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National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Characterization of 19.9%-Efficient 

CIGS Absorbers 

Preprint 

I. Repins, M. Contreras, M. Romero, Y. Yan, 

W. Metzger, J. Li, S. Johnston, B. Egaas, 

C. DeHart, and J. Scharf 

National Renewable Energy Laboratory 

B.E. McCandless 

Institute for Energy Conversion 

R. Noufi 

Solopower 

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42539 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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CHARACTERIZATION OF 19.9%-EFFICIENT CIGS ABSORBERS 

Ingrid Repins,1

 Miguel Contreras,1 Manuel Romero,1 Yanfa Yan,1 Wyatt Metzger,1 Jian Li,1 Steve Johnston,1 

Brian Egaas,1

 Clay DeHart,1 John Scharf,1 Brian E. McCandless,2 and Rommel Noufi3

1

National Renewable Energy Laboratory, Golden, CO 80401 2

Institute for Energy Conversion, Newark, DE 19716 3

Solopower, San Jose, CA 95138 

ABSTRACT 

We recently reported a new record total-area 

efficiency, 19.9%, for CuInGaSe2 (CIGS)-based 

thin-film solar cells [1]. Current-voltage analysis 

indicates that improved performance in the record 

device is due to reduced recombination. The 

reduced recombination was achieved by 

terminating the processing with a Ga-poor (In-rich) 

layer, which has led to a number of devices 

exceeding the prior (19.5%) efficiency record. This 

paper documents the properties of the highefficiency CIGS by a variety of characterization 

techniques, with an emphasis on identifying nearsurface properties associated with the modified 

processing.

INTRODUCTION 

Understanding the physical mechanisms that lead 

to champion CIGS devices is important for a 

number of reasons. First, this understanding helps 

define a pathway to future performance 

improvements. It also helps identify performance 

sensitivities that can improve yield and efficiency for 

CIGS devices deposited by related industrial 

techniques. 

In this paper, we document the properties of highefficiency (19.9%) CIGS by a variety of 

characterization techniques, with an emphasis on 

identifying near-surface properties associated with 

the modified processing. 

FILM GROWTH AND DEVICE PERFORMANCE 

The standard device layers and the processing 

used in the record CIGS devices have been 

described elsewhere [2,3,4,5,6]. The processing 

change responsible for the recent 0.4% (absolute) 

efficiency improvement is the termination of the 

three-stage process without Ga for the last ~100 Å 

of deposition. This modified termination is illustrated 

in Figure 1, which shows the In and Ga rates as 

measured by electron emission impact 

spectroscopy at the end of the three-stage CIGS 

deposition. Instead of maintaining the atomic ratio 

Ga/(In+Ga)~0.3 as throughout the rest of the 

deposition, about 30 Å of In (resulting in ~100 Å of 

chalcopyrite) is deposited without Ga at the end of 

the deposition. 

 

4

3

2

1

Rate (Angstroms/sec)

0

Gallium

Indium

-20 0

Time (sec)

20 40

Figure 1: In and Ga rates at the end of three-stage 

CIGS deposition process for 19.9%-efficient device. 

Zero-rate background signal has been removed. 

The processing variation illustrated in Figure 1 has 

led to the fabrication of a number of devices 

exceeding the previous efficiency record, 19.5% for 

a 0.42-cm2

 device [7]. Improved devices have been 

demonstrated in more than one evaporator and by 

several operators. The repetition of improved 

results is summarized in Table 1, which lists 

devices exceeding the previous efficiency records. 

Shown are the device name, device area, efficiency 

(η), open-circuit voltage (Voc), fill factor (FF), and 

short-circuit current density (Jsc). The column in 

Table 1 titled “Official Measurement?” is listed as 

“Yes” for current density-voltage (J-V) 

measurements made by the National Renewable 

Energy Laboratory (NREL) Device Performance 

Group [8]. This column is listed as “No” for J-V 

measurements performed at the NREL thin-film 

group user facility. 

1
Table 1: Summary of devices made using modified 

processing termination with performance exceeding 

prior record values for 0.42- and 1-cm2

 devices. 

Device 

Name 

Area 

(cm2

) 

η

 (%) 

Voc

(mV) 

FF 

(%) 

Jsc

(mA/ 

cm2

)

Official 

Measure

ment? 

M2992-

11#5 

0.419 19.9 690 81.2 35.4 Yes 

C2183-

12#5 

0.416 19.9 697 80.0 35.7 Yes 

C2219-

21#7 

0.417 19.8 714 79.1 35.1 Yes 

M2992-

11#4 

0.419 19.7 690 81.2 35.1 Yes 

M2992-

11#6 

0.419 19.7 690 81.1 35.3 Yes 

C2183-

12#4 

0.417 19.7 695 80.0 35.5 Yes 

C2200-

22#1 

0.420 19.6 725 80.6 33.6 No 

C2213-

22#2 

0.994 19.2 716 80.4 33.4 Yes 

MATERIALS CHARACTERIZATION 

Two features of the high-efficiency material are of 

particular interest: reduced recombination, and the 

effect of the Ga-poor process termination on the 

surface layer. We examined these features using 

grazing-incidence X-ray diffraction (GIXRD), timeresolved photoluminescence (TRPL), transmission 

electron microscopy (TEM), energy-dispersive 

spectroscopy (EDS), capacitance-voltage (CV) 

profiling, scanning tunneling luminescence 

spectroscopy (STL), and cathodoluminescence 

(CL) spectrum imaging. 

STL and CL 

We modified a scanning tunneling microscope at 

NREL to map STL in conjunction with CL [9]. The 

estimated penetration depth for tunneling electrons 

used to excite luminescence during STL 

measurements is estimated to be no more than 50 

nm [10]. On the other hand, the 15-keV electrons 

used to excite luminescence during CL 

measurements are estimated to penetrate about 1 

μm into the film. Thus, the STL probes the film very 

near the surface, whereas the CL probes into the 

bulk of the film. The luminescence from 19.9%-

efficient CIGS was characterized in terms of 

excitation source (CL vs. STL) and position. These 

data were compared with earlier data on 13%- and 

19.5%-efficient CIGS. 

When room-temperature CL mapping is performed, 

the 19.9%-efficient CIGS exhibits a 5%–10% 

reduction in luminescence intensity at the grain 

boundaries, compared to the grain interiors. This 

magnitude of intensity loss is typical of devices with 

efficiency greater than 18%. Lower-efficiency 

devices exhibit more nonradiative recombination 

(20%–30% intensity loss) at the grain boundaries. 

Scanning electron microscope (SEM) plan views and 

the corresponding CL intensity maps are shown in 

Figure 2 for 19.9%- and 13%-efficient absorbers. 

Figure 2: SEM plan view of a) 19.9%- and b) 13%-

efficient CIGS. Corresponding CL maps for c) 

19.9%- and d) 13%-efficient CIGS. 

STL, however, reveals a difference between the 

19.9% device and all earlier material measured by 

this technique. Figure 3 compares a roomtemperature CL spectrum from the 19.9% sample 

(pink), STL spectrum from the same point on the 

19.9% sample (dark blue), STL spectrum from an 

18% reference sample (yellow), and CL spectrum 

from the 18% reference sample (light blue). The 

reference sample exhibits the typical red shift 

between the CL and STL. Presumably, the red shift 

occurs because the surface of the CIGS contains a 

higher density of band-tailing defect states than the 

bulk. In the 19.9% sample, however, there is no 

red shift between the CL and STL. The lack of red 

shift implies that the material quality near the 

surface has been improved in the 19.9% material. 

a) b) 

c) d) 

2
0

10

20

30

40

50

60

70

80

90

100

0.9 1 1.1 1.2 1.3

Photon Energy (eV)

M2992 CL

REF STL REF CL

M2992 STL

Figure 3: Comparison of cathodoluminescence 

spectrum from 19.9%-efficient sample (in pink), 

tunneling luminescence spectrum from 19.9% 

sample (dark blue), cathodoluminescence spectrum 

from 18% reference sample (light blue), and 

tunneling luminescence spectrum from 18% 

reference sample (yellow). 

TEM 

Earlier measurements at NREL on CIGS that 

produced 19.5%-efficient devices indicated 

nanodomains when examined with high-resolution 

TEM (HRTEM), high-angle annular dark-field 

(HAADF) images, and EDS [11]. Similar 

measurements on the new 19.9%-efficient material 

show less evidence of nanodomains. Figure 4a 

shows an atomic number (Z) contrast TEM image 

of the 19.9% CIGS. The interior of the film is at the 

upper left corner of the image, and vacuum appears 

as the dark area in the lower right corner. A small 

gradual change in brightness is seen across the 

sample, most likely due to a gradient in cleaved 

sample thickness and gradual composition change 

toward the surface. However, no nanodomains are 

visible. For contrast, Figure 4b shows the same 

type of image from the early 19.5%-efficient 

material. Nanoscale composition domains are 

clearly visible. 

EDS data also indicate less pronounced 

nanodomains in the new material. In the 19.5%-

efficient CIGS, Cu ratio (Cu/(In+Ga)) and Ga ratio 

(Ga/(In+Ga)) exhibit large fluctuations (~2x) when 

measured by EDS in 5-nm steps over 70 nm. The 

new data exhibit somewhat smaller variations in the 

Cu and Ga ratios (~1.5x). Figure 5a shows Cu and 

Ga ratios as measured by EDS. The electron-probe 

size is estimated to be about 5 Å. The system 

detection error bar is estimated to be ±2%. 

Figure 5b is a TEM image that shows the path of 

the EDS measurements. The top of the red line 

corresponds to 0-nm displacement.

Figure 4: Z-contrast TEM image of a) 19.9%-

efficient CIGS, and b) earlier 19.5%-efficient CIGS. 

2 nm

a) 

b) 

3
Figure 5: a) Composition as a function of position 

in the 19.9%-efficient CIGS, and b) TEM image 

showing the line along which these measurements 

were taken. 

TRPL 

Carrier lifetime was measured by TRPL, both on 

the CIGS film and the device. The measurement 

and analysis technique has been described 

elsewhere [12]. The lifetime measured in the 

19.9%-efficient device, 5.5 ns, is somewhat longer 

than what has been measured on other devices. 

For example, Figure 6 shows efficiency versus 

lifetime for the record device, and for other recent 

samples made at NREL with variations in the Ga 

contents and composition profiles. The increased 

lifetime is consistent with improved diode quality 

factor in the device. However, one should not 

strictly associate the measured lifetime with 

reduced recombination, because several other 

factors—including charge separation in the 

depletion region, measurement intensity, and fitting 

algorithm—also influence the apparent lifetime 

value [13,14]. Furthermore, variations in 

measurement and analysis procedures from one 

study to the next preclude a straightforward 

comparison of the new results with many of those in 

the literature. 

0.0

1.0

2.0

3.0

4.0

5.0

10.0 12.0 14.0 16.0 18.0 20.0

Efficiency (%)

6.0

Figure 6: Efficiency versus lifetime measured in 

CIGS devices with a variety of Ga contents and 

composition gradings. 

CV 

CV measurements were used to examine carrier 

density. Data taken at 50 kHz are shown in Figure 

7. Zero-bias depletion width is marked with a 

diamond. Capacitance as a function of frequency 

varied little in the range from 1 to 200 kHz. The 

maximum carrier density of about 2 x 1016 cm-3 and 

zero-bias depletion width less than 0.5 μm are 

typical of CIGS devices made at NREL with 

efficiencies greater than 19% [15]. Lower-efficiency 

devices made at NREL tend to have a slightly lower 

carrier density (≤1 x 1016 cm-3) and slightly longer 

zero-bias depletion widths [12,16,17]. 

10 15

2

3

4

5

6

7

10 16

2

efect Density (cm

-3

) Carrier + D

0.35 0.40 0.45 0.50 0.55

Distance from Junction ( μm)

Figure 7: Carrier density versus distance from the 

junction as derived from CV profiling. The zero-bias 

depletion width is marked with a diamond. 

GIXRD 

GIXRD data were taken at the Institute for Energy 

Conversion (IEC). For incident angles from 0.5 to 4 

degrees (estimated penetration depths of 0.13 to 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 500 1000 1500 2000

Position (nm)

Cu/(In+Ga)

Ga/(In+Ga)

a) 

 b) 

4
1.0 μm), the (112) peak centered near 27 degrees 

shows the expected progression to lower values of 

2Θ, due to decreasing Ga content as the incident 

beam penetrates the notched Ga profile [18]. 

Typical of high-efficiency devices, the peaks are 

relatively narrow. For example, the half width at half 

maximum of the (112) peak was measured at 0.166 

degrees using a 4-degree incident angle. However, 

this narrow peak width does not distinguish the 

record material from other high-quality CIGS. 

Low incident angles were used to probe the 

material structure as near as possible to the 

surface. GIXRD measurements at 0.25-degree 

incident angle should probe about 50 nm into the 

film. Such measurements were compared for the 

record sample, a sample made with the same 

recipe but terminating the process with Ga 

(opposite of what is shown in Figure 1), and a 

sample made without any intentional Ga grading. 

These samples exhibited nearly identical 

compositions when measured by EDS. GIXRD 

scans of the (112) peak for the three samples are 

shown in Figure 8. All samples show a peak 

centroid shift to angles significantly higher than that 

expected from the 0.36 Ga ratio measured by EDS. 

These peak shifts are not associated with the 

alignment of the diffractometer. To account for the 

peak shifts of Figure 8 by Ga content alone, the 

ratio Ga/(In+Ga) near the surface would need to be 

0.46 for sample 34186, 0.60 for M2995, and 0.71 

for M2992. Instead, it is likely that the peak shifts 

are due to decreased Cu content near the film 

surface [19] or to lattice strain. The 19.9%-efficient 

CIGS exhibits the largest peak shift. 

Figure 8: GIXRD (112) scans of 19.9%-efficient 

CIGS film (M2992, black), similar film with Ga 

process termination (M2995, red), and CIGS film 

without Ga grading (IEC 34168, blue). 

CONCLUSIONS 

A modification to the termination of the three-stage 

co-evaporation process has recently improved the 

performance of CIGS devices at NREL. We applied 

a variety of characterization techniques to the 

19.9%-efficient CIGS. Several of the extracted 

properties are typical of CIGS with efficiency 

exceeding ~19%. These properties include carrier 

density around 2 x 1016 cm-3, zero-bias depletion 

width just under 0.5 μm, a relatively long PL decay 

time on devices, relatively high luminescence 

intensity at the grain boundaries, and narrow 

GIXRD peaks. Other properties appear to be new 

to the improved material: absence of a red shift in 

the luminescence spectrum near the CIGS surface, 

diminished manifestation of nanodomains, and 

increased shifting of the GIXRD (112) peak to 

smaller lattice parameters. 

ACKNOWLEDGEMENTS 

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

Energy under Contract No. DE-AC36-99GO10337 

with the National Renewable Energy Laboratory. 

The authors thank Craig L. Perkins and Bobby To 

for film composition data, the NREL Device 

Performance Group for J-V measurements, Falah 

Hasoon for discussions on film growth, and Tim 

Gessert and Ramesh Dhere for helpful review. 

REFERENCES 

 

[1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, 

J. Scharf, C.L. Perkins, B. To, and R. Noufi, 19.9% 

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6
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14. ABSTRACT (Maximum 200 Words)

We recently reported a new record total-area efficiency, 19.9%, for CuInGaSe2 (CIGS)-based thin-film solar cells. 

Current-voltage analysis indicates that improved performance in the record device is due to reduced recombination. 

The reduced recombination was achieved by terminating the processing with a Ga-poor (In-rich) layer, which has led 

to a number of devices exceeding the prior (19.5%) efficiency record. This paper documents the properties of the 

world-record-efficiency CIGS solar cell by a variety of characterization techniques, with an emphasis on identifying 

near-surface properties associated with the modified processing. 

15. SUBJECT TERMS

CIGS; PV; record efficiency; thin film; solar cells; current-voltage; open-circuit voltage; fill factor; grazing-incidence Xray diffraction; 

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