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Contract No. DE-AC36-99-GO10337 

K. Ramanathan, J. Keane, and R. Noufi

Prepared for the 31st IEEE Photovoltaics Specialists 

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Lake Buena Vista, Florida 

January 3-7, 2005 

February 2005 • NREL/CP-520-37404 

Properties of High-Efficiency 

CIGS Thin-Film Solar Cells 
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PROPERTIES OF HIGH-EFFICIENCY CIGS THIN-FILM SOLAR CELLS 

Kannan Ramanathan, James Keane, and Rommel Noufi 

National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden CO 80401 

ABSTRACT 

We present experimental results in three areas. Solar 

cells with an efficiency of 19% have been fabricated with 

an absorber bandgap in the range of 1.1-1.2 eV. Properties of solar cells fabricated with and without an undoped 

ZnO layer were compared. The data show that high efficiency cells can be fabricated without using the highresisitivity or undoped ZnO layer. Properties of CIGS solar 

cells were fabricated from thin absorbers (1 μm) deposited 

by the three-stage process and simultaneous codeposition of all the elements. In both cases, solar cells 

with efficiencies of 16%-17% are obtained. 

INTRODUCTION

CuInGaSe2 (CIGS) polycrystalline thin film photovoltaic cells are a realistic option for reaching the goal of 

low-cost, high efficiency power conversion from renewable 

energy sources. Several research groups have reported a 

steady increase in the efficiency of laboratory devices [1-

3]. Similar progress has been reported in the manufacturing arena, where the efficiency of champion modules has 

exceeded 13% and the yield is above 80% [4]. However, 

there is a large gap between the efficiency of laboratory

cells and commercial modules. This can be narrowed by a 

concerted effort at the fundamental science of thin film 

materials and interfaces. Historically, the community has 

concentrated its efforts on the fabrication and physical 

properties of the materials themselves and much less on 

the electronic processes that occur at the interfaces. More 

attention to this area can lead to a simplification of device 

structure, and improve the stability and yield of the products. At NREL, we have systematically improved the efficiency of the CIGS solar cells fabricated by the threestage process. Although the deposition method is somewhat difficult to implement in manufacturing, it offers advantages in control of morphology, defects, orientation, 

bandgap grading, and stoichiometry control. The most 

efficient CIGS thin film solar cells have been fabricated by

the three-stage process. It is now possible to compare the 

properties of the three-stage devices with those fabricated 

by other methods and identify areas for improvement. 

In this report, we summarize the properties of high efficiency cells fabricated by varying the Ga content of the 

absorber. Improvement in our understanding of the 

growth process has resulted in our ability to fabricate solar 

cells with efficiencies of 18%-19% in the bandgap range of 

1.1-1.2 eV. Next, we provide a comparison of the properties of solar cells fabricated using a bilayer ZnO and a 

single, conductive ZnO layer. Finally, we present the 

properties of solar cells fabricated on thin absorbers.

EXPERIMENTAL

CIGS absorbers were grown on soda-lime glass substrates with a sputter-deposited Mo layer. The absorber is 

grown by first depositing an (InGa)2Se3 layer and reacting 

it with Cu and Se. Compositional control was achieved by

detecting the temperature change of the substrate during 

Cu-poor to Cu-rich transition at the end of the second 

stage. The third stage consists of the evaporation of In 

and Ga in the presence of Se. Thinner CIGS absorbers 

were deposited by reducing the (In,Ga)2Se3 film thickness 

and making appropriate modifications to the process. Thin 

absorbers were also prepared by simultaneous deposition 

of all the elements in one step. CdS deposition was performed using a solution consisting of 0.0015M CdSO4, 

1.5M NH4OH, and 0.0075M thiourea. The samples were 

immersed in the bath at room temperature and the temperature of the bath was increased to 60 ̊C. CdS thin films 

in the thickness range of 50-60 nm were deposited in 16 

min. The ZnO layer was deposited in two stages. A 90- 

nm-thick, undoped layer was first deposited from a pure 

ZnO target using Ar/O2 working gas, and a second layer of 

about 120 nm was deposited from an Al2O3 doped ZnO 

target. The sheet resistance of the bilayer was about 65-

70 Ω/sq. Ni/Al grids were deposited by electron beam 

evaporation. A 100-nm thick MgF2 film was deposited to 

serve as an antireflection coating. Current-voltage characteristics of the devices were measured under AM1.5 

Global spectrum for 1000 W/m2

 irradiance. 

RESULTS AND DISCUSSION 

High Efficiency Solar Cells 

CIGS absorbers with a range of energy gaps were 

fabricated by adjusting the Ga content of the (In,Ga)2Se3

precursor layer. Compositional depth profiles show that 

the bandgap is graded through the depth of the film. The 

energy gap was evaluated from spectral response measurements, and the Ga/(In+Ga) ratio was derived from the 

electron microprobe data obtained using 10-kV beam energy. This is indicative of the average composition in a 

sample volume of approximately 1 μm. The energy gap 

ranged from 1.1 to 1.21 eV, and the corresponding 

Ga/(In+Ga) ratios ranged from 0.26 to 0.31. Table 1 

shows a summary of the properties of solar cells characterized under standard reporting conditions. Table 2 lists 

1 
the ideality factor and the reverse saturation current density of the solar cells. 

Table I. Parameters of high-efficiency CIGS solar cells. 

Sample 

Number

Voc 

(V)

Jsc

(mA/cm2

)

Fill 

factor 

(%)

Efficiency

(%)

S2212B1-3 0.701 34.60 79.65 19.3

S2212B1-4 0.704 34.33 79.48 19.2

S2212B1-5 0.703 34.08 79.23 19.0

S2213A1-3 0.740 31.72 78.47 18.4

S2213A1-4 0.737 31.66 78.08 18.2

S2229A1-3 0.720 32.86 80.27 19.0

S2229A1-5 0.724 32.68 80.37 19.0

S2229B1-2 0.731 31.84 80.33 18.7

S2229B1-4 0.728 31.87 80.16 18.7

S2232A1-3 0.703 33.94 79.67 19.0

S2232A1-4 0.704 33.83 80.09 19.1

S2232B1-2 0.717 33.58 79.41 19.1

S2232B1-3 0.713 33.38 79.54 18.9

Table II. Diode parameters determined from light currentvoltage curves. J0 is the reverse saturation current density, and n is diode ideality factor. 

Device Number Jo (A/cm2

) n

S2212-B1-3 5x10-11 1.35

S2212-B1-4 6x10-11 1.36

S2212-B1-5 6x10-11 1.35

S2213-A1-3 4x10-10 1.57

S2213-B1-4 5x10-10 1.62

The above results suggest that high efficiency CIGS solar 

cells can be fabricated up to a bandgap of about 1.2 eV. 

The advantage of using a higher band gap is the higher 

open-circuit voltage and a lower temperature coefficient. 

This also offers the possibility to reduce the number of 

cells in a module. High voltages can be maintained along 

with high fill factors, and the junctions are characterized by

low ideality factors and recombination (Jo). 

 

Simplifying Window Layer Structure

In the fabrication of CIGS solar cells, it is customary

to use a high-low, resistivity grading of the ZnO layer. An 

undoped layer of ZnO (high resisitivity) is first deposited on 

the CdS, and this is followed by the deposition of a doped 

layer. Maintaining adequate transparency and achieving 

resistivity control of films sputtered from two separate targets is not a trivial problem. Moreover, it is not clear 

whether the undoped layer is an essential part of the device and what role it plays in the junction. If it plays an 

insignificant role, it can be removed. To evaluate this, we

have fabricated solar cells using the bilayer ZnO and the 

doped layer only. Absorbers from five growth runs were 

used for this purpose. Ten samples of each type were 

prepared on adjacent parts of the absorbers. Each sample contained five solar cells. Hence, the properties of 

solar cells with bilayer ZnO could be compared against 

those with the doped layer fabricated on the same absorber and also against devices fabricated on four other 

absorber runs. Thus, the distribution of 50 cells of each 

type becomes a statistically significant population. Figure 

1 shows the distribution of Voc, Jsc, and Fill factor. 

0.72

0.68

0.64

0.60

Voc(V)

0 10 20 30 40

Cell Number

-34

-32

-30

-28

-26

Jsc(mA/cm2

)

0 10 20 30 40

Cell Number

85

80

75

70

65

Fill Factor (%)

0 10 20 30 40

Cell Number

Fig. 1. Voc, Jsc and fill factor of CIGS solar cells. 

Solid circles: doped ZnO layer only; open circles: 

bilayer ZnO. 

An inspection of the data in Fig. 1 reveals that the parameters of the solar cells made without the undoped ZnO layer 

are identical to those made with the bilayer. It must be 

pointed out that each sample in this experiment has a spatial variation in the Ga content along its length, and a similar run-to-run variation exists among the five samples. 

The experiment was repeated on another sample where 

the CdS layer was deliberately made much thinner than 

our standard process and similar results were obtained. 

The average values of Voc, Jsc, FF, and efficiency for the 

two cases are shown in Table 3. No AR coating was deposited on the above samples. 

2 
Table 3. Properties of CIGS solar cells with doped ZnO 

layer and bilayer ZnO. Values for the bilayer are given in 

parentheses. No AR coating was deposited. 

Voc (V) Jsc

(mA/cm2

) 

Fill Factor Efficiency

(%) 

0.686 

(0.685) 

32.08 

(32.06) 

0.77 

(0.76) 

17 

(16.8) 

Thin CIGS Solar Cells

The price and availability of Indium will become dominant concerns for the CIS PV industry when larger-scale 

production gets under way. One approach to mitigate this 

would be to reduce the thickness of the absorber layer to a 

half or quarter of the thickness presently used (2-2.5 μm). 

Indeed, several research groups have explored this issue 

in some detail [5,6]. We have also begun a study of this 

problem since it has been chosen as a priority research 

topic for our program. We have made minor modifications 

to the three-stage process to prepare thinner absorber 

layers. We have also deposited thin CIGS layers by a 

single-step co-deposition of all the elements. Absorbers 

prepared by the three-stage process may be graded band 

gap materials, whereas co-deposition can provide uniform 

band gap. A comparison of the materials and devices 

prepared by the two methods would then allow us to separate out the contributions due to bandgap grading. Fig. 2 

shows cross-sectional views of the samples obtained by

scanning electron microscopy. 

Solar cells fabricated from the three-stage absorber exhibited open circuit voltages in the range of 650-660 mV, current density of 32-33 mA.cm-2, fill factors of 77-78%, and 

the best conversion efficiency was 16.5%. When we compare the Jsc of these cells with those of the standard cells 

in Table 1, we find that the current loss due to thinning the 

cell is about 2-3 mA.cm-2. The external quantum efficiency

of a solar cell is shown in Fig. 3. The spectral response 

curve shows a decrease of the long wavelength collection. 

This is most likely due to incomplete absorption of the long 

wavelength photons. 

100

80

60

40

20

0

External QE(%)

400 600 800 1000 1200

Wavelength (nm)

1μm cell

Std cell

Fig. 3. Quantum efficiency of a thin CIGS solar cell. 

Also shown is the quantum efficiency of a standard 

cell. 

Fig. 2. SEM images of thin CIGS absorbers grown 

by three-stage (top) and co-deposition (bottom) 

processes. The bottom layer in each image is a 

two-layer Mo film. 

We have obtained a similar result with absorbers fabricated by co-deposition. The composition of the absorber, 

determined by electron probe microanalysis was similar to 

the three-stage absorber. In this case, we obtained an 

efficiency of nearly 17%. The parameters of the best cells 

obtained by the two methods are shown in Table 4, below. 

Further optimization of the growth processes could lead to 

a higher efficiency. We are also undertaking a study of the 

recombination processes. 

Table 4. Properties of 1-μm thick CIGS solar cells fabricated by three-stage process and co-deposition; the latter 

are shown in parentheses. 

Voc (V) Jsc

(mA/cm2

) 

Fill Factor Efficiency

(%) 

0.648 

(0.703) 

32.6 

(32.1) 

0.78 

(0.75) 

16.5 

(16.9) 

CONCLUSIONS

We have reported the properties of CIGS cells with 

bandgaps in the range of 1.1-1.2 eV and demonstrated the 

ability to achieve high efficiency. High open-circuit voltages are obtained in conjunction with high fill factors. A 

study of the properties of the solar cells with and without 

the undoped ZnO layer strongly suggests that highly efficient cells can be fabricated without using the undoped 

ZnO layer even when the CdS layer is made very thin. 

3 
[2] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. 

Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W.

Metzger, R. Noufi, J. Ward, and A. Duda, “Properties of 

19.2% efficiency ZnO/CdS/CuInGaSe2 thin film solar 

cells,” Prog. Photovolt: Res. Appl. 11, 2003, pp. 225-230. 

This might allow simplification of the device processing. 

We have also reported high efficiency cells (16%-17%) 

using CIGS absorbers in the thickness range around 1 

μm. 

[3] J. Kessler, M.Bodegard, J. Hedstrom, and L. Stolt, 

“Baseline Cu(In,Ga)Se2 device production: control and 

statistical significance,” Solar Energy Materials and Solar 

Cells, 67, 2001, pp. 67-76. 

ACKNOWLEDGMENTS 

This work was performed for the U.S. DOE PV Program 

under Contract No. DE-AC36-99GO10337 to NREL. The 

authors would like to thank M.A. Contreras, F.S. Hasoon, 

and R. Bhattacharya for helpful discussions; B. To, J.S. 

Ward, J. Dolan and J. Alleman for technical assistance; T.

Moriarty for cell characterization; and L.L. Kazmerski for 

his encouragement. 

[4] D.E. Tarrant, R. Gay, V. Probst, and F. Karg, “CIS thin 

film development and product status at Shell Solar, May

2003,” 3rd WCPEC, Osaka, Japan, May 2003. 

[5] T.Negami, S. Nishiwaki, Y. Hashimoto, and N. Kohara, 

“Effect of the absorber thickness on performance of 

Cu(In,Ga)Se2 solar cells,” Proceedings of the 2nd 

REFERENCES WCPEC, Vienna, 1998; pp. 1181–1184. 

[6] O. Lundberg, M. Bodegard, J. Malmstrom, and L. Stolt, 

Influence of the Cu(In,Ga)Se2 thickness and Ga grading 

on solar cell performance,” Prog. Photovolt: Res. Appl: 11, 

2003, pp. 77-88.

[1] T. Negami, Y. Hashimoto, and S. Nishiwaki, 

“Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 

18%,” Solar Energy Materials and Solar Cells, 67, 2001, 

pp. 331-335. 

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We present experimental results in three areas. Solar cells with an efficiency of 19% have been fabricated with an 

absorber bandgap in the range of 1.1-1.2 eV. Properties of solar cells fabricated with and without an undoped ZnO layer 

were compared. The data show that high efficiency cells can be fabricated without using the high-resisitivity or undoped 

ZnO layer. Properties of CIGS solar cells were fabricated from thin absorbers (1 μm) deposited by the three-stage 

process and simultaneous co-deposition of all the elements. In both cases, solar cells with efficiencies of 16%-17% are 

obtained. 

15. SUBJECT TERMS

PV; high efficiency; solar cells; window layer structure; absorber bandgap; three-stage process; 

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