases with increasing thickness more dramatically in nc-Si:H than in a-Si:H solar cells [43]. 

One explanation was the evolution of crystalline volume fraction with the intrinsic layer 

thickness. A similar trend is also observed in these nine samples as listed in Table XII. The 

reason for a small Voc change with the intrinsic layer thickness for the cells with no or thin i/p

buffer is probably due to the large shunt current density or a large recombination current density 

in the i/p interface. This is consistent with the results from the dark J-V measurements. With the 

increase of the i/p buffer layer thickness to 7 minutes, the shunt-current or interface 

recombination current was reduced, and the Voc monotonically decreased with the intrinsic layer 

thickness. Moreover, from the Raman measurements, we noticed that the increase of crystalline 

volume fraction with the intrinsic layer thickness is small, which could not be the dominant 

factor for the reduced Voc in the thicker cells made with hydrogen dilution profiling. Instead, the 

significantly increased dark current density could be the main reason for the reduced Voc in the 

thicker cells. Second, we found that the Voc increases with an increase in the i/p buffer layer 

thickness. This is because the a-Si:H i/p buffer layer reduces the shunt current density. This was 

confirmed by the dark J-V measurements as shown in Figure 28 (a), (b), and (c). We know that 

in the light J-V measurement, the Voc is obtained when the dark current equals photo current. 

The larger the dark current is, the smaller the Voc. We also found that the FF is not improved for 

these cells when the buffer layer was added. This is probably because the a-Si:H i/p buffer is too 

thick and introduces an extra series resistance. Another interesting observation is that the i/p

buffer has a larger effect on the thin cells than thick ones. It may indicate that the cell 

performance of the thin samples was dominated by the i/p interface, while the performance of the 

thick samples was dominated by the bulk properties. 

A stability study has been carried out on the nine nc-Si:H solar cells, and the results are 

listed in Table XI. As a comparison, four high rate a-Si:H solar cells with different intrinsic 

layer thicknesses were also studied, and the results are listed in Table XII. Three points have 

been observed from the tables. First, the light-induced degradation rate in nc-Si:H decreases 

with increasing intrinsic layer thickness. This is different from the case in a-Si:H based solar 

cells, where we usually found that the degradation rate increases with the intrinsic layer 

thickness. Second, adding an i/p buffer layer reduces the degradation rate, especially for the thin 

solar cells. For example, sample 14098 with no i/p buffer degrades ~14% in efficiency, whereas 

sample 14113 with a 7-minute i/p buffer layer degrades only ~2%. However, for the thick 

samples, the degradation rate does not change much when the i/p buffer layer is changed, which 

can be seen by comparing sample 14100 with no i/p buffer and sample 14106 with a 7-minute i/p

buffer layer. To clearly see this trend, we plotted the degradation rate versus the intrinsic layer 

thickness in Fig. 12. Third, the degradation (if any) happens mostly in Voc and FF. Jsc shows 

almost no degradation for all the samples. 

51
 As discussed above, the cell performance in thin cells is mainly dominated by the i/p

interface. Since light soaking causes more damage in the i/p interface than in the bulk due to 

more exposure, the thin cells will be subjected to more degradation than the thick cells. 

Moreover, according to our experience, the Voc and FF are more sensitive to the i/p interface 

than Jsc. Any change in the i/p interface will affect the Voc and FF. This is in agreement with the 

observation in the third point. Improving the interface by adding high quality amorphous i/p

buffer should also reduce the light-induced degradation in the i/p interface region. This explains 

the reduced light-induced degradation rate by adding an i/p buffer layer. Previously, it has been 

reported that the bulk properties are improved by using a hydrogen dilution profiling technique 

[23], but actually the main improvement occurs close to the i/p interface region due to decreased 

hydrogen dilution. Based on the same argument, it is not surprising that the thick cells with an 

optimized i/p buffer have a more stable performance. 

Table XI. Stability results of nc-Si:H solar cells with different intrinsic and i/p buffer layer 

thicknesses. 

Sample 

No. 

Time (i) 

(min) 

Time(i/p) 

(min) 

Status Voc

(V) 

FF Q 

(mA/cm2

) 

Pmax

(mW/cm2

) 

Initial 0.463 0.714 14.54 4.81 

Stable 0.422 0.672 14.60 4.14 

14098 7 0 

Deg.(%) -8.9% -5.9% 0 -13.9% 

Initial 0.455 0.641 18.71 5.46 

Stable 0.433 0.641 18.90 5.25 

14099 15 0 

Deg.(%) -4.8% 0 +1.0% -4.0% 

Initial 0.455 0.586 22.49 6.00 

Stable 0.443 0.586 22.83 5.93 

14100 30 0 

Deg.(%) -2.6% 0 +1.5% -1.2% 

Initial 0.481 0.700 14.18 4.77 

Stable 0.462 0.673 14.57 4.53 

14101 7 3 

Deg.(%) -4.0% -3.9% +2.8% -5.0% 

Initial 0.473 0.642 18.78 5.70 

Stable 0.448 0.646 18.92 5.48 

14111 15 3 

Deg.(%) -5.3% +0.6% +0.7% -3.9% 

Initial 0.473 0.590 22.87 6.38 

Stable 0.459 0.597 22.91 6.28 

14112 30 3 

Deg.(%) -3.0% +1.2% +0.2% -1.6% 

Initial 0.532 0.677 16.00 5.76 

Stable 0.516 0.679 16.04 5.62 

14113 7 7 

Deg.(%) -3.0% 0 0 -2.4% 

Initial 0.491 0.622 19.73 6.03 

Stable 0.492 0.646 19.51 6.20 

14114 15 7 

Deg.(%) 0 +3.9% -1.1% +2.8% 

Initial 0.483 0.590 22.82 6.50 

Stable 0.481 0.597 22.62 6.50 

14106 30 7 

Deg.(%) 0 +1.2% -0.9% 0 

52
Table XII. Stability results of high rate a-Si:H solar cells with different intrinsic layer 

thicknesses. The cells were deposited using MVHF on Ag/ZnO coated stainless steel substrates. 

Sample 

No. 

Status Q 

(mA/cm2

)

Voc

(V)

FF Pmax

(mW/cm2

) 

Thickness 

(nm) 

Initial 9.12 1.021 0.755 7.03 

Stable 8.93 0.977 0.711 6.20 

14672 

Deg. 2.1% 4.3% 5.8% 11.8% 

104 

Initial 11.11 0.987 0.744 8.16 

Stable 10.77 0.970 0.690 7.21 

14671 

Deg. 3.1% 1.7% 7.3% 11.6% 

156 

Initial 12.87 0.999 0.716 9.21 

Stable 12.32 0.973 0.666 7.98 

14673 

Deg. 4.3% 2.6% 7.0% 13.4% 

235 

Initial 13.77 1.001 0.690 9.51 

Stable 12.97 0.973 0.632 7.98 

14674 

Deg. 5.8% 2.8% 8.4% 16.1% 

307 

As previously discussed, high efficiency nc-Si:H solar cells also need optimized back 

reflectors. In Table VIII, we present a comparison of J-V characteristics of nc-Si:H singlejunction cells deposited on regular Ag/ZnO back reflectors made from one of our roll-to-roll 

production machines and on the newly optimized Ag/ZnO back reflectors. The results show that 

the improved back reflectors consistently improve the Jsc by 0.5-1.0 mA/cm2

, mainly in the long 

wavelength response as shown in Fig. 29. By combining the optimized nc-Si:H recipe and the 

optimized Ag/ZnO back reflector, we have attained a new high efficiency for a nc-Si:H singlejunction solar cell. Figure 30 shows the J-V characteristics and quantum efficiency of the best 

nc-Si:H single-junction solar cell, which has an initial active-area efficiency of 9.23%. We have 

used this optimized nc-Si:H single-junction solar cell in triple-junction structures to achieve the 

highest efficiency. 

Table VIII. Comparison of J-V characteristics of nc-Si:H solar cells deposited on regular 

Ag/ZnO back reflectors obtained from our manufacturing machine and on newly optimized 

Ag/ZnO back reflectors. 

Run 

No. 

BR Jsc 

 (mA/cm2) 

Voc

(V) 

FF Eff 

(%) 

15397 Improved 26.21 0.524 0.620 8.52 

15392 Regular 25.24 0.526 0.630 8.36 

15425 Improved 26.11 0.516 0.629 8.47 

15426 Regular 25.13 0.513 0.614 7.92 

15437 Improved 27.17 0.507 0.598 8.21 

15438 Regular 26.34 0.517 0.617 8.32 

53
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE

Improved BR

Regular BR

Figure 29. Comparison of quantum efficiency curves of two nc-Si:H single-junction cells 

deposited using the same recipe but on different Ag/ZnO back reflectors. 

54
-30

-25

-20

-15

-10

-5

0

5

-0.2 0 0.2 0.4 0.6

V (V)

J (mA/cm2

)

Eff=9.23 %

Jsc=27.62 mA/cm2

Voc=0.529 V

FF=0.632

Active area=0.25 cm2

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

QE

27.62 mA/cm2 (b)

Figure 30. (a) J-V characteristics and (b) quantum efficiency curve of a nc-Si:H single-junction 

cell with an initial active-area efficiency of 9.23%. 

55
4.3.4. Optimization of a-Si:H/a-SiGe:H/nc-Si:H Triple-junction design

 It is well known that the good quality of materials is a necessary but not sufficient 

condition for achieving high efficiency solar cells. An optimized device design is also very 

important for achieving high solar cell efficiency, especially for multi-junction solar cells. First, 

multi-junction solar cells need optimized tunnel junctions between the component cells. Second, 

the current mismatch between the component cells is also an important factor for high efficiency, 

especially for stable efficiency. In this project, we systematically studied the correlation between 

the current mismatch and solar cell efficiency of a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar 

cells. 

 We combine the J-V characteristics of component cells to obtain the J-V characteristics 

of multi-junction solar cells. The procedure includes measuring the component cell under a 

reduced light intensity to produce a current similar to the current when it is used in a multijunction solar cell, generating a new set of J-V data with fixed current values for all the 

component cells, and adding the voltages of the component cells at each given current value. For 

each current mismatch, the current values of component cells are multiplied by corresponding 

factors. Figure 31 (top plot) shows an example of an a-Si:H/a-SiGe:H/nc-Si:H triple-junction 

solar cell with a matched current of Jsc=9.25 mA/cm2

. The component cell J-V characteristics 

are listed in Table XIV. We have kept the total current the same and numerically created current 

mismatch defined by (JT-JB)/Jav, where JT is the top cell current, JB the bottom current, and Jav the 

average current (9.25 mA/cm2

). During the calculation, the middle cell current density was set to 

equal the highest current density. For example, when the cell was bottom cell limited, the top and 

middle cell were set to the same current. From Fig. 31 (bottom plot), one can see that both the aSi:H top and nc-Si:H bottom cell limited current mismatches result in high FFs, and the increase 

of FF has a clear correlation to the magnitude of the current mismatch. One notices that even 

through the nc-Si:H bottom cell has the smallest FF among the three component cells, the ncSi:H bottom cell limited current mismatch still results in high FFs. The cell with a small bottom 

cell limited current mismatch shows the highest efficiency. As we previously reported, the ncSi:H bottom cell limited current mismatch improved the stability of the cell after prolonged light 

soaking. Therefore, we can conclude that in general, a multi-junction solar cell with a nc-Si:H 

bottom cell should be designed to have a bottom cell limited current mismatch. 

Table XIV. J-V characteristics of a-Si:H/a-SiGe:H/nc-Si:H triple-junction cells and its 

component cells 

Cell Type Jsc 

(mW/cm2

) 

Voc

(V) 

FF Pmax 

(mW/cm2) 

a-Si:H top 9.25 1.032 0.767 7.32 

a-SiGe:H middle 9.25 0.754 0.712 5.00 

nc-Si:H bottom 9.25 0.469 0.685 2.97 

Triple-Junction 9.25 2.260 0.723 15.11 

56
-16

-12

-8

-4

0

4

-0.5 0.0 0.5 1.0 1.5 2.0 2.5

V (V)

J (mA/cm2) & P (mW/cm2

)

Top

Bottom

Middle

P

Triple

0.71

0.72

0.73

0.74

0.75

0.76

0.77

0.78

0.79

0.80

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Mismatch (JT-JB)/Jav

FF 

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2

15.4

Efficiency (%)

Figure 31. (upper): J-V characteristics of an a-Si:H/a-Si:H/nc-Si:H triple-junction cell and its 

component cells. The power versus voltage (P-V) characteristics of the triple-junction cell is also 

plotted; (lower): FF and efficiency of triple-junction cells as a function of current mismatch. 

57
4.3.5. High efficiency triple-junction cells

We have used the optimized component cells to make high efficiency a-Si:H/aSiGe:H/nc-Si:H and a-Si:H/nc-Si:H/nc-Si:H triple-junction cells. Last year, we reported an 

initial active-area efficiency of 15.1% using the same cell structure. In this quarter, we continued 

the optimization effort. We have made several a-Si:H/a-SiGe:H/nc-SiGe:H triple-junction cells 

with an initial active-area efficiency over 15%. Table XV lists the J-V characteristics of the high 

efficiency triple-junction cells. The highest record efficiency is 15.4% with J-V characteristics 

and QE curves shown in Fig. 32. 

Table XVI summarizes the J-V characteristics of several a-Si:H/nc-Si:H/nc-Si:H triplejunction solar cells in the initial and light-soaked states, where the light-soaked state was reached 

by light soaking under ~100 mW/cm2

 white light at 50 oC for over 1000 hours. The highest 

initial active-area efficiency of 14.1% was achieved with the initial J-V characteristics and QE 

shown in Fig. 33. Although the a-Si:H/nc-Si:H/nc-Si:H triple-junction structure does not yield 

the highest initial efficiency, it shows a very small light-induced degradation of 4.3%, resulting 

from a better stability of the nc-Si:H middle cell compared to the a-SiGe:H middle cell. A stable 

active-area efficiency of 13.3% is also achieved with this cell structure. 

Table XV. List of J-V characteristics of a-Si:H/a-SiGe:H/nc-Si:H triple junction solar cells with 

an initial active-area efficiency over 15%. 

Q (mA/cm2 Run ) 

No. 

Voc

(V) 

FF Jsc

(mA/cm2

) Total Top Middle Bottom 

Pmax

(mW/cm2

) 

16192 2.191 0.767 9.03 27.50 9.19 9.28 9.03 15.2 

16219 2.215 0.754 9.16 27.75 9.25 9.34 9.16 15.3 

16235 2.233 0.740 9.06 27.46 9.06 9.27 9.13 15.0 

16243 2.240 0.738 9.07 27.75 9.07 9.61 9.07 15.0 

16247 2.239 0.753 9.13 27.85 9.13 9.45 9.27 15.4 

Table XVI. J-V characteristics of high efficiency a-Si:H/nc-Si:H/nc-Si:H triple-junction solar 

cells. Deg. denotes the percentage of light-induced degradation. The bold numbers are the 

highest efficiencies and italic numbers are the limited current densities for Jsc.

QE (mA/cm2 Sample State Eff (%) Jsc ) 

(mA/cm2

) top middle bottom 

Voc

(V) 

FF 

Initial 14.14 9.11 9.11 9.72 9.11 1.965 0.790 

Stable 13.19 8.79 8.79 9.56 9.04 1.947 0.771 

13955-

33 

Deg. 6.7% 3.5% 3.5% 1.6% 0.8% 0.9% 2.4% 

Initial 13.86 8.89 9.02 9.52 8.89 1.981 0.787 

Stable 13.26 8.72 8.75 9.25 8.72 1.973 0.771 

13955-

24 

Deg. 4.3% 1.9% 3.0% 2.8% 1.9% 0.4% 2.0% 

Initial 13.67 8.99 9.44 9.54 8.99 1.944 0.782 

Stable 13.24 8.92 9.04 9.42 8.92 1.933 0.768 

14005-

33 

Deg. 3.1% 0.8% 4.2% 1.3% 0.8% 0.6% 1.8% 

58
4. 4. Summary 

 We have optimized the a-Si:H top cell, a-SiGe:H middle cell, and nc-Si:H bottom cell for 

high efficiency a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cells. The hydrogen dilution and 

hydrogen dilution profiling are the two important techniques for improving both amorphous and 

nanocrystalline silicon and silicon germanium solar cells. With the improved materials and 

optimized solar cell structures, we achieved a new record initial active-area efficiency of 15.4% 

using the a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure. Similarly, we used an a-Si:H/ncSi:H/nc-Si:H triple-junction structure to achieve a stable active-area efficiency of 13.3%. Both 

efficiencies exceeded those previously achieved with amorphous silicon and silicon germanium 

alloys [35,40]. 

59
-10

-8

-6

-4

-2

0

2

-0.5 0.0 0.5 1.0 1.5 2.0 2.5

V (V)

J (mA/cm2

)

Eff.=15.39 %

Jsc=9.13 mA/cm2

Voc=2.239 V

FF=0.753

(a) Line 16247-cell 32

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 500 700 900 1100

Wavelength (nm)

QE

9.13 9.45 9.27

(b) Line 16247-cell 32

Figure 32. (a) J-V characteristics and (b) quantum efficiency curves of an a-Si:H/a-SiGe:H/ncSi:H triple-junction cell with an initial active-area efficiency of 15.4%. 

60
-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

-0.4 0.0 0.4 0.8 1.2 1.6 2.0

V (V)

J (mA/cm2

)

Jsc=9.11 mA/cm2

Voc=1.965 V

FF=0.790

Eff.=14.14%

Area=0.25 cm2

(a

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100

Wavelength (nm)

QE

Top

Middle

Bottom

Total

9.11

9.11

9.72

27.94

mA/cm2 

(b)

Figure 33. (a) J-V characteristics and (b) quantum efficiency curves of an a-Si:H/nc-Si:H/ncSi:H triple-junction solar cell with an initial active-area efficiency of 14.1%. 

61
5. High rate deposition of a-Si:H and a-SiGe:H solar cells using modified 

very high frequency glow discharge 

5. 1. Introduction 

A high deposition rate for making solar cell intrinsic layers is always desirable to 

increase throughput and reduce production cost. However, a-Si:H solar cells made with the 

conventional radio frequency (RF) glow discharge at high rates exhibit poor quality. The 

materials contain a high defect density, microvoids, and dihydride structures, which lead to a low 

initial efficiency and poor stability in solar cells. In order to reduce manufacturing costs, new 

deposition techniques are needed to increase the deposition rate without compromising the 

material quality. Obviously, increasing the deposition rate will directly increase the annual 

capacity or reduce the initial capital investment on the machine. 

Very high frequency (VHF) glow discharge has been widely used in the deposition of aSi:H [44] and nc-Si:H [1,41-42,45-47] materials and devices. Compared to the conventional RF 

technique, VHF plasma has higher electron density and lower ion energy, which is believed to 

increase the deposition rate and improve the material quality. In our laboratory, we have used a 

modified VHF (MVHF) system to make a-Si:H, a-SiGe:H [48,49], and nc-Si:H [41,42] solar 

cells. An initial efficiency of 11.2% was obtained in an a-Si:H/a-SiGe:H double-junction solar 

cell with the top cell intrinsic layer deposited at 8 Å/s and the bottom cell at 6 Å/s [49]. We have 

achieved an initial active-area efficiency of 9.0% on a nc-Si:H single-junction solar cell and a 

stabilized active efficiency of 13.3% on an a-Si:H/nc-Si:H/nc-Si:H triple junction solar cell [34]. 

In this project, we have optimized the deposition condition of a-Si:H in a wider growth 

parameter space, mainly in the higher pressure and smaller gap spacing regime. We have 

systematically studied the initial cell performance and stability as a function of the deposition 

rate. 

5. 2. a-Si:H and a-SiGe:H single-junction solar cells made with MVHF at 

high deposition rates 

A series of a-Si:H n-i-p solar cells has been made using an MVHF high rate a-Si:H 

intrinsic layer and low rate RF doped layers. Under each deposition condition, two runs were 

made: one on a specular stainless steel (SS) substrate and one on a Ag/ZnO back reflector (BR) 

coated SS substrate. The deposition rate of the a-Si:H intrinsic layer was changed from 5 to 14 

Å/s by varying the VHF power and the pressure. The thickness of the intrinsic layer was 

controlled in the range of 200-220 nm. ITO dots with an active-area of 0.25 cm2

 were deposited 

on the p layer for J-V and QE measurements. 

The deposition rate of a-Si:H depends on many deposition parameters such as the 

excitation (VHF or RF) power, gas pressure, and gas dilution ratio. Under a given condition, the 

most common way to increase the deposition rate is to increase the excitation power. Figure 34 

shows the deposition rate of a-Si:H as a function of VHF power. One can see that the deposition 

rate continues to increase with the VHF power in the range of 9-14 Å/s. The deposition rate is 

also very sensitive to the gas pressure. In the high pressure regime, increasing the pressure leads 

to a decrease in the deposition rate. In this study, we focused on deposition rates in the range of 

5 to 14 Å/s. 

62
8

9

10

11

12

13

14

15

12 14 16 18 20 22

Power (W)

Deposition rate (A/s)

Figure 34. The deposition rate of a-Si:H as a function of VHF power. 

Table XVII lists the J-V characteristics of the cells on both SS and BR substrates, where 

the Jsc is obtained from the QE measurements. FFb and FFr denote the FF obtained from the 

measurements with weak blue and red lights, respectively. Normally, we believe that the FFb

probes the interface between the i and p layers, while the FFr detects the quality of the i layer. 

One can see that the cell performance does not have a strong dependence on the deposition rate. 

Most cells show a high FFr, indicating a high quality of the a-Si:H intrinsic layer. The average 

efficiency is around 7% for the cells on SS and 10% for the cells on BR. Compared to the cells 

on SS, the Voc and FF do not change much for the cells on BR, but the Jsc is increased by 40% 

due to light trapping, which directly improves the cell efficiency. The intrinsic layer thickness in 

all of the cells in the table is in the range of 200-220 nm. By increasing the intrinsic layer 

thickness to 330 nm, an efficiency of 10.6% (Jsc=15.94 mA/cm2

, Voc=0.993 V, and FF=0.674) 

has been obtained with an a-Si:H single-junction cell on BR deposited at 10 Å/s. 

Stability testing has been done on cells made with different deposition rates on SS and 

BR. The solar cells were light soaked under 100 mW/cm2

 white light at 50 oC for 1000 hours. 

As a reference, two a-Si:H cells made with RF at a low rate of ~1.0 Å/s (one on SS and one on 

Ag/ZnO BR) were light-soaked together with the high rate MVHF cells. The results are listed in 

Tables XVIII and XIX for the cells on BR and on SS, respectively. Figure 35 shows the initial 

and stable efficiencies as well as the light-induced degradation as a function of the deposition 

rate. The light-induced degradation rate of the efficiency varies from 15% to 17% for cells on 

both BR and SS, but it does not show any dependence on the deposition rate. The highest 

stabilized efficiency of 8.64% has been achieved with an a-Si:H single-junction cell made at 9.3 

Å/s on Ag/ZnO back reflector. 

63
Table XVII. Performance of a-Si:H solar cells made with MVHF on BR and SS substrates. FFb

and FFr represent the FF measured under weak blue and red lights, respectively. The values of 

Jsc are from QE measurements. 

Sample FF 

No. 

Voc

(V) FF FFb FFr

Jsc 

(mA/cm2)

Eff 

(%) 

Substrate Rate 

(Å/s) 

i layer time 

(seconds) 

14325 0.983 0.712 0.759 0.753 14.34 10.04 BR 

14166 1.000 0.735 0.767 0.749 9.71 7.14 SS 

5.2 420 

14324 1.001 0.723 0.763 0.750 13.68 9.90 BR 

14139 0.974 0.727 0.765 0.780 9.62 6.81 SS 

6.1 300 

14347 0.987 0.702 0.722 0.744 14.67 10.16 BR 

14336 0.988 0.710 0.760 0.770 10.36 7.27 SS 

9.2 288 

14323 0.993 0.709 0.741 0.741 14.69 10.34 BR 

14318 0.978 0.697 0.754 0.763 10.48 7.14 SS 

9.3 240 

14346 0.986 0.696 0.716 0.742 14.46 9.92 BR 

14335 0.995 0.698 0.764 0.747 10.43 7.24 SS 

11.4 222 

14338 0.997 0.704 0.743 0.751 13.68 9.60 BR 

14330 0.985 0.714 0.755 0.768 9.96 7.00 SS 

11.9 180 

14345 0.984 0.697 0.717 0.742 14.20 9.74 BR 

14342 0.986 0.713 0.760 0.767 9.89 6.95 SS 

12.6 175 

14348 0.982 0.710 0.717 0.743 14.11 9.84 BR 

14333 0.995 0.713 0.769 0.763 9.64 6.84 SS 

13.8 156 

64
Table XVIII. Stability results of a-Si:H solar cells on Ag/ZnO BR coated SS made by MVHF at 

high rates. For comparison, a sample (14028) made by RF at a low rate of 1.0 Å/s has been 

light-soaked with the high rate samples. The thickness for all of the samples is in the range of 

200-220 nm. 

Sample FF 

No. 

State Voc

(V) AM1.5 FFb FFr

Jsc 

(mA/cm2) 

Eff 

(%) 

Rate 

(Å/s) 

Initial 1.020 0.695 0.740 0.719 13.78 9.77 

Stable 0.998 0.640 0.636 0.653 13.12 8.38 

14028 

Deg(%) 2.2% 7.9% 4.8% 14.2% 

1.0 

Initial 0.983 0.712 0.759 0.753 14.34 10.04 

Stable 0.951 0.645 0.677 0.665 13.92 8.54 

14325 

Deg(%) 3.3% 9.4% 2.9% 14.9% 

5.2 

Initial 1.001 0.723 0.763 0.750 13.68 9.90 

Stable 0.965 0.668 0.697 0.676 13.05 8.41 

14324 

Deg(%) 3.6% 7.6% 4.6% 15.1% 

6.1 

Initial 0.987 0.702 0.722 0.744 14.67 10.16 

Stable 0.958 0.645 0.647 0.667 13.67 8.45 

14347 

Deg(%) 2.9% 8.1% 6.8% 16.8% 

9.2 

Initial 0.993 0.709 0.741 0.741 14.69 10.34 

Stable 0.956 0.649 0.644 0.639 13.92 8.64 

14323 

Deg(%) 3.7% 8.5% 5.2% 16.4% 

9.3 

Initial 0.986 0.696 0.716 0.742 14.46 9.92 

Stable 0.960 0.655 0.663 0.684 13.49 8.48 

14346 

Deg(%) 2.6% 5.9% 7.2% 14.5% 

11.4 

Initial 0.997 0.704 0.743 0.751 13.68 9.60 

Stable 0.959 0.645 0.681 0.672 13.20 8.15 

14338 

Deg(%) 3.8% 8.4% 3.5% 15.1% 

11.9 

Initial 0.984 0.697 0.717 0.742 14.20 9.74 

Stable 0.960 0.648 0.656 0.675 13.41 8.34 

14345 

Deg(%) 2.4% 7.0% 5.6% 14.4% 

12.6 

Initial 0.982 0.710 0.717 0.743 14.11 9.84 

Stable 0.959 0.659 0.662 0.673 13.34 8.43 

14348 

Deg(%) 2.3% 7.2% 5.5% 14.3% 

13.8 

65
Table XIX. Stability results of MVHF high rate a-Si:H solar cells on SS. As a reference, a RF 

low rate a-Si:H cell RF (13899) is also included. The thicknesses for all of the samples are in the 

range of 200-220 nm. 

Sample FF 

No. 

State Voc

(V) AM1.5 Blue Red 

Jsc 

(mA/cm2) 

Eff 

(%) 

Rate 

(Å/s) 

Initial 1.014 0.748 0.782 0.760 9.82 7.45 

Stable 0.974 0.676 0.720 0.677 9.37 6.17 

13899 

Deg. (%) 3.94% 9.63% 4.58% 17.2% 

1.0 

Initial 1.002 0.732 0.775 0.760 9.71 7.14 

Stable 0.959 0.661 0.697 0.655 9.41 5.96 

14166 

Deg. (%) 4.3% 9.7% 3.1% 16.5% 

5.2 

Initial 0.997 0.733 0.782 0.751 9.61 7.07 

Stable 0.958 0.663 0.721 0.669 9.30 5.91 

14308 

Deg. (%) 3.9% 9.5% 3.2% 16.4% 

7.5 

Initial 0.979 0.699 0.776 0.752 10.09 6.94 

Stable 0.944 0.636 0.702 0.636 9.71 5.83 

14309 

Deg. (%) 3.6% 9.0% 3.8% 16.0% 

9.0 

Initial 0.979 0.702 0.757 0.762 10.48 7.14 

Stable 0.943 0.626 0.688 0.648 10.05 5.93 

14318 

Deg. (%) 3.7% 10.8% 4.1% 16.9% 

9.3 

Initial 0.984 0.713 0.756 0.769 10.14 7.11 

Stable 0.950 0.673 0.687 0.666 9.45 6.04 

14370 

Deg. (%) 3.5% 5.6% 6.8% 15.0% 

11.3 

Initial 0.986 0.713 0.762 0.765 9.89 6.95 

Stable 0.952 0.647 0.678 0.670 9.33 5.75 

14342 

Deg. (%) 3.2% 9.3% 5.7% 17.3% 

12.6 

Initial 0.995 0.713 0.760 0.754 9.64 6.84 

Stable 0.957 0.650 0.694 0.653 9.17 5.70 

14333 

Deg. (%) 3.8% 8.8% 4.9% 16.7% 

13.8 

66
2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14

Deposition rate (A/s)

Efficiency (%)

0

2

4

6

8

10

12

14

16

18

20

Degradation rate (%)

Degradation rate 

Initial efficiency 

Stable efficiency

(a)

2

3

4

5

6

7

8

9

10

11

12

0 2 4 6 8 10 12 14

Depositon rate (A/s)

Efficiency (%)

0

2

4

6

8

10

12

14

16

18

20

Degradation rate (%)

Initial efficiency

Stable efficiency

(b) Degradation rate 

Figure 35. Initial and stable efficiencies of MVHF-deposited a-Si:H, as well as their degradation 

ratios, as a function of the deposition rate for the cells on (a) BR and (b) SS. 

A series of a-SiGe:H n-i-p single-junction cells was made using a MVHF high rate aSiGe:H intrinsic layer and RF doped layers on stainless steel substrates. The n/i and i/p buffer 

layers were made using RF at a low rate of ~1 Å/s. Specular stainless steel substrates were used. 

It is known that the a-SiGe:H cell performance strongly depends on the Ge/Si ratio in the 

intrinsic layer and this ratio depends on the deposition rate even if the same GeH4/SiH4 ratio in 

the gas flow rate is used. The reason for the Ge/Si ratio dependence on the deposition rate is due 

to the difference in the power dependence of the decomposition rate of SiH4 and GeH4 in the 

67
plasma. In order to simplify the study, the deposition time of the a-SiGe:H intrinsic layer was 

fixed at 4 minutes, which corresponds to a deposition rate of ~10 Å/s. The major techniques to 

optimize the recipe include: (1) a proper GeH4 ramping to enhance the hole collection, (2) an 

optimized i/p buffer to reduce the shunt current and interface recombination, and (3) optimized p

and n layers to reduce the series resistance. 

Table XX lists the J-V characteristics of typical a-SiGe:H component cells made at high 

rates. In this study, we focused on medium bandgap materials, which give an AM1.5 Voc around 

0.75 to 0.80 V for the a-SiGe:H component cell in a high efficiency a-Si:H/a-SiGe:H doublejunction structure. In order to investigate the long wavelength response and simulate the 

performance when the a-SiGe:H cell is placed in an a-Si:H/a-SiGe:H double-junction structure, 

we measured the cell performance under an AM1.5 solar simulator with a 530-nm cut-on filter. 

From previous experience, a Pmax of 4 mW/cm2

 is a benchmark for a good a-SiGe:H component 

cell on a stainless steel substrate. In this study, we achieved an active-area Pmax of 4.4 mW/cm2

under AM1.5 illumination with a 530-nm cut-on filter. 

Table XX. J-V characteristics of a typical a-SiGe:H component cells made with MVHF at high 

rates. The intrinsic layer deposition time for all the cells was 4 minutes. The results in 

parentheses were measured under AM1.5 with a 530 nm cut-on filter. 

Sample 

No. 

Voc (> 530 nm) 

(V) 

FF (> 530 nm) Jsc (> 530 nm) 

(mA/cm2

) 

Pmax (> 530 nm) 

(mW/cm2

) 

14407 0.795(0.775) 0.667(0.696) 13.49(7.82) 7.15(4.22) 

14583 0.768(0.751) 0.648(0.671) 14.41(8.66) 7.17(4.36) 

14584 0.768(0.750) 0.664(0.681) 14.23(8.60) 7.26(4.39) 

Table XXI. Stability of a-SiGe:H component cells on SS made by MVHF at a deposition rate of 

~10 Å/s. Light soaking was carried out under a 100 mW/cm2

 white light with a 515 nm cut-on 

filter at 50 o

C for 1095 hours. The cells were measured under an AM1.5 solar simulator with a 

530-nm cut-on filter at 25 o

C. 

Sample 

No. 

Status Voc

(V) 

FF Jsc 

(mA/cm2) 

Pmax

(mW/cm2

) 

Initial 0.784 0.693 7.67 4.17 

Stable 0.728 0.553 7.06 2.84 

14382 

Degradation (%) 7.1% 20.2% 8.0% 31.9% 

Initial 0.780 0.677 7.67 4.05 

Stable 0.722 0.542 7.05 2.76 

14395 

Degradation (%) 7.4% 19.9% 8.1% 31.9% 

Initial 0.774 0.682 7.82 4.13 

Stable 0.718 0.545 7.36 2.88 

14407 

Degradation (%) 7.2% 20.1% 5.9% 30.3% 

Initial 0.785 0.675 7.50 3.97 

Stable 0.727 0.538 6.92 2.71 

14397 

Degradation (%) 7.4% 20.3% 7.7% 31.7% 

68
We also studied the stability of the a-SiGe:H component solar cells. Light soaking was 

done under a 100 mW/cm2

 white light with a proper long wavelength pass filter, which results in 

a Jsc in the a-SiGe:H cells similar to the Jsc in an a-Si:H/a-SiGe:H double-junction structure. The 

temperature of the light soaking stage was controlled at 50 °C during the light soaking. After 

over 1000 hours of light soaking the solar cell performance was stabilized. Table XXI lists the 

initial and stable J-V characteristics measured under an AM1.5 solar simulator with a 530-nm 

cut-on filter. It appears that the cell performance degrades about 31% on average, which is 

similar to the light-induced degradation observed in the a-SiGe:H component cells made with RF 

glow discharge at 3 Å/s. 

5. 3. a-Si:H/a-SiGe:H double-junction solar cells made with MVHF at high rates 

Having optimized the a-Si:H top and a-SiGe:H bottom cells, we proceeded to combine 

them to make a-Si:H/a-SiGe:H double-junction solar cells on Ag/ZnO coated stainless steel 

substrates. The deposition time for both the a-Si:H intrinsic layer in the top cell and a-SiGe:H 

intrinsic layer in the bottom cell was fixed at 4 minutes. The J-V characteristics of typical 

double-junction cells are listed in Table XXII. An initial active-area efficiency of 11.7% has 

been achieved. The J-V curves and QE spectrum of the best a-Si:H/a-SiGe:H double-junction 

cell are plotted in Fig. 35. 

Table XXII. J-V characteristics of the a-Si:H/a-SiGe:H double-junction cells made at high rates. 

The deposition time for both top and middle cell intrinsic layers was 4 minutes. 

QE (mA/cm2 Sample ) 

No. 

Voc

(V) 

FF 

Top Bottom Total 

Pmax

(mW/cm2

) 

14520 1.719 0.717 9.50 10.94 20.44 11.71 

14530 1.696 0.664 10.23 10.16 20.39 11.44 

14534 1.700 0.679 9.78 10.48 20.26 11.29 

5. 4. Summary 

In summary, we have found that the MVHF-deposited a-Si:H solar cells showed good 

initial efficiency and stability. The most important result is that the cell performance and 

stability do not depend on the deposition rate up to 14 Å/s. This phenomenon is quite different 

from the cells made using RF at high rates. The degradation rate of the RF-cells usually 

increases with the deposition rate. We have also optimized the a-SiGe:H component cells at high 

deposition rates on SS substrate. An active-area Pmax of 4.4 mW/cm2

 has been achieved under 

AM1.5 with a 530 nm cut-on filter. The light-induced degradation of the a-SiGe:H component 

cells is about 31%, which is similar to the cells made with RF glow discharge at 3 Å/s. By 

combining this cell with an optimized high rate a-Si:H top cell, we have made an a-Si:H/aSiGe:H double-junction cell on Ag/ZnO coated SS with an initial active-area efficiency of 

11.7%. 

69
-10

-8

-6

-4

-2

0

2

-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

V (V)

J (mA/cm2

)

Eff.=11.71 %

Voc=1.719 V

Jsc=9.5 mA/cm2

FF=0.717

(a)

0.0

0.2

0.4

0.6

0.8

1.0

300 400 500 600 700 800 900

Wavelength (nm)

QE

20.44 mA/cm2

9.50 10.94

(b)

Figure 35. (a) J-V characteristics and (b) QE spectrum of an a-Si:H/a-SiGe:H double-junction 

cell made at 10 Å/s using MVHF. 

70
6. Large-area a-Si:H/a-SiGe:H/a-SiGe:H triple-junction and a-Si:H/ncSi:H double-junction solar cells 

6. 1. Introduction

One of our major objectives for this project is to develop new deposition parameters for 

improving solar cell efficiency and throughput, and hence reducing the cost of manufacturing. 

The spectrum splitting a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure on Al/ZnO back 

reflector is used in our current 118 MW manufacturing lines in Auburn Hills and Greenville, 

Michigan. Therefore, one task of the project is to optimize the deposition parameters of aSi:H/a-SiGe:H/a-SiGe:H triple-junction solar cells under the manufacturing constraints, mainly 

the deposition time of each layer. Previously, we have achieved a stable total-area (0.268 cm2

) 

cell efficiency of 9.1% and stable aperture-area (460 cm2

) module efficiency of 8.9% using our 

large-area batch machine. Those cells were deposited with Si2H6 and GeH4 mixtures under the 

manufacturing constraints on Al/ZnO back reflectors. In order to reduce cost, we have switched 

to SiH4 and GeH4 mixtures in the manufacturing process. In the research and development 

department, we have also studied the optimization of the deposition conditions using the SiH4

and GeH4 mixtures. We achieved a stable total-area (0.268 cm2

) triple-junction cell efficiency of 

9.1%, which is similar to the result obtained with the Si2H6 and GeH4 mixture. This result 

showed that, in principle, one can achieve a similar performance using inexpensive SiH4 instead 

of Si2H6. In this project, we have carried out optimization of large-area deposition of a-Si:H/aSiGe:H/a-SiGe:H triple-junction solar cells using SiH4 and GeH4 gas mixtures. Al/ZnO back 

reflectors made in the production machines were used. 

For the future a-Si:H based thin film manufacturing technology, improving the module 

efficiency and reducing the cost are the two major challenges. One of the promising new 

technologies is using nc-Si:H in the bottom cell of multi-junction structures. As presented 

above, we have studied the material and device optimization of nc-Si:H solar cells using small 

area machines. In addition, we also studied large-area a-Si:H/nc-Si:H double-junction solar 

modules at a low rate [50]. In this project, we have investigated a-Si:H/nc-Si:H double-junction 

solar cells made with RF glow discharge at high deposition rates. 

6. 2. Experimental details 

For a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures, we used one of our large-area 

(14×15 in2

) batch machines (2B) for the deposition of a-Si:H and a-SiGe:H intrinsic layers as 

well as the doped layers. The deposition time of each layer is fixed according to the calculation 

with the web speed and the chamber lengths of the production line. The triple-junction solar 

cells and modules were deposited on Al/ZnO back reflector coated stainless steel substrates. For 

RF deposited a-Si:H/nc-Si:H double-junction solar cells and modules, we used the 2B machine 

with RF glow discharge at a high rate of ~3 Å/s. 

The optimization of cell performance and uniformity were carried out using small Indium 

Tin Oxide (ITO) dots with an active-area of 0.25 cm2

. Large-area (>400 cm2) modules were 

made using the center area of the substrate with wires and bus bars. The large-area modules 

were encapsulated with a procedure similar to that used in the manufacturing process. The 

small-area cells were measured using an AM1.5 ORC solar simulator with quantum efficiency 

71
72

(QE) correction for the short-circuit current density. The large-area modules were measured at 

various states of light soaking using a Spire solar simulator with an NREL calibrated c-Si solar 

cell with a proper filter. The light soaking experiments were done under 100 mW/cm2

 white 

light at 50 °C for over 1000 hours. 

6. 3. a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells under manufacturing 

constraints 

The uniformities of thickness and cell performance are major concerns for large-area 

deposition. In order to obtain high module efficiency, the cell performance has to satisfy certain 

uniformity requirements. We deposited the samples on a large-area (14×15 in2

) substrate and 

then cut them into 1.85×1.85 in2

 pieces at different locations. Since we normally make 8.5×8.5 

in2

 modules, we cut nine pieces from an area 9.2×9.2 in2. One is in the center of the substrate, 

one at the middle of each of the four edges, and one at each of the four corners of the 9.2×9.2 in2

area. Sixteen small ITO dots, with an active area of 0.25 cm2

, were deposited on the nine pieces 

for J-V and QE measurements. Table XXIII lists the cell performance from one run (2B 11147). 

The J-V data for each location are averages of the measurements on four cells. The uniformity 

of the cell performance in the given area is reasonably good. The highest initial J-V measured 

active-area efficiency is 11.06% (QE corrected 10.92%), and the average is 10.62% with a nonuniformity of ±5%. This uniformity is acceptable for large-area modules. 

Table XXIII. J-V characteristics of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells at different 

locations in a large-area cell, where the Jsc was taken directly from the J-V measurements. QE 

current densities and QE corrected efficiency (EffQ) of five cells are listed as a reference. 

QE (mA/cm2 Location in ) 

2B 11147 

Jsc

(mA/cm2)

Voc 

(V) 

FF Eff 

(%) 

EffQ

(%) Top Mid. Bot. 

Left North 7.41 2.195 0.656 10.67 

Left Center 7.40 2.220 0.659 10.82 

Left South 7.27 2.238 0.663 10.78 10.62 7.28 7.16 7.36 

Central North 7.18 2.238 0.688 11.06 10.92 7.09 7.20 7.22 

Central Center 7.24 2.237 0.679 10.98 10.66 7.02 7.31 7.28 

Central South 7.22 2.207 0.676 10.77 10.49 7.03 7.49 7.32 

Right North 7.47 2.161 0.623 10.05 

Right Center 7.49 2.122 0.634 10.28 9.83 7.31 7.41 7.48 

Right South 7.38 2.159 0.636 10.14 

Average 7.34 2.197 0.657 10.62 

Several large-area modules have been made with the same deposition recipe as the cells 

shown in Table XXIII. The modules were measured at the states before and after lamination, as 

well as various light soaked states. Table XXIV lists the aperture-area performance of four 

modules. The data presented here are as-measured data without spectral mismatch corrections. 

The encapsulation causes a decrease of 1-4% in Jsc, 0.4-1.0% in Voc, but an increase of 0-1.5% in 

FF. The major light-induced degradation appears in the first 500 hours. The change in the last 

500 hours is very small. The average light-induced degradation is about 15.7%, which is similar 

to that observed in our previous experiments of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar 

cells. The major change is in the FF, where an average of 10.6% reduction was observed aft
73

Table XXIV. Summary of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction module performance in different states. 

corrected efficiency at 25 °C. 

Sample 

2B# State 

Area 

(cm2) T (oC) Isc (A) 

Jsc

(mA/cm2) Voc (V) FF 

Pmax

(W) 

10386 Before lamination 420.0 24.2 2.960 7.05 2.190 0.654 4.24 

After lamination 413.8 24.2 2.882 6.96 2.180 0.662 4.16 

Change due to lamination -1.3% -0.5% 1.2% 

585-hour light soaking 413.8 24.2 2.820 6.81 2.100 0.606 3.58 

1004-hour light soaking 413.8 25.1 2.813 6.80 2.090 0.599 3.52 

Change due to light soaking -2.3% -4.1% -9.5% 

10383 Before lamination 420.0 23.9 2.947 7.02 2.210 0.662 4.31 

After lamination 416.0 25.1 2.876 6.91 2.180 0.671 4.20 

Change due to lamination -1.6% -1.4% 1.3% 

585-hour light soaking 416.0 24.4 2.833 6.81 2.100 0.616 3.66 

1004-hour light soaking 416.0 24.9 2.811 6.76 2.100 0.609 3.59 

Change due to light soaking -2.2% -3.7% -9.2% 

11156 Before lamination 462.0 23.7 3.382 7.32 2.230 0.645 4.87 

After lamination 458.0 24.2 3.239 7.07 2.220 0.654 4.70 

Change due to lamination -3.4% -0.4% 1.4% -3.5% 

585-hour light soaking 458.0 24.7 3.236 7.07 2.13 0.584 4.02 

1004-hour light soaking 458.0 24.9 3.231 7.05 2.12 0.569 3.89 

Change due to light soaking -0.3% -4.5% -13.1% 

11166 Before lamination 462.0 23.7 3.349 7.25 2.230 0.650 4.85 

After lamination 458.0 24.4 3.215 7.02 2.210 0.651 4.63 

Change due to lamination -3.2% -0.9% 0.2% -4.5% 

585-hour light soaking 458.0 24.4 3.199 6.98 2.110 0.581 3.92 

1004-hour light soaking 458.0 25.4 3.176 6.93 2.110 0.578 3.88 

Change due to light soaking -1.3% -4.5% -11.2% 
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.4 0.8 1.2 1.6 2.0 2.4

V (V)

I (A)

Before Lamination

After Lamination

After Light soaking

Figure 36. I-V characteristics of an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cell (2B 

10383) before and after lamination, as well as after light soaking. 

light soaking. The best stable aperture-area efficiency is 8.6%, which is slightly lower than the 

best module efficiency of 8.9% achieved using a Si2H6 and GeH4 mixture. Figure 36 shows the 

I-V characteristics of the best module various different states. 

It is well known that the efficiency measured under a given solar simulator with a 

calibrated reference cell is not necessarily the same as under the standard AM1.5 illumination 

due to the difference in their spectra. Normally two major errors appear in Jsc and FF. First is 

the difference between the spectrum of the light source and the ideal AM1.5 spectrum as well as 

the difference in the QE spectra of the testing and reference cells, both of which cause an error in 

Jsc. Second, the non-ideal AM1.5 spectrum of the light source causes a difference in the current 

mismatching of the component cells, which results in an error in FF. 

In principle, the error in the Jsc of a single-junction solar cell can be corrected by 

adjusting the light intensity of the solar simulator based on a spectrum mismatching (M) factor 

[51]. The calculation of the M factor involves the reference light source (AM1.5) spectrum, the 

test light source (solar simulator) spectrum, the QE curve of reference cell and the QE curve of 

the test cell. In order to obtain a corrected Jsc, one can change the test light intensity by a factor 

of 1/M or use the measured Jsc under the standard setting condition and divide it by the M factor. 

Theoretically, the M-factor correction can provide a more accurate measurement of Jsc. 

74
However, in practice, more errors could be introduced by the errors in the QE of the testing cell. 

The major problem is that it is hard to measure the QE of large-area modules. An alternative 

way is to use the QE measured from a different cell with a small area but made under the same 

condition. In most cases, one uses QE data from a small-area cell without encapsulation. It is 

well known that the encapsulation changes the QE curves and therefore, the M factor. In this 

study, we tried to make the measurement more accurate by using QE data that are close to the 

QE of real modules. We used adjacent pieces from the module fabrication and made small-area 

(1.2 cm2

) cells with the same wires and bus bars as used in the module fabrication. We also 

encapsulated the small-area cells and light soaked them using the same procedures as the largearea modules. The QE curves were measured at different states for calculating the M factor. It 

appears that the small-area cells have a slightly smaller Voc and Jsc, but a slightly larger FF than 

the large-area modules at each corresponding state, which could be due to the non-uniformity of 

the deposition. Figure 37 gives an example of the QE changes from the initial non-encapsulated 

state to the encapsulated and light-soaked states. Corresponding reflection curves are also 

presented. The QE curves change dramatically at short wavelengths below 400 nm. We 

calculated the M-factors of the four cells at the initial non-encapsulated state and the 

encapsulated and light soaked states using the measured QE data and the QE of our reference cell 

as well as the spectrum of our Spire solar simulator. On average, the M factor is about 0.965 in 

the initial non-encapsulated state, and 0.983 in the final encapsulated and light-soaked state. If 

we take the M factor into account, the highest stable encapsulated module efficiency should be 

8.78%. 

6. 4. Large-area a-Si:H/nc-Si:H double-junction modules 

We have continued to work on large-area a-Si:H/nc-Si:H double-junction cells using one 

of our large area multi-chamber batch systems (2B) using RF glow discharge decomposition of 

SiH4 diluted with H2. In this project, the effort has been mainly focused on optimizing 

processing parameters under a lower pressure and a larger spacing between the cathode and 

sample substrate, which may ease process control in large volume manufacturing. The results 

summarized in Table XXV show that the cell performance made with the new processing 

conditions is comparable to the best cell performance achieved at the higher pressure and 

narrower spacing. 

Table XXV. J-V characteristics of newly made a-Si:H/nc-Si:H double-junction solar cells (first 

two rows). The cells previously made with higher pressure and narrower spacing are also listed 

for the comparison (last two rows). 

Sample 

No. 

Area 

(cm2

) 

Voc

(V) 

FF Jsc 

(mA/cm2

) 

Eff. 

(%) 

Comment 

2B11770 462 1.48 0.762 10.40 11.73 

2B11774 462 1.49 0.743 10.53 11.66 

Wider spacing 

2B10582 420 1.44 0.729 11.21 11.77 

2B10587 420 1.44 0.689 11.68 11.59 

Narrow spacing 

75
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE & R

Top Before Lam

Middle Before Lam

Bottom Before Lam

QE Total Before Lam

R Before Lam

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800 900 1000

Wavelength (nm)

QE & R

Top After Lam & LS

Middle After Lam & LS

Bottom After Lam & LS

QE Total After Lam & LS

R After Lam & LS

(b)

Figure 37. Quantum efficiency (QE) and reflection (R) curves of an a-Si:H/a-SiGe:H/a-SiGe:H 

triple-junction cell on Al/ZnO (a) before lamination and (b) after lamination and light soaking. 

76
6. 5. Summary 

We have optimized large-area deposition of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction 

solar cells under the manufacturing constraints with a SiH4 and GeH4 gas mixture on Al/ZnO 

back reflectors from the manufacturing line. A stable aperture-area (416 cm2

) efficiency of 8.6% 

has been achieved. With the spectrum mismatching correction, this efficiency could be 8.8%, 

which is similar to the highest module efficiency previously achieved using a Si2H6 and GeH4

gas mixture. 

We also optimized a-Si:H/nc-Si:H double-junction solar cells deposited on Ag/ZnO back 

reflectors with RF glow discharge at high deposition rates. An initial aperture-area efficiency of 

11.7% has been achieved using a nc-Si:H deposition time of 50 minutes. 

References: 

1. For a review: A. V. Shah, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and 

U. Graf, Sol. Energy Mater. & Sol. Cells 78, 469 (2003). 

2. A. Matsuda, Thin Solid Films 337, 1 (1999). 

3. H. Fujiwara, J. Koh, P. I. Rovira, and R. W. Collins, Phys. Rev. B61, 10832 (2000) 

4. R. W. Collins, A. S. Ferlauto, G. M. Ferreira, C. Chen, J. Koh, R. J. Koval, Y. Lee, J. M. 

Pearce, and C. R. Wronski, Sol. Energy Mater. & Sol. Cells 78, 143 (2003). 

5. H. Fujiwara, Y. Toyoshima, M. Kondo, and A. Matsuda, Phys. Rev. B60, 13598 (1999). 

6. E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, and J. Pohl, J. Appl. Phys. 87, 3137 

(2000). 

7. T. Mates, P.C.P. Bronsveld, A. Fejfar, B. Rezek, J. Kocka, J.K. Rath, and R.E.I. Schropp, 

J. Non-Cryst. Solids 352, 1011 (2006). 

8. B. Yan, C.-S. Jiang, C.W. Teplin, H.R. Moutinho, M.M. Al-Jassim, J. Yang, and S. Guha, 

J. Appl. Phys. 101, 033711 (2007). 

9. C.-S. Jiang, B. Yan, H.R. Moutinho, M.M. Al-Jassim, J. Yang, and S. Guha, Mater. Res. 

Soc. Symp. Proc. 989, 15 (2007). 

10. C.-S. Jiang, H.R. Moutinho, M.M. Al-Jassim, L.L. Kazmerski, B. Yan, J.M. Owens, J. 

Yang, and S. Guha, Proc. of 4th World Conf. on Photovoltaic Energy Conversion, Hawaii, 

USA, 2006, p. 1552. 

11. J. Kočka, A. Fejfar, H. Stuchlíková, J. Stuchlík, P. Fojtík, T. Mates, B. Rezek, K. 

Luterová, V. Švrček, and I. Pelant, Sol. Energy Mater. & Sol. Cells 78, 493 (2003). 

12. J. Yang, A. Banerjee, K. Lord, and S. Guha, Proc. of 28th IEEE Photovoltaic Specialists 

Conference, Anchorage, AK, September 15-22, 2000 (IEEE New York, 2000), p. 742. 

13. T. Roschek, T. Repmann, J. Müller, B. Rech, H. Wagner, Proc. of 28th IEEE Photovoltaic 

Specialists Conference, Anchorage, AK, September 15-22, 2000, (IEEE New York, 

2000), p. 150. 

77
14. K. Lord, B. Yan, J. Yang, and S. Guha, Appl. Phys. Lett. 79, 3800 (2001). 

15. J. Yang, K. Lord, B. Yan, A. Banerjee, S. Guha, D. Han, and K. Wang, Mat. Res. Soc. 

Symp. Proc. 715, 601 (2002). 

16. D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977). 

17. B. Yan, J. Yang, and S. Guha, Proc. of 3rd World Conference on Photovoltaic Energy 

Conversion, Osaka, Japan, 2003, p. 1627. 

18. T.I. Kamins, in Polycrystalline Silicon for Integrated Circuit Applications (Kluwer 

Academic Publishers, Boston USA, 1997), p. 32. 

19. J. Bailat, E. Vallat-Sauvain, L. Feitknecht, C. Droz, and A. Shah, J. Non-Cryst. Solids, 

299-302, 1219 (2002). 

20. Y. Nasuno, M. Kondo, and A. Matsuda, Proc. of 28th IEEE Photovoltaic Specialists 

Conference (IEEE, Anchorage, Alaska, 2000), p.142. 

21. F. Finger, S. Klein, T. Dylla, A. L. Baia Neto, O. Vetterl, and R. Carius, Mater. Res. Soc. 

Symp. Proc. 715, 123 (2002). 

22. B. Yan, K. Lord, J. Yang, S. Guha, J. Smeets, and J.-M. Jacquet, Mater. Res. Soc. Symp. 

Proc. 715, 629 (2002). 

23. B. Yan, G. Yue, J. Yang, S. Guha, D. L. Williamson, D. Han, and C.-S. Jiang, Appl. 

Phys. Lett. 85, 1955 (2004). 

24. A. Gordijn, J. Francke, L. Hodakova, J. K. Roth, and R. E. I. Schroop, Mat. Res. Soc. 

Symp. Proc. 862, 87 (2005). 

25. A. Gordijn, J.K. Rath, and R.E.I. Schropp, Prog. Photovoltaics 14, 305 (2006). 

26. C. Das. X. Cao, W. Du, X. Yang, Y. Ishikawa, and X. Deng, Conf. Record of the 2006 

IEEE 4th World Conf. on Photovoltaic Energy Conversion, (Hawaii, USA, May, 2006), 

p1504. 

27. G. Yue, B. Yan, G. Ganguly, J. Yang, S. Guha, and C. Teplin, Appl. Phys. Lett. 88, 

263507 (2006). 

28. A. F. Halverson, J. J. Gutierrez, D. Cohen, B. Yan, J. Yang, and S. Guha, Mater. Res. 

Soc. Symp. Proc. 862, 481 (2005). 

29. M. H. Brodsky, M. Cardona, and J. J. Cuomo, Phys. Rev. B 16 3556 (1977). 

30. E. Bustarret, M. A. Hachicha, and M. Brunel, Appl. Phys. Lett. 52, 1675 (1988). 

31. E. Vallat-Sauvain, C. Droz, F. Meillaud, J. Bailat, A. Shah, and C. Ballif, J. Non-Cryst. 

Solids 352, 1200 (2006). 

32. A. Banerjee and S. Guha, J. Appl. Phys. 69, 1030 (1991). 

33. B. Yan, G. Yue, J.M. Owens, J. Yang, and S. Guha, Conf. Record of the 2006 IEEE 4th 

World Conf. on Photovoltaic Energy Conversion, (Hawaii, USA, May 7-12, 2006), p. 

1477. 

78
34. G. Yue, B. Yan, G. Ganguly, J. Yang, S. Guha, C. Teplin, and D. L. Williamson, Conf. 

Record of the 2006 IEEE 4th World Conf. on Photovoltaic Energy Conversion, (Hawaii, 

USA, May 7-12, 2006), p. 1588. 

35. J. Yang, A. Banerjee, and S. Guha, Appl. Phys. Lett. 70, 2975 (1997). 

36. B. Yan, J. M. Owens, C.-S. Jiang, J. Yang, and S. Guha, Mater. Res. Soc. Symp. Proc. 

862, 603 (2005). 

37. H.J. Simon, D.E. Mitchell, and J.G. Watson, Am. J. Phys. 43, 630 (1975). 

38. E. Yablonovitch, J. Opt. Soc. Am. 72, 899 (1982). 

39. J. Yang, A. Banerjee, and S. Guha, Sol. Energy Mater. & Sol. Cells 78, 597 (2003). 

40. J. Yang, A. Banerjee, K. Lord, and S. Guha, Proc. of 2nd World Conference and 

Exhibition on Photovoltaic Solar Energy Conversion, July 6-10, 1998, Vienna, Austria, p. 

387. 

41. B. Yan, J. M. Owens, J. Yang, and S. Guha, Proc. of 31st IEEE Photovoltaic Specialists 

Conference, Lake Buena Vista, FL, January 3-7, 2005, p. 1456. 

42. B. Yan, G. Yue, J. Yang, A. Banerjee, and S. Guha, Mater. Res. Soc. Symp. Proc. 762, 309 

(2003). 

43. T. Dylla, F. Finger, and E. A. Schiff, App. Phys. Lett. 87, 032103 (2005). 

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

Hollenstein, Mater. Res. Soc. Symp Proc. 258, 15 (1992). 

45. J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J. A. Anna Selvan, N. Pellaton Vaucher, 

Ch. Hof, D. Fischer, H. Keppner, A. Shah, K.-D. Ufert, P. Giannoulès, and J. Koehler, 

Mater. Res. Soc. Symp. Proc. 420, 3 (1990). 

46. Y. Mai, S. Klein, R. Carius, J. Wolff, A. Lambertz, F. Finger, and X. Geng, J. Appl. 

Phys. 97, 114913 (2005). 

47. T. Matsui, A. Matsuda, and M. Kondo, Mater. Res. Soc. Symp Proc. 808, 557 (2004). 

48. J. Yang, S. Sugiyama, and S. Guha, Mater. Res. Soc. Symp Proc. 507, 157 (1998). 

49. J. Yang, B. Yan, J. Smeets, and S. Guha, Mater. Res. Soc. Symp Proc. 664, A11.3 

(2001). 

50. B. Yan, G. Yue, A. Banerjee, J. Yang, and S. Guha, Mater. Res. Soc. Symp Proc. 808, 

581 (2004). 

51. K. A. Emery, C. R. Osterwald, T. W. Cannon, D. R. Myers, J. Burdick, T. Glatfelter, W. 

Czubatyj, and J. Yang, Proceeding of 18th IEEE Photovoltaic Specialist Conference, 

(IEEE, 1985), p. 623. 

79
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1. REPORT DATE (DD-MM-YYYY)

May 2008 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

30 January 2006 - 29 January 2008 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

High-Efficiency Amorphous Silicon and Nanocrystalline SiliconBased Solar Cells and Modules: Final Technical Progress Report, 

30 January 2006 - 29 January 2008 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-43191 

5e. TASK NUMBER

PVB74101 

6. AUTHOR(S)

S. Guha and J. Yang 

5f. WORK UNIT NUMBER

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

United Solar Ovonic, LLC 

Troy, Michigan 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

ZXL-6-44205-14 

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NREL 

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14. ABSTRACT (Maximum 200 Words)

United Solar Ovonic LLC successfully used its spectrum-splitting a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure in 

their manufacturing plants, achieiving manufacturing capacity of 118 MW in 2007 from its Auburn Hills and 

Greenville, Michigan plants. United Solar has a very aggressive expansion plan to achieve grid parity by improving 

its solar panel efficiency, improving manufacturing throughput, and reducing manufacturing cost. In the Thin Film 

Partnership Program, Uni-Solar identified three areas of research: i) Optimize a-Si:H and a-SiGe:H deposition 

parameters under the current manufacturing constraints for improving solar module efficiency and manufacturing 

throughput, and reducing the manufacturing cost; ii) Explore new deposition methods for a-Si:H and a-SiGe:H 

materials to improve a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell efficiency at high deposition rates; and iii) Explore 

new materials and new cell structures for higher efficiency at high deposition rates. 

15. SUBJECT TERMS

PV; manufacturer; spectrum splitting; grid parity; module efficiency; triple junction; solar cells; devices; mixed-phase 

silicon; back reflector; deposition rate; large-area; amorphous silicon 

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