Optimization of Copper Indium Gallium

Diselenide Thin Film Solar Cell (CIGS)

A. Aissat1,2(&), A. Bahi Azzououm1, F. Benyettou2,

and A. Laidouci1,2
1
LATSI Laboratory, Faculty of Technology, University Saad Dahlab,
BP270, 09000 Blida, Algeria
sakre23@yahoo.fr
2
FUNDAPL Laboratory, Faculty of Sciences, University Saad Dahlab,
BP270, 09000 Blida, Algeria

Abstract. We performed modeling and simulation of copper indium gallium

diselenide (CIGS) thin film solar cell. CIGS absorbers today have a typical
thickness of about 1–2 µm. However, on the way toward mass production, it
will be necessary to reduce the thickness even further. We investigated the
influence of the alloy compositions x = ½Ga=f½In þ ½Gag, the CIGS absorbers
thickness and the temperature. Optimal results are obtained with a thickness of
about 2 µm and a temperature of 318 K. It was also shown that, the short-circuit
current density (Jsc) decreases when the x composition increases. Very high Jsc
of 40.72 mA/cm2 was obtained, when x = 0.2. In contrary, the open-circuit
voltage (Voc), the fill factor (FF) and the efficiency ðgÞ of the solar cell are
increasing with the increase of the x composition. An optimal efficiency of about
30.34% was obtained with x = 0.9. Moreover, a comparison with published data
for the Cu(In,Ga)Se2 cells have shown an excellent agreement.

Keywords: Semiconductor  CIGS  Thin film  Solar cell  Photovoltaic

1 Introduction

Thin film solar cells have the potential for low-cost and large-scale terrestrial photo-
voltaic applications. A number of semiconductor materials including polycrystalline
CdTe, CIGS and amorphous silicon (a-Si) materials have been developed for thin-film
photovoltaic solar cells [1]. CuIn1-xGaxSe2 (CIGS) has attracted great interest as an
absorber layer in thin film photovoltaic (PV) devices because of the high power con-
version efficiency that it provided [2–6]. The polycrystalline copper indium gallium
diselenide (CIGS) thin film is an element of the I–III–VI2 group of chalcopyrite
semiconductors, compared to other Cu-chalcopyrite thin film solar cells as well as
CdTe and amorphous Si thin film solar cells [1], CIGS gives the highest conversion
efficiency.
Recently, the conversion efficiency of CIGS thin film solar cells has been improved
to 20.3% by ZSW (Centre for Solar Energy and Hydrogen Research) [4]. The optical
and electrical properties of CIGS are in function of the alloy composition x measured as
the atomic ratio x = ½Ga=f½In þ ½Gag. The efficiency of PV devices exhibits a

© Springer International Publishing AG 2018

M. Hatti (ed.), Artificial Intelligence in Renewable Energetic Systems, Lecture Notes
in Networks and Systems 35, https://doi.org/10.1007/978-3-319-73192-6_50
480 A. Aissat et al.

dependence on CIGS composition, and the maximum is found to occur at the value of
x * 0.3, when considered as an average through the depth of the absorber layer [7].
This paper addresses a simulation study to optimize the CIGS based thin film solar
cells through the investigation of the influence of different alloy compositions
½Ga=f½In þ ½Gag, also the influences of CIGS absorbers thickness and the
temperature.

2 Structure of CIGS Solar Cells and Simulation

The structure of CIGS solar cells include five layers, from bottom to up: Substrate:
CIGS solar cells can use glass, foil, and more flexible material such as Polyimide as
substrates. Back contact: molybdenum is the back contact of choice because it can
tolerate the harsh reactive ambient of the selenization processes at high temperature and
form an Ohmic contact with CIGS. Absorption layer: p type CIGS thin film is formed
with a little Cu-poor in the total composition. It should be noted that the ratio
Ga=In þ Ga was controlled. Buffer layer: CdS is usually used as the buffer material.
Front contact: Transparent and contacting oxide (TCO) window bilayers include
intrinsic ZnO and Al doped ZnO (AZO) (Fig. 1).

Fig. 1. Structure of the CIGS solar cell [8–10].

 Optimization of Copper Indium Gallium Diselenide Thin Film 481

The device characteristics of the CIGS solar cells are studied numerically using the
Silvaco-Atlas software. To investigate the effect of the variation of the Ga=ðIn þ GaÞ
ratio, x, in a CIGS solar cell, physical parameters are required, and are obtained from
measurements and previous literatures to make the accurate model [8–10]. Table 1
summarizes the physical parameters used in the simulation.

Table 1. Base parameters for CIGS solar cells.

ZnO CdS CIGS
Thickness (µm) 0.055 0.050 2
Eg ðeVÞ 3.3 2.4 1.2
er 9 10 13.4
ve ðeVÞ 4 3.75 3.89
ln ðcm2 =VsÞ 50 10 300
lp ðcm =Vs)
2 5 1 10
NA ð1=cm3 Þ 0 0 8:1016
ND ð1=cm3 Þ 5:1017 5:1017 5:1017
NCð1=cm3 Þ 2:2:10 2:2:1018
18
2:2:1018
NVð1=cm3 Þ 1:8:1019 1:8:1019 1:8:1019

2.1 Effects of Ga/(In + Ga) on CIGS Thin Films Solar Cell

To investigate the effect of the variation of the Ga=ðIn þ GaÞ ratio, x, in a CIGS solar
cell, firstly, we will report the modeling and simulation results of CIGS solar cells with
x = 0.3, in comparison with the previous reported experimental results as shown in
Table 2. Secondly, we will show the simulation results of CIGS solar cell with absorber
layer of 2 lm for variable x ratio. The I–V curve obtained using ATLASTM is shown in
Figs. 2 and 3.

Table 2. CIGS cell parameters and characteristics.

Simulation Experimental [11, 12]
x = Ga=ðIn þ GaÞ 0.3 0.3
Eg ðeVÞ 1.2726 1.27
JSC ðmA=cm2 Þ 39.8959 31.2
VOC ðVÞ 0.737769 0.752
FF 78.2406 77.73
Efficiency (%) 23.0293 18.3

The difference between the simulation and the experimental results with absorber
layer of 2 lm for x = 0.3 were due to different values of the basic parameters used and
especially to the thickness of the absorber. The large difference in the short-circuit
current is because ATLASTM uses the entire surface of the superior layer as a contact,
482 A. Aissat et al.

0,040 x=0.3
0,035

Cathode Current (A)

0,030

0,025

0,020

0,015

0,010

0,005
0,0 0,2 0,4 0,6 0,8
Anode Voltage (V)

Fig. 2. I–V curve for a CIGS solar cell with x = 0.3.

0,045

0,040
Cathode Current (A)

0,035

0,030

0,025
CIGS_X = 0,4
0,020 CIGS_X = 0,6
CIGS_X = 0,8
0,015 CIGS_X = 0,9

0,010

0,005
0,0 0,2 0,4 0,6 0,8 1,0 1,2
Anode Voltage (V)

Fig. 3. I–V curve for a CIGS solar cell with variable x composition.

in the other hand the actual cells presents a grid of contacts at the top are used as a
contact of the cell. The difference in the efficiencies was due to large difference in
short-circuit current.
According to the results of Table 3, we observe that the short-circuit current density
(Jsc) decreases when the compositions x increase, a very high Jsc of 40.72 mA/cm2 was
obtained when x = 0.2. In contrary, the open-circuit voltage (Voc), the fill factor
(FF) and the efficiency ðgÞ of the solar cell increase when the composition x increase.
An optimal efficiency of about 30.34% was obtained with x = 0.9.

Table 3. Characteristics of CIGS cells with absorber layer of 2 lm for variables compositions x.
CIGS absorber = 2 µm
x = Ga=ðIn þ GaÞ 0.2 0.4 0.6 0.8 0.9
JSC ðmA=cm2 Þ 40.7226 38.3248 35.279 33.4837 32.5563
VOC ðVÞ 0.67045 0.804901 0.940859 1.07906 1.14862
FF 77.1967 78.9545 80.0911 80.893 81.1564
Efficiency (%) 21.0766 24.3553 26.5843 29.2274 30.3483
 Optimization of Copper Indium Gallium Diselenide Thin Film 483

2.2 Effects of CIGS Absorber Thickness on CIGS Thin Films Solar Cell
To reduce production time and reduce cost. The thickness of a solar cell based on a thin
layer is a very important parameter, so the choice of an optimum thickness is the goal
of several researchers. The standard thickness of the CIGS absorber layer in CIGS
thin-film solar cells is presently 1.5–2 lm. If this thickness could be reduced with no
loss in performance, it would lead to even more effective solar cells and in boosting
efficiencies to new record levels. To investigate the effect of CIGS absorbers thickness,
firstly, we report the modeling and simulation of CIGS solar cells with absorber layer
of 1.2, 1.6 and 1.8 lm at x = 0.3, and secondly at x = 0.9 as shown in Table 4.

Table 4. Characteristics of CIGS cells for variables absorbers thickness.

Ga=ðIn þ GaÞ ¼ 0:3 Ga=ðIn þ GaÞ ¼ 0:9
CIGS absorber thickness (µm) 1.2 1.5 1.8 1.2 1.5 1.8
JSC ðmA=cm2 Þ 38.6222 39.3271 39.7289 30.8465 31.7628 32.3169
VOC ðVÞ 0.7345 0.7364 0.7374 1.1445 1.1469 1.1481
FF 78.0754 78.1867 78.2281 80.982 81.0866 81.1409
Efficiency (%) 22.1479 22.6441 22.918 28.592 29.5391 30.1068

The I–V curve obtained is shown in Fig. 4.

0,050

0,045

0,040
Cathode Current (A)

0,035

0,030

0,025 CIGS absorbers thickness 1 = 1,2 µm

CIGS absorbers thickness 2 = 1,5 µm
0,020 CIGS absorbers thickness 3 = 1,8 µm

0,015
(a) x=0.3
0,010

0,005
0,0 0,2 0,4 0,6 0,8
Anode Voltage (V)

Fig. 4. I–V curve for a CIGS solar cell with variable CIGS absorbers thickness and for fixed x
compositions.

According to the results in Table 4, we observe that, whatever the ratio x, the
short-circuit current density increase when the thickness increase, a very high Jsc of
39.72 mA/cm2 was obtained at x = 0.3 when the thickness is about 1.8 µm. Similarly,
the Voc, FF and the g of the solar cell increase when the thickness increase. An optimal
efficiency of about 30.10% was obtained at x = 0.9 when the thickness is about 1.8 µm.
484 A. Aissat et al.

2.3 Effects of Temperature on CIGS Thin Films Solar Cell

Temperature is an important parameter in the behavior of the solar cells. According to
the results of Table 5, we observe that, whatever the ratio x, the increase in temperature
results in a net decrease in the open circuit voltage, as well as a decrease in the
maximum power (a variation of 15 K results in a decrease of 10 mW of the maximum
power). The optimum operating temperature used in our calculations is 318 K.

Table 5. Characteristics of CIGS cells for variables temperature.

Ga=ðIn þ GaÞ ¼ 0:3 Ga=ðIn þ GaÞ ¼ 0:9
Temperature (K) 288 303 318 288 303 318
JSC ðmA=cm2 Þ 40.1625 40.0283 39.8959 32.835 32.6914 32.5563
VOC ðVÞ 0.7952 0.7665 0.737769 1.2066 1.1776 1.14862
FF 80.556 79.4144 78.2406 82.9127 82.045 81.1564
Efficiency (%) 25.7284 24.3675 23.0293 32.8492 31.5894 30.3483

The I–V curve obtained using ATLASTM is shown in Fig. 5. At higher temperature,
the band gap energy has been slightly reduced, which may accelerate recombination
between the valence band and the conduction band. Although more free electrons are
produced in the conduction band, the high temperature band gap energy is unstable
which can lead to the recombination of electrons and holes while traversing across the
regions.

0,05 0,040

0,035
0,04
Cathode Current (A)

Cathode Current (A)

0,030

0,03 0,025 T1 = 288 K

T2 = 303 K
0,020
(a) x=0.3 T3 = 318 K
0,02
0,015
T1 = 288 K (b) x=0.9
T2 = 303 K 0,010
0,01 T3 = 318 K
0,005

0,00 0,000
0,0 0,2 0,4 0,6 0,8 1,0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
Anode Voltage (V) Anode Voltage (V)

Fig. 5. I–V curve for a CIGS solar cell with variable temperature and for fixed x compositions:
x = 0.3 (b) x = 0.9

3 Conclusion

Based on the Silvaco-Atlas software, we presented numerical simulations of CIGS thin

film solar cells under AM1.5. An optimal results with conversion efficiency around
23.02% were obtained at x = 0.3 with a thickness of about 2 µm and a temperature of
 Optimization of Copper Indium Gallium Diselenide Thin Film 485

318 K, it’s in agreement with the high record conversion efficiency found experi-
mentally in the CIGS solar cell. From this study, we found that the open-circuit voltage
(Voc), the fill factor (FF) and the efficiency ðgÞ of the solar cell are increasing with the
increase of the x composition. In contrary, we observe that, whatever the ratio x, the
increase in temperature results in a net decrease in the open circuit voltage, as well as a
decrease in the maximum power (a variation of 15 K results in a decrease of 10 mW of
the maximum power).

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