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J. AbuShama, R. Noufi, S. Johnston, S. Ward, 

and X. Wu 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37411 

Improved Performance in 

CuInSe2 and Surface-Modified 

CuGaSe2 Solar Cells
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IMPROVED PERFORMANCE IN CuInSe2 AND SURFACE-MODIFIED 

CuGaSe2 SOLAR CELLS 

Jehad AbuShama*, Rommel Noufi, Steve Johnston, Scott Ward, and X. Wu

National Renewable Energy Laboratory, Golden, Co 80401, U.S.A. 

*Physics Department, Colorado School of Mines, Golden, CO 80401, U.S.A. 

ABSTRACT

In this paper, we present an update and review on the

progress made in the development of low-bandgap

CuInSe2 (CIS) and wide-bandgap CuGaSe2 (CGS) solar

cells. Our research project is primarily concerned with the

optimization of the bottom and top cells of the tandem

solar cell. This past year, we established new record totalarea efficiencies of 15.0% and 10.2% for CIS and surfacemodified CGS solar cells, respectively. These

achievements were possible by modifying the growth

process for CIS and CGS absorbers. We attempt to

modify the surface region of the CGS absorber to be

CIGS-like in composition. We also have designed a

mechanical-stacked tandem solar cell where the 15% CIS

cell serves as the bottom cell and transparent CdTe cell

serves as the top cell. The NREL-confirmed total-area

efficiency for this CdTe/CIS tandem device is 15.31%. 

INTRODUCTION

Polycrystalline thin-film solar cells based on CuIn1-

xGaxSe2 (CIGS) (where 0 ≤ x ≤ 1) have great potential for

photovoltaic applications. Total-area efficiencies 

exceeding 19% have been established for CIGS solar

cells [1,4]. In this contribution, we report new record totalarea efficiencies of 15.0% and 10.2% for CIS and surfacemodified CGS solar cells, respectively. These improved

efficiencies were possible by modifying the growth process

for CIS and CGS absorbers. CIS and CGS are important

candidates for bottom- and top-cell absorber materials for

thin-film tandem devices. We designed a 4-terminal

polycrystalline thin-film mechanical-stacked tandem cell

where our 15% CIS cell serves as the bottom cell and a 

newly developed transparent CdTe cell serves as the top

cell. The NREL-confirmed efficiency for this tandem cell is

15.3%. In this paper, we present material and device

properties for the new CIS and surface-modified CGS 

films and devices. 

EXPERIMENTAL

 Our technical approach is based on the following: 

 Growth: We investigated the dependence of the device

performance of CIS and CGS solar cells on the growth

conditions of the respective absorber layers. We deposited

CIS and CGS absorbers on Mo-coated soda-lime glass

substrates using the three-stage process. We deposited

the new CIS absorber (S2279) at temperatures about 15

oC higher than those used during the growth of the

previous one (S2044) [2]. We deposited the new CGS film

(S2194) under more Cu-rich conditions and with the

addition of a very small amount of In at the end of the 

growth (with In/(In+Ga) ≤ 0.8%). Therefore, S2194 will be

referred to as surface-modified CGS. 

 Device Fabrication: We deposited 500-600 Å of CdS

using chemical-bath deposition (CBD) at a bath

temperature of about 65 oC. We deposited the ZnO layer

using sputtering in Ar/O2 working gas. An undoped ZnO

layer 90 nm thick was deposited from a pure ZnO target, 

followed by an Al-doped ZnO layer about 120 nm thick 

deposited from an Al2O3-doped ZnO target. The ZnO 

bilayer has a sheet resistance of about 65–70Ω/square.

We deposited the Ni/Al grids by electron-beam

evaporation. We defined the cell area of our CIS cells by

photolithographic procedure and by chemical etching, 

whereas for the modified CGS cells, the cell area was

defined using mechanical scribing. We then deposited a

100-nm-thick MgF2 antireflection coating using sputtering. 

 Characterization: We analyzed our devices using a

number of device characterization techniques. We used

deep-level transient spectroscopy (DLTS) and admittance

spectroscopy (AS) techniques to study the defects in our

solar cells. We also used a number of materials

characterization techniques (e.g. X-ray diffraction (XRD),

transmission electron microscopy (TEM), scanning Kelvin

probe microscopy (SKPM), photoluminescence (PL), 

cathodoluminescence (CL)) to examine our absorbers.

Due to lack of space here, we will not include all data. 

In this paper, we present a comparison in device

performance between the previous and new record CIS 

and CGS cells. 

RESULTS AND DISCUSSION 

 We demonstrated improved efficiency of 10.2% for 

surface-modified CGS solar cells. The NREL-confirmed 

device operating parameters for this cell are given in Table

1. For comparison, the device parameters for the previous 

CGS cell (S2087) [3] are also given. Figure 1 shows the

current-voltage (I-V) data for the new modified CGS cell.

CGS is a candidate top cell absorber material for thin- film

tandem devices. Its bandgap is ideal at 1.67 eV. This

particular device had a bandgap of 1.64 eV. Improving

CGS device efficiency has proven to be a challenge over 

the past several years. The recent understanding of the

differences in structural and electronic properties between

CIGS and CGS thin films and devices has led to varying

the growth process in a way that is likely to make the CGS

surface region similar to that of CI(G)S and to minimize

1 
defects in the material. This change led to a gain in the

current density of about 3.7 mA/cm2 versus the previous 

Fig. 2. Total-area current-voltage characteristics of

the 15.0% ZnO/CdS/CuInSe2 solar cell. Cell

measured under standard reporting conditions (1000

W/m2

, AM1.5 Global spectrum at 25 oC. Total area of

the cell is 0.403 cm2

). 

record cell. 

We also demonstrated an improved total-area efficiency 

of 15.0% for CIS solar cells. The NREL-confirmed device 

parameters for this cell (S2279) are given in Table 1. 

Figure 2 shows the I-V data for this cell. For comparison, 

device parameters for the previous CIS cell (S2044) [2]

and record CIGS [4] cell are also given in Table 1. 

Figure 3 shows the absolute external quantum

efficiency (QE) curves for the previous (S2087) and new

(S2194) CGS cells. It also shows the absolute external QE 

for the new CIS cell (S2279). The better QE gain and the 

higher currents for sample S2194 resulted from a modified

deposition scheme and a modified surface of the CGS

absorber. This figure reveals higher overall collection for 

the 10.2% cell compared to the 9.53% one. About 30% of 

the increase in current comes from a slight decrease inthe

energy bandgap, whereas the remaining current (~70%) 

comes from better overall collection. In sample S2194, 

the Voc and the fill factor decreased compared to sample

S2087, which indicates that the recombination 

mechanisms are different in the two devices. 

Table 1. Device parameters for previous (S2044 and

S2087) and improved (S2279 and S2194) CIS and CGS 

cells, respectively. Device parameters for record CIGS cell

are also included [4]. 

Sample 

Number 

Voc 

(V) 

Jsc 

(mA/cm2) 

Fill 

factor 

(%) 

Totalarea 

efficiency 

(%) 

S2194-B23 0.823 18.61 66.77 10.2

S2087-B33 [3] 0.905 14.88 70.79 9.53 

S2279-B31 0.491 40.58 75.15 15.0

S2044-A35 [2] 0.491 41.1 71.9 14.5 

CIGS [4] 0.692 35.22 79.87 19.5 

80

60

40

20

0

Absolute External QE (%)

400 600 800 1000 1200 1400

 Wavelength (nm)

S2087

S2194

S2279

Fig. 1. Total-area current-voltage characteristics 

of the 10.2% surface-modified CuGaSe2 solar

cell. Cell measured under standard reporting

conditions (1000 W/m2

, AM1.5 Global spectrum 

at 25oC. Total area of the cell is 0.419 cm2

). 

Fig. 3. Absolute external quantum efficiency (QE) for 

surface-modified CGS cell, S2194-B2#3. For

comparison, absolute external QEs for new CIS cell [2]

and our previous [3] CGS cell are also included.

2 
For the new record CIS cell (S2279), the absolute

external QE is almost identical to that of the previous one

(S2044) [2]. The major improvement in device 

performance comes from the fill factor. The increase in fill

factor is 4.4% (70.79% for S2044, compared to 75.15% for 

S2279). The series and shunt resistance for sample

S2044-A35 are ~1.2 and ~1893 Ω-cm, and those for

sample S2279-B31 are ~1.1 and ~3000 Ω-cm,

respectively. This higher shunt resistance for sample

S2279-B31 may be responsible for the higher fill factor.

The Voc and Jsc for samples S2044-A35 and S2279-B31

are almost the same. 

We designed a 4-terminal polycrystalline thin-film

mechanical-stacked tandem cell where our 15% CIS cell

serves as the bottom cell and a newly developed 

transparent CdTe with an NREL-confirmed total-area

efficiency of 13.8% [5] serves as the top cell. This

mechanical-stacked tandem has an NREL-confirmed new

record efficiency of 15.31%, which exceeds a U.S. 

Department of Energy program milestone of 15%. The

newly designed mechanical-stacked tandem will be the

subject of another publication. 

Our group is also focusing on developing CGS/TCO as 

the top cell for the tandem device. Given the CIS and CGS 

bandgaps of 1.0 and 1.67 eV, respectively, we expect the

photovoltaic performance of the CIS bottom cell under a

CGS top cell to be better than if it is under a CdTe cell. We

designed a mechanical-stacked tandem where CIS serves

as the bottom cell and CGS serves as the top cell. The 

NREL-confirmed efficiency for this tandem is 9.7%. 

Figure 4(a) shows the cross-sectional scanning electron

microscopy (SEM) images for the surface-modified CGS 

film (S2194), previous CGS film (S2171), and a thicker

CGS film (S1820). The thickness of these films are 2.3, 

1.8, and 3.0 μm, respectively. Figure 4(a) shows large 

columnar grains of the surface-modified CGS film. This

image also shows distinct surface structure, as compared

to the cross-sectional SEM image of the previous CGS

film shown in Fig. 4(b). We believe that this different

surface structure of the surface-modified CGS film is due

to a modified growth process and the addition of a small

amount of In at the end of the growth. Figure 4(c) shows

the cross-section SEM image of a CGS film grown using 

the three-stage process, but with a thickness of 3.0 μm,

which is comparable to that of CIGS [1]. This figure shows

a different grain structure compared to the structures 

shown in Fig. 4(a) and Fig. 4(b). The grains for sample

S1820 are smaller in size, and the image shows a layer of

smaller grains toward the back of the film (close to the Mo

layer). Therefore, reducing the thickness of the CGS film 

caused better grain growth, which resulted in better device

performance. 

Figure 5 shows the cross-sectional SEM image for the

CIS film used in the 15.0% new record CIS device. This 

image shows larger grains compared to those we reported 

or the CIS film used in our previous CIS record device [2]. 

It also shows better grain growth compared to our

pervious film [2]. This better grain growth and larger grain

size in the new CIS film may be due to the elevated 

substrate temperature we used during the growth of the

new CIS film, as compared to our previous film [2].

(a) 

(b) 

(c) 

S1820

Fig. 4. Cross-sectional scanning electron microscopy

images for (a) modified CGS film used in 10.2% device

(S2194), (b) CGS film used in previous CGS device

(S2171) [3], and (c) a CGS film (with ~8% efficiency)

(S1820). 

3 
250x1012

200

150

100

50

0

Defect density (cm-3) 

10-5 10-4 10-3 10-2 10-1 100 101

Filling Pulse Width (s)

 S2171

(previous CGS)

 S2194 

(modified CGS)

 

We examined our modified and previous CGS cells

using deep level transient spectroscopy (DLTS). The

DLTS data for the modified CGS sample exhibit deeper

minority traps compared to our previous CGS. Figure 6

shows the trap density versus filling pulse width for

improved CGS cell and our previous one. As the pulse

duration varies from 10 μs-10 s, the activation energy

changes from EC-0.3 to EC-0.6 eV (where Ec is the energy

of electrons at the conduction band minimum) in the

modified CGS cell and from EC-0.06 to EC-0.1 eV in the

previous CGS cell. We also observed the gradual increase

in the amplitude of the DLTS signal until it apparently 

CGS

is 

hort pulse durations. Figure 6 shows lower defect

py analysis. This work is 

upported by DOE Contract No. DE-AC36-99GO10337. 

. AbuShama was supported by NREL subcontract No.

XEA-3-33607-03 to the Colorado School of Mines. 

REFERENCES 

] K. Ramanathan, M. Contreras, C. Perkins, S. Asher, F.

asoon, J. Keane, D. You . Metzger, R. 

, J. Ward 19.2%

oved

Performance in ZnO/CdS/CuGaSe in-Film Solar Cells”, 

Prog. Photo volt: Res 003). 

Fig. 5. Cross-sectional scanning electron microscopy

image for CIS film used in 15% new record CIS device. 

saturates at pulse durations of ~1s in the modified

and ~0.5s in the previous CGS (as shown in figure 6). Th

gradual increase is due to partial filling of the states with

s

density in the new modified CGS cell compared to the 

previous one. This decrease in defect density in the 

modified CGS cell is due to improved quality of the surface

region of the modified CGS absorber layer as a result of a 

modified deposition scheme and a modified surface of the

CGS absorber layer. 

SUMMARY 

We modified the growth process for CIS and CGS thin

films. This modification resulted in new record total-area

efficiencies of 15.0% and 10.2%, respectively, for these

cells. The improved device performance in the new record

CGS cell resulted, in part, from varying the growth process

in a way that is likely to make the CGS surface region

similar to that of CI(G)S and to minimize defects in the

aterial. The new CIS record efficiency may have resulted

from a modified temperature profile, in which we used

elevated temperatures during the three stages of the 

three-stage growth process. 

ACKNOWLEDGEMENTS 

 

The authors acknowledge the technical support of J. 

Dolan and J. Keane. The authors thank T. Moriarty for the

I-V and QE measurements and K. Ramanathan for his

support. The authors thank M. Young for the QE

measurements. The authors also thank B. To for the 

canning electron microsco

Fig. 6. Defect density vs. pulse width for improved

modified CGS cell (S2194) and for previous CGS cell

(S2171). 

s

s

J

K

[1

H ng, M. Romero, W

Noufi , and A. Duda, ”Properties of 

Efficiency ZnO/CdS/CuInGaSe2 Thin-film Solar Cells”,

Prog. Photovolt. Res. Appl. 11, 225 (2003). 

[2] J. A. M. AbuShama, S. Johnston, T. Moriarty, G.

Teeter, K. Ramanathan, and R. Noufi, “Properties of

ZnO/CdS/CuInSe2 Solar Cells with Improved

Performance”, Prog. Photovolt: Res. Appl. 12, 39 (2004). 

[3] D. Young, J. Keane, A. Duda, J. AbuShama, C.

Perkins, M. Romero, and R. Noufi, “Impr

2 Th

. Appl. 11: 535 (2

[4] M. Contreras and J. AbuShama (private

communication). 

[5] X. Wu and J. AbuShama (private communication). 

4 
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Improved Performance in CuInSe2 and Surface-Modified CuGaSe2

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14. ABSTRACT (Maximum 200 Words)

In this paper, we present an update and review on the progress made in the development of low-bandgap CuInSe2 

(CIS) and wide-bandgap CuGaSe2 (CGS) solar cells. Our research project is a primarily concerned with the 

optimization of the bottom and top cells of the tandem solar cell. This past year, we established new record totalarea efficiencies of 15.0% and 10.2% for CIS and surface-modified CGS solar cells, respectively. These 

achievements were possible by modifying the growth process for CIS and CGS absorbers. We attempt to modify the 

surface region of the CGS absorber to be CIGS-like in composition. We also have designed a mechanical-stacked 

tandem solar cell where the 15% CIS cell serves as the bottom cell. The NREL-confirmed total-area efficiency for 

this tandem device is 15.31%. 

15. SUBJECT TERMS

PV; CuInSe2 (CIS); CuGaSe2 (CGS); tandem solar cell; device; low-band gap; wide-band gap; surface-modified; 

absorbers; total-area efficiency; 

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