April 2003 • NREL/CP-520-33949 

Evolution of Electronic 

Properties of Cu(In,Ga)Se2 

(CIGS)-Based Solar Cells

During a 3-Stage Growth 

Process 

Preprint 

J. A. AbuShama 

Colorado School of Mines 

S. Johnston, R. Ahrenkiel, R. Crandall, D. Young, 

and R. Noufi 

National Renewable Energy Laboratory 

To be presented at the 2003 Materials Research 

Society Spring Meeting 

San Francisco, California 

21–25 April 2003 

National Renewable Energy Laboratory 

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Evolution of Electronic Properties of Cu(In,Ga)Se2 (CIGS)-Based 

Solar Cells During a 3-stage Growth Process 

Jehad A. AbuShama*, S. Johnston, R. Ahrenkiel, R. Crandall, D. Young, and R. Noufi 

National Renewable Energy Laboratory, Golden, CO 80401 

*Physics Department, Colorado School of Mines, Golden, CO 80401 

ABSTRACT 

We investigated the electronic properties of ZnO/CdS/CIGS /Mo/SLG polycrystalline thinfilm solar cells with compositions ranging from Cu-rich to In(Ga)-rich by deep-level transient 

spectroscopy (DLTS) and capacitance-voltage (C-V) measurements. This compositional change 

represents the evolution of the film during growth by the 3-stage process. Two sets (four samples 

each) of CIGS thin films were prepared with Ga/(In+Ga) ratios of ~0.3 (low Ga) and ~0.6 (high 

Ga). The Cu/(In+Ga) ratio ranges from 1.24 (Cu-rich) to 0.88 (In(Ga)-rich). The films were 

treated with NaCN to remove the Cu2-xSe phase where needed. Key results include: (1) For lowGa devices, DLTS data show that acceptor-like traps dominate in samples where CIGS grains do 

not go through the Cu-rich to In(Ga)-rich transition, whereas donor-like traps dominate in 

In(Ga)-rich samples. Therefore, we see a clear transformation of defects from acceptor-like to 

donor-like traps. The activation energies of these traps range from 0.12 to 0.63 eV. We also 

observed that NaCN treatment eliminates a deep minority trap in the In(Ga)-rich devices, (2) For 

high-Ga devices, only majority-carrier traps were detected. These traps again range from shallow 

to deep, (3) The carrier concentration around the junction and the density of traps decrease as the 

CIGS becomes more In(Ga)-rich. 

INTRODUCTION 

Cu(In,Ga)Se2 (CIGS)-based polycrystalline thin-film solar cells are promising for 

photovoltaic applications. The highest conversion efficiency for small laboratory devices is 

19.2% [1]. The four elements of this multinary polycrystalline film can be alloyed to form 

different CIGS phases as dictated by the pseudo-binary CIS phase diagram [2]. Even though this 

multiplicity makes the material complicated, CIGS nevertheless tolerates defects and impurities 

by self-adjusting its chemistry and microstructure. In our laboratory, we are investigating the 

thin-film growth mechanisms using our so-called “3-stage process” as influenced by the specific 

dynamics of this process. The electronic properties of thin-film CuxIn1-yGaySe2 devices made 

from films as they transition from Cu-rich to In(Ga)-rich, and for different Ga contents (i.e., 

different y values), are the subject of this work. Devices made from these films were analyzed 

using deep-level transient spectroscopy (DLTS) and capacitance-voltage measurements (C-V). 

The phase and microstructure evolution of Cu(In,Ga)Se2 from Cu-rich to In(Ga)-rich with the 

addition of Cu has been discussed in our previous publications (e.g. ref. [3]). 

For comparison of our results with previous studies, see references [4,5]. 

1
• •

EXPERIMENTAL 

The CIGS thin films were deposited on Mo-coated soda lime glass substrates by physical 

vapor deposition in a multisource bell jar system using the three-stage process (see Figure 1). In 

this process, a precursor of (In,Ga)2Se3 is reacted with Cu+Se to produce Cu(In,Ga)Se2 plus 

Cu2-xSe as a secondary phase, followed by the addition of In+Ga+Se to adjust the composition to 

slightly Cu-poor. 

Our approach is to interrupt the film growth at predetermined points along the film growth 

pathway. The thin-film samples from which the devices were made are labeled a through d and 

are indicated in Figure 1. Sample a is Cu-rich and sample d is In(Ga)-rich. Thin-film samples ad were treated by NaCN for about 5 min to remove the Cu2-xSe secondary phase where needed. 

Devices were made from these NaCN-treated thin-film samples. CdS was deposited by CBD and 

ZnO, and the front contacts were deposited by sputtering. Other devices were made from the 

untreated In(Ga)-rich thin-film sample (which will be referred to as d'). 

40 

12 

30 

8 

20 

4 

In 

Ga 

Cu 

Se 

1st -stage 2nd

-stage 3rd-stage 

• •

a

b In 

Ga 

Monitor T 

Substrate T (Right scale) 

d

c 10 

0 0 

600 

602 

400 

598 

200 594 

0 590 

0 10 20 30 40 50 60 70 

Run time (min) 

Figure 1. Schematic profile of the “three-stage growth process. 

Temperature (°C) 

Metals rates (Å/s) 

Temperature (°C) Se rate (Å/s) 

2
RESULTS AND DISCUSSION 

(I) Low Ga Samples 

The electronic properties of ZnO/CdS/CuxIn1-yGaySe2 (CIGS)/Mo/SLG polycrystalline thinfilm solar cells made from samples a through d (where the film transitions from Cu-rich to 

In(Ga)-rich) were investigated by DLTS and C-V measurements. For DLTS measurements, 

carriers are introduced using a bias pulse, changing the electron occupation of a trap. The 

junction capacitance increases as the electron occupation of the trap increases [6]. In this study, 

we used the DLTS technique to measure the activation energy (which is approximately the 

energy distance from the respective band to the trap level) and the trap concentration. The DLTS 

spectra were measured with a reverse voltage V=-0.4 V, trap-filling pulses of amplitude 0.5 V, 

and a saturation pulse width of 1 ms. Figure 2a shows the DLTS data for low-Ga (i.e. 

Ga/(In+Ga)=~30%) samples a-d'. For sample a, the signal shows a negative peak (at T=~120 K), 

identifying the shallow defect A1 as a hole trap with activation energy of 0.12 eV from the 

valence band. Also, it shows a beginning of a positive peak (at T ≅300 K), identifying a defect 

D1 as a deeper electron trap. For sample b, the signal shows a negative peak (at T ≅210 K), 

identifying a defect A2 as a hole trap with activation energy of 0.28 eV from the valence band. 

For sample c, the signal shows a positive peak (at T ≅230 K), identifying a defect D2 as an 

electron trap with activation energy of 0.47 eV from the conduction band. For sample d, the 

signal has two positive peaks (at temperatures of about 120 and 260 K, respectively), identifying 

the defects D3 and D4 as electron traps with activation energies of 0.12 and 0.25 eV, respectively, 

from the conduction band. For sample d', the signal shows a positive peak (at T ≅230 K), 

identifying a defect D5 as an electron trap with activation energy of 0.63 eV from the conduction 

band. Comparing samples d and d', we see that NaCN treatment eliminates a deep minority trap 

in the In(Ga)-rich devices. The heights of the peaks in Figure 2 are proportional to their 

respective trap concentration [6]. The traps in samples a-d' have densities in the range 

6.6 ×1011 − 2.0 ×1014 cm−3. The low values of the trap densities may be due to the incomplete 

filling of the traps. 

Figure 3(a) shows the C-V measurements. The average carrier concentrations (N values) 

were deduced from Figure 3b. These concentrations were found to be: 4.8x1016, 7.5x1015, 

4.8x1015, 7.9x1016, and 1.4x1016 cm-3 (for samples a, b, c, d, and d', respectively, see Figure 1). 

(II) High Ga Samples 

Now, consider the high-Ga (i.e., Ga / (In + Ga) ≈ 0.6) samples a-d'. Figures 4 (a) and (b) 

show the DLTS data and Arrhenius plots for these samples, whereas figure 4(c) shows the C-V 

data. Some of the traps were seen using a rate window of 20 ms, however to see the other traps, 

shorter rate windows were used; thus, two DLTS plots are shown in figure 4 (a). All the DLTS 

peaks for these samples are negative, and therefore belong to majority (or hole) traps. Traps A3, 

A4, A5, A6, A7, A8, A9, and A10 (figure 4) have activation energies of 0.09, 0.72, 0.07, 0.25, 0.23, 

1.17, 0.84, and 0.57 eV, respectively. The activation energies are measured relative to the 

valence band. The traps have densities in the range of 0.12 − 24 ×1014 

cm−3. The carrier 

3
concentrations around the junction of the cell (from C-V) for samples a-d and d' are in the range 

of 0.14 − 4.8 ×1017 

cm−3 . These concentrations are one order of magnitude higher than in the low 

Ga case. The carrier concentration is the highest for sample a and decreases as the film becomes 

more In(Ga)-rich. This is true for low- and high-Ga cases. 

(III) CIS and CGS Samples 

We also fabricated CIS- and CGS-based devices and analyzed them using DLTS and C-V. 

The CIS-based device has a new world-record efficiency of 14.5% [7]. In this device, 

Figure 3b. [A/C]2 vs. applied voltage for 

samples (a-d'). (A: area, C: capacitance) 

4 

3 

2 

1 

Voltage (volts) 

 a 

b 

d 

d' 

-0.2 0.0 0.2 0.4 0.6 

[A/C]2 x1015 (c

m2/F)2 

Figure 2b. Arrhenius plots of ln(T2τ) vs. 1000/T 

for samples (a-d'). (T: temperature, τ: time 

constant). 

5 

4 

3 

2 

1 

ln(T 2

τ) 

2 4 6 8 10 12 

1000/T (K -1 ) 

Majority Trap 

Minority Trap 

C b

d 

d' 

a 

d 

0.63 eV 

0.25 eV 

0.47 eV 

0.28 eV 

0.12 eV 

0.12 eV 

Capacitance (nF) 

∆C (pF) 

0.6 

a (Cu-rich) 

d (In-rich) 

a 

b 

c 

d 

d' 

rate window = 0.05 ms 

b 

d' 

C 

A1 

A2 

D3 

D1 

D5 

D4 

D2 

1.0 

0.4 0.8 

0.2 

0.6 0.0 

-0.2 0.4 

-0.4 

0.2 

a 

b 

c 

d 

d' 

100 150 200 250 300 -1.0 -0.5 0.0 0.5 

Temperature (K) Voltage (V) 

Figure 2a. DLTS data for samples (a-d'). (') Figure 3a. C-V data for samples a-d'. before NaCN treatment. 

4

 c 
we detected a hole trap with activation energy of 0.17 eV from the valence band and another 

electron trap with activation energy of 0.42 eV from the conduction band. In the CGS-based 

device, which has an efficiency of about 8%, a hole trap with activation energy of 0.22 eV from 

the valence band was detected. 

Figure 5 shows the relative defect density for CGS, and low- and high-Ga devices for the 

different defect states shown in figures 2 and 4 above. The defect density is the highest for 

sample a and decreases as the film becomes more In(Ga)-rich. 

Conclusions 

(1) For low-Ga devices, both acceptor-like traps dominate in samples where CIGS grains do not 

go through the Cu-rich to In(Ga)-rich transition, whereas donor-like traps dominate in In(Ga)-

Fi.gure 4. (a) DLTS data, (b) Arrhenius plots of ln (T2τ) vs. 1000/T, and (c) C-V 

data for high-Ga samples a-d and d'. 

-2.5 

-2.0 

-1.5 

-1.0 

-0.5 

0.0 

∆ C (pF) 

100 150 200 250 300

Temperature (K) 

a (Cu-rich) 

b 

c 

d (In-rich) 

d' (No CN) 

A5 

A6 

A3 

A8 

-0.8 

-0.6 

-0.4 

-0.2 

0.0 

∆ C (pF) 

50 100 150 200 250 300

Temperature (K) 

20 ms rate window 

a 

b 

c 

d 

d' 

A6 

A4 

A7 

A9 A10 

(a) 

10 

8 

6 

4 

2 

ln(T2

τ) 

4 6 8 10 12

1000/T (K-1) 

a (Cu-rich) 

b 

c 

d (In-rich) 

d' (In-rich) 

0.092 eV 

0.72 eV 

0.25 eV 

0.070 eV 

0.84 eV 

1.17 eV 

0.23 eV 

0.57 eV 

a 

c 

d 

b 

d d' 

b 

a 

1.6 

1.2 

0.8 

0.4 

Capacitance (nF) 

Voltage (V) 

a 

b 

c 

d 

d' 

c 

a b 

d 

d' 

High Ga 

-0.5 0.0 0.5 1.0 

(b) (c) 

5
rich samples. Therefore, we see a clear transformation of defects from acceptor-like to donor-like 

traps. The activation energies of these traps range from 0.12 to 0.63 eV. We also observed that 

NaCN treatment eliminates a deep minority trap in the In(Ga)-rich devices. (2) For highA3 A4 A5 A6 A7 A8 A9 A10 

16 Low Ga 

15 

14 

13 

High Ga CGS 

12 

11 

A1 A2 D2 D3 D4 D5 

Defect states 

Figure 5. Defect densities (logarithmic scale) for the different defect 

states shown in figures 2 and 4 above. 

Ga devices, only majority-carrier traps were detected. These traps again range from shallow to 

deep. (3) The carrier concentration and the density of point defects decrease as the CIGS 

becomes more In(Ga)-rich. 

ACKNOWLEDGMENTS 

The authors acknowledge the technical support of J. Dolan, J. Alleman and J. Keane. This 

work is supported by DOE Contract No. DE-AC36-99GO16337. J. AbuShama was supported 

by the Islamic Development Bank of Saudi Arabia and NREL subcontract No. KCQ-0-30600-08 

to the Colorado School of Mines. 

REFERENCES 

1. K. Ramanathan, M. Contreras, C. Perkins, S. Asher, F. Hasoon, J. Keane, D. Young, M. 

Romero, W. Metzger, R. Noufi, J. Ward, and A. Duda, Prog. Photovolt: Res. Appl. 11, 

225 (2003). 

2. U.C. Boehnke and G. Kuhn, J. Mat. Sci. 22, 1635 (1987). 

3. Jehad AbuShama, R. Noufi, Y. Yan, K. Jones, B. Keyes, P. Dippo, M. Romero, M. AlJassim, J. Alleman, and D. Williamson, Mat. Res. Soc. Symp. 668, H7.2.1 (2001). 

4. A. Jasenek, U. Rau, V. Nadenau, and H. W. Schock, J. Appl. Phys. 87, 594 (2000). 

5. R. Herberholz, M. Igalson, and H. W. Schock, J. Appl. Phys. 83, 321 (1998). 

6. D. V. Lang, J. Appl. Phys. 45, 3023 (1974). 

7. Jehad A. AbuShama et al., to be published. 

6 

Defect density (Log scale) 
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Evolution of Electronic Properties of Cu(In,Ga)Se2 (CIGS)-Based Solar Cells During a 

3-Stage Growth Process: Preprint 

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PVP34301 

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J.A. AbuShama,* S. Johnston, R. Ahrenkiel, R. Crandall, D. Young, and R. Noufi 

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*Colorado School of Mines, Department of Physics, Golden, CO 80401 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401-3393 

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13. ABSTRACT (Maximum 200 words): We investigated the electronic properties of ZnO/CdS/CIGS /Mo/SLG polycrystalline thin-film 

solar cells with compositions ranging from Cu-rich to In(Ga)-rich by deep-level transient spectroscopy (DLTS) and 

capacitance-voltage (C-V) measurements. This compositional change represents the evolution of the film during growth 

by the 3-stage process. Two sets (four samples each) of CIGS thin films were prepared with Ga/(In+Ga) ratios of ~0.3 

(low Ga) and ~0.6 (high Ga). The Cu/(In+Ga) ratio ranges from 1.24 (Cu-rich) to 0.88 (In(Ga)-rich). The films were treated 

with NaCN to remove the Cu2-xSe phase where needed. Key results include: (1) For low-Ga devices, DLTS data show that 

acceptor-like traps dominate in samples where CIGS grains do not go through the Cu-rich to In(Ga)-rich transition, 

whereas donor-like traps dominate in In(Ga)-rich samples. Therefore, we see a clear transformation of defects from 

acceptor-like to donor-like traps. The activation energies of these traps range from 0.12 to 0.63 eV. We also observed that 

NaCN treatment eliminates a deep minority trap in the In(Ga)-rich devices. (2) For high-Ga devices, only majority-carrier 

traps were detected. These traps again range from shallow to deep. (3) The carrier concentration around the junction and 

the density of traps decrease as the CIGS becomes more In(Ga)-rich. 

14. SUBJECT TERMS: PV; polycrystalline; thin film; solar cells; deep-level transient 

spectroscopy (DLTS); capacitance-voltage (C-V); acceptor-like traps; CIGS; 3-stage 

process 

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