Progress in Indium (III) Sulfide (In2S3) Buffer Layer Deposition Techniques for CIS, CIGS, and CdTe-based Thin Film Solar Cells

Review
Progress in indium (III) sulfide (In2S3) buffer layer deposition
techniques for CIS, CIGS, and CdTe-based thin film solar cells
Maqsood Ali Mughal a,⇑
, Robert Engelken a
, Rajesh Sharma b
a
Optoelectronic Materials Research Laboratory (OMRL), Electrical Engineering Program, Arkansas State University-Jonesboro, State University,
AR 72467, USA
b
Technology Program, Arkansas State University-Jonesboro, State University, AR 72467, USA
Received 13 May 2015; received in revised form 22 June 2015; accepted 8 July 2015
Communicated by: Associate Editor Takhir M. Razykov
Abstract
Recent progress with indium (III) sulfide (In2S3)-buffered thin film solar cells (TFSC) was briefly reviewed. In2S3 has emerged as a
promising low-hazard buffer (or window) material, and has proven to improve the properties of the solar cells, while reducing toxicity.
Various deposition techniques have been employed to synthesize In2S3 films on different types of substrates. Until now, atomic layer
deposition (ALD) and ionic layer gas atomic reaction (ILGAR) techniques have been the two most successful, yielding maximum energy
conversion efficiencies up to 16.4% and 16.1%, respectively. The impact of varied deposition parameters upon the In2S3 film properties
and performance of cadmium (Cd)-free solar cells has been outlined. A comparative/operational analysis (solar cell efficiencies above 9%
reported for cell area 6 1 cm2
) of various buffer layers used in two primary types of TFSC technology: chalcopyrite (CIS/CIGS)- and
CdTe-based solar cells was also performed to measure the progress of In2S3 compared to its counterparts.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Indium (III) sulfide (In2S3); Buffer layer; Efficiency; Solar cell; Thin Film Solar Cell (TFSC)
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
2. Thin film market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
3. Cadmium-free buffer Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4. In2S3-buffered thin film solar cells with record efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
4.1. Atomic Layer Deposition (ALD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
4.2. Physical Vapor Deposition (PVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
4.3. Ultrasonic Spray Pyrolysis (USP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
4.4. Chemical Spray Pyrolysis (CSP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
4.5. Sputtering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.6. Atomic Layer Epitaxy (ALE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
http://dx.doi.org/10.1016/j.solener.2015.07.028
0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +1 (870)819 9043; fax: +1 (870)972 3539.
E-mail addresses: maqsoodali.mughal@smail.astate.edu (M.A. Mughal),
bdengens@astate.edu (R. Engelken), rsharma@astate.edu (R. Sharma).
www.elsevier.com/locate/solener
Available online at www.sciencedirect.com
ScienceDirect
Solar Energy 120 (2015) 131–146

4.7. Ionic Layer Gas Atomic Reaction (ILGAR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.8. Electrodeposition (ED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.9. Chemical Bath Deposition (CBD). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.10. Metal Organic Chemical Vapor Deposition (MOCVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5. Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
1. Introduction
Indium (III) sulfide (In2S3), an indium chalcogenide, is a
III–VI semiconductor compound (Mughal et al., 2014)
important for optoelectronic (Cansizoglu et al., 2010;
Mughal et al., 2015), photoelectric (Ho, 2011), and photo-
voltaic (PV) applications Haleem et al., 2012 due to its
stable chemical composition (Newell et al., 2011;
Strausser et al., 1995), photoconductivity (Gilles et al.,
1962), and luminescent characteristics (Springford, 1963)
at ambient conditions. It functions as an n-type semicon-
ductor with an optical bandgap of 2.1–2.3 eV (Mughal
et al., 2014; Dutta et al., 2007); however, there is still con-
troversy about whether it is has a direct or indirect band-
gap. In2S3 crystallizes into three allotropic forms, namely,
a-In2S3 (cubic structure between 420 °C and 754 °C),
b-In2S3 (tetragonal structure below 420 °C), and c-In2S3
(trigonal structure above 754 °C) (Lee et al., 2008).
Among these crystallographic phases, b-In2S3 has the
widest applications (Mughal et al., 2015) due to its defec-
tive spinal structure (most stable) Tao et al., 2008, large
photosensitivity (Warrier et al., 2013), and physical charac-
teristics (Cansizoglu et al., 2010). With optimal physical
properties, it can meet the requirement of a suitable buffer
layer in TFSCs.
In2S3 has been deposited onto different types of sub-
strates (ITO, FTO, etc.) Mughal et al., 2014;
Dimova-Malinovska, 2010 by various deposition tech-
niques (both wet and dry), with diverse morphologies
(Dutta et al., 2007; Sheng et al., 2011). Techniques such
as chemical bath deposition (CBD) Hariskos et al., 1996,
electrodeposition (ED) Mughal et al., 2013, atomic layer
deposition (ALD) Naghavi et al., 2003, physical vapor
deposition (PVD) Hossain, 2012, ultrasonic spray pyrolysis
(USP) Buecheler et al., 2009, and ionic layer gas atomic
reaction (ILGAR) Allsop et al., 2005 have yielded efficien-
cies above 9% at the laboratory scale. Efficiencies up to
16.4% (Hariskos et al., 2005) have been achieved using
the ALD technique. Hence, scientists worldwide are con-
sidering In2S3 as an effective non-toxic substitute for cad-
mium sulfide (CdS), which has successfully been used as
a buffer layer in copper indium gallium selenide
(CIGS)-based solar cells for many years (Rusu et al.,
2005; Repins et al., 2008).
Recently, TFSC technology has gained momentum, now
demonstrating energy conversion efficiencies above 20%
(Jackson et al., 2015), and successfully replacing crystalline
silicon (c-Si) solar cells (see Fig. 1) (Dimova-Malinovska,
2010). Soon it will be the leading PV technology.
Typically, a TFSC is comprised of an absorber layer
(p-type), a buffer (or window) layer (n-type), a transparent
conductive oxide film (front contact), and an anti-reflective
coating (Shah et al., 1999), all stacked on top of each other
on a conductive substrate (for example, molybdenum
(Mo)-coated glass) (see Fig. 2). The absorber layer, in
which photons are efficiently absorbed resulting in elec-
tron–hole pair generation, constitutes the core of the device
(Wu¨rfel, 2005). However, in this paper, we focus upon the
buffer layer (In2S3) whose principal goal is to form a reli-
able p–n junction and establish good interface properties
with the absorber layer, while allowing maximum transmis-
sion of light (minimum absorption loss) to both the junc-
tion region and the absorber layer (Vallejo et al., 2010).
In addition, it passivates the junction region, allowing
absorber materials better suited for environmental expo-
sure, hence providing stability to the solar cell (Roedern,
2001). Appropriate thickness could also result in reducing
the overall reflectance and thus improving the energy con-
version efficiency. In2S3 films have also been reported to
have no conduction band discontinuity at the interface
with CIGS absorber layer (Afzaal and O’Brien, 2006).
The reader is encouraged to consult the glossary fre-
quently for acronym definitions.
2. Thin film market
Currently, as TFSC technology reaches large
industrial-scale production, it is crucial for further growth
to adopt processing measures that are low-cost,
contaminant-free, and industrially applicable. In 2013, the
solar market share for all TFSC technology was 11% with
n-CdS/p-CdTe heterojunction solar cells leading the annual
production by 2 GWp (Gigawatt peak) ISE, 2014. The solar
PV share for TFSC is expected to grow at an annual rate of
24%, reaching 22 GW by the end of 2020 (Wood, 2020). In
the past, chemical bath deposited (CBD)-CdS regularly fea-
tured as a buffer material in CIGS- and CdTe-based solar
cells, yielding maximum energy conversion efficiencies up
to 21.7 (Jackson et al., 2015) and 21.5% (Solar Inc, 2015),
respectively. However, from an environmental/health/eco-
nomic standpoint, scientists are seeking a buffer material
(for example, In2S3), which can serve as an alternative to
hazardous CdS, in order to reduce or eliminate its environ-
mental impact (Dimova-Malinovska, 2010; Hamakawa,
2004), consequently, avoiding climate change and human
health-risks, which potentially poses risk to the economy.
In addition, because of the prohibition of toxic Cd and
increase in stringent legislation relating to its use and
132 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146

disposal, several countries are holding restrictions upon
solar PV market share for Cd-containing solar cells
(Nordic Council of Ministers, 2003). This opens the gates
for In2S3 to enter the TFSC technology market. Although
some reservations have surfaced regarding the availability
and high price of indium (In), the primary annual reported
production of In in 2011 was 550–650 MT (Metric Tons)
Woodhouse et al., 2012. According to Indium
Corporation, In is quite abundant in the crust of the earth
and there is enough available to meet the present and future
needs. In is more abundant than silver (Ag), which has
annual production of about 20,000 MT, nearly 40 times
more than that of In. The currently-observed price fluctua-
tions are primarily due to a time lag between emerging
demand and available supply (Gowans, 2010).
For this reason, we have been studying In2S3. Our work
at the Optoelectronic Materials Research Laboratory
(OMRL)-ASU focuses upon electrodeposition of CuInS2
(Newell et al., 2014) and In2S3 (Mughal et al., 2014,
2015) films using organic electrolytes, and the ultimate goal
is to fabricate n-In2S3/p-CuInS2 heterojunction TFSC. In
this paper, we highlight the progress and development of
In2S3-buffered TFSCs by various deposition techniques.
3. Cadmium-free buffer Layers
The important performance characteristics of any buffer
material include bandgap energy (Sankapal et al., 2004),
absorption coefficient (Roedern, 2001), transport of
Fig. 1. PV technologies and their respective growth in record efficiencies from 1977 to 2015. (Image courtesy of NREL, available at http://www.nrel.gov/
ncpv/; Accessed June 3, 2015).
Fig. 2. Cross-sectional scanning electron microscope (SEM)-view of n-
CdS/p-CIGS heterojunction TFSC from Zentrum fu¨r Sonnenenergie-und
Wasserstoff-Forschung (ZSW) Baden-Wu¨rttemberg. The top layer, usually
n-type, is a CdS buffer layer that allows almost all of the light to transmit
through to the absorbing layer (CIGS), usually p-type, which converts light
into energy. A transparent conductive oxide (TCO) layer, ZnO, carries
excited electrons to the top of the solar cell while still letting the light
through. A back (ohmic) contact is used to provide a good electrical
connection to the substrate, whereas, the front contact carries electrons out
to an external load, thus completing an electric circuit. Source: P. Jackson,
D. Hariskos, et al., March 2014; Accessed April 27, 2015.
M.A. Mughal et al. / Solar Energy 120 (2015) 131–146 133

photo-generated carriers to the outer circuit with minimum
electrical resistance, carrier lifetime/mobility/concentra
tion, recombination rate, thickness (Nayak et al., 2012),
refractive index (Ramli et al., 2013), diffusion length, lattice
mismatch (Hossain, 2012), etc. These characteristics indi-
vidually and collectively play a key role in improving the
performance parameters of the solar cell including open
circuit voltage (Voc), fill factor (FF), current density (Jsc),
etc. The current understanding is that the buffer layer
should have a small thickness (25–300 nm) Naghavi
et al., 2011 and large energy bandgap (Sankapal et al.,
2004) for high optical transmission. Materials and deposi-
tion techniques, which have the capabilities to provide the
alignment of conduction band with the absorber layer and
passivate surface states, can yield higher efficiencies
(McCandless et al., 1996). In the past, various metal sul-
fides, oxides, and oxy-sulfides such as CdS, In2S3, InxSey,
ZnS, ZnSe, ZnO, SnO2, Zn1ÀxMgxO, and Inx(OH,S)y. have
been investigated and used in manufacturing TFSCs
(Dimova-Malinovska, 2010; Ahn et al., 2008; Vallejo
et al., 2010).
A comparative/operational analysis (solar cell efficien-
cies above 9% reported for cell area 6 1cm2
) of various
buffer layers used in two primary types of TFSC technol-
ogy: chalcopyrite (CIS/CIGS)- and CdTe-based solar cells
was made to measure the progress of In2S3 compared to its
counterparts (see Table 1). We made our best efforts to
include as many buffer layers as we were able to find in
the literature review. A closer look at the analysis revealed
that the buffer materials tested at different laboratories and
institutions are mainly chalcogenides (oxides, selenides,
and sulfides) of In, Zn, Cd, Al, and Sn. The FF’s and
Voc’s for CdS-buffered solar cells have been slightly higher
than for alternate buffer-based solar cells. In addition, the
lower energy conversion efficiencies for the solar cells seem
to be caused either by technological scale-up problems, the
need for special post-treatment, or the need for further pro-
cess optimization to improve interface properties between
absorber and buffer layers.
Until now, CBD, sputtering, co-evaporation, and ALD
techniques have been the four most widely used/studied
deposition techniques at both the laboratory and industrial
scale. In the early 1990s, there was much focus on materials
like SnO2, Sn(O, S)2, ZnSe, ZnIn2Se4, Cd1ÀxZnxS, and
InxSey to fabricate Cd-free solar cells. The major advantage
of these buffer materials was that their bandgap energies
were larger than that of CdS (except for InxSey), which
improved the light transmission in the blue wavelength
region, resulting in higher Voc (up to 652 mV) and FF
(76.3%) Ohtake et al., 1997. Depending upon the Zn/Cd
ratio, Cd1ÀxZnxS alloys yielded conversion efficiencies up
to 19.52% due to better lattice match with the CIGS and
favorable conduction band offset at the heterojunction
interface (Bhattacharya et al., 2006). In early 2000s, the
focus started shifting toward zinc/indium compounds
including Inx(O, OH, S)y, Zn(S, OH), ZnO1ÀxSx, ZnO,
etc. However, scientists have been particularly interested
in In2S3 because of its stability, wider bandgap (2.3 eV)
Hariskos et al., 2005, and photoconductive behavior
(Naghavi et al., 2010). Until now, In2S3 has been synthe-
sized by 10 different deposition techniques, the most for
any buffer material, and incorporated with both CIS and
CIGS absorbers.
Fig. 3 shows the timeline (in green) and efficiency trend
(in red) of various buffer layer-based TFSCs since 1987. In
the last 7 years, In2S3 and CdS have been studied the most;
however, CdS takes a slight lead, leaving In2S3 behind by
5% efficiency and higher FF’s. Fig. 4 summarizes the high-
est efficiencies reported in the literature for In2S3- and
CdS-based solar cells. It was clearly evident that In2S3 buf-
fer layers grown by different techniques can lead to high
efficiencies, while each technique confers unique film prop-
erties. Fig. 5 is a graphical representation of conversion
efficiencies from Table 1 for various buffer layer-based
chalcopyrite (CIS/CIGS) and CdTe TFSCs. It illustrates
that several deposition techniques and materials have
resulted in energy conversion efficiencies equal or higher
than those of corresponding CdS-based solar cells.
Zn/In-based buffer materials have been exceptional, yield-
ing efficiencies above 15% on multiple occasions (Nakada
and Mizutani, 2002; Eisele et al., 2003; Minemoto et al.,
2000; Hultqvist et al., 2007; Zimmermann et al., 2006;
Ohtake et al., 1997; Nakada and Yagioka, 2009;
Bhattacharya et al., 2004; Pistor et al., 2009; Saéz-Araoz
et al., 2012).
4. In2S3-buffered thin film solar cells with record efficiencies
In the following section, we provide more detailed
description of the In2S3-buffered TFSCs that yielded record
efficiencies above 9%. In2S3 films were deposited using var-
ious deposition techniques (both wet and dry) onto differ-
ent types of substrates. The films exhibited diverse
structural, morphological, compositional, and electrical
properties, depending upon the treatment and varying
deposition parameters. The section is focused upon the
synthesis of In2S3 films exhibiting characteristics that had
significant impacts upon the performance of the solar cells.
4.1. Atomic Layer Deposition (ALD)
ALD is one of the most successful deposition techniques
reported for In2S3 buffer layers. In 2010, ENSCP and ZSW
reported a world record efficiency of 16.4%
(Jsc = 31.5 mA/cm2
, Voc = 665 mV, and FF = 78% under
a light intensity of 100 mW/cm2
) (Naghavi et al., 2003)
incorporating In2S3 with in-line evaporated CIGS sub-
strates. The solar cell structure was completed by
RF-sputtering a bi-layer of zinc oxide (ZnO) used as a
front contact. Efficiencies up to 12.9% were recorded for
30 Â 30 cm2
solar cell (Mo/CIGS/In2S3/ZnO/ZnO:Al)
modules. In2S3 films were synthesized via surface reactions
by the sequential introduction of the precursors, indium
acetylacetonate (In(acac)3) and hydrogen sulfide (H2S), in

Table 1
Summary of various buffer layers used in two primary types of TFSC technology: chalcopyrite (CIS/CIGS)- and CdTe-based solar cells with their record efficiencies above 9% for cell area 6 1cm2
.
Buffer layer Deposition technique Absorber
layer
Efficiency
(%)
Current
density
Jsc (mA/
cm2
)
Open
circuit
voltage
Voc (mV)
Fill
factor
FF
(%)
Area
(cm2
)
Institution/Year Ref.
ZnS CBD CIGS 18.1 34.9 671 77.6 0.155 AGU/2002 Nakada and Mizutani
(2002)
ILGAR CIGS 14.2 35.9 559 70.7 0.5 SIEMENS-HMI/2000 Muffler et al. (2000)
USP CIGS 10.8 38 482 59.2 0.5 LTFP-SFLMST/2010 Fella et al. (2010)
Co-evaporation CIGS 9.1 29.1 560 71.2 0.48 ETHZ/2004 Romeo et al. (2004)
ZnSe CBD CIGS 15.7a
35.2 570 72.3 1.08 HMI-Siemens/2003 Eisele et al. (2003)
MOPVE CIGS 11.6 35.8 469 69.2 1.85 UO-IPE-UR-Siemens/2000 Engelhardt et al. (1999)
MOCVD CIS 14.1 41.9 506 66.3 0.06 WSU-NREL/1994 Olsen et al. (1993)
ALD CIGS 11.6 35.2 502 65.4 0.172 Showa Shell-TIT/1994 Ohtake et al. (1994)
CVD CIGS 9.6 31 482 64 0.5 HMI/2000 Rumberg et al. (2000)
DC-sputtering CIGS 10.74 31.4 635.2 64.6 0.537 MVU-FhG-ISE/2000 Ennaoui (2000)
Zn1ÀxMgxO Sputtering CIGS 16.2a
37.6 632 68.1 0.3 Matsushita Electric Industrial/2000 Minemoto et al. (2000)
ALD CIGS 18.1 35.7 668 75.7 0.5 ASC-Uppsala University/2005 Hultqvist et al. (2007)
ZnO CBD CIGS 14.3 35.50 557 72.1 0.18 TIT/2003 Mikami et al. (2003)
RF magnetron
sputtering
CIGS 14.48 34.88 581 71.38 0.18 KIER/2000 Lee et al. (2000)
ALD CIGS 13.2 36.6 409 73.5 0.189 TIT/2000 Shimizu et al. (2000)
ILGAR CIGS 14.6 34.6 578.7 72.1 0.5 HMI/2006 Ba¨r et al. (2006)
ALD/MOCVD CIGS 13.9 34.5 581 69.2 0.135 UD/2000 Shimizu et al. (2000)
ALE CIGS 11.7 32.6 512 70 0.42 Uppsala University/2006 Sterner et al. (1998)
ED CIGS 11.4 28.7 557 71.2 0.5 WIS-ENSCP-IPE/2000 Gal et al. (2000)
ZnO1ÀxSx ALD CIGS 18.5a
35.5 689 75.8 0.5 ASC/2006 Zimmermann et al. (2006)
CBD CIGS 14.9 39 560 68.3 0.5 HZB/2010 Naghavi et al. (2010)
CBD CIS 10.4 22.5 700 65.8 0.5 HZB/2006 Ennaoui et al. (2006)
Zn(Se, OH) CBD CIGS 13.67 36.1 535 70.7 0.537 HMI-Shell Solar GmbH/2003 Ennaoui et al. (2003)
ZnIn2Se4 Co-evaporation CIGS 15.1 30.4 652 76.3 0.5 TIT/1997 Ohtake et al. (1997)
Zn(S, O, OH) CBD CIGS 17.9 37.4 645 74 0.503 AGU/2009 Nakada and Yagioka
(2009)
CBD CIGS 18.5 36.105 660.7 78.16 0.408 NREL/2004 Bhattacharya et al. (2004)
Zn(S, OH) CBD CIGS 14.2 34.9 569 71.3 0.475 HMI-TFPL/2004 Sankapal et al. (2004)
Zn1ÀxSnxOy ALD CIGS 18 35.2 684 74.9 0.5 ASC/2012 Lindahl et al. (2012)
In2S3 ALD CIGS 16.4 31.5 665 78 0.5 ENSCP-ZSW-IREM/2005 Hariskos et al. (2005)
PVD CIGS 15.2 33.7 628 72.7 0.528 HMI-Wu¨rth Solar-ZSW-UoB/2009 Pistor et al. (2009)
USP CIGS 13.4 33.4 585 69 0.5 LTFP-SFLMTR/2009 Buecheler et al. (2009)
CBD CIGS 15.7a
37.4 574 68.4 0.5 IPE-ASC/1996 Hariskos et al. (1996)
Sputtering CIS 16.4(12.2a
) 31.5 665 78 0.1 ZSW-HMI-WS/2010 Naghavi et al. (2010)
ILGAR CIGS 16.1 35.5 631 72 0.4894 HMI-SINGULUS-BHT/2012 Saéz-Araoz et al. (2012)
ALE CIS 13.5 30.6 604 73 0.1 ENSMP-ZSW-ASM Microchemistry Ltd./
2000
Yousfi et al. (2000)
MOCVD CIGS 12.3 NR NR NR NR ZSW- Wu¨rth Solar-AIX/2008 Spiering et al. (2009)
M.A.Mughaletal./SolarEnergy120(2015)131–146135

ED CIGS 10.2 32 569 56 0.528 IRDEP-IREM/2011 Naghavi et al. (2011)
CSP CIS 9.5a
48.2 588 33.5 0.05 CUSAT-IU/2005 John et al. (2005)
InxSey Co-evaporation CIGS 13.3 30.5 600 72.5 0.5 TIT-EP/1997 Ohtake et al. (1997)
In(OH)3-
based
CBD CIGS 14 32.1 575 75.8 0.5 TIT/2003 Tokita et al. (2003)
Inx(O, OH,
S)y
CBD CIGS 12.55 33.17 574 65.89 0.19 KAIST/2008 Ahn et al. (2008)
CBD CIS 9.1 21.5 685 61.8 0.5 Oxford Uni-HMI-CIEMAT/2000 Kaufmann et al. (2000)
CBD CIS 11.4a
23.2 735 67 0.38 Stuttgart Uni/1996 Braunger et al. (1996)
CBD CIGS 14.9 32.4 630 73 0.5 Stuttgart Uni-NSC-Uppsala Uni/1996 Allsop et al. (2005)
CdS CBD CIGS 21.7 36.6 746 79.3 0.5 ZSW/2015 Jackson et al. (2015)
CdTe 21.5 30.25 875.9 79.4 1.0623 First Solar Inc./2015 Solar Inc (2015)
CIGS 19.9 35.4 690 81.2 0.42 NREL-Solopower/2008 Repins et al. (2008)
CIS 12.5a
21.42 728 70.9 0.5 HMI/1998 Klaer et al. (1998)
Co-evaporation CIGS 18.7 34.8 712 75.7 0.582 IPE/2011 Chirila˘ et al. (2011)
Sputtering CdTe 14 23.6 814 73.25 0.3 UoT/2004 Gupta and Compaan
(2004)
PVD CIGS 14.1 31.4 610 73 0.5 HMI/2005 Rusu et al. (2005)
ALD CdTe 16.7 32.8 671 75.8 0.5 ZSW-HMI- BOSCH Solar CISTech GmbH/
2001
Hultqvist et al. (2007)
USP CIGS 12.5 30.3 576 73 0.3 ETHZ-CREST/2005 Fella et al. (2010)
ED CdTe 10.8 23.6 753 61 0.5 UQ/1993 Dimova-Malinovska
(2010)
ILGAR CIGS 14.7 35.3 599 69.8 0.5 HMI/2005 Saéz-Araoz et al. (2012)
Sn(O, S)2 CBD CIGS 12.2a
31.8 567 68 0.5 IPE-Stuttgart Uni/1995 Hariskos et al. (1995)
SnO2 CBD CIGS 10.1 36.6 430 64 0.5 IPE/1995 Hariskos et al. (1995)
Al2O3 ALE CIS 9 30 572 53 0.1 ENSMP-ZSW-ASM Microchemistry Ltd./
2000
Yousufi et al. (2000)
Cd1ÀxZnxS CBD CIGS 19.52 35.15 705.2 77.9 NR NREL-CSU/2006 Bhattacharya et al. (2006)
E-Gun Evaporation CIS 9.6 35.3 432 62.6 0.23 Boeing/1987 Lindahl et al. (2012)
E-Beam Evaporation CIS 10.5 37.8 419 55.4 0.08 Fuji Electric Corporate R&D Ltd./1991 Hariskos et al. (2005)
NR – Not Reported
a
Active area efficiency.
136M.A.Mughaletal./SolarEnergy120(2015)131–146

the reactor chamber by varying deposition temperature
between 160 and 220 °C. Nitrogen gas (N2) was used as a
carrier and purging gas. A pulse sequence of In(acac)3/N2
purge/H2S/N2 purge with a pulse duration of
700/1000/500/800 ms was optimized by using in-situ quartz
crystal microgravimetry (QCM). Deposition temperature
and thickness were demonstrated to have significant
impacts upon the properties of the In2S3 films and perfor-
mance of the solar cells. The optimal deposition tempera-
ture was recorded in the range of 200–220 °C, which
yielded 30–50 nm thick b-In2S3 films with an indirect
bandgap of 2.7–2.8 eV (external quantum efficiencies
(EQE) of 80–95% between 550 and 900 nm). The growth
rate was observed to decrease for higher deposition temper-
atures, indicating a decrease of reactivity at the surface
with respect to the adsorption and reaction of the precur-
sors. Increase in deposition temperature resulted in low
FF for films with thicknesses below 30 nm, whereas, the
30–50 nm thick films exhibited improvement in FF and
short circuit current (Isc). Hence, during synthesis of
In2S3 films, the numbers of cycles were adjusted at each
temperature to reach the optimal thickness.
Fig. 3. Timeline (in green) and efficiency trend (in red) of various buffer layer-based TFSCs since 1987. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Fig. 4. CdS vs. In2S3 buffered TFSCs by different deposition techniques.

ZnS
ZnSe
Zn1-xMgxO
ZnO
ZnO1-xSx
Zn(Se,OH)
ZnIn2Se4
Zn(S,O,OH)
Zn(S,OH)
Zn1-xSnxOy
In2S3
InxSey
In(OH)3-based
Inx(O,OH,S)y
CdS
Sn(O,S)2
SnO2
Al2O3
Cd1-xZnxS
BBuffer Layers
Fig. 5. Energy conversion eﬃciencies (>9% for cell area 6 1cm2
) reported for various buﬀer layer-based chalcopyrite (CIS/CIGS) and CdTe TFSCs by
various deposition techniques.

Photoelectron spectroscopy (XPS) surface analysis
detected the diffusion of Cu and Na at high deposition tem-
peratures, indicating the possibility of forming a p–n
homojunction inside the CIGS.
Furthermore, annealing of ALD In2S3 films improved
and enhanced the solar cell efficiency to 16.4%. The devices
did not show any metastability, or any significant loss in
the performance, even after a few months of indoor stor-
age. Devices were also tested according to the IEC 61646
standard in damp heat (1000 h, 85 °C, and 85% relative
humidity), but exhibited no significant loss in the output
power, and passed the allowed limit of 5% (Hariskos
et al., 2005; Naghavi et al., 2003).
4.2. Physical Vapor Deposition (PVD)
The PVD technique first had success with In2S3 in 1997,
reaching a solar cell efficiency of 11.2% by co-evaporation
of In and sulfur (S). Further work at IPE improved the effi-
ciency to 12.4% by varying deposition parameters. In 2009,
direct evaporation of In2S3 powder resulted in the highest
energy conversion efficiency of 15.2% (Jsc = 29.8 mA/cm2
,
Voc = 677 mV, and FF = 75.6% under a light intensity of
100 mW/cm2
) (Rumberg et al., 2000). In2S3 powder from
four different suppliers was evaporated onto both soda lime
glass and CIGS absorbers (from both HMI and Wu¨rth
Solar). All four powders were characterized to study the
differences in their stoichiometry, purity, crystal size, and
crystal phase. 50-nm thick In2S3 films were grown by
PVD (base pressure of chamber 5.0EÀ5 mbar) with cru-
cible temperature set to 720 °C, while the substrate was
not heated and stayed below 50 °C. Films were character-
ized to measure the impact of different powders upon the
performance of the buffer layer and the solar cell. Best solar
cell (Mo/CIGS/In2S3/ZnO/Al/Ni) efficiency was achieved
from powder that exhibited b-In2S3 crystallinity,
long-term stability, and no impurities/contamination (for
example, Cl or O), and CIGS absorbers from HMI.
Furthermore, multiple depositions were carried-out with
the same crucible filling, extending the range from fresh
In2S3 powder to nearly complete evaporation (710 min of
accumulated deposition time) to measure the stability.
No degradation in film or device performance was
observed. Post-annealing treatment ($35–45 min at
200 °C in air) further optimized the performance. With this
single source approach, evaporation of elemental S was
avoided, which could be extremely beneficial if the tech-
nique is applied to industrial-scale production. However,
the quality of evaporated In2S3 powder, which may vary
after a certain period of time, is a concern. XPS surface
analysis indicated a significant loss of S in the evaporated
In2S3 powder, changing the stoichiometry after a few runs
(Pistor et al., 2009).
The reason for the difference in device performance with
co-evaporated and direct-evaporated buffer layers is still
confusing and not fully investigated. Nevertheless, investi-
gation had indicated that the junction quality was
dependent upon the interfacial diffusion mechanism that
occurs while depositing In2S3 films.
4.3. Ultrasonic Spray Pyrolysis (USP)
USP-deposited In2S3 buffer layers were used to fabricate
solar cells (Mo/CIGS/In2S3/i-ZnO/ZnO:Al), which
achieved the maximum conversion efficiency of 13.4%
(Jsc = 33.4 mA/cm2
, Voc = 585 mV, and FF = 69% under
a light intensity of 1000 mW/cm2
) (Buecheler et al., 2009)
in 2009, a joint work by LTFP and SFLMTR. USP is a
low-cost, non-vacuum, and industrially applicable tech-
nique (Fella et al., 2010; Buecheler et al., 2009). It consisted
of an ultrasonic atomizer and a droplet transporting sys-
tem, which were used to deposit In2S3 films onto a heated
substrate. A chamber-based ultrasonic atomizer was used
to transfer droplets (from liquid solution in the chamber,
droplet size 0.9–5.2 lm) using N2 gas projected into a fun-
nel. The heated CIGS substrate was positioned close to the
funnel, creating a closed deposition area above the sub-
strate to prevent oxidation. The chamber was vented with
N2 gas in order to decrease the oxygen content in the reac-
tion area. Indium chloride (InCl3) and thiourea CS(NH2)2
were used as precursor chemicals, while methanol (CH4O)
and acetone (C3H6O) were the most suitable solvents with
respect to the excitation frequency (1.7 MHz), vapor pres-
sure, and boiling point. The solution was prepared by dis-
solving the precursors in an organic solvent. Complete
evaporation of the solvent above the substrate surface
was confirmed by SEM. The solution was sprayed for
300 s onto the substrate to yield 30 nm thick In2S3 films,
which exhibited homogenous layer formation without
any evidence of droplet formation. It was observed that
In2S3 buffer layers formed through the reaction between
InClx fragments and CS2. An increase in growth rate was
observed with increase in temperature of the substrate.
The SEM cross-sectional view of the solar cell revealed
no voids between the layers. It was also reported that mod-
ified CIGS layers could further improve the performance
(Buecheler et al., 2009).
4.4. Chemical Spray Pyrolysis (CSP)
In 2005, an In2S3-buffered solar cell
(ITO/CuInS2/In2S3/Ag) reached the record active area effi-
ciency of 9.5% (Jsc = 48.2 mA/cm2
, Voc = 588 mV, and
FF = 33.5% under a light intensity of 100 mW/cm2
)
(John et al., 2005). Both buffer and absorber layers
(CIGS) were deposited using CSP technique (with Ag elec-
trode), a simple, easy-to-control, and low-cost deposition
technique that can easily be up-scaled for industrial pro-
duction (Aydin et al., 2014). Copper indium sulfide
(CuInS2) films were prepared from an aqueous solution
containing copper chloride (CuCl2.2H2O), InCl3, and
CS(NH2)2, whereas, In2S3 films required InCl3 and
CS(NH2)2 as precursors. The spray rate and substrate tem-
perature were 20 ml/min and 300 ± 5 °C. Initially, two

layers of both CuInS2 and In2S3 were sprayed in two steps
(spraying 375 ml of the solution first and 300 ml later for
depositing CuInS2, and similarly, spraying 200 ml of the
solution first and 150 ml later for depositing In2S3) to
increase the thickness of the layers and avoid pinholes.
However, the performance of the device improved with a
single layer of CuInS2 and a double layer of In2S3. The
thickness for In2S3 and CuInS2 films were $0.85 lm and
1.1 lm, respectively. The thicker In2S3 films prevented Cu
diffusion that historically has degraded the performance
of devices (Pistor et al., 2009). The solar cell was kept at
the preparation temperature for 1 h after deposition.
X-ray diffraction (XRD) analysis revealed that a Ag layer
coating deposited by vacuum evaporation over the surface
of In2S3 improved the crystallinity of the In2S3 buffer layer.
There was no oxygen in the bulk of the solar cell, which
resulted in better collection of photogenerated carriers at
the electrode. The performance of the device was influenced
by the surface chemistry of the absorber layer. It was also
observed that when the Cu/In ratio decreased, the CuInS2
films became more photosensitive (John et al., 2005).
4.5. Sputtering
Magnetron sputtering is a well-known deposition tech-
nique applicable to industry since it allows large area depo-
sition with reasonable control. It has the capability to be
implemented to production lines for CIGS modules
(Ennaoui, 2000; Lee et al., 2000). The In2S3 buffer layer
was deposited using two different sputtering systems. The
first was from a ceramic In2S3 target in argon (Ar) and
the second from a metallic In target in a hydrogen sulfide
(H2S)/Ar gas mixture. Power densities were in the range
of 1 W/cm2
. Sputtered films from the ceramic target exhib-
ited excellent results, yielding maximum solar cell
(CIGS/In2S3/i-ZnO/ZnO:Al) energy conversion efficiencies
up to 12.2% (Jsc = 31.5 mA/cm2
, Voc = 665 mV, and
FF = 78% under a light intensity of 100 mW/cm2
)
(Naghavi et al., 2010). Deposition parameters were varied
to find the optimal results. Films were deposited at temper-
atures in the range of 200–250 °C. Films deposited at tem-
peratures higher than 250 °C exhibited deteriorated device
performance. The performance was also limited with low
deposition temperatures, which eventually improved after
annealing treatment. The stability of the devices was deter-
mined by accelerated lifetime tests and was found to be sat-
isfactory. Solar cells were also tested according to the IEC
61646 standard in damp heat (1000 h, 85 °C, and 85% rel-
ative humidity) exhibiting no significant loss in the output
power and no transient effects, and passing the test with
allowed limit of 5% loss in power (Naghavi et al., 2010).
This work was a joint venture between ZSW and HMI.
4.6. Atomic Layer Epitaxy (ALE)
ALE provides precise control and uniform coverage
over a large substrate (Sterner et al., 1998; Bhattacharya
et al., 2006). ALE deposited In2S3-buffered solar cells
(Mo/CIGS/In2S3/ZnO:Al) achieved a maximum efficiency
of 13.5% (Jsc = 30.6 mA/cm2
, Voc = 604 mV, and
FF = 73% under a light intensity, of 100 mW/cm2
)
(Yousfi et al., 2000) in 2000 (a joint venture by ENSMP,
ZSW, and ASM Microchemistry Ltd.). The buffer and win-
dow layers were grown in an ALE reactor, whereas, the
absorber layer was co-evaporated over ITO-coated glass
substrates. The deposition temperature in the reactor was
kept constant at 160 °C. N2 gas was used as a carrier and
purging gas. Pulse duration was 300 ms for indium
acetyleacetonate (In(acac)3) and 500 ms for N2 purge
pulses (flow rate: 700 sccm). In was deposited using
In(acac)3, which formed a thin In2S3 layer on the substrate
when reacted with H2S gas. ZnO was used as a window
layer to increase optical transmission and reduce resistivity.
The films were grown with different thicknesses (1–70 nm),
exhibiting changes in performance with change in thick-
ness. The deposition parameters (pulse duration) were opti-
mized by using in-situ QCM. Results were excellent with an
energy conversion efficiency of 13.5%, Voc in excess of
600 mV, and FF approaching that of reference
CdS-buffered solar cells for In2S3 films with thickness
around 30 nm. The In2S3 buffer layer was found to have
an optical bandgap of 3.25 eV, which was confirmed, as
there was no loss in the UV region due to increased collec-
tion of photons (Yousfi et al., 2000).
4.7. Ionic Layer Gas Atomic Reaction (ILGAR)
Recently, significant work on In2S3 buffer layers used
the spray-ILGAR method, a sequential and cyclic tech-
nique, which enables thin film deposition via
aerosol-assisted chemical vapor deposition (AACVD) Ba¨r
et al., 2006; Saéz-Araoz et al., 2012. In 2012, solar cells
(Mo/CIGS/In2S3/ZnO) containing ILGAR-deposited
In2S3 films yielded the maximum energy conversion effi-
ciency of 16.1% (Jsc = 35.5 mA/cm2
, Voc = 631 mV, and
FF = 72% under a light intensity of 100 mW/cm2
)
(Saéz-Araoz et al., 2012) at HMI. An ethanol solution con-
taining In(acac)3 or InCl3 was sprayed onto CIGS-coated
substrates (DC sputtering of Cu–Ga–In precursors fol-
lowed by chalcogenization; from BOSCH Solar CISTech
GmbH), which were heated to 250 °C. The generated aero-
sols were carried in a N2 gas stream through a narrow glass
tube to form an In(Cl, O, OH) precursor film, which was
converted to In2S3 after reacting with H2S gas. The
sequential-ILGAR process cycle consists of the four steps:
(1) spraying-N2; (2) N2-purging; (3) H2S sulfurization; (4)
N2 purging. 30–35 nm thick In2S3 films were deposited by
repeating the cycle multiple times. Interestingly, the chlo-
rine (Cl) content in the buffer layers had a significant
impact upon the performance of the device. Deposition
parameters were varied by changing precursor, H2S con-
centration, and step duration to change the Cl content.
Voc was maximum for Cl-free buffer layers, whereas, FF
showed a decrease with increasing Cl-content. However,

Jsc was nearly independent of the Cl content (up to 22 at.%
chlorine). At H2S flow rates as low as 77 ml/min, good
working solar cells were achieved. Lower H2S flow rates
resulted in poor device performance due to the incomplete
sulfurization reaction of the precursor layer during the
spray step, which resulted in formation of In(O,OH,C,S)
rather than In2S3. In a few experiments, 1% of water was
added to the solution and resulted in an increase in deposi-
tion rate. The Cl-free buffer layers exhibited a bandgap of
2.0 eV, whereas, the layer containing 14 at.% Cl had a
bandgap of 2.4 eV. Films were expected to potentially
increase the absorption in the blue wavelength region. In
addition, it is also believed that increasing Cl content in
buffer layers also influences Cu diffusion into the In2S3 film,
which is beneficial in obtaining optimal performance
(Saéz-Araoz et al., 2012).
4.8. Electrodeposition (ED)
In 2011, electrodeposited In2S3-buffered solar cells
(Mo/CIGS/In2S3/i-ZnO/Zn:Al) reached the maximum
energy conversion efficiency of 10.2% (Jsc = 32 mA/cm2
,
Voc = 569 mV, and FF = 56% under a light intensity of
100 mW/cm2
) (Naghavi et al., 2011). This was a significant
breakthrough for In2S3, as it proved to be the most success-
ful attempt to synthesize In2S3 films using electrodeposi-
tion. Experiments were carried out in a three-electrode
electrochemical cell setup using a saturated mercurous sul-
fate (Hg2SO4) electrode as reference electrode, Mo-coated
glass substrates as working electrode, and platinum (Pt)
as counter electrode. The aqueous solution used for elec-
trodeposition contained InCl3, sodium thiosulfate
(Na2S2O3, used as S source), and potassium chloride
(KCl, used as a supporting electrolyte). Thin In2S3 films
were electrodeposited at 60 °C onto co-evaporated CIGS
absorbers (provided by Wu¨rth Solar) with complete surface
coverage. i-ZnO/ZnO:Al top-window layers were deposited
by radio-frequency (RF) sputtering to complete the solar
cell structure. Films deposited at less negative electrode
potential (<À0.9 V/MSE) onto CIGS absorbers were
dense, homogenous, and uniform. Increase in potential
and thickness of the buffer layer resulted in a transition
in the morphology from nanocolumn arrays to disordered
nanorods. Regardless of the electrode potential, composi-
tional analysis revealed that the films contained oxygen.
Furthermore, the cells were annealed at 200 °C for
10 min and light-soaking was performed at room tempera-
ture for 1 h. Deposition potential and thickness had the
most significant impact upon the performance of the
device. However, the device performance was limited by
low FF and Voc. Based upon results obtained at
IRDEP/IREM, the energy efficiency can be improved if
the interface quality between the CIGS and In2S3 layer is
improved, and there is a further need to optimize the pro-
cess parameters (Naghavi et al., 2011).
4.9. Chemical Bath Deposition (CBD)
In 2005, IPE reported that In2S3 films synthesized by
CBD were employed as a buffer layers in CIGS solar cells
along with Inx(OH, S)y layers, which achieved a remark-
able efficiency of 15.7% (Jsc = 37.4 mA/cm2
,
Voc = 574 mV, and FF = 68.4% under a light intensity of
100 mW/cm2
) (Hariskos et al., 1996). In2S3 deposition
started with deposition of In(OH)3 buffer layers grown in
an aqueous solution containing InCl3 and CS(NH2)2 at
70 °C. In2S3 buffer layers containing oxide/hydroxide
formed when CS(NH2)2 was replaced with thioacetamide
(C2H5NS). The stoichiometry of both layers was dependent
upon the concentration of InCl3 and C2H5NS, deposition
temperature, and time. The solar cell (CIGS/Inx(OH,
S)y/In2S3/ZnO) initially yielded 9.5% efficiency, which
was then increased to 15.7% (active area) after optimizing
the deposition time at 20 min and thickness at 10 nm.
Efficiencies up to 9.7% resulted with CBD In2S3 buffer lay-
ers over 30 Â 30 cm2
solar cell modules. The devices
reached the highest efficiencies after annealing and illumi-
nation, however, efficiencies dramatically dropped as FF
decreased when devices were stored in the dark. Further
studies on growth mechanisms and nucleation of crystal
growth were conducted by CIEMAT, revealing degrada-
tion of the performance of the device due to the impurities
from absorption of species like oxygen, which reacted with
photogenerated carriers. The presence of acceptor defects
at the CBD In2S3/ZnO interface was also deemed responsi-
ble for the poor performance (Hariskos et al., 1996).
4.10. Metal Organic Chemical Vapor Deposition
(MOCVD)
MOCVD is another vacuum-based deposition technique
that uses metalorganic precursors. In the 1980’s, the
MOCVD technique was first used to deposit In2S3 using
single source precursors, which failed due to limited pro-
cess control and high decomposition temperatures. In
2008, MOCVD had its first major success in synthesizing
In2S3 films using the metalorganic precursors, trimethyl
indium (TMIn) and t-butyl-thiol (tBuSH). 60-nm thick
films were deposited over CIGS/Mo/glass (co-evaporated
CIGS from ZSW and Wu¨rth Solar) and Mo/glass sub-
strates, respectively. The source materials (TMIn and
tBuSH) were transported using N2 gas into the process
chamber. The process pressure was 450 mbar. The sub-
strate temperature was varied between 300 and 500 °C,
whereas the deposition time was 20 min. The solar cell
(glass/Mo/CIGS/InxSy/i-ZnO/ZnO:Al/Ni) yielded the
record efficiency of 12.3% (under standard test conditions
of 100 mW/cm2
AM 1.5 illumination and 25 °C) after
5 min post-annealing treatment at 200 °C in air (Spiering
et al., 2009). However, the reproducibility of the results
was not investigated due to poor homogeneity of the films

at low deposition temperatures of 300–325 °C. In order to
eliminate or reduce the diffusion between buffer and absor-
ber, the growth studies focused on the lowest possible tem-
peratures (since former studies have shown Cu
incorporation within the buffer layer at high temperatures).
No growth was observed on either CIGS or Mo substrates
at temperatures below 300 °C, while at 350 °C, the growth
of In2S3 on CIGS absorbers started with a thin seed layer,
followed by tetrahedral structure formation. This structure
disappeared in films which were synthesized at tempera-
tures above 400 °C. As expected, the depth profile measure-
ment revealed a considerable amount of Cu diffusion into
the buffer layer, probably forming Cu–In–S compounds,
which disordered the In2S3 structure and its electrical prop-
erties. Similarly, sodium (Na) diffusion from the absorber
into the buffer layer was significant at higher temperatures.
The efficiency of the device decreased with increasing depo-
sition temperature. Quantum efficiency (QE) measurements
revealed that an enhanced current collection in the blue
wavelength region (300–500 nm) was observed for cells
with In2S3 used as a buffer layer compared to reference
CBD-CdS cells. However, the current collection was
slightly below the reference cell for longer wavelengths.
Further improvement in the performance of the device will
depend heavily upon the improved homogeneity of the
In2S3 films at low temperatures (Spiering et al., 2009).
Major development in the last 2–3 years include in-situ
doping (using Ag) of the CSP-In2S3 thin films to improve
photosensitivity, control structural phase, and tailor opti-
cal and electrical properties (Aydin et al., 2014).
CSP-In2S3 films are now being incorporated as buffer lay-
ers with copper zinc tin sulfide (CZTS) absorbers reaching
conversion efficiency of 1.85% (Rajeshmon et al., 2013).
Diffusion of evaporated metallic In on In2S3 films followed
by annealing improved the crystallinity and carrier collec-
tion. IRDEP studied the high oxygen content in In2S3 films
deposited by plasma enhanced ALD to find correlation
between the species detected in vapor phase and film prop-
erties (Bugot et al., 2015). The stoichiometry and bandgaps
were controlled by varying the plasma power.
Furthermore, there is also research focusing on fabricating
devices using different phases of In2S3 (for example InS)
incorporated with different polymers (Chen et al., 2014),
while a group in Turkey is working on inverted organic
solar cells from sol–gel derived In2S3 films that exhibited
conversion efficiencies of 3.04 ± 0.14% (Aslan et al., 2014).
5. Discussion and outlook
Considerable progress has been made in the develop-
ment of In2S3-buffered TFSCs. We reviewed various depo-
sition techniques utilized in fabrication of In2S3 films on
different types of substrates. Influence of various process
parameters upon the properties of TFSCs was discussed.
A comparative assessment of different buffer layers
revealed that the performance of In2S3-buffered TFSCs is
very similar to that of the reference CdS-buffered TFSCs.
Therefore, In2S3 is among the forerunners of buffer layers
with potential to replace hazardous CdS. In2S3 films incor-
porating CIS and CIGS absorbers have yielded conversion
efficiencies above 9%, with highest being 16.4% by the
ALD technique.
The study of synthesis of In2S3 films by various deposi-
tion techniques revealed that the solar cell efficiency was
linked with the judicious selection of deposition parameters
and post-deposition annealing treatments. Composition,
structure, morphology, and thickness of the films were
highly sensitive to deposition parameters (for example,
bath chemistry and nature of the substrate). Diffusion
(Cu, Na, O, etc.) processes at the buffer layer interface,
dependent upon deposition temperature and annealing
temperature, had a significant favorable impact upon the
performance. However, Cu was a major diffusing element.
The presence of these contaminants in In2S3’s crystalline
matrix (as third atoms) can induce broad changes in the
optical and electrical properties of the films. Grain size of
In2S3 films was significantly affected by the substrate type
and film thickness. Lower Voc for In2S3-buffered TFSC
compare to CdS-buffered TFSC could be attributed to bulk
recombination via states generated by intrinsic defects
induced by a lattice mismatch at the heterojunction inter-
face. In addition, the performances of the solar cells were
not only impacted by the buffer/absorber interface recom-
bination, but also seem to be impacted by the defects at the
buffer/window interface. We also confirmed that the per-
formance of In2S3-buffered solar cells was highly absorber
dependent and there is an obvious need to use the window
layer.
It was observed that in preparation of In2S3 films, the
vacuum-based deposition techniques offer slight advantage
over solution-based techniques in terms of processing of
large-area substrates, better uniformity, and high control-
lability of the deposition parameters (composition, temper-
ature, etc.). In addition, solar cells made from
vacuum-based techniques, with the exception of ILGAR
exhibited better FF’s. This performance could be attribu-
ted to formation of a heterojunction providing highest
interface crystalline quality, i.e. good lattice matching that
offered minimum charge carrier recombination and suffi-
cient carrier transport. Comparatively, solution-based
techniques were cost effective, however, they haven’t been
able to match the performance of solar cells synthesized
by vacuum-based techniques, which are often associated
with low deposition rates and need of expensive equipment.
The quality and long-term stability of the source material
had also been a major factor in accomplishing the highest
solar cell performance. Although CBD-CdS is reproducible
and yields good performance, there are drawbacks con-
cerning industrial-upscaling from use of the carcinogen
thiourea and hazardous cadmium in large amounts. The
spray-ILGAR method yielded an efficiency of 16.1%. It
offers several advantages: no plugging of a nozzle, smaller
droplet size, a narrower size distribution leading to better
homogeneity, greater material yield, and easy recovery of

the residuals, unlike existing Cd-based CBD technology for
buffer deposition. One significant advantage of using
solution-based techniques to synthesize In2S3 films was that
the tetragonal structure is achievable by varying deposition
parameters, without the need for annealing. On the other
hand, most vacuum-deposited In2S3 films exhibited amor-
phous structures and required post-deposition annealing
in order to achieve the same crystalline structure.
Moreover, all deposition techniques offered capabilities to
control/engineer In2S3’s electrical, optical, thermal, and
structural properties, which helped improve the perfor-
mance of solar cells.
In2S3 buffer layers were stable and offered opportunity
for device preparation at high temperatures compared to
its counterparts, which could allow formation of tandem
junctions in the future. Solar cells with In2S3 buffer layers
exhibited excellent lifetime measurements and long-term
stability, further reinforcing In2S3 as a buffer material for
Cd-free devices. However, with the aim of an industrial
implementation, future studies need to focus on interface
formation during deposition and post-deposition treatment
in order to avoid defects and interdiffusion. A concerted
effort in optimizing the process parameters would further
improve the performance parameters of In2S3-buffered
TFSCs, making possible the widespread adoption of this
technology.
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Glossary
AGU: Aoyama Gakuin University, Setagaya-ku, Tokyo, Japan
AIX: Aixtron (Semiconductor Industry Company), Herzogenrath,
Germany
ASC: A˚ ngstro¨m Solar Centre, Uppsala University, Uppsala, Sweden
ALD: Atomic Layer Deposition
ALE: Atomic Layer Epitaxy
BHT: Beuth University of Applied Sciences, Berlin, Germany
CBD: Chemical Bath Deposition
CdTe: Cadmium Telluride
CIEMAT: Spanish Research Centre for Energy, Environment, and
Technology, Madrid, Spain
CIGS: Copper Indium Gallium Selenide
CIS: Copper Indium Disulfide
CREST: Center for Renewable Energy Systems and Technology,
Leicestershire, UK
CSU: Colorado State University, Fort Collins, CO, USA

CSP: Chemical Spray Pyrolysis
CUSAT: Cochin University of Science and Technology, Kochi, India
CVD: Chemical Vapor Deposition
CZTS: Copper Zinc Tin Sulfur
DC-sputtering: Direct Current-Sputtering
ED: Electrodeposition
ENSCP: National Chemical Engineering Institute in Paris, France (E´ cole
Nationale Supe´rieure De Chimie De Paris)
ENSMP: MINES Paris Tech (Ecole Nationale Supe´rieure des Mines de
Paris), France
ETHZ: Swiss Federal Institute of Technology, Zurich, Switzerland
(Eidgeno¨ssische Technische Hochschule Zu¨rich)
FhG-ISE: Fraunhofer Institute for Solar Energy Systems, Freiburg,
Germany
FTO: Fluorine-doped Tin Oxide
GmbH: Gesellschaft mit beschra¨nkter Haftung
HMI: Hahn-Meitner-Institute, Berlin, Germany
ITO: Indium Tin Oxide
HZB: Helmholtz-Zentrum Berlin for Materials and Energy, Berlin,
Germany
KAIST: Korea Advanced Institute of Science and Technology, Daejeon,
South Korea
ILGAR: Ionic Layer Gas Atomic Reaction
IPE: Institute for Physical Electronics, Universita¨t Stuttgart, Stuttgart,
Germany
IRDEP: Institute of Research and Development on Photovoltaic Energy,
Chatou Cedex, France
IREM: Institute of Research for Energy Materials, Lavoisier Institute,
Versailles Cedex, France
IU: Iwate University, Morioka, Japan
KIER: Korea Institute of Energy Research, Taejon, South Korea
LTFP: Laboratory for Thin Films and Photovoltaics, Empa, Switzerland
MOCVD: Metal Organic Chemical Vapor Deposition
MOPVE: Metal Organic Physical Vapor Epitaxy
MVU: Michigan Virtual University, Lansing, MI
NREL: National Renewable Energy Laboratory, Golden, CO, USA
NSC: Nordic Solar Energy, Kista, Sweden
PVD: Physical Vapor Deposition
SCCM: Standard Cubic Centimeter/Minute
SINGULUS: Technologies AG, Kahl am Main, Germany
SFLMTR: Swiss Federal Laboratories for Materials Testing and
Research, Empa, Switzerland
SSG: Sunshine Solar Group
TFPL: Thin Film Physics Laboratory, Shivaji University, Kolhapur,
India
TFSC: Thin Film Solar Cell
TIT: Tokyo institute of technology, Tokyo, Japan
UD: University of Delaware, Newark, DE, USA
UO: University of Oldenburg, Oldenburg, Germany
UQ: University of Queensland, Brisbane, Australia
UR: University of Regensburg, Regensburg, Germany
UB: University of Barcelona, Barcelona, Spain
UT: University of Toledo, Toledo, Ohio, USA
CSU: Colorado State University
USP: Ultrasonic Spray Pyrolysis
WIS: Weizmann Institute of Science, Rehovot, Israel
WSU: Washington State University, Pullman, WA, USA
ZSW: Center for Solar Energy and Hydrogen Research, Stuttgart,
Germany (Zentrum fuer Sonnenenergie-und Wasserstoﬀ-Forschung)

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Progress in Indium (III) Sulfide (In2S3) Buffer Layer Deposition Techniques for CIS, CIGS, and CdTe-based Thin Film Solar Cells