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Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Improved Energy Conversion 

Efficiency in Wide-Bandgap 

Cu(In,Ga)Se2 Solar Cells

Preprint

Miguel Contreras, Lorelle Mansfield, 

Brian Egaas, Jian Li, Manuel Romero, 

and Rommel Noufi 

National Renewable Energy Laboratory

Eveline Rudiger-Voigt and Wolfgang Mannstadt 

Schott AG

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50669

July 2011
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1 

IMPROVED ENERGY CONVERSION EFFICIENCY IN WIDE BANDGAP Cu(In,Ga)Se2

SOLAR CELLS

Miguel A. Contreras1, Lorelle M. Mansfield1, Brian Egaas1, Jian Li1, Manuel Romero1, Rommel Noufi1

Eveline Rudiger-Voigt2

, Wolfgang Mannstadt2 

1

National Renewable Energy Laboratory, Golden, CO, USA 2

Schott AG, Mainz, Germany

ABSTRACT

This report outlines improvements to the energy 

conversion efficiency in wide bandgap (Eg>1.2 eV) solar 

cells based on CuIn1-xGaxSe2. Using (a) alkaline 

containing high temperature glass substrates, (b) elevated 

substrate temperatures 600 ̊C-650 ̊C and (c) high vacuum 

evaporation from elemental sources following NREL’s 

three-stage process, we have been able to improve the 

performance of wider bandgap solar cells with 

1.2<Eg<1.45 eV. Initial results of this work have led to 

efficiencies >18% for absorber bandgaps ~1.30 eV and 

efficiencies ~16% for bandgaps up to ~1.45 eV. In 

comparing J-V parameters in similar materials, we 

establish gains in the open-circuit voltage and, to a lesser 

degree, the fill factor value, as the reason for the improved 

performance. The higher voltages seen in these wide gap 

materials grown at high substrate temperatures may be 

due to reduced recombination at the grain boundary of 

such absorber films. Solar cell results, absorber materials 

characterization, and experimental details are reported.

INTRODUCTION

CuIn1-xGaxSe2 (CIGS)-based solar cells have achieved the 

highest energy conversion efficiency (>20%) among 

photovoltaic (PV) thin-film materials. Those solar cells are 

typically made from CIGS alloys with low Ga content 

(x~0.3) resulting in absorber energy bandgap values ~1.1-

1.2 eV. It is desirable to develop wider bandgap absorbers 

for a variety of reasons and not just to tune Eg to 1.4 eV, 

as suggested by the theoretical calculations for optimum 

conversion efficiency.

In practice, low Eg solar cells suffer from a great loss of 

power output when their operating temperature becomes 

>>25 ̊C. This is referred to as the thermal coefficient of 

efficiency (or power) loss and sometimes is quoted as a 

percentage loss of power (or voltage) in commercial 

modules. Crystalline silicon (Eg~1.1 eV), for example, has 

a -0.50%/ ̊C efficiency loss, which is a little worse than 

CIGS at -0.44%/ ̊C (Eg~1.2 eV). On the other hand, other 

thin-film PV technologies like a-Si and CdTe (Eg~1.45 eV) 

show coefficients of power/efficiency loss of -0.17%/ ̊C 

and -0.25%/ ̊C respectively. Since modules under real 

operating conditions can get as hot as 50  ̊C -75 ̊C 

depending on location and evidently time of the year, it 

means that the efficiency (or power output) of low 

bandgap cells and modules are effectively de-rated by at 

least 11% and in some instances as high as 25%. It is also 

desirable to have solar cells (and modules) that produce 

higher output voltages and a lower current (as wider gap 

solar cells should) in order to minimize the intrinsic 

resistive losses (R*I2

) in a module or cell.

As mentioned above, historically the high efficiency regime 

for CIGS has been limited to the low Eg materials and 

efficiencies attained for CIGS materials with Eg>1.2 eV 

have been low [1,2]. In this paper, we report on recent 

improvements to CIGS materials with Eg>1.2 eV and 

discuss the solar cell parameters, processes, and 

materials physical properties that have enabled high 

efficiency (>15%) for Eg values (so far) up to ~1.45 eV. 

CIGS GROWTH AND SOLAR CELL FABRICATION

The standard growth of high efficiency CIGS thin-films 

requires the use of high substrate temperatures (~600 ̊C) 

and a clear correlation between this parameter and the 

performance attained from those films has been 

established: in brief, the higher the substrate temperature 

the better the performance, see for instance ref [3]. The 

CIGS community has been limited to the use of soda-lime 

glass (SLG) or stainless steel as a substrate choice for 

reasons of cost, but such substrates also limit the degree 

of freedom in this important parameter. In such substrates, 

processing temperatures of 550 ̊C or less are commonly 

used to avoid deformation and/or adhesion problems in 

the case of the glass or to limit the diffusion of unwanted 

impurities from the steel into the CIGS absorber film. 

Other glasses and oxides, such as borosilicate glass 

(BSG) or alumina plates have been used in CIGS R&D, 

but the lack of sodium and a non-optimum thermal 

coefficient of expansion relative to the Mo back contact 

make them a less attractive choice.

The development and availability of a high temperature 

glass tailored to CIGS technological needs, such as: (1) a 

much higher softening point than that of SLG, (2) a 

matching of the thermal coefficient of expansion to the 

Molybdenum back contact, and (3) a source of sodium, 

has enabled a revisit of the CIGS performance for 0<x<1 

with an emphasis on x>0.3. We have studied the behavior 

and quantified the performance of solar cells fabricated on 

SCHOTT specialty glass. And, we have used the NREL 

three-stage process for the fabrication of the absorbers 

and selected substrate temperatures in the range of 600 ̊C 

to 650 ̊C. In the following sections, we will refer to the 

CIGS materials obtained in this study as high temperature

CIGS to help in our presentation of data and discussion.
2 

All solar cells fabricated have a standard 

ZnO/CdS/CIGS/Mo layered structure where the 

molybdenum back contact of the solar cells is deposited 

by DC sputtering; the CdS by chemical bath deposition 

and the ZnO by RF sputtering from ceramic targets. The 

small area (~0.4 cm2

) solar cells fabricated in this study 

also incorporate a top contact grid with ~5% obscuration 

loss made of nickel and aluminum and deposited by 

electron beam evaporation. Selected champion solar cells 

were coated with an antireflection layer of MgF2. 

SOLAR CELL RESULTS

CIGS solar cells made in this study were analyzed by 

current density-voltage (J-V), external quantum efficiency 

(EQE) and capacitance-voltage (C-V) measurements. The 

J-V characteristics obtained for the many cells (>300) 

fabricated in this work are perhaps best summarized by

certified J-V measurements for a selected few champion 

cells made with high temperature absorbers of varied 

absorber bandgap values (see Fig.1). Additionally, Table I 

shows the main J-V parameters for selected solar cells 

from this study. For comparison, Fig.1 includes the 

efficiency values published in the literature for other 

related works in this area of wide gap chalcogenides 

(references [4,5,6,7,8,9]). The energy bandgap values for 

the NREL data in Fig. 1 were determined from quantum 

efficiency (QE) curves by arbitrarily assigning an 

“effective” bandgap value to the energy calculated from 

the wavelength value where a 20% QE value is observed 

for the long wavelengths.

Figure 1 Solar cell efficiency vs. CIGS absorber 

bandgap value.

From the J-V parameters obtained, we can attribute the 

enhancement in performance for CIGS solar cells with 

1.20 eV<Eg<1.45 eV to the attainment of higher opencircuit voltage (Voc) values as shown in the graph of Voc vs. 

Eg in Fig. 2. The data in Fig. 2 show not just the higher 

absolute values for the voltage but also:

(a) an improvement to the (expected) linear 

behavior of Voc as a function of Eg for solar cells with 

energy gaps in the range 1.10 eV<Eg<1.35 eV

(b) the breakdown of that linear behavior of Voc

as a function of Eg for solar cells with Eg>1.35 eV

(c) a saturation of the Voc values at ~830 mV for 

solar cells with Eg>1.35 eV.

Table I. J-V parameters of high temperature wide gap 

CIGS solar cells.

Voc

(V)

Jsc

(mA/cm2)

FF 

(%)

Efficiency 

(%)

Eg

(eV)

0.743 31.9 78.75 18.7 1.21

0.752 31.2 77.73 18.3 1.27

0.801 28.5 77.21 17.7 1.32

0.817 27.5 76.65 17.2 1.38

0.801 24.4 75.55 14.8 1.43

0.813 26.9 72.17 15.8 1.44

0.829 20.2 65.32 11.0 1.53

0.766 20.9 71.02 11.4 1.60

0.795 16.4 69.22 9.0 1.67

The fill factor values (FF) and their trend with of Eg (or Ga 

content) for the high temperature CIGS solar cells are not 

any different than those values seen in standard CIGS. 

The maximum FF values (77%-79%) are attained for 

bandgaps 1.20 eV<Eg<1.35 eV and for higher energy gap 

values the FF values decrease gradually to values ~70% 

for Eg~1.67 eV (or x=1.0) in the pure CuGaSe2 case.

Figure 2 Voc vs. CIGS absorber bandgap value. Data 

symbols as noted in Fig.1.

Similarly to the FF data, the short-circuit current density 

(Jsc) of the high temperature CIGS solar cells do not differ 

from what has been seen and reported for standard CIGS 

absorber materials: our experimental Jsc values do follow 

the expected linear behavior of decreasing Jsc values with 

increased bandgap (or Ga content).
3 

The EQE data for selected devices for which the absorber 

layers were grown at the highest substrate temperatures 

of this study (Ts~650 ̊C) are shown in Fig.3. The insert in 

the figure shows the CIGS absorber bulk atomic chemical 

composition (x) values that were obtained by x-ray 

fluorescence (XRF) analysis. It is important to note that 

when using the NREL three-stage evaporation process, 

and due to the different diffusivity values for In and Ga, a 

direct correlation of bandgap to bulk Ga content is not 

quite correct because of the graded bandgap nature of 

those films. We find a more realistic estimate of an 

effective bandgap value for three-stage CIGS films can be 

obtained from QE data and in a manner explained earlier: 

20% QE value in long wavelengths (shown by the small 

arrow in Fig. 3 for the pure CuGaSe2 case).

Figure 3 External quantum efficiency curves for CIGS 

solar cells with absorbers grown at 650 ̊C.

From the EQE data, another shortcoming of the wider gap 

CIGS solar cells is revealed: as the Ga content is 

increased, the overall collection of the cells decreases 

predominantly for the longer wavelengths indicative of

reduced diffusion lengths in those cells. In other words, 

photons that penetrate deep into the wide gap absorber 

are not contributing effectively to current generation and 

are lost to recombination. The highest bandgap materials, 

such as the CuGaSe2 case in Fig.3, also show an overall 

lower collection efficiency (<90%) in the visible 

wavelengths, an indication additional recombination is 

further limiting the performance of such cells.

It was mentioned earlier that the gains in efficiency 

reported in this work for high temperature CIGS solar cells 

with 1.2 eV<Eg<1.45 eV were due to higher voltages as 

compared to results from standard CIGS absorber 

materials. For this reason, it became imperative in our 

work to establish any significant or measurable differences 

in the diode behavior and the absorber physical properties 

of high temperature and standard CIGS samples 

fabricated in the course of this work.

From C-V and dark J-V measurements on selected solar 

cells, we were able to establish two important 

observations about these devices:

(a) the carrier concentration values—C-V data 

not shown—for these new high temperature CIGS 

absorbers are similar to that of earlier CIGS materials. On 

the other hand,

(b) the diode reverse saturation current density 

(Jo) for these new high temperature absorbers is 

measurably and significantly lower (by more than 1 order 

magnitude) than values seen previously in materials with 

similar bandgap values.

Fig. 4 shows the dark J-V curves for representative CIGS 

wide gap solar cells with Eg~1.5 eV from this study 

illustrating the lower Jo values in solar cells made with high 

temperature wide gap CIGS absorbers. The lower values 

measured in the Jo parameter for such cells is significantly 

lower for materials with 1.2 eV<Eg<1.5 eV; however, in

solar cells made with high temperature CIGS and Eg>>1.5 

eV the Jo values are similar to those in standard CIGS 

materials with similar bandgaps.

Figure 4 Dark J-V curves for high temperature CIGS 

(lower curves) and standard CIGS (upper curves) solar

cells with Eg ~1.5 eV.

HIGH TEMPERATURE CIGS ABSORBER 

CHARACTERIZATION

Because of the interesting features seen in J-V, C-V, EQE 

and room temperature dark-JV data of the solar cells 

made with high temperature CIGS, we investigated 

several physical properties of the CIGS absorbers used in 

those cells and compared data and observations to 

standard CIGS materials in order to establish more 

differentiating characteristics between materials.

The chemical composition of the CIGS absorbers 

fabricated were characterized by XRF, Auger electron 

spectroscopy (AES) and secondary ion mass 
4 

spectroscopy (SIMS). Morphological and structural 

information was obtained from scanning electron 

microscopy (SEM) and x-ray diffraction (XRD). Lastly, 

cathode luminescence (CL) spectroscopy and imaging 

were also carried out on representative samples to study 

grain boundary phenomena [10]. 

Figure 5 Morphology of CIGS grown at 650  ̊C for 

x~0.4 (top), x~0.5 (middle) and x=1 (bottom).

Let us begin with microstructure and morphological 

observations. Here, the SEM data indicates there is a 

reduction in grain size as Ga content is increased (for the 

same substrate temperature). Even the highest substrate 

temperatures in our experiments (650 ̊C) fail to achieve 

grain sizes that are of the order of film thickness (~2 μm) 

for Ga contents above x~0.5. The morphology of CIGS 

with x>0.5 is characterized by small grain size in the back 

of the film (near the Mo layer) and larger grains towards 

the surface. Eventually, for x>0.9 the films become 

homogeneous in grain size but with submicron grain sizes. 

Figure 5 shows three representative CIGS materials 

grown at ~650 ̊C depicting these morphological 

differences. 

From XRD data (not shown), we establish that all films 

grown in this study have a preferential orientation in the 

(220) direction. The use of higher than standard 

processing temperatures does not seem to hinder this 

structural characteristic and, in fact, high temperature 

CIGS films are highly textured in that crystallographic 

orientation. On the other hand, the high processing 

temperatures lead to more uniform films in terms of 

chemical composition regarding the Ga content. That is, 

the graded bandgap nature of three-stage films is less 

pronounced as the substrate temperature is increased. In 

other words, the typical “notch” shape to the Ga profile 

seen in compositional depth profiles such as those 

obtained from SIMS and/or AES becomes less prominent 

(see Fig.6). 

Figure 6 SIMS Ga/(In+Ga) ratio as a function of film 

depth. 

Structural and chemical properties of the high temperature 

CIGS materials are not different than expected from the 

three-stage process and the effect of higher processing 

temperatures. But the grain boundary (GB) activity we see 

and report here from CL measurements is in our 

experience something new in these materials. 

From our earlier work on CL as applied to wide bandgap 

materials, we learned that the elevated temperatures help 

in minimizing a certain pair of radiative defect states with 

an emission localized 1.41-1.46 eV within the gap of 

materials such as pure CuGaSe2 [11]. The wide-gap high 

temperatue CIGS materials reported here lack that 

particular emission and we can assert that those defect 

levels have been reduced. Additionally, when we look at 

the GB CL emission, we find the high tenperature CIGS 

materials have emissive properties at the GB similar to 

those we have previously associated with high efficiency 

materials [12, 13]. That is, all high efficiency CIGS 

materials we studied in the past displayed what we have 

previously refered to as the “red shift” in the GB CL 

emission (relative to the CL emission at the grain interior) 
5 

of such materials: there is a small, yet measurable shift in 

the donor-to-acceptor (DAP) emission peak intensity 

energy towards lower energy emission values, hence, “red 

shift” (as shown in Fig. 7). We have previously associated 

this CL emission shift with the precense of a large neutral 

barrier leading to reduced recombination at the GB.

(a) 

(b) 

Figure 7 Typical CL emission (a) linescan and (b) 

energy map at 1.130 eV, 80 meV energy window at 

grain interior (G1,G2) and at GB in high efficiency 

CIGS.

In the past, we only observed that behaviour for standard 

low Ga-content CIGS materials with bandgaps <1.2 eV 

and higher Eg standard CIGS materials did not show this 

”red shift” behavior; that is, the energy of the GB emission 

did not differ significantly from that of the grain interior 

(see Fig. 8). The high temperature CIGS solar cells we 

report here do show such “red shift” in the CL emission at 

their grain boundaries as can be seen from the CL photon 

energy maps we obtained for a selected number of high 

temperature (and standard) CIGS materials. Figures 7-9 

show the spectra and CL energfy maps (color coded) for 

three cases that exemplify our findings.

Figure 8 Energy map map of the CL emission (at 1.260 

eV, 80 meV energy window) from standard CIGS with 

x~0.60 from year 2003. 

Figure 9 Energy map of the CL emission from high 

temperature CIGS with x~0.60 (similar conditions as in 

Fig.8 were used). 

DISCUSSION

Considering standard solar cell equations and based on 

data presented above, we can deduce that the higher 

voltages seen in the high temperature CIGS solar cells 

arise from a reduced reverse saturation current density 

value and not to an increase of carrier concentration. It is 

also clear to us that the grain boundary activity in high 

temperature CIGS absorber materials has been improved 

and now match the behavior seen in standard high 

efficiency solar cells. As an upshot, we can say that the 

defects present at grain boundary of these new materials 

provide for a reduced recombination at grain boundary. 

The true nature of those beneficial defects at the grain 

boundary may still be a matter of discussion but whatever 

their nature is, we are inclined to think they are the main 

reason for the improvements to wider gap CIGS solar cells 

reported here.

We note that the “red shift” is present in all samples 

fabricated, even in the widest bandgap materials such as 

CuGaSe2. However, as it has been shown, the solar cells 

with Eg>1.45 eV still suffer from a limited value in Voc. It is 

G1

G2

GB
6 

very likely other limiting factors are present (aside from 

grain boundary recombination) that limit the Voc of the 

widest gap materials (Eg> 1.45 eV). We speculate that 

factors such as band alignment between the CdS and the 

wider gap CIGS materials may not be optimum and 

perhaps alternatives to CdS, wider gap “buffer” layers may 

be needed to discern more limiting factors of the widest 

gap solar cells (Eg>1.5 eV). There could be other reasons 

as well, such as the nature of the electrical conductivity 

type at the usually Cu-depleted surface region of these 

materials. Naturally, more investigations are necessary, 

but within the scope of this work we can already see and 

report significant improvements to the performance of 

wide gap CIGS solar cells. 

CONCLUSIONS 

We have experimented with higher than standard 

substrate temperatures for the growth of CIGS materials 

with 0.3<x<1.0. The much more elevated processing 

temperatures than the standard 550 ̊C-600 ̊C typically 

used have led to significant improvements to wide gap 

CIGS solar cells with bandgaps up to 1.45 eV. The main 

parameter that has improved is the output voltage of the 

cells as compared to previous (and standard) CIGS 

materials. A reduction in the reverse saturation current 

density value is responsible for the enhanced voltages 

attained. The high temperature CIGS materials grown 

show improvements to their grain boundary characteristics 

and now show properties similar to those found in high 

efficiency CIGS materials with low bandgaps (Eg<1.2 eV). 

The widest bandgap solar cells (Eg >1.5 eV) are still 

limited in performance (efficiency <15%). Additional 

experimental and theoretical work are necessary to finally 

isolate and identify the most significant limiting factors in 

the efficiency and voltage for those solar cells. 

ACKNOWLEDGEMENT 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory. Also, many 

thanks to our colleagues at NREL: Ingrid Repins, Bobby 

To and Paul Ciszek for data and input to this work. 

REFERENCES 

[1] R. Herberholz, V. Nadenau, U. Rühle, C. Köble, H.W. 

Schock , B. Dimmler. Solar Energy Materials and Solar 

Cells 49, 1997, pp. 227-237. 

[2] M. Gloeckler and J.R. Sites. Thin Solid Films, Volumes 

480-481, 1 June 2005, Pages 241-245. 

[3] Li Zhang, Qing He, Wei-Long Jiang, Fang-Fang Liu, 

Chang-Jian Li, Yun Sun. Solar Energy Materials & Solar 

Cells 93, 2009, pp. 114–118. 

[4] Miguel A. Contreras, K. Ramanathan, J. AbuShama, F. 

Hasoon, D. L. Young, B. Egaas and R. Noufi. Prog. 

Photovolt: Res. Appl., 2005; 13, pp. 209–216. 

[5] W.N. Shafarman, R. Klenk, and B.E. McCandless. 

Twenty fifth IEEE PVSC, 1996; pp. 763. 

[6] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, 

R. Menner, W. Wischmann and M. Powalla. Twenty fifth 

EU PVSEC, WCPEC-5, Valencia, Spain, 2010. 

[7] T. Eisenbarth, T. Unold, R. Caballero, C.A. Kaufmann, 

D. Abou-Ras, H.-W. Schock. Thin Solid Films 517, (2009) 

2244–2247. 

[8] G. M. Hanket, J. H. Boyle, and W. N. Shafarman. 

“Characterization and device performance of 

(Ag,Cu)(In,Ga)Se2 Absorber layers”, IEEE PVSC, 2009, 

pp. 1240. 

[9] S. Merdes, R. Mainz, J. Klaer, A. Meeder, H. 

Rodriguez-Alvarez, H.W. Schock, M.Ch. Lux-Steiner, R. 

Klenk. “12.6% efficient CdS/Cu(In,Ga)S2-based solar cell 

with an open circuit voltage of 879 mV prepared by a rapid 

thermal process”, Solar Energy Materials & Solar Cells 95

2011, pp. 864–869. 

[10] M. J. Romero, K. Ramanathan, M. A. Contreras, M. 

Al-Jassim, R. Noufi, P. Sheldon. Appl. Phys. Lett. 83(23), 

4770 (2003). 

[11] M. A. Contreras, M. Romero, and D. Young, 

“Optimization of CuGaSe2 for Wide-bandgap Solar Cells”, 

Proceedings of the 3rd World Conference in Photovoltaic 

Energy Conversion, Osaka, Japan. 

[12] M. A. Contreras, I. Repins, W. K. Metzger, M. Romero 

and D. Abou-Ras, “Se activity and its effect on 

Cu(In,Ga)Se2 photovoltaic thin films”, Phys. Status Solidi A 

206, No. 5, 1042–1048 (2009). 

[13] M. A. Contreras, M. J. Romero, R. Noufi, 

“Characterization of Cu(In,Ga)Se2 materials used in record 

performance solar cells”, Thin Solid Films 511–512 (2006), 

pp. 51–54.