Abstract--This paper analyzes the Copper-Indium-Selenium (CIS)

and Copper-Indium-Gallium-Selenium (CIGS) based solar cell

performance by AMPS-1D numerical modeling. Various factors

which affect the solar cell’s performance are investigated,

carefully referring to practical cells, to obtain the optimum

parameters for the CIS and CIGS solar cells. Among the factors

studied are thickness and bandgap energy of absorber layer,

thickness of buffer layer of the cells. In this study, an efficiency of

19.4% has been achieved with CdS based buffer layer with

performance parameters of 0.68 V for open circutit voltage (Voc),

35 mA/cm2 for short circuit current (Jsc) and 0.82 for fill factor

(FF). This solar cell has been used as a base case for simulation. It

is found that the optimum solar cell, regardless whether it is CIS

or CIGS type, has the absorber thickness between 2000 nm and

3000 nm. Moreover, the optimum bandgap of the CIS and CIGS

absorber layer are found to be 1.04 eV and 1.15 eV, respectively.

The thickness of buffer layer has been found in the range of 40

nm to 50 nm as the optimum value.

Index Terms – Numerical Analysis, AMPS-1D, Solar Cells,

CIS, CIGS.

I. INTRODUCTION

HIN film CuInSe2 (CIS) and Cu(In, Ga)Se2 (CIGS) solar

cells possess major potentials as the source of low-cost,

high-efficiency solar electricity [1]. In a CIS/CIGS model, the

absorber acts as a p-type doped region with typical thickness

of 2 to 3 µm [2]. The band gap, Eg for CIS absorber layer is

around 1 eV (0.98 eV-1.04 eV) [3]. By adding the gallium

content into the CIS model, the resulting bandgap of CIGS is

ranged from 1.0-1.7eV [5]. However, the optimum CIGS

bandgap is ranged from 1.16 eV [6] to 1.38 eV. In this study,

CIGS bandgap is simulated from 1eV to 1.6eV to show the

effect of bandgap in the properties of the solar cell. Apart from

that, it is also found that the thickness of CdS buffer layer is

used within the range of 40 nm to 50 nm.

Besides, Nakada and Mizutani [7] have shown that the use

of chemical bath deposition (CBD) ZnS (Eg>3 eV) as buffer

N. Amin is with the Department of Electrical, Electronic & System

Engineering, Faculty of Engineering, National University of Malaysia,

Malaysia (e-mail: nowshad@eng.ukm.my).

K. Sopian is the director of Solar Energy Research Institute (SERI),

National University of Malaysia, Malaysia (email: ksopian@eng.ukm.my).

layer has reached higher efficiency. ZnS is a very promising

material as a replacement for CdS as it has recorded the

second highest efficiency after CdS in the thin film industry.

Although the CBD technique is very efficient in forming a

heterojunction in the cell with good rectifying properties, but it

does not perform well for technique which is based on vacuum

technology [3].

Another replacement for CdS buffer layer is the Zn1−xMgxO.

The best Zn1−xMgxO cell efficiency of 16.2% (active area) was

achieved by Matsushita Electric Industrial [8]. Zn1−xMgxO is

used as a barrier layer for the modulation of bandgap to

enhance light transmission. It also helps to increase the current

generation of the solar cell [9].

Another buffer layer material which is used in this paper is

zinc oxide (ZnO). ZnO has a wider bandgap of 3.3eV than

CdS. ZnO which is grown by chemical bath deposition (CBD)

method has recorded an efficiency of 14.3%. Furthermore, the

solar cells with this new buffer layer didn't show the light-

soaking effect [10]. On the other hand, if the ZnO based buffer

layer is grown by atomic layer deposition (ALD) technique,

the light-soaking effect exist and it is mainly associate with the

CIGS surface properties. This phenomenon can be overcome

by surface etching or doping CIGS surface with Zinc [11]. The

solar cell which is grown by ALD technique has achieved an

efficiency of 13.9% without the light soaking effect.

II. EXPERIMENTAL

In this study, a one dimensional numerical analysis tools

that stands for Analysis of Microelectronic and Photonic

Structures (AMPS-1D) is used to construct various solar cell

models as well as to obtain performance results once the

design parameters are adopted from various practical

references. Four different layers of CIS/CIGS solar cell are

required for the modeling. More layers can be added as long as

the width does not exceed the limitation of 400 grid points or 6

µm. However, it is not necessary to increase the layer

thickness as this project is about thin film CIS/CIGS solar cell

and the aim is to find the optimum efficiency with a narrow

width solar cell. The four layers that are emphasized in this

modeling is the n-type ZnO, i-ZnO, CdS and CIS/CIGS. Table

I and Table II show the description for the parameters used in

the simulation and the base parameter that is used throughout

the study [12].

Numerical Modeling of the Copper-Indium-Selenium (CIS) based Solar Cell Performance

by AMPS-1D Nowshad Amin, Member, IEEE, Michael Tang and Kamaruzzaman Sopian

T

The 5th

Student Conference on Research and Development –SCOReD 2007

11-12 December 2007, Malaysia

1-4244-1470-9/07/$25.00 ©2007 IEEE.

Table I: Overall electronic properties.

Table II: Base parameters for CIS/CIGS solar cells.

III. RESULTS AND DISCUSSION

A. Modeling with Variable CIS Absorber Thicknesses

The efficiency the of the solar cell, as can be seen in Figure

1, is increasing with the thickness of the CIS absorber layer,

but almost saturates while exceeds 5000 nm. At 1000 nm,

3000 nm and 5000 nm of the absorber layer, the efficiency

obtained are 14.64%, 16.92% and 17.83%, respectively. A

2000 nm decrease of absorber layer thickness from the

optimum layer thickness would result in 2.28% decrease in

efficiency. Open circuit voltage (Voc) and short circuit current

density (Jsc) of the solar cell are also shown in Figure 1. Both

values are increasing with the thickness of the absorber layer.

This was mainly due to the increasing of the absorber layer,

which was the p-type region in solar cell. This allows the

longer wavelengths of the illumination to be collected which in

turn contribute to electron-hole pair generation. If the absorber

layer thickness is reduced, the back contact will be very near

to the depletion region. Thus electrons will be captured easily

by the back contact for the recombination process. Therefore,

fewer electrons will contribute to the quantum efficiency of the

solar cell and the values for Voc and Jsc will be low. The J-V

characteristics of the solar cell is shown in Figure 2. It shows

an increase in Voc and Jsc when the absorber layer was

increased.

Spectral response of the solar cells with different

thicknesses of absorber layers is shown in Figure 3. It has been

found that the quantum efficiency of the solar cell is increasing

with the increasing absorber layer thickness. For thicker

absorbers, a greater percentage of electron-hole pairs would be

generated from the absorbed photons. Therefore, the quantum

efficiency would be increasing with the increment of the

absorber layer thickness.

Figure 1: Cell performance with variable CIS absorber layer

thickness.

Figure 2: J-V characteristic of solar cell with variable CIS

absorber layer thickness.

V (v)

J (m

A/c m

2)

Figure 3: Spectral response of solar cells with variable CIS

absorber layer thickness.

B. Modeling with Variable CIGS Absorber Thicknesses

In this part, the numerical modeling for variable CIGS

absorber thickness is presented. The parameters used in this

part of simulation are the same as those used in Section A.

The only difference would be the bandgap energy as bandgap

energy for CIS used in previous simulation was 1.04 eV [4]

while the bandgap energy for CIGS used for current

simulation was 1.15 eV. At the recommended thickness of the

absorber layer (3000 nm) [4], an efficiency of 19.39%,

almost equal to the 19.5% of the highest recorded efficiency

[13] has been obtained. From the simulated result at 1000 nm

and 5000 nm, conversion efficiencies of 16.98% and 20.2%

are achieved, respectively. Comparing the results with the

19.39% efficiency at 3000 nm, it was found that a decrease in

2000 nm of the absorber thickness resulted in 2.42% decrease

of efficiency.

For the CIGS absorber layer, a much higher efficiency was

achieved as compared to CIS absorber layers as shown in

Figure 4. This is mainly due to the introduction of gallium in

CIGS. Gallium is introduced to increase the bandgap energy

of the material. CIS without gallium concentration has

bandgap value in the range of 0.98 eV to 1.04 eV [3]. But

with the introduction of gallium into CIS, the range of the

CIGS bandgap value is 1.0 eV to 1.7 eV [5]. Thus an increase

in bandgap value would result in increase of solar cell’s

efficiency, as it approaches to the optimum value of 1.5 eV.

The simulated results showed that the optimum thickness

of absorber layer should be within the range of 2000 nm to

3000 nm, which was consistent with Pudov’s finding [14]. It

would not be cost effective if a solar cell with the absorber

width greater than 3000 nm were fabricated. Furthermore it

would lead to material wastage and would not comply with

conservation of solar cell material.

Voc and Jsc values were increasing with CIGS absorber

layer. Besides, the spectral response of the solar cell is also

illustrated in Figure 5, where the absorber thickness shows a

major role in the increase of photocurrent.

Figure 4: Cell performance with variable thickness of CIGS

absorber layer.

Figure 5: Spectral response of solar cell with variable

thickness of CIGS absorber layer.

C. Effect of CIS Bandgap on Solar Cell Performance

CIS which stands for Copper-Indium-Selenium has

bandgap value in the range of 0.98 eV to 1.04 eV [3]. Studies

have shown that the optimum bandgap energy for CIS

absorber layer is 1.04eV [4]. Besides, it is also known that

higher bandgap energy will result in high efficiency solar cell.

Thus in order to increase the bandgap energy for CIS solar

cell, indium is needed to be added. Although the introduction

of indium may increase the CIS bandgap energy, it is not cost

effective to do so. This is because indium is a rare material.

Thus, most of the studies conducted have introduced gallium

into the solar cell to form CIGS, Copper-Indium-Gallium-

λ (µm)

QE

λ (µm)

QE

Selenium. Gallium is added to replace as much indium as

possible due to gallium's relative availability to indium.

In this section, CIS bandgap energy ranged from 0.95eV to

1.05eV is chosen for simulation. Performance for the CIS

solar cell is observed. It is found that Voc of the solar cell is

increasing if the bandgap energy is increased. On the other

hand, Jsc is found decreasing as shown in Figure 7. This may

be due to the higher bandgap energy of the solar cell which

prevents the absorbed photons to excite the electrons of the

valence band into conduction band. Efficiency which is

shown in Figure 6 also indicates that higher bandgap energy

will result in higher efficiency.

Figure 6: Cell performance with variable CIS bandgap.

Figure 7: Energy band diagram of solar cell with variable

CIGS bandgap.

D. Effect of CIGS Bandgap on Solar Cell Performance

In order to achieve high efficiency CIGS solar cell, two

different criterions need to be considered; the bandgap

grading in a CIGS absorber layer and also the conduction

band offset at hetero-junction [15]. Studies have shown that

the optimum bandgap energy for CIGS absorber layer is

1.40eV [16], but the range for the bandgap energy is 1.0eV to

1.7eV [5]. The bandgap can be varied by varying the gallium

concentration in CuIn1-xGaxSe2 (x is ranged from 0 to 1)

[18]. If x is 0, then the bandgap energy would be 1.0eV which

means that CIGS has no concentration of gallium. On the

other hand, the bandgap energy would be 1.7eV if x is equal

to 1. This means that CIGS has the highest concentration of

gallium as forming CGS.

Figure 7 shows the energy band diagram for the simulation.

The bandgap energy of 1.0eV and 1.6eV is shown. It was

found out that the conduction band for the 1.6eV is much

higher than the conduction band for the 1.0eV. This means

that for higher bandgap energy, more photons energy is

required to free the electrons into the conduction band. On

the other hand, high bandgap energy creates fast

recombination of electrons and holes at the p-type region in

solar cell. The free carriers are being recombined before they

are diffused to depletion region to contribute to the

generation of electricity. Thus a decrease in Jsc was recorded

with an increasing value of bandgap energy. Besides, high

bandgap energy have a small positive value of conduction

band offset between the absorber layer and buffer layer where

it eventually became negative when the bandgap energy is

further increased.

Figure 8: Cell performance with variable CIGS bandgap.

The CIGS bandgap ranges from 1.0eV to 1.6eV has been

chosen for simulation. From Figure 8, it has been found out

that the Voc is increasing with bandgap. However the Voc

becomes constant at around 0.895 V over the bandgap value

of 1.48 eV. Jsc is decreasing with the bandgap value. Due to

the high bandgap value of the CIGS, the photons that are

position (µm)

eV

being absorbed at the p-type region of the solar cell do not

have enough energy to surpass the bandgap of CIGS solar

cell. If the bandgap energy is 1.5eV, this means that only the

photons which carried energy greater than 1.5eV contributes

to electron hole pair generation. All the other photons which

are carrying lesser energy would be wasted. As a whole,

efficiency is increasing with the increment of bandgap value,

but it decreases afterwards. The highest efficiency that was

obtained from this simulation is 21.297% at 1.36eV, which is

quite near to the optimum bandgap energy of CIGS absorber

layer of 1.4eV. The bandgap energy of 1.36eV is the

optimum bandgap energy for our simulation as both the Jsc

and efficiency are decreasing after this optimum value.

However, currently the laboratory scale best CIGS solar cells

are having bandgap values of 1.15eV to 1.20eV [17].

Figure 9 shows the J-V characteristics of the solar cell

where an increase of CIGS bandgap value results in a

decrease of short circuit current density. Besides, it also

results in an increase of open circuit voltage until a saturation

point.

Figure 9: J-V characteristics of solar cells with variable CIGS

bandgap values.

E. Effect of CdS BufferLayer Thicknesses

Generally the thickness of the optimum CdS buffer layer

should be within 50 nm and 60 nm [19]. The buffer layer

thickness of 50 nm has been used as the base parameter for

all of the simulation in this study. In this section, the effect of

the thickness variation of the CdS buffer layer on the

parameters of solar cell is shown. Here, the thickness of the

CdS buffer layer was varied from 10 nm to 100 nm.

Voc of the solar cell is shown in Figure 10 where the values

are almost constant for all the varying thicknesses of the CdS

buffer layer. Beside, Jsc values of the solar cell are found

decreasing with the increase of CdS buffer layer thickness.

This is because a thicker buffer layer will result in higher

photon absorption as loss [14]. When the buffer layer is

increased, more photons which carry the energy are being

absorbed by this layer. Therefore it would lead to a decrease

in the photons which could reach the absorber layer. A

decrease in the numbers of photons at the absorber layer

would decrease the quantum efficiency of the solar cell as

shown in Figure 12.

Figures 10 and 11 also show the efficiency of the solar cell

and the J-V characteristics with increasing CdS buffer layer

thickness. As explained earlier, both the values were found

decreasing because most of the photons are being absorbed

by the buffer layer. From Figure 10, it was found that buffer

thickness of 10 nm gives the highest efficiency. But it is

impossible to be fabricated practically because of the

complexity in fabrication. It is found that the optimum buffer

thickness would be from 40 nm to 50 nm as Jsc is found

decreasing after 50 nm.

Figure 10: Cell performance with variable thickness of CdS

buffer layer.

Figure 11: J-V characteristics of solar cells with variable

thicknesses of CdS buffer layer.

V (v)

J (m

A/c m

2)

J (m

A/c m

2)

V (v)

Figure 12: Spectral response of solar cells with variable

thicknesses of CdS buffer layer.

IV. CONCLUSION

Optimum parameters for the CIS and CIGS based solar

cells are obtained from numerical modeling. From the

simulation results, it is found that the increasing thickness of

the absorber layer results in higher efficiency of solar cell,

where the optimum thickness is around 3000 nm. Moreover,

it is found that the energy bandgap of 1.48eV was the

optimum for the CIGS, as the bandgap value greater than this

affects the efficiency. Furthermore, thickness of the CdS

buffer layer is needed to be at an optimum value of 50 nm to

reduce the photon loss. All these optimization results give

helpful indication for practical usage in fabrication process.

V. REFERENCES

[1] James R. Sites, Tokio Nakada, Hans-Werner Schock, Sigeru Niki,

Akira Yamada, Miguel A. Contreras, “Research and Development of

High-Voltage CIS-Based Thin Film Solar Cells for Industrial

Technology (2005)”.

[2] A. O. Pudov, “Impact of Secondary Barriers on CuIn1−xGaxSe2

Solar-Cell Operation”, Department of Physics, Colorado State

University, 2005.

[3] 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, “Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe2 Thin-

film Solar Cells”, Prog. Photovolt: Res. Appl. 2003; 11:225-230.

[4] M. Gloeckler, J. R. Sites, “Bandgap grading in Cu (In. Ga) Se2 solar

cells”, Department of Physics, Colorado State University, pp 4,

September 2004.

[5] Clas Persson, “Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells:

anomalous physical properties of the CuIn1-xGaxSe2 absorber”,

Brazilian Journal of Physics. v.36 n.3b São Paulo sep. 2006.

[6] C. H. Huang, Sheng S. Li, T. J. Anderson, “Modeling and Parameter

Optimization of CuInSe2-based Solar Cells”, University of Florida,

2000.

[7] T. Nakada and M. Mizutani. Jpn. J. Appl. Phys. 41 (2002), p. 165.

Source: http://jjap.ipap.jp/journal/html/JJAP-41-2B/L165/

[8] T. Negami, T. Aoyagi, T. Satoh, S. Shimakawa, S. Hayashi and Y.

Hashimoto, Proceedings 29th IEEE Photovoltaic Specialist

Conference, New Orleans, USA (2002), p. 656.

[9] Jae Won Kim, Hong Seong Kang, Jong Hoon Kim, and Sang Yeol Lee,

Jung-Kun Lee and Michael Nastasi, “Variation of structural, electrical,

and optical properties of Zn1−xMgxO thin films”, Journal of Applied

Physics pp100, 033701 2006

[10] Rui Mikami, Hisashi Miyazaki, Tatunobu Abe, Akira Yamada and

Makoto Konagai, “Chemical Bath Dposited (CBD)-ZnO Buffer Layer

for CIGS Solar Cells”, 3rd World Confrre,ice on Photovolraic Energy

Conversion May 11-18. 2003, Pages 519.

[11] Sutichai Chaisitsak, Akira Yamada, and Makoto Konagai,

“Comprehensive Study of Light-Soaking Effect in ZnO/Cu(InGa)Se2

Solar Cells With Zn-Based Buffer Layers”, MRS.

[12] M. Gloeckler, A.L. Fahrenbruch, and J.R. Sites, “Numerical Modeling

of CIGS and CdTe Solar Cells: Setting the Baseline”, Photovoltaic

Energy Conversion, 2003. Proceedings of 3rd World Conference on

Volume 1, Issue, 11-18 May 2003 Page(s): 491 - 494 Vol.1.

[13] 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, “Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe2 Thin-

film Solar Cells”, Prog. Photovolt: Res. Appl. 2003; 11:225-230.

[14] A. O. Pudov, “Impact of Secondary Barriers on CuIn1−xGaxSe2

Solar-Cell Operation”, Department of Physics, Colorado State

University, 2005.

[15] G. Cernivec, J. Krc, F. Smole and M. Topic, “Band-gap engineering in

CIGS solar cells using Nelder-Mead simplex optimization algorithm,

Thin Solid Films”, Volumes 511-512, EMSR 2005 - Proceedings of

Symposium on Thin Film and Nanostructured Materials for

Photovoltaics - EMRS 2005, 26 July 2006, Pages 60-65.

[16] W. Shockley, H. J. Queisser, J. Appl. Phys., 32:510–519, 1961.

[17] C. H. Huang, S. S. Li, T. J. Anderson, “Device Modeling And

Simulation of CIS-Based Solar Cells”, University of Florida, 2002.

[18] G. Hanna, A. Jasenek, U. Rau and H. W. Schock, “Influence of the

Ga-content on the bulk defect densities of Cu(In,Ga)Se2”, Thin Solid

Films, Volume 387, Issues 1-2, 29 May 2001, Pages 71-73.

[19] Y. Hashimoto, T. Satoh, S. Shimakawa and T. Negami, “High

efficiency CIGS solar cell on flexible stainless steel”, Living

Environment Development Center, Matsushita Electric Ind. Co., Ltd.

3-4 Hikaridai, Seika-cho, Soraku-gun, 2003.

VI. BIOGRAPHIES

Nowshad Amin was born in Chittagong,

Bangladesh, on January 5, 1972. He graduated

from the Comilla Cadet College, Bangladesh,

and later studied at the Universities of Gunma

National College of Technology for diploma in

Electrical Engineering, Toyohashi University of

Technology for BS, Tokyo Institute of

Technology for MS and PhD in Electronics, in

Japan. His special fields of interest include

Semiconductor Physics and Devices,

Optoelectronics.

Michael Tang was born in Sarawak, Malaysia, on

February 23, 1984. He graduated from Multimedia

University, Malaysia in 2007. His field of interest

includes electronics, optoelectronics.

Kamaruzzaman Bin Sopian was born in the

historical town of Batu Gajah, Perak in 1962.

He obtained his BSc in Mechanical Engineering

from the University of Wisconsin-Madison in

1985, MSc in Energy Resources from the

University of Pittsburgh in 1989 and PhD. in

Mechanical Engineering from the Dorgan Solar

Laboratory, University of Miami in 1997.

Currently, he is the Director of the Solar

Energy Research Institute, a center of excellence

for the research and development in solar energy

technology.

QE

λ (µm)