Subcontract Report

NREL/SR-520-45544

April 2009

High Throughput, Low Toxic 

Processing of Very Thin, High 

Efficiency CIGSS Solar Cells 

Final Report

December 2008

N.G. Dhere

Florida Solar Energy Center 

Cocoa, Florida
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-45544

April 2009

High Throughput, Low Toxic 

Processing of Very Thin, High 

Efficiency CIGSS Solar Cells 

Final Report

December 2008

N.G. Dhere

Florida Solar Energy Center 

Cocoa, Florida

NREL Technical Monitor: Bolko von Roedern

Prepared under Subcontract No. XXL-5-444205-08
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL

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iii

TABLE OF CONTENTS

1. INTRODUCTION ...................................................................................................................... 1

2. SELENIZATION/SULFURIZATION IN THE CONVENTIONAL FURNACE ..................... 2

3. RAPID THERMAL PROCESSING ........................................................................................... 4

4. CIGS2 THIN FILM SOLAR CELLS ......................................................................................... 5

5. ZnxCd1-xS AS ALTERNATIVE BUFFER LAYER FOR CIGS2 SOLAR CELLS ................... 8

6. ULTRA THIN CIGS/CIGSS ABSORBER LAYER ................................................................ 10

7. HUMAN RESOURCE DEVELOPMENT ............................................................................... 12

8. SUMMARY AND CONCLUSION ......................................................................................... 13

9. REFERENCES ......................................................................................................................... 14
1 

1. INTRODUCTION

This is the Final Report for the project “High Throughput, Low Toxic Processing of Very 

Thin, High Efficiency CIGSS Solar Cells” NREL contract no. XXL-5-44205-08, UCF/FSEC 

Account no. 2012 8098. 

During this project CuIn1-xGaxSe2-ySy (CIGSeS)/CdS thin-film solar cells were prepared 

on molybdenum coated sodalime glass (SLG) substrates. Low toxicity selenium precursors, 

diethylselenium (DESe) or other organometallic precursors were used for selenization of metallic 

precursors. Rapid Thermal Processing (RTP) technique was developed as an alternative to 

conventional selenization and sulfurization to reduce the process time and thermal budget thus 

enhancing throughput. CuIn1-xGaxS2 (CIGS2) thin-film solar cells were also developed. Easily 

scaleable magnetron sputtering technique was used for deposition of back contact, metallic 

precursors and transparent and conducting oxides. A compact and large-grain, 0.9-2.75 μm thick 

absorber layers were prepared by selenization/sulfurization of elemental precursors at 475-515oC 

in diluted DESe or other organometallic precursors and diluted H2S. Experiments were 

performed to reduce absorber thickness so as to minimize the indium consumption using both 

conventional as well as the RTP approach. Experiments were carried out on thinner i.e. 1.2 μm1.5 μm CIGS2 absorbers. Effect of sodium addition on copper poor CIGS2 absorbers and thinfilm solar cells was studied. Alternate heterojunction partner layers were developed for CIGS2 

thin-film solar cells.
2 

2. SELENIZATION/SULFURIZATION IN THE CONVENTIONAL 

FURNACE

Mo-back contact was deposited by DC magnetron sputtering over 15 cm x 10 cm 

sodalime glass substrates. Presence of small amount of sodium during CIGSeS formation has 

proven to be beneficial. Sodium containing precursor, NaF was deposited over the back contact 

prior to the precursor deposition. Experiments were carried out with various NaF thicknesses. 

The Cu-In-Ga precursors were deposited by DC magnetron sputtering. The thickness of layers 

was adjusted by varying the linear speed of substrate movement over the target. Selenization and 

sulfurization were carried out by heating the metallic precursors in diluted DESe or other 

organometallic precursors and diluted H2S respectively. Materials characterization was carried 

out by scanning electron microscopy (SEM), x-ray diffraction (XRD), electron probe 

microanalysis (EPMA), Auger electron spectroscopy (AES), secondary ion mass spectroscopy 

(SIMS) and transmission electron microscopy (TEM).Electrical/photovoltaic/device 

characterization was carried out by current-voltage (I-V), external quantum efficiency (EQE), 

capacitance-voltage (C-V), and light beam induced current (LBIC). Both of these 

characterizations were mentioned in earlier reports. Cells were completed by deposition of CdS 

heterojunction partner layer by chemical bath deposition, i:ZnO/ZnO:Al window bilayer by RF 

magnetron sputtering and Cr/Ag contact fingers by thermal evaporation. 

Initially, the thickness of absorber layers was maintained in the range of 2.5 - 2.7 μm. 

Experiments were carried out to optimize the process parameters of DC magnetron sputtering of 

metallic precursors and temperature, time, and reactant quantities of selenization/sulfurization for 

absorber preparation. Addition of minute quantities of sodium was found to improve the 

morphology of the absorber films [1]. Moreover, the grains were well faceted and compactly 

packed (Fig. 1). 
3 

Figure 1: SEM micrographs (a) No NaF (b) 120 Å NaF.

Initially CuIn1-xGaxSe2 (CIGS) absorbers were prepared by selenization of the precursors. 

Thin film solar cells prepared by using CIGS layers having Cu/(In+Ga) ratio of 0.83 had an 

efficiency of 12.33%. At this stage, the Cu/(In+Ga) ratio was further optimized to 0.86. Side-byside, selenization/sulfurization process was optimized to prepare CIGSeS layers in the 

conventional furnace. An efficiency of 13.73%, as measured at NREL under AM1.5 irradiance, 

was achieved with cells prepared using these CIGSeS layers (Fig 2). At the time, this was the 

highest officially measured efficiency for a small-area CIGSeS thin film solar cell prepared by a 

two-stage technique. 

FSEC # 44D 

CIGSS Cell 

-2

0

2

4

6

8

10

12

14

16

18

-0.2 0.0 0.2 0.4

Voltage (V)

Current (mA)

Area = 0.42 cm2

Voc = 0.5397 V

Jsc = 38.369 mA/cm2

Fill Factor = 66.33%

Efficiency = 13.73%

FSEC 44D

CIGSS

0

20

40

60

80

100

200 400 600 800 1000 1200 1400

Wavelength (nm)

Quantum Efficiency 

(%)

Device Area: 

0.442 cm2

Light Bias Density:

30.60 mA/cm2

Jsc (Global) =

43.1 mA/cm2

Figure 2: a) Current –voltage characteristics (I-V), b) Quantum efficiency (QE) characteristics of 

the cell having 8 nm layer of NaF and selenized/sulfurized
4 

3. RAPID THERMAL PROCESSING

CIGSeS thin-films were also prepared by rapid thermal processing (RTP). Glass 

substrates were coated with Mo back contact and Cu-In-Ga precursor layers by DC magnetron 

sputtering. Selenium and NaF were deposited by thermal evaporation. RTP was carried out in a 

mixture of H2S and nitrogen at a temperature of 550°C to prepare CIGSeS absorber films. 

CIGSeS thin film solar cells were completed similar to cells for CIGSeS absorber films prepared 

by selenization/sulfurization in a conventional furnace described earlier. Small-area cells with an 

efficiency of 12.78% were prepared by a two stage process using RTP for 

selenization/sulfurization (Fig 3). This is the highest, officially measured efficiency till date 

achieved by any University or National Lab, for small-area CIGSeS thin film solar prepared by 

two-stage technique using RTP. 

RTP CIGSS # 12E 

-2

0

2

4

6

8

10

12

14

16

-0.2 0.3

Voltage (V)

Current (mA)

Area = 0.42 cm2

Voc = 0.573 V

Jsc = 32.69 mA/cm2

Fill Factor = 68.25%

Efficiency = 12.78%

RTP CIGSS # 12E 

0

20

40

60

80

100

200 400 600 800 1000 1200 1400

Wavelength (nm)

Quantum Efficiency 

(%)

Device Area: 

0.4200 cm2

Light Bias Density:

 22.18 mA/cm2

Jsc (Global) =

36.9 mA/cm2

 

Figure 3: a) Current-Voltage (I-V) and b) Quantum efficiency (QE) Characteristics of cell 12E
5 

4. CIGS2 THIN FILM SOLAR CELLS

CIGS2 thin-films were prepared on Mo-coated sodalime glass substrates. Cu-In-Ga 

precursor layers were deposited on Mo back contact layers by DC magnetron sputtering. 

Sulfurization of these precursor layers was carried out in a mixture of H2S and nitrogen. Cells 

were completed by etching of the CIGs2 film in 10% KCN solution, deposition of CdS 

heterojunction partner layer by chemical bath deposition, i:ZnO/ZnO:Al window bilayer 

deposition by RF magnetron sputtering and Cr/Ag contact fingers by thermal evaporation. World 

record Voc of 830 mV was obtained on CuIn1-xGaxS2 (Fig 4). The efficiency (11.99 %) is the 

highest ever demonstrated using sputtering technique.

FSEC # 1020 CIGS2 Cell 

-2

0

2

4

6

8

10

-0.2 0.3 0.8

Voltage (V)

Current (mA)

Area = 0.43 cm2

Voc = 0.8305 V

Jsc = 20.88 mA/cm2

Fill Factor = 69.13%

Efficiency = 11.99%

Figure 4: Current-voltage (I-V) characteristics of CIGS2 solar cell

The CIGS2 thin-film solar cells development work was continued for thinner absorber 

layers. Figure 5 shows the current-voltage (I-V) characteristics measured at the National 

Renewable Energy Center (NREL) under AM1.5 conditions of CIGS2 solar cell prepared using a 

1.5 μm thick CIGS2 absorber layer. Photovoltaic characteristics of the CIGS2 thin film solar 

cells were as follows: short circuit current density, Jsc, of 19.46 mA/cm2

, open circuit voltage, 
6 

Voc, of 765.9 mV, fill factor, FF, of 67.85% and photovoltaic conversion efficiency, η, of 

10.12%. Figure 6 shows quantum efficiency (QE) characteristics. Figure 7 and Figure 8 exhibit 

current-voltage (I-V) and quantum efficiency (QE) characteristics for a 1.2 μm absorber 

respectively. Photovoltaic characteristics of the CIGS2 thin film solar cells were as follows: short 

circuit current density, Jsc, of 18.94 mA/cm2, open circuit voltage, Voc, of 775.9 mV, fill factor, 

FF, of 65.40% and photovoltaic conversion efficiency, η, of 9.62%. 

FSEC # 2387 ZnO/CdS/CIGS2/Cell

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6 0.8

Voltage (V)

Current (mA)

Area = 0.450 cm2

Voc = 0.766 V

Jsc = 19.46 mA/cm2

Fill Factor = 67.85 %

Efficiency = 10.12 %

Figure 5: Current-voltage (I-V) characteristics for a 1.5 μm CIGS2 absorber

Quatum Efficiency-2387

0

10

20

30

40

50

60

70

80

90

100

200 400 600 800 1000

Wavelength(nm)

Normalized QE(%)

Figure 6: Quantum efficiency (QE) characteristics for a 1.5 μm CIGS2 absorber
7 

FSEC # 1267 ZnO/CdS/CIGS2/Cell

0

1

2

3

4

5

6

7

8

9

0 0.2 0.4 0.6 0.8 1

Voltage (V)

Current (mA)

Area = 0.442 cm2

Voc = 0.776 V

Jsc = 18.948 mA/cm2

Fill Factor = 65.40 %

Efficiency = 9.62 %

Figure 7: Current-voltage (I-V) characteristics for a 1.2 μm CIGS2 absorber

Quatum Efficiency-1267

0

20

40

60

80

100

200 400 600 800 1000

Wavelength (nm)

Normalized QE(%)

Figure 8: Quantum efficiency (QE) characteristics for a 1.2 μm CIGS2 absorber
8 

5. ZnxCd1-xS AS ALTERNATIVE BUFFER LAYER FOR CIGS2 SOLAR 

CELLS

The aim of this work is to develop an alternative heterojunction partner for CIGS2 solar 

cells. ZnxCd1-xS has been shown to be a strong contender as an alternative heterojunction partner

[2-7]. Effect of various chemical bath deposition parameters on device performance was studied 

and attempts were made to optimize the deposition parameters in order to improve the device 

performance. Device properties were studied using current–voltage characteristics. The device 

performance of CIGS2/ ZnxCd1-xS solar cells was comparable to CIGS2/CdS solar cells. This 

indicates that with further optimization of the deposition parameters of the heterojunction partner 

and improvement in the absorber layer might lead to better device performance of 

CIGS2/ZnxCd1-xS solar cells as compared to CIGS2/CdS solar cells. Moreover, it should be 

noted that here the optimization of the deposition parameters were carried out on a ~2 μm thick 

CIGS2 absorber layer.

The current-voltage(I-V) characteristics (Figure 9) shows the highest efficiency cell 

fabricated with CdS and ZnxCd1-xS as heterojunction partners for 2 μm CIGS2 absorber. 

CIGS2/ZnxCd1-xS cell had an efficiency of 5.45%, whereas CIGS2/CdS cell had efficiency of 

6.21%. The CIGS2/CdS cell has series resistance value of 18.1 Ω, and shunt resistance value of 

2500 Ω. The CIGS2/ZnxCd1-xS cell has series resistance value of 25.9 Ω, and shunt resistance 

value of 1100 Ω. CIGS2/ZnxCd1-xS has higher VOC of 0.73 volts as compared to 0.71 V of 

CIGS2/CdS cell. Short circuit current density for CIGS2/ ZnxCd1-xS cell was 12.57 mA/cm2

. 

This value is lower than that of the CIGS2/CdS cell of 12.8 mA/cm2. It is expected that CIGS2 

solar cell with ZnxCd1-xS as the heterojunction partner should provide higher current density due 

to lower blue photon losses as compared to one with CdS as the heterojunction partner. 

However, an increase in the current density is not observed probably due to the highly resistive 

ZnS film as compared to CdS. Due to the higher resistance the fill factor of CIGS2/ZnxCd1-xS 

cell is 29.45%, lower than the fill factor value of 68.35% for CIGS2/CdS cell. 
9 

0

2

4

6

8

10

12

14

-0.2 0 0.2 0.4 0.6 0.8

Voltage (V)

Current density (mA/cm2

)

a

b

Figure 9: The J-V characteristics for (a) SLG/Mo/CIGS2/CdS/i-ZnO/ZnO:Al and (b) SLG/Mo/CIGS2/ 

ZnxCd1-xS / i-ZnO/ZnO:Al devices
10

6. ULTRA THIN CIGS/CIGSS ABSORBER LAYER

Experiments were carried out for reducing the thickness of the absorber layer from the 

present ~2.5 μm to ~ 9000 Å. The metallic precursors Cu-In-Ga were deposited using DC 

magnetron sputtering technique. The thickness of the metallic precursors was reduced

proportionately. Selenization and sequential selenization/sulfurization of these metallic 

precursors were carried out using the conventional and rapid thermal processing approaches 

respectively. Cells were completed by deposition of CdS heterojunction partner layer by 

chemical bath deposition, i:ZnO/ZnO:Al window bilayer by RF magnetron sputtering and Cr/Ag 

contact fingers by thermal evaporation. 

The cells prepared by rapid thermal processing were sent to NREL for current-voltage (IV) and quantum efficiency (QE) measurements which are shown in Figure 10. An efficiency of 

8.65% was achieved on CIGSeS thin-film solar cells using 0.9 μm thick CIGSeS absorber layer. 

The open circuit voltage was measured to be 560 mV which suggests that there was sulfur 

incorporation into the bulk of the material also the bandgap calculated from the quantum 

efficiency curve is about 1.13 eV which again suggests the bandgap shift due to sulfur 

incorporation. The short circuit current density measured using the current-voltage 

characteristics is 25.57 mA/cm2

. This lower value of short circuit current density could be due to 

the diffusion of sulfur into the bulk of the absorber rather than just the surface. This could also be 

because of poor absorber quality as can be seen from the quantum efficiency curve. The red 

response shows that the recombination losses are very high which results in the low current 

density. This suggests poor absorber film material.

The sulfur diffusion can be controlled by further optimization of the sulfur and sodium 

content which could lead to better currents. 
11

FSEC # 0.9μm RTP CIGSeS Cell 

-2

0

2

4

6

8

10

-0.2 0.0 0.2 0.4

Voltage (V)

Current (mA)

Area = 0.451 cm2

Voc = 0.5606 V

Jsc = 25.565 mA/cm2

Fill Factor = 60.4%

Efficiency = 8.65%

FSEC #0.9μm RTP CIGSeS Cell

-20

0

20

40

60

80

100

200 400 600 800 1000 1200 1400

Wavelength (nm)

Quantum Efficiency (%)

Device Area: 

0.451cm2

Light Bias Density:

12.00 mA/cm2

Jsc (Global) =

33.4 mA/cm2

Figure 10: a) Current-voltage (I-V) and b) Quantum efficiency (QE) Characteristics of 0.9 μm 

RTP CIGSeS cell
12

7. HUMAN RESOURCE DEVELOPMENT

During this project, Anant H. Jahagirdar and Ankur A. Kadam have completed their 

Ph.D. Anil U.Pai, Vinay V. Hadagali, Upendra S. Avachat, Jyoti S. Shirolikar, and Bhaskar 

Kumar have completed their master’s degrees. Two students, Sachin S. Kulkarni and Parag S. 

Vasekar are completing their Ph.D. dissertation this semester. Vinay V. Hadagali will also be 

completing his Ph.D. dissertation. Shirish Pethe has already completed his candidacy exam. 

Ashwani Kaul will be completing his candidacy exam during the spring 2009 semester. A new 

student, Eigo Takahashi has joined for M.S. (Materials Science and Engineering). 
13

8. SUMMARY AND CONCLUSION

The work carried out during this project presents the use of DESe or other organometallic 

precursors as a low toxicity alternative selenium sources for preparation of a high quality 

absorber. The addition of small quantity of sodium in the form of NaF layer was beneficial in 

improving the morphology of the film which showed large, well faceted and compactly packed 

CIGSeS grains. A small area cell with an efficiency of 13.73 % was prepared by sputtering and 

selenization/sulfurization in a conventional furnace. At the time, this was the highest officially 

measured efficiency for a small-area CIGSeS thin film solar cell prepared by a two-stage 

technique. 12.78 % efficiency with 2.0 μm absorber thickness has been achieved on CIGSeS 

films prepared by RTP. This is the highest, officially measured efficiency till date achieved by 

any University or National Lab, for small-area CIGSeS thin film solar prepared by two-stage 

technique using RTP. An efficiency of 8.65% with 0.9 μm absorber thickness has been prepared 

by RTP. 11.99% efficiency CIGS2 solar cells have been prepared with a world record Voc value 

of 830.5 mV. An efficiency of 10.2% was obtained on CIGS2 solar cell having 1.5 μm thick 

absorber and 9.62% on CIGS2 solar cell having a 1.2 μm thick absorber. ZnxCd1-xS has been 

demonstrated as an alternate heterojunction partner layer for CIGS2 cell.

Two approaches for reducing the cost of photovoltaics have been demonstrated. First, 

indium quantity can be reduced by preparing ultra-thin CIGSeS layer while maintaining device 

quality and second, high throughput can be obtained by RTP. 

Recently, the Florida Solar Energy Center has received substantial funding for 

development of renewable energy technologies. Part of this funding will be used to upgrade the 

equipment and facilities at the FSEC Photovoltaic Materials Laboratory. It may be noted that 

most of the equipment and facilities at the FSEC Photovoltaic Materials Laboratory were built 

several years ago, and hence the frequency of outage has increased recently. Now using the 

newly available funds the maintenance and, if necessary, replacement of the equipments and 

facilities will be carried so as to put the PV Materials lab back in good shape for carrying out 

future research and development. 
14

9. REFERENCES

[1] N.G. Dhere, and R.G. Dhere, JVST A, Vol.23, No.4, 1208-1214 (2005) 

[2] T.L. Chu, S.S. Chu, J. Britt, C. Ferekides, C.Q. Wu, J. Appl. Phys. 70,2688, (1991) 

[3] Von Hortmann and H treptow, Z. Naturforshg., 16a, 910 (1961) 

[4] D.W.G. Ballentyne and B. Ray, Physica, 27, 337(1961) 

[5] V. D. Vankar, S.R. Das, P.Nath and K.L. Chopra, Phys. Status.Solidi, 45, 665 (1978) 

[6] C.M.Mbow, D.LaPlaze and A. Cachard, Thin Solid Films, 88, 203(1982) 

[7] Y.F. Nicolau, M. Dupuy and M. Brunel, J. Electrochem. Soc. Vol. 137, No. 9 (1990), 2915. 
F1146-E(10/2008) 

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High Throughput, Low Toxic Processing of Very Thin, High 

Efficiency CIGSS Solar Cells: Final Report, December 2008

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

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N.G. Dhere

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NREL/SR-520-45544

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PVB91199

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Florida Solar Energy Center

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XXL-5-44205-08

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NREL

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NREL Technical Monitor: Bolko von Roedern

14. ABSTRACT (Maximum 200 Words)

During this project CuIn1-xGaxSe2-ySy (CIGSeS)/CdS thin-film solar cells were prepared on molybdenum-coated 

soda lime glass substrates. Low-toxicity selenium precursors, diethylselenium (DESe) or other organometallic 

precursors, were used for selenization of metallic precursors. Rapid thermal processing (RTP) was developed as an 

alternative to conventional selenization and sulfurization to reduce the process time and thermal budget, thus 

enhancing throughput. CuIn1-xGaxS2 (CIGS2) thin-film solar cells were also developed. As easily scaleable 

magnetron sputtering technique was used for deposition of back contact, metallic precursors, and transparent and 

conducting oxides. Compact, large-grain, 0.9-2.75-μm-thick absorber layers were prepared by 

selenization/sulfurization of elemental precursors at 475o-515oC in diluted DESe or other organometallic precursors 

and diluted H2S. Experiments were performed to reduce absorber thickness so as to minimize the indium 

consumption using both conventional, as well as the RTP, approach. Experiments were carried out on thinner (i.e., 

1.2 - 1.5 μm) CIGS2 absorbers. The effect of sodium addition on copper-poor CIGS2 absorbers and thin-film solar 

cells was studied. Alternate heterojunction partner layers were developed for CIGS2 thin-film solar cells.

15. SUBJECT TERMS

PV; CIGSS; high throughput; low toxicity; thin film; high efficiency; solar cells; organometallic precursors; rapid 

thermal processing; transparent conducting oxides; diethyselenium (DESe) 

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