Enhancing the efficiency of CIGS thin film solar

cells by inserting novel back surface field

(SnS) layer
S. Benabbas 1*, H. Heriche1, Z. Rouabah1, N. Chelali1
1
Materials and Electronic Systems Laboratory, University of BordjBordjArréridj, El-Anasser, 34265
Bordj-Bou-Arreridj, Algeria
*Corresponding author: benabbessabrina@gmail.com

Abstract— In this work, we propose a novel structure of The contribution of the study here in aims to improve the
solar cell based on copper-indium-gallium-diselenide performances of ultra-thin CIGS solar cells, with an
CuIn1-xGaxSe2 (CIGS) absorber layer by using SCAPS-1D ZnO/CdS/CIGS/SnS/Mo structure using SCAPS code. This
(Solar Cell Capacitance Simulator of the University of has improved the devices efficiency which exceeds 25 %
Gent). This numerical simulation has been used to explore bearing in mind that without using the layer SnS the efficiency
the possibility of higher efficiency and stable CdS/CIGS was 17.99 %. Various thicknesses of CIGS absorber layer
cell structures with (ZnO) as window layer, and (CdS) a ranging from 0.3 -1.0 μm have been used so as to investigate
buffer layer, (CIGS) absorber layer and (SnS) BSF layer. the performance.
The optimal values to give maximum performance of the In the present research study, numerical modeling of CIGS
structure ZnO/CdS/CIGS/SnS, without and with the BSF thin film solar cell has been carried out by SCAPS-1D,
layer (SnS), were determined. The study shows potential SCAPS is a one-dimensional computer software developed at
results for improvement of efficiency of solar cell when the University of Gent under Marc Burgelman [8]. It is
using the back surface field (BSF). It was observed that developed especially to simulate the AC and DC
the proposed cell provided conversion efficiency of 25.29% characteristics of hetero-junction thin film solar cells,
(Voc = 0.79 V, Jsc = 36.43 mA/cm2, FF = 84.83). However especially CIGS and CdTe solar cells.
the efficiency of cell reference (without BSF) is 17.99%
(Voc = 0.62 V, Jsc = 36.03 mA/cm2, FF = 80.23 %). II. RESULTS AND DISCUSSIONS

Keywords— CIGS, Solar cells, BSF, efficiency This research is a leading work to develop ultra-thin CIGS PV
cell. However, the use of back surface field (BSF) material is
I. INTRODUCTION found to be better, effective and easier one among several
methods still used to overcome the problem with the back
Copper-indium-gallium-diselenide (CIGS) is one of the contact.
promising absorbing material in thin-film solar cells, which This paper numerically explores the possibility of high
exhibit record efficiencies of 20.3 % in the Centre of Solar efficiency, ultra-thin and stable CIGS cells with SnS (BSF).
Energy and Hydrogen Research (ZSW) [1], CIGS is a direct The cell performances (Voc, Jsc, FF, efficiency, temperature
band gap material with the band gap (Eg) ranging from 0.98 stability) are investigated by recognized simulator (SCAPS-
to 1.68 eV for x = 0 (CIS) and x=1 (CGS), respectively [2 ]. 1D).
In a CIS/CIGS model, the absorber acts as a p-type doped A modified structure of CIGS based PV cell
region with typical thickness of 2–3 µm [3]. ZnO/CdS/CIGS/SnS/Mo has been proposed over reference
The problem of the reduction in thickness of the CIGS structure ZnO/CdS/CIGS/Mo. The simulated results have
absorbers layer was first time addressed in the works of shown that the use of SnS as BSF layer enhances the cell
Shafarman [4], Negami [5], NREL [6] and the University of performances. In this analysis the highest conversion
Uppsala [7], in ordre to reducing the material cost in large- efficiency of CIGS based photovoltaic cell without BSF has
volume production of CIGS, especially that of In. These been found around 18% using CIGS absorber thickness of
works undertaken in detail the effect of reduction of the 1µm, on the other hand with the BSF the conversion
thickness of absorber layer on the photovoltaic properties of efficiency has been found to reach high performance up to 25
solar cell CIGS, with the aim to reducing the cost. This has % using only 1 µm thick CIGS absorber.
prompted researchers to turns to ultra-thin CIGS.

978-1-4799-6503-8/14/$31.00 ©2014 IEEE

 A. Simulation of ZnO/CdS/CIGS /Mo structure
A.1. Thickness optimization of CIGS absorber layer
n-ZnO (10 nm) The aim of this work is to produce CIGS cells with high
conversion efficiency while reducing the thickness of the
n-CdS (10nm)
absorbing layer 1 µm. To determine the influence of thickness
p-CIGS (1µm) of the CIGS absorber layer on the photovoltaic properties of
the solar cells. The thickness of the CIGS layer was varied
Back contact (Mo) from 0.3 to 1 μm.
Fig.2 shows that, for the reference cell all the solar cell output
Glass mA
(a) parameters (( , 2
, FF (%), efficiency (%)
increases when the thickness of the absorbing layer is thicker,
n-ZnO (10 nm) with 1 µm can achieve an efficiency of 17.99 %.

n-CdS (10 nm)

p-CIGS (1µm)
0,65

p-SnS (30nm)
Back contact (Mo) 0,60

Glass

Voc(v)
0,55

(b)
0,50
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Fig. 1 Structures of the CIGS solar cells:

(a) Conventional baseline case structure

(b) Modified structure for higher conversion efficiency 36

34

The table 2 shows the different values of the physical 32

Jsc(mA/cm)
2

parameters of the materials in the CIGS solar cell 30

[3,9, 10, 11]. 28

In the study these physical parameters were chosen. A gap of 26

1.2 eV for CIGS, the junction is made between the Cu (In, 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

Ga) 2 and P-type and N-type CdS has a gap of 2.4 eV. The
window layer is formed of ZnO with a gap equal to 3.3 eV. In
the new structure we added a new layer p-SnS which has a 82

gap of 1.25. eV 80

78

TABLE 1
FF(%)

76

Base parameters for CIGS 74

Parameters n-ZnO n-CdS p-CIGS p-SnS 72

3.3 2.4 1.2 1.25 70

0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

(eV) 4.6 4.4 4.5 4.2

(1/m) 9 10 13.6 12.5
19

(1/ ) 2 ×10 2.2×10 2.2 ×10 1×10 18

(1/ ) 1.8×10 1.8×10 1.8×10 4.13×10 17

Efficiency(%)

16

/ 100 100 100 25 15

/ 25 25 25 100 14

n,p(1/ ) 1 10 1 10 1.1 10 6 10 13
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Thikness of CIGS absorber layer (µm)

Fig.2 Effect of CIGS thickness layer variation on reference

cell parameters
The performances of the respective cells are shown in terms of
Voc, Jsc, FF and efficiency in Fig. 2. All the performance 0,762

parameters illustrate increasing trend as the CIGS absorber 0,760

layer is increased from 0.3 μm to 1.0 μm.

0,758
For a 0.3 μm CIGS absorber layer thickness, the obtained

Voc(V)
efficiency is 11.43 %, the highest efficiency obtained is 17.99 0,756

% which corresponds to 1 μm CIGS absorber layer solar cell. 0,754

The increase in the conversion efficiency is mainly due to the 0,752

increase in the p-type CIGS absorber layer. As the p type layer 0,750
is increased more photons with longer wave length can be 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

collected in the absorber layer [12].

B. Simulation of ZnO/CdS/CIGS/SnS /Mo structure

B.1. Thickness optimization of CIGS absorber layer 40

38

In the new structure, we have inserted a BSF layer (SnS)

between the back contact Mo and CIGS absorber layer, in the

Jsc(mA/cm )
36

2
reference structure. 34
The Tin sulfide (SnS) prepared in thin film with several
features (the photovoltaic conversion efficiency, the optical 32

gap, the absorption coefficient, low cost and non-toxic nature)

[13]. To see the influence of the thickness of the absorbing 30
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

Layer on the performance of the solar cell of the ZnO / CdS /

CIGS / SnS /Mo structure; the thickness of this layer from
0.3 μm to 1 μm. 85
The CIGS thickness has been varied from 0.3 μm to 1 μm to
80
investigate thinner absorber layer and the results obtained
from SCAPS simulation are shown in Fig. 3. 75

Fig.3 shows that the Cell efficiency increase when the 70

FF(%)

thickness of the absorbing layer is thicker, with 1 µm an 65

efficiency of 25.29% can be achieved [3, 11]. 60

The results emphasize that the cell output parameters can be 55

improved while having a thinner CIGS layer adding an extra
50
layer of SnS with 30 nm only. The highest conversion 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

efficiency of the proposed cell is 25.29% with a CIGS

absorber layer thickness of just 1 µm.

26

25
Efficiency%

24

23

22
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Thikness of CIGS absorber layer (µm)

Fig.3 Effect of CIGS film thicknesses on cell parameters

C. Comparison between CIGS and new structure CIGS/SnS reference structure 17.99% efficiency , with the form factor
of 80.23%, current density 36.03 (mA/ cm2) and the open
circuit voltage of 0.6224 V.
R e f e r e n c e c e ll
0 ,8 0 P r o p o s e d c e ll D. Effects of operating temperature on the reference cell and
0 ,7 5
the proposed cell with SnS (BSF)
0 ,7 0

0 ,6 5
Voc(V)

0 ,6 0

26 P r o p o s e d c e ll
0 ,5 5
R e fe r e n c e c e ll
0 ,5 0 24

0 ,4 5 22

0 ,4 0
0 ,2 0 ,3 0 ,4 0 ,5 0 ,6 0 ,7 0 ,8 0 ,9 1 ,0 1 ,1 20

Efficiency(%)
18

16

14
40

38 12
36

34 10
300 320 340 360 380 400
Jsc(mA/cm )

32
2

30
O p e r a tin g te m p e r a tu r e ( C ° )
28

26

24

22
Fig .5 Effects of operating temperature on the reference cell and the proposed
20
0 ,2 0 ,3 0 ,4 0 ,5 0 ,6 0 ,7 0 ,8 0 ,9 1 ,0 1 ,1
cell with SnS (BSF)

The efficiency of solar cells is decreasing when the

temperature is increased. At higher temperature, parameters
85
such as the electron and hole mobility, carrier concentrations
and band gaps of the materials where negatively affected the
80
resulting in lower efficiency of the cells. This behavior due the
75
effect of the temperature regarding the efficiently is validated
70
in previous study [14].
FF(%)

65

E. Comparison of the obtained results of the present work with

60

55
previous studies
50
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

For a better illustration of validation of the obtained results,

the table 2 presents the comparison between our results with
the results obtained by other researchers. It can be observed
26 that the values obtained are higher than those proposed by
24 previous studies.
22

TABLE 2
Efficiency(%)

20

18 Summary of photovoltaic parameters for different CIGS solar cells

16 Value PV our work [3] [11]
14

12
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Thickness CIGS (µm) 1 4 3
Thikness of CIGS absorber layer (µm)
Efficiency(%) 25.29 21.32 24.97
Fig.4 Effect of CIGS thickness layer variation on reference cell parameters
FF(%) 84.83 82 88.15
and proposed cell
mA/ 36.43 33.5 24.97
The proposed structure ZnO / CdS / CIGS / SnS/ Mo with
SnS layer gave a better performance of a 25.29% efficiency, 0.79 0.78 1.13
the form factor was 84.83%, the density of current 0.7623V
and voltage of open circuit 36.43 (mA/ cm2) compared to
 III. CONCLUSION

The performance of CIGS solar cells has been improved as far [13] Yuanzheng Yang JiaxiongXu, "Study on the performances of
as the absorber layer thickness is increased. It was found that SnSheterojunctions by numerical analysis," Energy Conversion and
the best structure could have a window layer(ZnO), a buffer Management, vol. 78, pp. 260–265, 2014.
layer (CdS), an absorbent layer (CIGS) and a SnS layer with [14] T.Nakada, M. Mizutan, "18% efficieny Cd- free Cu(In.Ga)Se2 thin-
film Solar cells fabricated using chemical bath deposition (CBD)-ZnS
thicknesses of 0.01, 0.01, 1 and 0.03 μm, respectively. Cells buffer layers," Jpn. J. Appl. Phys, vol. 41 , no. 165, 2002.
with these features give conversion efficiencies of 25.29%.
Our results showed that new ultra-thin CIGS solar cells
structure has comparable performance parameters with the
conventional CIGS solar cells, but with lower cost.

ACKNOWLEDGEMENT
The authors gratefully acknowledge Dr. Marc Burgelman,
University of Gent, Belgium, for providing the SCAPS
simulation software.

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