Copper-Indium-Gallium-Diselenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current Technological Trend and Optimization

Abstract

The Copper Indium Gallium diSelenide (CIGS) thin film solar cells are considered in this chapter. The interest in Cu(In1-x, Gax)Se2 thin film solar cells has increased significantly due to its promising characteristics for high performance and low cost. It is aimed to present an extensive evaluation on CIGS nanocrystalline bulk semiconductor and its application as an absorber layer for thin film solar cells. It is also aimed to improve the CIGS thin film solar cell efficiency through finding optimum ranges of material properties. The first section of this chapter gives an extensive overview on CIGS nanocrystalline bulk semiconductor background and technological trend. In the middle section, a brief review on CIGS Solar Cell processing and challenges are highlighted and the last section a numerical simulation results on the effects of each of constructive nano layer properties on cell performance are shown and compared with valid experimental results.

The energy band gap is also an important criterion for choosing a proper material for buffer and window layers. In this case, the energy band gap (Eg) needs to be adequately larger than absorber band gap to absorb as less photons as possible. It is assumed that photons with energies equal to and greater than the semiconductor material band gap can be absorbed in that material. In a CIGS solar cell, the light first passes through the window layer and those photons with energies more than the window layer’s band gap will be absorbed in this layer. The remaining photons then pass through the buffer layer, and similarly, a part of incoming photons that have energies higher than the buffer band gap will be absorbed in the buffer layer. Most of the photogenerated carriers in the window and buffer layer cannot be collected due to their low mobility. Figure 4 shows the photon flux at standard AM1.5 solar spectrum as a function of wavelength and maximum possible short circuit current density (Jsc) that can be generated in a solar cell as function of absorber material band gap. As it can be seen, the photogenerated current loss is less than 1 mA/cm due to the absorption of photons in ZnO transparent conductive oxide (TCO) layer. Therefore, the loss of current caused by the absorption of light in the buffer layer is about 4 and 8 mA/cm² for In2S3 and CdS buffer materials, respectively.

Full text

Chapter 3
Copper-Indium-Gallium-diSelenide (CIGS)
Nanocrystalline Bulk Semiconductor as the Absorber
Layer and Its Current Technological Trend and
Optimization
Nima Khoshsirat and Nurul Amziah Md Yunus
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/64166
Abstract
The Copper Indium Gallium diSelenide (CIGS) thin film solar cells are considered in this
chapter. The interest in Cu(In1-x, Gax)Se2 thin film solar cells has increased significantly
due to its promising characteristics for high performance and low cost. It is aimed to
present an extensive evaluation on CIGS nanocrystalline bulk semiconductor and its
application as an absorber layer for thin film solar cells. It is also aimed to improve the
CIGS thin film solar cell efficiency through finding optimum ranges of material properties.
The first section of this chapter gives an extensive overview on CIGS nanocrystalline bulk
semiconductor background and technological trend. In the middle section, a brief review
on CIGS Solar Cell processing and challenges are highlighted and the last section a
numerical simulation results on the effects of each of constructive nano layer properties
on cell performance are shown and compared with valid experimental results.
Keywords: copper-indium-gallium-diselenide (CIGS), indium sulfide (In2S3), nano‐
crystalline, thin film, solar cell, SCAPS
1. Introduction
Thin film solar cells are introduced and developed as the second generation of solar cells to
provide high production capacity at lower energy and material consumption [1]. Main
motivations for the growth of thin film photovoltaic (PV) are their potential for high-speed and
high-throughput manufacturing and minimum material requirements that lead to cost reduction
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.

[2]. This type of solar cell is made on cheap, large area and nonsilicon substrates such as glass,
metal foil or plastic.
In 1976, three separate research groups, Panasonic Matsushita, University of Maine and RCA
research group had formed the thin-film cell. Each group had used different absorber mate‐
rials. Consequently, thin film solar cells were divided into three main subcategories based on
their absorber materials such as amorphous silicon (a-Si), cadmium telluride (CdTe) and
copper-indium-gallium-diselenide (CIGS). All these materials are direct-gap semiconductors
that can absorb incoming solar radiation at a thickness much thinner than the required
thickness for the silicon wafers in crystalline silicon (Si) solar cells. It is worth noting that the
absorption of a-Si, CdTe and CIS/CIGS materials are significantly different. The highest value
of solar radiation absorption belongs to CIS/CIGS that can absorb almost complete incoming
radiation at first 3–4 μm of the material thickness and 95% of the radiation in its first 0.4–0.5
μm [3].
Figure 1. Best research solar cell efficiency released on April 2016 (Lawrence L. Kazmerski, National Renewable Ener‐
gy Laboratory (NREL) [4].
Figure 1, which was recently published by US National Renewable Energy Laboratory (NREL),
shows the best researched solar cell efficiencies that have been reported so far [4]. It can be
observed that there is an efficiency gap between mono (single)-crystalline Si and poly-
(multi)crystalline Si cells. Both of these silicon-based solar cells show higher levels of efficiency
in comparison with other generations of solar cell. As a third-generation solar cell, the dye-
sensitized cells have reached to over 10% efficiency for laboratory sample, but in general, their
efficiency is less than other types of solar cells and are still known as an emerging PV tech‐
nology. Among thin film solar cells, the CIGS cell has the highest record by the efficiency of
over 20% (22.3% for a cell with glass substrate [4] and 20.4% for a cell with polyimide foil
substrates [5]). The highest CdTe efficiency record is 22.1% [6], which is 0.2% lower than CIGS
Nanoelectronics and Materials Development
42

on glass substrate. Nevertheless, it needs to be studied and developed more in order to be
successfully transferred from the laboratory stage to commercial stage and stay competitive
in the PV market.
2. CIGS cell structure
Copper-indium-gallium-diselenide (CIGS) thin-film solar cells are multilayer thin film devices
with Cu(In1-x Gax)Se2 nanocrystalline bulk semiconductor as the absorber material. The cheap
substrate and monolithic interconnection of individual cells in a module are some initial
advantages of CIGS thin film solar cells in comparison with silicon wafer-based solar cell. The
suitable energy band gap of CIGS is another benefit of this compound semiconductor.
Theoretically to ensure a sufficient absorption of the solar irradiation, the band gap of the
absorber should be within the range of 1.0–1.8 eV with the optimum value of 1.5 eV [7]. The
energy band gap of Cu(In1-x Gax)Se2 can be varied from 1.06 to 1.7 eV depending on the
Ga/(Ga+In) ratio. Thus, the CIGS quaternary compound semiconductor is a good option to be
used as an absorber material in solar cells.
Figure 2. Typical CIGS thin-film solar cell structure [9].
Figure 2 shows the structure of CIGS solar cell. In this structure, the device is formed on low
price substrates such as glass, polyimide foils, stainless steel, etc. The most common material
used for substrate in CIGS technology is soda lime glass (SLG) due to its smooth surface,
stability, electrical insulating features and more importantly its affordable price [8].
2.1. Back contact
The metal back contact is the first layer to be deposited on the substrate. This layer plays the
role of an optical reflector as well as a contact for delivering the carriers to the load. Several
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
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materials have been proposed for the back contact such as molybdenum (Mo), tungsten (W),
tantalum (Ta), manganese (Mn), titanium (Ti), etc. The most common material used for the
back contact is molybdenum (Mo) (especially for soda lime glass substrates). This is because
of molybdenum has low contact resistance to the absorber layer and it is stable at the processing
temperature. These can limit the diffusion of atoms and hence minimize the destructive
reactions during CIGS growth [10]. The process condition and growth parameters in the
electrical and structural properties of Mo contact are important in the deposition method. For
instance, although the resulting electrical resistivity of the Mo layer deposited at the low-
pressure process is low, its adhesion to the substrate is poor and can be easily peeled off from
the substrate’s surface [11]. Thus, a compromise between the layer’s adhesion and its resistivity
is required during the back contact deposition.
2.2. Absorber layer
The absorber layer is deposited on the top of back contact layer as shown in Figure 2. Cu(In1-
x Gax)Se2 is one of the most promising absorber material for PV applications. It is a direct band
gap semiconductor. Its energy band gap (Eg) is within the range of 1.06–1.7 eV. The exact value
of Eg depends on Ga/(Ga+In) ratio. The Cu concentration in the composition can also cause
changes to CIGS band-gap value. A decrease in Cu concentration can cause an increase in band
gap [12]. CIGS application in solar cells started with growth and the structural characterization
of CuInS2 (CIS) with the energy band gap of 1–1.06 eV in 1953. Then, some researches were
conducted to increase the CIS band gap to the theoretical optimum value of around 1.4–1.5 eV.
It was found that by adding gallium (Ga) to CIS will keep the overall number of group-III
atoms, while In + Ga constant could lead to an increase in band gap [13]. This finding opens a
new path for the enhancement of CIGS solar cell through adjusting the absorber layer band
gap.
2.3. Buffer layer
The next layer in the CIGS solar cell structure is the buffer layer. This layer is deposited on the
top of the absorber layer. The role of a buffer layer in the heterojunction is to form a junction
with the absorber layer while leading maximum amount of incoming light to the absorber
layer. The buffer layer should have a minimal absorption loss, low surface recombination and
electrical resistance in photogenerated carriers driving out [14]. The most important features
of the buffer layer are to protect the junction against chemical reactions and mechanical
damage. It is also to optimize the band alignment of the cell, electrical properties and making
a wide depletion region with p-type absorber layer. This will eventually minimize the carriers
tunneling and maintain higher open circuit voltage value and later establish a higher contact
potential [15]. In order to satisfy such desired features, the buffer layer should have a wider
band gap in comparison with CIGS layer. In addition, the deposition process for the buffer
layer should passivate the surface states of the absorber layer and provide a suitable conduc‐
tion band alignment with the absorber to achieve higher efficiency [16]. The first experimental
thin-film solar cell device using CIS absorber layer was an heterojunction between a p-type
CuInSe layer and a thin layer of n-type CdS compound semiconductor. Further development
Nanoelectronics and Materials Development
44

of this structure was the vacuum evaporation of an undoped CdS layer, followed by indium-
doped CdS layer to increase cell’s performance and efficiency. Furthermore, the doped CdS
would play the role of a transparent conductor layer [17]. The most efficient CIGS solar cell
fabricated so far used CdS buffer layer. Although CdS buffer layer has yielded high-efficiency
cell, its toxicity [18], incompatibility with in-line vacuum-based production method [19] and
low-performance level of CdS/CIGS cells in short wavelength domain [20] led researchers to
think about replacing CdS with a nontoxic material. At present, the development of Cd-free
wide-band gap buffer layer is one of the main objectives in the field of CIGS thin film solar
cells. The development of Cd-free device was initiated in 1992, and many different materials
were proposed for it until now. These materials generally can be categorized in two main
groups of zinc (Zn) based such as ZnS, ZnSe, ZnO, (Zn,Mg)O and indium (In) based including
In(OH)3, In2S3, In2Se3 [21].
2.4. Window layer
A thin layer of transparent conductive oxide (TCO) as front contact is just next to a buffer layer.
This TCO layer should have a sufficient transparency to let most part of incoming light through
the underlying layers. It must also have sufficient conductivity in order to transport the
photogenerated current to the external circuit with minimum resistivity loss [22]. The most
common used TCO material in CIGS solar cells is highly doped zinc oxide (ZnO) with the
energy band gap of above 3.3 eV [23]. A doping of the ZnO layer is usually obtained by group
III elements, exclusively with aluminum [24].
3. Criteria of material selection for the CIGS thin-film solar cell
There are some parameters that should be taken into consideration while choosing the material
for different layers of a CIGS thin-film solar cell. The parameters such as the band gap, electron
affinity, absorption coefficient, carrier concentration and many more can affect solar cell’s
characteristics and its output performance. The most effective parameters are the band gap
and the electron affinity that will be discussed in the following sections.
3.1. Band gap
The most important parameter in CIGS solar cell is the absorber layer band gap. According to
the theoretical considerations proposed by Loferski [25], the band gap values within the range
of 1.4–1.5 eV is ideal for the absorber layer. Figure 3 shows the maximum theoretical conversion
efficiency as the function of semiconductor band gap for different absorber materials. Fur‐
thermore, the absorber layer should be a p-type and direct band gap semiconductor to have
maximum conversion efficiency. This p-type semiconductor is proposed due to its longer
electron diffusion length [26].
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
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Figure 3. Theoretical maximum achievable efficiency versus absorber material band gap [25].
The energy band gap is also an important criterion for choosing a proper material for buffer
and window layers. In this case, the energy band gap (Eg) needs to be adequately larger than
absorber band gap to absorb as less photons as possible. It is assumed that photons with
energies equal to and greater than the semiconductor material band gap can be absorbed in
that material. In a CIGS solar cell, the light first passes through the window layer and those
photons with energies more than the window layer’s band gap will be absorbed in this layer.
The remaining photons then pass through the buffer layer, and similarly, a part of incoming
photons that have energies higher than the buffer band gap will be absorbed in the buffer layer.
Most of the photogenerated carriers in the window and buffer layer cannot be collected due
to their low mobility. Figure 4 shows the photon flux at standard AM1.5 solar spectrum as a
function of wavelength and maximum possible short circuit current density (Jsc) that can be
generated in a solar cell as function of absorber material band gap. As it can be seen, the
Figure 4. Photon flux at standard AM1.5 solar spectrum; maximum possible short circuit current density (Jsc).
Nanoelectronics and Materials Development
46

photogenerated current loss is less than 1 mA/cm due to the absorption of photons in ZnO
transparent conductive oxide (TCO) layer. Therefore, the loss of current caused by the
absorption of light in the buffer layer is about 4 and 8 mA/cm2 for In2S3 and CdS buffer
materials, respectively.
In a cell with CIS absorber material in which its band gap is 1.06 eV as shown in Figure 4, the
maximum photogenerated short-circuit current density (Jsc) is about 46 mA/cm2, which is
around 22 mA/cm2 higher than maximum short-circuit current density in a cell with CGS
absorber layer. Thus, although the band gap is not the only parameter, which can affect the
cell performance, the selection of material for different layers in a solar cell should be under
the band gap theoretical considerations in order to have the maximum absorption of light in
absorber layer and minimum current loss.
3.2. Electron affinity
Another important parameter is electron affinity (Xe). The difference between electron affinity
of absorber and buffer layer has an important role in the band alignment and shaping the
discontinuity of energy band at the buffer/absorber interface. The discontinuity of conduction
band at the interface that is called conduction band offset (CBO) can be positive (spike) or
negative (cliff). This is due to the difference between absorber and buffer electron affinity.
Figure 5 shows the band alignment and formation of cliff and spike conduction band offset in
CIGS solar cell. A cliff CBO at absorber/buffer interface can cause a reduction in open-circuit
voltage (Voc) because of the lack of barrier height [27]. A positive CBO (spike) inhibits the flow
of photogenerated carriers from the absorber to the buffer. A large spike makes a large barrier
for carriers and therefore reduces the Jsc. However, a small spike does not act as a barrier [28].
Thus, the electron affinity of absorber and buffer layer should be compatible based on their
energy band gap to maintain suitable levels of open-circuit voltage and short-circuit current
density.
Figure 5. Band alignment between absorber and buffer layer. Conduction band offset: (a) Cliff and (b) Spike [20].
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
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3.3. Quantum efficiency (QE)
Quantum efficiency is used as a tool for measuring the spectral response of the device. It gives
a detail information about the absorption of photons and creation of carriers at different
wavelength or photon energy levels. It is defined by the ratio of electrons collected from the
device per incident photons at each wavelength:
   ( ) /
( )  
   ( )
p
Number of collected electrons I q
QE Number of incident photons
l
lj l
= =
where the I(λ) and φp(λ) are photogenerated current and photon flow, respectively. Quantum
efficiency is a relative value and its optimum number is 1 (i.e., 100%) for all wavelengths below
the corresponding wavelength to the absorber band gap and zero for wavelength above it, but
in reality, it is always less than 100%. Figure 6 shows a typical CIGS cell’s quantum efficiency
(QE) curve and the loss mechanisms that can cause decreases in quantum efficiency. As shown,
the reflection is one mechanism that decreases the quantum efficiency of the cell. It can be
caused by light reflection at material interfaces or partial coverage cells’ front surface that is
made by front electrode.
Figure 6. A typical cell’s quantum efficiency curve and loss mechanism.
The other loss mechanism, which has destructive effect on cells’ quantum efficiency, is the
absorption of photons in the short-wavelength (UV) region. This arises from the absorption of
light in buffer and window layer. The absorption in transparent conductive oxide window
layer is typically low. This is due to the high-energy band gap of the materials that are usually
used for this layer. But the buffer layer absorption is one of the major losses source in CIGS
solar cells. As can be seen, there is a limitation at long wavelength because of the limit of the
absorber layer absorption, which is based on its energy band gap. The short-circuit current
Nanoelectronics and Materials Development
48

and consequently the short-circuit current density of the cell can be obtained from the quantum
efficiency:
 ( ). ( )
sc
J G QE d
l
ll l l
¥
=ò
where Gλ is the spectral irradiance of the reference distribution.
3.4. Recombination rate
The generation of carriers is counteracted by the carriers’ recombination. The recombination
phenomenon is the process, which acts to bring the solar cell back to equilibrium by the
combination of exited electrons and holes. During the recombination, an electron relaxes back
to the valence band from the conduction band by giving its energy to a photon or a phonon.
While the electron transfers its energy to a photon in a single step, its energy will be divided
and transferred to several phonons in several steps. There are three main mechanisms of
recombination in a solar cell that can relax back the electrons from the conduction band to the
valence band including radiative recombination, auger recombination and Shockley-Read-
Hall (SRH) recombination [29]. The radiative mechanism happens when an electron gives its
energy to a photon and emits it with this excess energy. Auger recombination mechanism
includes the recombination of electron and hole and transferring the excess energy from the
electron to the third carrier. The third recombination mechanism is Shockley-Read-Hall (SRH)
recombination that includes emission of one or several phonons by transferring the excess
energy of electron to them. In this mechanism, since the phonons’ energy is ≤0.1 eV, band-to-
band recombination requires simultaneous multiple phonon involvement. The SRH recombi‐
nation usually involves defects states in the volume and at surfaces of the material. It is
significant that the SRH recombination is the dominant recombination mechanism for CIGS
solar cells due to the existence of defects in CIGS material. The recombination rate, R, is given
by the classical Shockley-Read-Hall (SRH) description:
Figure 7. Band diagram and recombination paths in a CIGS solar cell: (1) Recombination at back contact. (2) Recombination
at quasi neutral region. (3) Recombination in the space charge region (SCR). (4) Recombination at the interface.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
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1

(  )  (  )
-
=+ + +
0
p
τ
0
2
i
srh
n 1
np n
Rτ n n p p
where the τn0 and τp0, respectively, are the electron and hole minimum lifetime in the case of
completely unoccupied defect states. The n1 and p1 are the electron and hole density, respec‐
tively, when the Fermi level lay at the energetic position of the defect. Furthermore, the
recombination in thin film chalcopyrite solar cells (e.g., CIGS solar cells) can be categorized
based on the place where it happens. There are four main recombination regions, including
the recombination at the back contact (BC), quasi-neutral bulk, space charge region (SCR) and
recombination at the interfaces (IFR). Figure 7 represents the recombination regions in the
band diagram of a CIGS solar cell. Region 1 represents the recombination at back contact and
region 2 shows the quasi-neutral recombination (QNR) in the absorber layer. Region 3 is the
place, where the recombination at space charge region (SCR) occurs, while region 4 is the place
of recombination at the absorber/buffer interface happens. At each operation condition and
cell structure, one of these recombination mechanisms is dominant. The basis of determination
of the dominant recombination path in a CIGS solar cell is that, for all recombination paths,
the diode saturation current density (J) can be written as below.
( )
0 00

a
E
AKT
J J e
-
=
Figure 8. Interface barrier for holes at the absorber/buffer interface that is the energy distance between Fermi level and
valence band edge at the absorber/buffer interface.
Nanoelectronics and Materials Development
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The J00 is the temperature-independent prefactor, Ea is the activation energy, “A” is the diode
ideality factor that is the function of temperature and KT is the thermal energy. The most
important parameter among these parameters is the activation energy that is dependent on
the path of recombination. For the recombination in quasi-neutral balk (QNR) and recombi‐
nation in space charge region (SCR), the activation energy (Ea) is equal to the energy band gap
(Eg). In case of recombination at the absorber/buffer interface (IFR), Ea is equal to interface
barrier for holes (φp
b). The interface barrier for holes normally is lower than the band gap. The
(φp
b) that is shown in Figure 8 is actually the energy distance between Fermi level (EF) and
valence band edge at the absorber/buffer interface.
4. A brief review on CIGS solar cell processing
CIGS thin-film solar cell fabrication process starts from the deposition of back contact layer
onto the substrate. The condition of the process and growth parameters plays an important
role in the electrical and structural properties of back contact layer. Therefore, the processing
setup should maintain the desired resultant layers’ features such as low resistivity and high
adhesion to the substrate. The compatible deposition methods that are used to deposit
molybdenum (Mo) as the common used back contact material are evaporation and sputtering
[30, 31].
The second layer that should be deposited is the CIGS absorber layer. The essential criteria to
select the processing technique of absorber layer are growth controllability, high deposition
rate, reproducibility and low cost. Thus, several deposition techniques and growth methods
are used for the absorber layer since the beginning of CIGS thin-film technology. There are
advantages and disadvantages for each technique particularly for the criteria that are men‐
tioned earlier. The CIGS deposition techniques can be divided into two main categories of
high-cost vacuum-based technique and low-cost nonvacuum-based processing technique [32].
The vacuum-based deposition techniques, which are commonly used in CIGS deposition, are
as follows [33]:
•Thermal evaporation such as coevaporation from pure elemental metals.
•Sputtering from metal selenide targets.
•Selenization of metal precursors.
•A hybrid of the above processes.
Although the vacuum deposition technique is applied to fabricate the commercialized CIGS
cells and also the best laboratory-scale CIGS cell, there are some other low-cost CIGS thin film
deposition techniques that are nonvacuum processes. Some of these techniques, which lead to
fabricating a cell with an acceptable efficiency level, are the hydrazine-based deposition [34],
screen printing [35], spray deposition [36], dip-coating [37], pulsed laser-assisted deposition
(PLAD) [38], electrodeposition [39], etc. The considerable challenges in each alternative
deposition and formation methods are producing CIGS layer, free from destructive concen‐
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
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trations of impurities and controlling the Ga concentration profile across the CIGS absorber.
Both are effective factors on device efficiency [40].
The most commonly used deposition technique for the buffer layer in CIGS thin-film solar is
the chemical-bath deposition (CBD) that is a low-cost, large-area process [41–43]. Since the
chemical bath deposition technique is incompatible with in-line vacuum-based production
method, which is used for commercialized CIGS solar cell fabrication in industry, several
alternative deposition techniques were proposed for buffer layer deposition process. These
alternative methods are sputtering, atomic layer deposition (ALD), metal organic chemical
vapor deposition (MOCVD), an ion layer gas reaction (ILGAR), a molecular beam epitaxy
(MBE), etc. Each method may have some advantages and disadvantages, so a compromise
between desired features and possible drawback is required. For instance, the atomic layer
deposition (ALD) is a chemical vapor deposition method with very good thickness controlla‐
bility and film uniformity; however, its weakness is the low growth rate in comparison with
the other techniques [44]; while, the MOCVD is known as a fast and reliable deposition method.
It can be integrated into in-line processes, but the MOCVD involve low controllability of
impurity concentration across the layer. It is used for the deposition of zinc (Zn)-based buffer
layers’ deposition and the 13.4% efficiency was obtained from a Cu(In,Ga)(S,Se)2 solar cell with
a 20-nm MOCVD deposited ZnSe buffer layer [45].
Various deposition methods have been tested for TCO films such as RF or DC magnetron
sputtering [46, 47], the sol-gel method [48], chemical vapor deposition (CVD) [49], pulsed laser
deposition [50] and electrodeposition [51]. The TCO layers’ deposition technique should be in
low-temperature i.e. lower than 150°C. This is to avoid the detrimental interdiffusion across
the underlying chalcopyrite layers. It also should be compatible with CIGS cells’ in-line
processing steps. Hence, magnetron sputtering of ZnO is the most commonly used deposition
technique of TCO films among the above-mentioned processing methods. Magnetron sput‐
tering is in a moderate price and consumes low temperature along with well controllable
thickness and doping concentration. The RF magnetron sputtering is usually used for small
area and laboratory-scale cell. But for the large-scale industrial production, the DC magnetron
sputtering is used [52, 53].
5. CIGS thin-film solar cell challenge
Although the CIGS thin-film solar cell has already reached a technical maturity level that made
it able to enter a mass production, there is still a large gap between the best commercial CIGS
module efficiency of 12% [54] and the highest laboratory-scale efficiency of over 22%. Besides,
there are some questions about the optimum cell structure, material properties and many more.
These should be answered and explained in order to develop further the CIGS thin-film solar
cell. Those unanswered questions arise obviously from an incomplete theoretical understand‐
ing about the cells.
In comparison with other types of solar cell, the CIGS cell is much more complex. This
complexity comes from the nature of materials that are usually used in this type of solar cell.
Nanoelectronics and Materials Development
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Those materials are all compound semiconductors with tunable material properties. The other
reason that makes the CIGS solar cells much more complex is the number of layers used in the
cell structure. Thus, the complex fundamental semiconductor equations should be applied to
all these layers with different material properties. That is why the study of this type of solar
cell seems to be difficult and the theoretical understanding about some phenomenon in the
CIGS cell’s formation and operation are still under study. There are two major challenges that
are briefly discussed below.
5.1. Defect nature
CIGS is a p-type semiconductor that is doped by intrinsic defects. In a CIGS bulk, there are
vacancies such as VCu, VIn, VSe, etc. and antisite defects like InCu, CuIn, GaCu, CuGa. Some of these
defects cause p-type doping such as VCu and some others may add n-type doping such as VSe
or InCu. Theoretical and experimental studies have been done on CIGS intrinsic defects [55–
57]. The results show that the copper vacancies dominate in CIGS and the p-type nature of this
compound semiconductor arises from this defect. Although it is known that gallium (Ga)
content can improve the CIGS electro-optical properties, but high Ga ratio could cause
efficiency degradation. The physics behind this phenomenon is still under debate but one of
the most possible reasons in antisite defects related to Ga. It is also identified that the diffusion
of Na from the glass substrate to the absorber layer increases the carrier concentration in CIGS
layer. This leads to a formation useful defect cluster such as the upgrading of CIGS material
properties. However, more comprehensive understanding from the origin of defects and their
contribution to CIGS electro-optical properties are still required.
5.2. Buffer layers
The highest demonstrated efficiency of CIGS solar cell has been obtained by a cell with a
chemical bath-deposited (CBD) cadmium sulfide (CdS) buffer layer. Nevertheless, the
cadmium is classified as a toxic material. Thus, a Cd-free buffer layer needs to be investigated
and developed. Although, with the appropriate safety rules in fabrication process, the
exposure to the cadmium can be avoided, due to breaking or disposal of cadmium containing
products, the release of cadmium in the cell has a destructive effect on the environment and
human health. Therefore, from July 1, 2006, two directives were regulated on the use of
toxicants and heavy metals such as Hg, Cd, Pb in electronic products within the European
Union: Directive on the Restriction of the Use of certain Hazardous Substances in Electrical
and Electronic Equipment (RoHS) and Waste Electrical and Electronic Equipment Directive
(WEEE) [58, 59]. Hence, in order to avoid the toxic heavy metal containing waste in solar cells,
examining Cd-free buffer layers is necessary. Nontoxicity will ensure that stringent legislation
relating to the use and disposal of toxic material cannot be an obstacle to improve the CIGS
thin-film solar technology. In addition to the environmental problems, CdS buffer layer causes
poor short-wavelength response due to absorption of the UV lights. This prevents high energy
photons to reach the absorber layer and consequently reduces the quantum efficiency of the
cell in UV region. Therefore, using an alternative buffer material or elimination of buffer layer
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
53

in such a way can enhance the quantum efficiency with environmental friendly materials.
These are other challenges of CIGS thin-film solar cells.
Absorber Window η (%) Voc (v) Jsc (mA/cm2) FF (%) Area (cm2) Ref.
CuInS2 i-ZnO/ZnO/Al 13.5 0.604 30.6 73 25 [63]
CuInS2 i-ZnO/ZnO/Al 14.7 0.574 37.4 68.4 0.5 [64]
Cu(In,Ga)Se2*i-ZnO/ZnO/Al 13.3 0.606 29.6 74 0.5 [65]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 11.1 0.652 24.7 69.1 0.5 [66]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 13.3 0.637 28.8 72.3 0.5 [67]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 16.4 0.665 31.5 78 0.1 [68]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 2.88 0.5136 30.83 47.65 3.75 [69]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 12.1 0.66 26.9 68.4 25 [70]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 12.3 0.525 31.8 73.6 0.5 [71]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 14.1 0.648 34.3 63.3 0.5 [72]
Cu(In,Ga)Se2*i-ZnO/ZnO/Al 12.4 0.556 31 72 6.25 [73]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 10.8 0.592 29.5 62 900 [74]
Cu(In,Ga)Se2i-ZnO/ZnO/Al 12.9 0.662 26.8 72.6 900 [75]
CuGaSe2ZnO/Al 6 0.625 11.5 83 0.5 [76]
CuGaSe2ZnO/Al 3.9 0.785 14.5 34.3 0.5 [76]
Table 1. Summary of the Cd-free CIGS solar cell and modules with In2S3 buffer layer.
Development of Cd-free buffer layer for CIGS solar cell started in 1992 with an efficien‐
cy level of about 9% [60, 61]. Investigations during last decades show that some Cd-
free material such as In2S3, ZnS and Zn1-xMgxO can potentially be used as alternative buffer
layers in CIGS solar cells. The major advantage of these materials is that their band gap
is larger than CdS band gap. It is worth noting that the In2S3 present a wide range of
energy band gap from 2.1 to 2.9 eV [62]. In comparison with CdS, cells that use buffer
materials with higher band gap have better spectral response in short wavelength due to
less blue absorption loss in the buffer layer. Table 1 shows the summary of Cd-free CIGS
solar cells and modules that used In2S3 as one of the most promising alternative materi‐
als for the buffer layer.
The data shown in Table 1 indicate that wide ranges of efficiency have been reported for the
cells, which used same materials in absorber, buffer and window layer. These differences in
efficiency value of the cells arise from differences in processing techniques, cells’ geometrical
and electro-optical properties of constituent layers. For instance, the cells that are shown with
star mark (*) in Table 1 are made by Cu(In, Ga)Se2 absorber, In2S3 buffer and i-ZnO/ZnO/Al
window layer. Both cells are made by same processing techniques but represent different
efficiency (12.4 and 13.3%). The differences that are clearly reported for these samples are in
Nanoelectronics and Materials Development
54

buffer layer thickness and band gap. The cell with the efficiency of 12.4% used a 50-nm buffer
layer and the other one used 90-nm buffer layer. In one sample, they tried to control the oxygen
content in buffer layer, and in another, they neglected it. These cells may have other differences
in terms of absorber or window layer’s properties. Thus, in parallel with ongoing efforts for
mass production, it is necessary to optimize the cells in terms of layers’ geometrical and
materials’ electro-optical properties in order to develop the CIGS thin film solar cells further.
Solar cells’ modeling and simulation are those beneficial approaches to reach an optimized
cell. In other words, the cell performance can be simulated under different conditions by
considering the independent and dependent parameters and the optimized cell can be
concluded from superposition of simulation results.
6. Performance of the optimized CIGS solar cell through simulation
In this section, all the obtained optimized ranges that are reported in published results [9, 77–
81] are used to evaluate the performance of a CIGS solar cell with optimum layer properties.
This evaluation was done for the cells with uniform and graded band structure separately.
Figure 9(a) shows the output parameters and the J–V characteristics of the cell with uniform
band structure and optimized material properties, under dark and illuminated conditions. As
shown, the efficiency of 20.16% is achieved through the simulation from this CIGS solar cell,
which is higher than the best reported efficiency of a CIGS/In2S3/i-ZnO/ZnO/Al solar cell is
16.4% [68]. According to the simulation results and analysis, which are given in previous
sections, the cell’s open circuit voltage is enhanced by setting the absorber layer band gap at
optimum value (1.2 eV). The cell’s short-circuit current density is improved by setting the
absorber layer thickness at optimum value (2±0.5 μm) and also by decrease in buffer and
window layers’ thickness that leads to the increase in absorber layer’s absorption rate and
decrease in buffer and window layers’ recombination rate. The fill factor is also upgraded by
setting the absorber layer band gap and thickness at optimum value and decrease in buffer
Figure 9. J–V characteristics and the output parameters of the optimized cell under dark and illuminated conditions:
(a) cell with uniform band structure, and (b) cell with graded band structure.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
55

and window layers’ thickness. A cell with optimum parameters and graded band absorber
and buffer layer was also examined. The output characteristics of the cell with graded band
structure and optimized material properties are shown in Figure 9(b). As expected, it repre‐
sents higher performance in comparison with the cell that has uniform band structure.
Cell Material properties Cells’ output measurements
CIGS layer
thickness
(μm)
CIGS
layer
band
gap (eV)
In2S3
layer
thickness
(μm)
In2S3
layer
band
gap
(eV)
ZnO/Al
layer
thickness
(μm)
ZnO/Al
layer
band
gap (eV)
Voc
(V)
Jsc
(mA/c
m2)
FF (%)Efficiency, ȵ
(%)
Best cell reported
in literatures [67]
2 N.A 0.03 2.7 0.1 N.A 0.665 31.5 78 16.4
Simulation results
for
the best cell
reported in
literatures
2 1.14–1.270.03 2.7 0.1 3.42–3.680.665 31.5 78 16.4
Simulated cell
with uniform
band structure
2 1.2 0.04 2.5 0.08 3.86 0.762 32.28 81.99 20.16
Simulated cell
with graded
band structure
2 1.2–1.7 0.04 2.5 0.08 3.86 0.762 33.84 82.08 21.18
Table 2. Summary of simulated cells’ properties in comparison with the best reported cell.
Table 2 shows the summary of material properties that were used in simulated cells plus the
best experimental and laboratory-scale CIGS/In2S3 cell’s property, reported in the literature
with their output measurements under the standard AM1.5 solar spectrum and environment
temperature of 300°K. The differences in open-circuit voltage and the efficiency may arise from
the difference in absorber and TCO layers’ band gap, which is not reported for the experimental
cell in the reference [68]. A cell with same parameter settings as used and reported in experi‐
mental reference was simulated and the results are also shown in Table 2.
As the proof of the present study, the best experimental cell with the settings that are mentioned
in reference [68] was simulated to find approximate and possible ranges of unknown variables;
CIGS layer and ZnO layer energy band gap. The simulation was performed by varying the
CIGS and ZnO layers’ band gap while the other independent parameters were kept constant.
Figure 10 shows the simulation results in the form of efficiency as the function of absorber and
window layers’ energy band gap in a color map.
Nanoelectronics and Materials Development
56

Figure 10. Simulation of best experimental cell: efficiency as the function of absorber and window layers’ energy band
gap.
Layer parameters CIGS In2S3i-ZnO n-ZnO
Thickness (μm) 2 0.04 0.07 0.08
Eg (ev) 1.2 2.5 3.3 3.68
Xe (ev) 4.25 4.25 4.6 4.24
13.6 13.5 9 9
Nc (1/cm3)2.2E+18 1.8E+19 2.2E+18 2.2E+18
Nv (1/cm3)1.8E+19 4.0E+13 1.8E+19 1.8E+19
μn (cm2/Vs) 100 400 100 100
μp (cm2/Vs) 25 210 25 25
NA (1/cm3)1.0E+16 0 0 0
ND (1/cm3)0 1.0E+18 1.0E+16 1.0E+18
Table 3. Parameter set for the optimized CIGS/In2S3/ZnO solar cell.
According to the simulation results, the absorber and window layer energy band gaps of the
best experimental CIGS/In2S3/i-ZnO/ZnO/Al solar cells with 16.4% output efficiency should
have been in the range of 1.14–1.27 eV and 3.42–3.68 eV, respectively. In the color map, the cell
performance was defined as an order of triple made of absorber layer’s band gap, window
layer’s band gap and the cell’s efficiency. Thus, each efficiency value in color map has specific
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
57

corresponding value for absorber and window layer’s band gap. Consequently, some partic‐
ular points were specified in Figure 10 with the efficiency value of 16.4%, equal to the efficiency
reported for the cell in the reference. For example, if the measured value for the CIGS layer
band gap in the best experimental cell is 1.2 eV, its window layer’s energy band gap should
be 3.42 eV and 16.4% efficiency. In other case, using absorber layer with 1.14 or 1.27 eV band
gap for the cell would lead to having a window layer with energy band gap of 3.68 eV and
hence obtains 16.4% efficiency from the cell. These outcomes can be considered as the proof
of simulation result validity (Table 3).
7. Conclusion
CIGS thin film solar cells background, its technological trend and current challenges are
presented in this chapter. The optimized CIGS/In2S3/i-ZnO/ZnO/Al solar cell material prop‐
erties are proposed based on simulation results. As the proof of simulation results’ validity,
the best cells experimented and reported with their parameters are collected. These collections
of cell information from literature are simulated and investigated. Some material properties
from experimental cells are not reported directly in the literatures but have been calculated
and proposed in a range through SCAPS simulation. The analyses, modeling and examination
of the simulations results have shown that the optimization of material properties are prom‐
ising and can improve the cell efficiency.
Author details
Nima Khoshsirat1 and Nurul Amziah Md Yunus2*
*Address all correspondence to: amziah@upm.edu.my
1 Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD,
Australia
2 Micro and Nano Electronic Systems Unit (MiNES) and Advanced Material Synthesis and
Fabrication Laboratory (AMSF), Department of Electrical and Electronic Engineering, Facul‐
ty of Engineering, Universiti Putra Malaysia (UPM), Serdang, Malaysia
References
[1] J. Poortmans and V. Arkhipov, Thin film solar cells: fabrication, characterization and
applications, vol. 5. John Wiley & Sons, West Sussex, England, 2006.
Nanoelectronics and Materials Development58

[2] J. W. Arnulf, “Progress in chalcopyrite compound semiconductor research for photo‐
voltaic applications and transfer of results into actual solar cell production,” Solar
Energy Materials and Solar Cells, vol. 95, pp. 1509–1517, 2011.
[3] L. A. Kosyachenko, Solar cells thin-film technologies. Rijeka, Croatia: InTech, 2011.
[4] L. L. Kazmerski. Best research solar cell efficiencies, 2016. Available: http://www.nrel.gov/
ncpv/images/efficiency_chart.jpg
[5] M. Powalla, W. Witte, P. Jackson, S. Paetel, E. Lotter, R. Wuerz, et al., “CIGS cells and
modules with high efficiency on glass and flexible substrates,” Photovoltaics, IEEE
Journal of, vol. 4, pp. 440–446, 2014.
[6] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency
tables (version 48)”, Progress in Photovoltaics: Research and Applications, vol. 24, pp. 905–
913, 2016.
[7] N. H. Rafat and S. E. D. Habib, “The limiting efficiency of band gap graded solar cells,”
Solar Energy Materials and Solar Cells, vol. 55, pp. 341–361, 9/4/1998.
[8] S. Niki, M. Contreras, I. Repins, M. Powalla, K. Kushiya, S. Ishizuka, et al., “CIGS
absorbers and processes,” Progress in Photovoltaics: Research and Applications, vol. 18, pp.
453–466, 2010.
[9] N. Khoshsirat, N. A. Md Yunus, M. N. Hamidon, S. Shafie, and N. Amin, “Analysis of
absorber layer properties effect on CIGS solar cell performance using SCAPS,” Optik –
International Journal for Light and Electron Optics, vol. 126, pp. 681–686, 4/2015.
[10] L. Assmann, J. C. Bernède, A. Drici, C. Amory, E. Halgand, and M. Morsli, “Study of
the Mo thin films and Mo/CIGS interface properties,” Applied Surface Science, vol. 246,
pp. 159–166, 2005.
[11] S.-Y. Kuo, L.-B. Chang, M.-J. Jeng, W.-T. Lin, Y.-T. Lu, and S.-C. Hu, “Effects of Growth
Parameters on Surface-morphological, Structural and Electrical Properties of Mo Films
by RF Magnetron Sputtering”, MRS Online Proceedings Library, vol. 1123, pp. 1123–P05,
2008.
[12] W. Witte, R. Kniese, and M. Powalla, “Raman investigations of Cu(In,Ga)Se2 thin films
with various copper contents,” Thin Solid Films, vol. 517, pp. 867–869, 11/28/2008.
[13] O. Lundberg, J. Lu, A. Rockett, M. Edoff, and L. Stolt, “Diffusion of indium and gallium
in Cu(In,Ga)Se2 thin film solar cells,” Journal of Physics and Chemistry of Solids, vol. 64,
pp. 1499–1504, 2003.
[14] B. McCandless and S. Hegedus, “Influence of CdS window layers on thin film CdS/
CdTe solar cell performance,” in Photovoltaic Specialists Conference, Conference Record of
the Twenty Second IEEE, Las Vegas, NV, US, 1991, pp. 967–972.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
59

[15] M. A. Contreras, M. J. Romero, B. To, F. Hasoon, R. Noufi, S. Ward, et al., “Optimization
of CBD CdS process in high-efficiency Cu(In,Ga)Se2-based solar cells,” Thin Solid
Films, vol. 403–404, pp. 204–211, 2002.
[16] U. Rau and H. W. Schock, “llc-4 – Cu(In,Ga)Se2 thin-film solar cells,” in Solar Cells,
M. Tom and C. Luis, Eds. Oxford: Elsevier Science, 2005, pp. 303–349.
[17] R.A. Mickelsen and WS Chen, “Development of a 9.4% efficient thin-film CuInSe2/CdS
solar cell,” presented at the 15th IEEE Photovoltaic Specialists Conference, New York, USA,
1981.
[18] S. Siebentritt, “Alternative buffers for chalcopyrite solar cells,” Solar Energy, vol. 77, pp.
767–775, 2004.
[19] A. Romeo, M. Terheggen, D. Abou-Ras, D. L. Bätzner, F. J. Haug, M. Kälin, et al.,
“Development of thin-film Cu(In,Ga)Se2 and CdTe solar cells,” Progress in Photovoltaics:
Research and Applications, vol. 12, pp. 93–111, 2004.
[20] R. Klenk and M. C. Lux-Steiner, "Chalcopyrite based solar cells," in Thin Film Solar
Cells: Fabrication, Characterization and Applications, John Wiley & Sons, Ltd, West
Sussex, UK, pp. 237-275, 2006.
[21] D. Hariskos, S. Spiering, and M. Powalla, “Buffer layers in Cu(In,Ga)Se2 solar cells and
modules,” Thin Solid Films, vol. 480–481, pp. 99–109, 2005.
[22] D. Ginley. and J. Perkins., “Transparent conducting oxides for advanced photovoltaic
applications,” Photovoltaics International Journal, 3rd edition, pp. 95–102, February 2009.
[23] M. Izaki and T. Omi, “Transparent zinc oxide films prepared by electrochemical
reaction,” Applied Physics Letters, vol. 68, pp. 2439–2440, 1996.
[24] T. Minami, H. Sato, H. Nanto, and S. Takata, “GROUP III Impurity Doped Zinc Oxide
Thin Films Prepared By RF Magnetron Sputtering,” Japanese Journal of Applied Physics,
Part 2: Letters, vol. 24, pp. 781–784, 1985.
[25] J. J. Loferski, “Theoretical considerations governing the choice of the optimum semi‐
conductor for photovoltaic solar energy conversion,” Journal of Applied Physics, vol. 27,
pp. 777–784, 1956.
[26] V. Avrutin, N. Izyumskaya, and H. Morkoç, “Semiconductor solar cells: recent progress
in terrestrial applications,” Superlattices and Microstructures, vol. 49, pp. 337–364, 2011.
[27] R. Scheer, “Activation energy of heterojunction diode currents in the limit of interface
recombination,” Journal of Applied Physics, vol. 105, p. 4505, 2009.
[28] A. Niemegeers, M. Burgelman, and A. De Vos, “On the CdS/CuInSe2 conduction band
discontinuity,” Applied Physics Letters, vol. 67, pp. 843–845, 1995.
Nanoelectronics and Materials Development
60

[29] L. A. Kosyachenko, “A Theoretical Description of Thin-Film Cu(In,Ga)Se2 Solar Cell
Performance,” in Solar Cells - New Approaches and Reviews, ed: Leonid A. Kosyachenko,
INTECH, Croatia, 2015.
[30] Z.-H. Li, E.-S. Cho, and S. J. Kwon, “Molybdenum thin film deposited by in-line DC
magnetron sputtering as a back contact for Cu(In,Ga)Se2 solar cells,” Applied Surface
Science, vol. 257, pp. 9682–9688, 2011.
[31] M. Edoff, N. Viard, J. T. Wätjen, S. Schleussner, P.-O. Westin, and K. Leifer, “Sputtering
of highly adhesive Mo back contact layers for Cu(In,Ga)Se2 solar cells,” presented at
the 24th European Photovoltaic Solar Energy Conference, München, Germany, 2009.
[32] H.-W. Schock, “Properties of chalcopyrite-based materials and film deposition for thin-
film solar cells,” in Thin-Film Solar Cells, ed: Springer, Berlin, 2004, pp. 163–182.
[33] K. Seshan, Handbook of thin film deposition: William Andrew, Elsevier, UK, 2012.
[34] D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, L. Gignac, et al., “Hydrazine-based
deposition route for device-quality CIGS films,” Thin Solid Films, vol. 517, pp. 2158–
2162, 2009.
[35] M. G. Faraj, K. Ibrahim, and A. Salhin, “Fabrication and characterization of thin-film
Cu (In, Ga) Se2 solar cells on a PET plastic substrate using screen printing,” Materials
Science in Semiconductor Processing, vol. 15, pp. 165–173, 2012.
[36] D.-Y. Lee, S. Park, and J. Kim, “Structural analysis of CIGS film prepared by chemical
spray deposition,” Current Applied Physics, vol. 11, pp. S88–S92, 2011.
[37] M. Kaelin, D. Rudmann, F. Kurdesau, H. Zogg, T. Meyer, and A. N. Tiwari, “Low-cost
CIGS solar cells by paste coating and selenization,” Thin Solid Films, vol. 480, pp. 486–
490, 2005.
[38] T. Nakada and S. Shirakata, “Impacts of pulsed-laser assisted deposition on CIGS thin
films and solar cells,” Solar Energy Materials and Solar Cells, vol. 95, pp. 1463–1470, 2011.
[39] V. S. Saji, I.-H. Choi, and C.-W. Lee, “Progress in electrodeposited absorber layer for
CuIn(1–x)GaxSe2 (CIGS) solar cells,” Solar Energy, vol. 85, pp. 2666–2678, 2011.
[40] Y. Hamakawa, Thin-film solar cells: next generation photovoltaics and its applications
vol. 13: Springer Science & Business Media, Berlin 2004.
[41] Y. Hashimoto, “Chemical bath deposition of CdS buffer layer for CIGS solar cells,”
Solar Energy Materials and Solar Cells, vol. 50, pp. 71–77, 1998.
[42] A. Ennaoui, M. Weber, R. Scheer, and H. Lewerenz, “Chemical-bath ZnO buffer layer
for CuInS 2 thin-film solar cells,” Solar Energy Materials and Solar Cells, vol. 54, pp. 277–
286, 1998.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
61

[43] J. F. Trigo, B. Asenjo, J. Herrero, and M. T. Gutiérrez, “Optical characterization of In2S3
solar cell buffer layers grown by chemical bath and physical vapor deposition,” Solar
Energy Materials and Solar Cells, vol. 92, pp. 1145–1148, 2008.
[44] N. Naghavi, R. Henriquez, V. Laptev, and D. Lincot, “Growth studies and characteri‐
sation of In2S3 thin films deposited by atomic layer deposition (ALD),” Applied Surface
Science, vol. 222, pp. 65–73, 2004.
[45] S. Siebentritt, P. Walk, U. Fiedeler, I. Lauermann, K. Rahne, M. C. Lux-Steiner, et al.,
“MOCVD as a dry deposition method of ZnSe buffers for Cu(In,Ga)(S,Se)2 solar cells,”
Progress in Photovoltaics: Research and Applications, vol. 12, pp. 333–338, 2004.
[46] L. Gao, Y. Zhang, J.-M. Zhang, and K.-W. Xu, “Boron doped ZnO thin films fabricated
by RF-magnetron sputtering,” Applied Surface Science, vol. 257, pp. 2498–2502, 2011.
[47] R. Menner, D. Hariskos, V. Linss, and M. Powalla, “Low-cost ZnO:Al transparent
contact by reactive rotatable magnetron sputtering for Cu(In,Ga)Se2 solar modules,”
Thin Solid Films, vol. 519, pp. 7541–7544, 2011.
[48] K.-m. Lin and P. Tsai, “Parametric study on preparation and characterization of ZnO:Al
films by sol–gel method for solar cells,” Materials Science and Engineering: B, vol. 139,
pp. 81–87, 2007.
[49] D. Kim, I. Yun, and H. Kim, “Fabrication of rough Al doped ZnO films deposited by
low pressure chemical vapor deposition for high efficiency thin film solar cells,” Current
Applied Physics, vol. 10, pp. S459–S462, 2010.
[50] L. Cao, L. Zhu, J. Jiang, R. Zhao, Z. Ye, and B. Zhao, “Highly transparent and conducting
fluorine-doped ZnO thin films prepared by pulsed laser deposition,” Solar Energy
Materials and Solar Cells, vol. 95, pp. 894–898, 2011.
[51] J. Rousset, F. Donsanti, P. Genevée, G. Renou, and D. Lincot, “High efficiency cadmium
free Cu(In,Ga)Se2 thin film solar cells terminated by an electrodeposited front contact,”
Solar Energy Materials and Solar Cells, vol. 95, pp. 1544–1549, 2011.
[52] O. Kluth, G. Schöpe, B. Rech, R. Menner, M. Oertel, K. Orgassa, et al., “Comparative
material study on RF and DC magnetron sputtered ZnO: Al films,” Thin Solid Films,
vol. 502, pp. 311–316, 2006.
[53] M. Saad and A. Kassis, “Effect of rf power on the properties of rf magnetron sputtered
ZnO: Al thin films,” Materials Chemistry and Physics, vol. 136, pp. 205–209, 2012.
[54] R. Noufi, CIGS PV technology – challenges, opportunities, and potential. National Renew‐
able Energy Laboratory. National Center for Photovoltaics, USA. 2013.
[55] Q. Cao, O. Gunawan, M. Copel, K. B. Reuter, S. J. Chey, V. R. Deline, et al., “Defects in
Cu (In, Ga) Se2 chalcopyrite semiconductors: a comparative study of material proper‐
ties, defect states, and photovoltaic performance,” Advanced Energy Materials, vol. 1, pp.
845–853, 2011.
Nanoelectronics and Materials Development
62

[56] J. Pohl, T. Unold, and K. Albe, “Antisite traps and metastable defects in Cu (In, Ga)
Se2 thin-film solar cells studied by screened-exchange hybrid density functional
theory,” arXiv preprint arXiv:1205.2556, 2012.
[57] B. Huang, S. Chen, H.-X. Deng, L.-W. Wang, M. A. Contreras, R. Noufi, et al., “Origin
of reduced efficiency in Cu (In, Ga) Se solar cells with high Ga concentration: alloy
solubility versus intrinsic defects,” Photovoltaics, IEEE Journal of, vol. 4, pp. 477–482,
2014.
[58] EC-2002a, “Directive 2002/95/EC of the European Parliament and of the Council of 27
January 2003 on the restriction of the use of certain hazardous substances in electrical
and electronic equipment,” Official Journal of the European Union, vol. L 37, pp. 19–23,
2003.
[59] EC-2002b, “Directive 2002/96/EC of the European Parliament and of the Council of 27
January 2003 on waste electrical and electronic equipment,” Official Journal of the
European Union, vol. L 37, pp. 24–38, 2003.
[60] R. Ortega Borges, D. Lincot, and J. Vedel, “Chemical bath deposition of zinc sulfide thin
films,” Proceedings of the 11th EC PVSEC, Montreux, Switzerland, pp. 862–865, 1992.
[61] C. Huang, S. S. Li, W. Shafarman, C.-H. Chang, E. Lambers, L. Rieth, et al., “Study of
Cd-free buffer layers using In x (OH, S) y on CIGS solar cells,” Solar Energy Materials
and Solar Cells, vol. 69, pp. 131–137, 2001.
[62] N. Barreau, S. Marsillac, D. Albertini, and J. Bernede, “Structural, optical and electrical
properties of β-In2S3-3xO3x thin films obtained by PVD,” Thin Solid Films, vol. 403, pp.
331–334, 2002.
[63] E. Yousfi, T. Asikainen, V. Pietu, P. Cowache, M. Powalla, and D. Lincot, “Cadmium-
free buffer layers deposited by atomic later epitaxy for copper indium diselenide solar
cells,” Thin Solid Films, vol. 361, pp. 183–186, 2000.
[64] N. A. Allsop, A. Schönmann, H. J. Muffler, M. Bär, M. C. Lux-Steiner, and C. H. Fischer,
“Spray-ILGAR indium sulfide buffers for Cu(In,Ga)(S,Se)2 solar cells,” Progress in
Photovoltaics: Research and Applications, vol. 13, pp. 607–616, 2005.
[65] S. Gall, N. Barreau, F. Jacob, S. Harel, and J. Kessler, “Influence of sodium compounds
at the Cu (In, Ga) Se 2/(PVD) In2S3 interface on solar cell properties,” Thin Solid Films,
vol. 515, pp. 6076–6079, 2007.
[66] D. Hariskos, R. Menner, S. Spiering, A. Eicke, M. Powalla, K. Ellmar, et al., “In2S3 buffer
layer deposited by magnetron sputtering for Cu (InGa) Se2 solar cells,” in Proc. Solar
Energy Conference, New Orleans, US, 2004.
[67] M. R. Hariskos D, Lotter E, Spiering S, Powalla M., “Magnetron sputtering of indium
sulphide as the buffer layer in Cu(InGa)Se2-based solar cells.,” in 20th European
Photovoltaic Solar Energy Conference, Barcelona, Spain, 2005.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
63

[68] N. Khoshsirat and N. A. M. Yunus, “Numerical Simulation of CIGS Thin Film Solar
Using SCAPS-1D,” presented at the 2013 IEEE Conference on Sustainable Utilization and
Development in Engineering and Technology, Putrajaya, Malaysia, 2013.
[69] M. I. Hossain, “Fabrication and characterization of CIGS solar cells with In2S3 buffer
layer deposited by PVD technique,” Chalcogenide Letters, vol. 9, pp. 185–191, 2012.
[70] J. Sterner, J. Malmstr[??]m, and L. Stolt, “Study on ALD In2S3/Cu(In,Ga)Se2 interface
formation,” Progress in Photovoltaics: Research and Applications, vol. 13, pp. 179–193, 2005.
[71] S. Spiering, L. Bürkert, D. Hariskos, M. Powalla, B. Dimmler, C. Giesen, et al., “MOCVD
indium sulphide for application as a buffer layer in CIGS solar cells,” Thin Solid Films,
vol. 517, pp. 2328–2331, 2009.
[72] H. A. Maksoud, M. Igalson, and S. Spiering, “Influence of post-deposition heat
treatment on electrical transport properties of In2S3-buffered Cu (In, Ga) Se 2 cells,”
Thin Solid Films, vol. 535, pp. 158–161, 2013.
[73] S. Gall, N. Barreau, S. Harel, J. Bernede, and J. Kessler, “Material analysis of PVD-grown
indium sulphide buffer layers for Cu(In, Ga)Se2-based solar cells,” Thin Solid Films, vol.
480, pp. 138–141, 2005.
[74] S. Spiering, D. Hariskos, M. Powalla, N. Naghavi, and D. Lincot, “CD-free
Cu(In,Ga)Se2 thin-film solar modules with In2S3 buffer layer by ALCVD,” Thin Solid
Films, vol. 431–432, pp. 359–363, 2003.
[75] S. Spiering, A. Eicke, D. Hariskos, M. Powalla, N. Naghavi, and D. Lincot, “Large-area
Cd-free CIGS solar modules with In2S3 buffer layer deposited by ALCVD,” Thin Solid
Films, vol. 451, pp. 562–566, 2004.
[76] W. Vallejo, J. Clavijo, and G. Gordillo, “CGS based solar cells with In2S3 buffer layer
deposited by CBD and coevaporation,” Brazilian Journal of Physics, vol. 40, pp. 30–37,
2010.
[77] N. Khoshsirat, N. A. Md Yunus, M. N. Hamidon, S. Shafie, and N. Amin, “Analysis of
absorber and buffer layer band gap grading on CIGS thin film solar cell performance
using SCAPS,” Pertanika Journal of Science an Technology, vol. 23, pp. 241–250, 2015.
[78] N. Khoshsirat and N. A. M. Yunus, “Numerical Simulation of CIGS Thin Film Solar
Using SCAPS-1D,” presented at the 2013 IEEE Conference on Sustainable Utilization and
Development in Engineering and Technology, Putrajaya, Malaysia, 2013.
[79] N. Khoshsirat, N. A. Md Yunus, H. M. Nizar, S. Shafie, and N. Amin, “Optimization
of CIGS Thin Film Solar Cells via Numerical Simulation,” in INTERNATIONAL
SYMPOSIUM ON APPLIED ENGINEERING AND SCIENCES (SAES 2013), UPM,
Serdang Malaysia, 2013.
[80] N. Khoshsirat, N. Md Yunus, M. N. Hamidon, S. Shafie, and N. Amin, “ZnO doping
profile effect on CIGS solar cells efficiency and parasitic resistive losses based on cells
Nanoelectronics and Materials Development
64

equivalent circuit,” in 2013 IEEE International Conference on Circuits and Systems
(ICCAS), Kuala Lumpur, Malaysia, 2013, pp. 86–91.
[81] J. Pettersson, T. Torndahl, C. Platzer-Bjorkman, A. Hultqvist, and M. Edoff, “The
influence of absorber thickness on Cu (In, Ga) Se solar cells with different buffer layers,”
IEEE Journal of Photovoltaics, vol. 3, pp. 1376–1382, 2013.
Copper-Indium-Gallium-diSelenide (CIGS) Nanocrystalline Bulk Semiconductor as the Absorber Layer and Its Current
Technological Trend and Optimization
http://dx.doi.org/10.5772/64166
65