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Solar Energy 120 (2015) 131–146
Review
Progress in indium (III) sulfide (In2S3) buffer layer depositiontechniques 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, USAb 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 apromising 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 layerdeposition (ALD) and ionic layer gas atomic reaction (ILGAR) techniques have been the two most successful, yielding maximum energyconversion efficiencies up to 16.4% and 16.1%, respectively. The impact of varied deposition parameters upon the In2S3 film propertiesand 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)- andCdTe-based solar cells was also performed to measure the progress of In2S3 compared to its counterparts.� 2015 Elsevier Ltd. All rights reserved.
Keywords: Indium (III) sulfide (In2S3); Buffer layer; Efficiency; Solar cell; Thin Film Solar Cell (TFSC)
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1322. Thin film market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1323. Cadmium-free buffer Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334. In2S3-buffered thin film solar cells with record efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
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4.1. Atomic Layer Deposition (ALD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1344.2. Physical Vapor Deposition (PVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1394.3. Ultrasonic Spray Pyrolysis (USP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1394.4. Chemical Spray Pyrolysis (CSP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1394.5. Sputtering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1404.6. Atomic Layer Epitaxy (ALE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
x.doi.org/10.1016/j.solener.2015.07.028
92X/� 2015 Elsevier Ltd. All rights reserved.
rresponding author. Tel.: +1 (870)819 9043; fax: +1 (870)972 3539.ail addresses: [email protected] (M.A. Mughal),
[email protected] (R. Engelken), [email protected] (R. Sharma).
132 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146
4.7. Ionic Layer Gas Atomic Reaction (ILGAR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1404.8. Electrodeposition (ED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414.9. Chemical Bath Deposition (CBD). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414.10. Metal Organic Chemical Vapor Deposition (MOCVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5. Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
1. Introduction
Indium (III) sulfide (In2S3), an indium chalcogenide, is aIII–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 itsstable 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 (Mughalet 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 thewidest applications (Mughal et al., 2015) due to its defec-tive spinal structure (most stable) Tao et al., 2008, largephotosensitivity (Warrier et al., 2013), and physical charac-teristics (Cansizoglu et al., 2010). With optimal physicalproperties, it can meet the requirement of a suitable bufferlayer 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 suchas chemical bath deposition (CBD) Hariskos et al., 1996,electrodeposition (ED) Mughal et al., 2013, atomic layerdeposition (ALD) Naghavi et al., 2003, physical vapordeposition (PVD) Hossain, 2012, ultrasonic spray pyrolysis(USP) Buecheler et al., 2009, and ionic layer gas atomicreaction (ILGAR) Allsop et al., 2005 have yielded efficien-cies above 9% at the laboratory scale. Efficiencies up to16.4% (Hariskos et al., 2005) have been achieved usingthe 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 asa 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, nowdemonstrating energy conversion efficiencies above 20%(Jackson et al., 2015), and successfully replacing crystallinesilicon (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 transparentconductive oxide film (front contact), and an anti-reflectivecoating (Shah et al., 1999), all stacked on top of each otheron a conductive substrate (for example, molybdenum(Mo)-coated glass) (see Fig. 2). The absorber layer, inwhich photons are efficiently absorbed resulting in elec-tron–hole pair generation, constitutes the core of the device(Wurfel, 2005). However, in this paper, we focus upon thebuffer layer (In2S3) whose principal goal is to form a reli-able p–n junction and establish good interface propertieswith 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, allowingabsorber materials better suited for environmental expo-sure, hence providing stability to the solar cell (Roedern,2001). Appropriate thickness could also result in reducingthe overall reflectance and thus improving the energy con-version efficiency. In2S3 films have also been reported tohave no conduction band discontinuity at the interfacewith 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 largeindustrial-scale production, it is crucial for further growthto adopt processing measures that are low-cost,contaminant-free, and industrially applicable. In 2013, thesolar market share for all TFSC technology was 11% withn-CdS/p-CdTe heterojunction solar cells leading the annualproduction by 2 GWp (Gigawatt peak) ISE, 2014. The solarPV share for TFSC is expected to grow at an annual rate of24%, reaching 22 GW by the end of 2020 (Wood, 2020). Inthe past, chemical bath deposited (CBD)-CdS regularly fea-tured as a buffer material in CIGS- and CdTe-based solarcells, yielding maximum energy conversion efficiencies upto 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 tohazardous CdS, in order to reduce or eliminate its environ-mental impact (Dimova-Malinovska, 2010; Hamakawa,2004), consequently, avoiding climate change and humanhealth-risks, which potentially poses risk to the economy.In addition, because of the prohibition of toxic Cd andincrease in stringent legislation relating to its use and
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 fur Sonnenenergie-undWasserstoff-Forschung (ZSW) Baden-Wurttemberg. The top layer, usuallyn-type, is a CdS buffer layer that allows almost all of the light to transmitthrough to the absorbing layer (CIGS), usually p-type, which converts lightinto energy. A transparent conductive oxide (TCO) layer, ZnO, carriesexcited electrons to the top of the solar cell while still letting the lightthrough. A back (ohmic) contact is used to provide a good electricalconnection to the substrate, whereas, the front contact carries electrons outto 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
disposal, several countries are holding restrictions uponsolar PV market share for Cd-containing solar cells(Nordic Council of Ministers, 2003). This opens the gatesfor In2S3 to enter the TFSC technology market. Althoughsome reservations have surfaced regarding the availabilityand high price of indium (In), the primary annual reportedproduction of In in 2011 was 550–650 MT (Metric Tons)Woodhouse et al., 2012. According to IndiumCorporation, In is quite abundant in the crust of the earthand there is enough available to meet the present and futureneeds. In is more abundant than silver (Ag), which hasannual production of about 20,000 MT, nearly 40 timesmore than that of In. The currently-observed price fluctua-tions are primarily due to a time lag between emergingdemand and available supply (Gowans, 2010).
For this reason, we have been studying In2S3. Our workat 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 goalis to fabricate n-In2S3/p-CuInS2 heterojunction TFSC. Inthis paper, we highlight the progress and development ofIn2S3-buffered TFSCs by various deposition techniques.
3. Cadmium-free buffer Layers
The important performance characteristics of any buffermaterial include bandgap energy (Sankapal et al., 2004),absorption coefficient (Roedern, 2001), transport of
134 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146
photo-generated carriers to the outer circuit with minimumelectrical resistance, carrier lifetime/mobility/concentration, recombination rate, thickness (Nayak et al., 2012),refractive index (Ramli et al., 2013), diffusion length, latticemismatch (Hossain, 2012), etc. These characteristics indi-vidually and collectively play a key role in improving theperformance parameters of the solar cell including opencircuit voltage (Voc), fill factor (FF), current density (Jsc),etc. The current understanding is that the buffer layershould have a small thickness (25–300 nm) Naghaviet 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 thealignment of conduction band with the absorber layer andpassivate 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, Zn1�xMgxO, and Inx(OH,S)y. havebeen investigated and used in manufacturing TFSCs(Dimova-Malinovska, 2010; Ahn et al., 2008; Vallejoet al., 2010).
A comparative/operational analysis (solar cell efficien-cies above 9% reported for cell area 6 1cm2) of variousbuffer layers used in two primary types of TFSC technol-ogy: chalcopyrite (CIS/CIGS)- and CdTe-based solar cellswas made to measure the progress of In2S3 compared to itscounterparts (see Table 1). We made our best efforts toinclude as many buffer layers as we were able to find inthe literature review. A closer look at the analysis revealedthat the buffer materials tested at different laboratories andinstitutions are mainly chalcogenides (oxides, selenides,and sulfides) of In, Zn, Cd, Al, and Sn. The FF’s andVoc’s for CdS-buffered solar cells have been slightly higherthan for alternate buffer-based solar cells. In addition, thelower energy conversion efficiencies for the solar cells seemto be caused either by technological scale-up problems, theneed for special post-treatment, or the need for further pro-cess optimization to improve interface properties betweenabsorber and buffer layers.
Until now, CBD, sputtering, co-evaporation, and ALDtechniques have been the four most widely used/studieddeposition techniques at both the laboratory and industrialscale. In the early 1990s, there was much focus on materialslike SnO2, Sn(O, S)2, ZnSe, ZnIn2Se4, Cd1�xZnxS, andInxSey to fabricate Cd-free solar cells. The major advantageof these buffer materials was that their bandgap energieswere larger than that of CdS (except for InxSey), whichimproved the light transmission in the blue wavelengthregion, resulting in higher Voc (up to 652 mV) and FF(76.3%) Ohtake et al., 1997. Depending upon the Zn/Cdratio, Cd1�xZnxS alloys yielded conversion efficiencies upto 19.52% due to better lattice match with the CIGS andfavorable conduction band offset at the heterojunctioninterface (Bhattacharya et al., 2006). In early 2000s, thefocus started shifting toward zinc/indium compoundsincluding Inx(O, OH, S)y, Zn(S, OH), ZnO1�xSx, 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 forany buffer material, and incorporated with both CIS andCIGS absorbers.
Fig. 3 shows the timeline (in green) and efficiency trend(in red) of various buffer layer-based TFSCs since 1987. Inthe last 7 years, In2S3 and CdS have been studied the most;however, CdS takes a slight lead, leaving In2S3 behind by5% efficiency and higher FF’s. Fig. 4 summarizes the high-est efficiencies reported in the literature for In2S3- andCdS-based solar cells. It was clearly evident that In2S3 buf-fer layers grown by different techniques can lead to highefficiencies, while each technique confers unique film prop-erties. Fig. 5 is a graphical representation of conversionefficiencies from Table 1 for various buffer layer-basedchalcopyrite (CIS/CIGS) and CdTe TFSCs. It illustratesthat several deposition techniques and materials haveresulted in energy conversion efficiencies equal or higherthan those of corresponding CdS-based solar cells.Zn/In-based buffer materials have been exceptional, yield-ing efficiencies above 15% on multiple occasions (Nakadaand 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; Saez-Araozet al., 2012).
4. In2S3-buffered thin film solar cells with record efficiencies
In the following section, we provide more detaileddescription of the In2S3-buffered TFSCs that yielded recordefficiencies above 9%. In2S3 films were deposited using var-ious deposition techniques (both wet and dry) onto differ-ent types of substrates. The films exhibited diversestructural, morphological, compositional, and electricalproperties, depending upon the treatment and varyingdeposition parameters. The section is focused upon thesynthesis of In2S3 films exhibiting characteristics that hadsignificant impacts upon the performance of the solar cells.
4.1. Atomic Layer Deposition (ALD)
ALD is one of the most successful deposition techniquesreported for In2S3 buffer layers. In 2010, ENSCP and ZSWreported a world record efficiency of 16.4%(Jsc = 31.5 mA/cm2, Voc = 665 mV, and FF = 78% undera 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 byRF-sputtering a bi-layer of zinc oxide (ZnO) used as afront contact. Efficiencies up to 12.9% were recorded for30 � 30 cm2 solar cell (Mo/CIGS/In2S3/ZnO/ZnO:Al)modules. In2S3 films were synthesized via surface reactionsby the sequential introduction of the precursors, indiumacetylacetonate (In(acac)3) and hydrogen sulfide (H2S), in
Table 1Summary 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 Absorberlayer
Efficiency(%)
CurrentdensityJsc (mA/cm2)
OpencircuitvoltageVoc (mV)
FillfactorFF(%)
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)
Zn1�xMgxO 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 magnetronsputtering
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 Bar 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)
ZnO1�xSx 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)
Zn1�xSnxOy 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-Wurth 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 Saez-Araoz et al. (2012)ALE CIS 13.5 30.6 604 73 0.1 ENSMP-ZSW-ASM Microchemistry Ltd./
2000Yousfi et al. (2000)
MOCVD CIGS 12.3 NR NR NR NR ZSW- Wurth Solar-AIX/2008 Spiering et al. (2009)
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13
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14
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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/
2001Hultqvist 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 Saez-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)
Cd1�xZnxS 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 Reporteda Active area efficiency.
136M
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46
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 thisfigure legend, the reader is referred to the web version of this article.)
Fig. 4. CdS vs. In2S3 buffered TFSCs by different deposition techniques.
M.A. Mughal et al. / Solar Energy 120 (2015) 131–146 137
the reactor chamber by varying deposition temperaturebetween 160 and 220 �C. Nitrogen gas (N2) was used as acarrier and purging gas. A pulse sequence of In(acac)3/N2
purge/H2S/N2 purge with a pulse duration of700/1000/500/800 ms was optimized by using in-situ quartzcrystal microgravimetry (QCM). Deposition temperatureand thickness were demonstrated to have significantimpacts 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, whichyielded 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 growthrate was observed to decrease for higher deposition temper-atures, indicating a decrease of reactivity at the surfacewith respect to the adsorption and reaction of the precur-sors. Increase in deposition temperature resulted in lowFF for films with thicknesses below 30 nm, whereas, the30–50 nm thick films exhibited improvement in FF andshort circuit current (Isc). Hence, during synthesis ofIn2S3 films, the numbers of cycles were adjusted at eachtemperature to reach the optimal thickness.
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 byvarious deposition techniques.
138 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146
M.A. Mughal et al. / Solar Energy 120 (2015) 131–146 139
Photoelectron spectroscopy (XPS) surface analysisdetected the diffusion of Cu and Na at high deposition tem-peratures, indicating the possibility of forming a p–nhomojunction inside the CIGS.
Furthermore, annealing of ALD In2S3 films improvedand enhanced the solar cell efficiency to 16.4%. The devicesdid not show any metastability, or any significant loss inthe performance, even after a few months of indoor stor-age. Devices were also tested according to the IEC 61646standard in damp heat (1000 h, 85 �C, and 85% relativehumidity), but exhibited no significant loss in the outputpower, and passed the allowed limit of 5% (Hariskoset 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-evaporationof 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 highestenergy conversion efficiency of 15.2% (Jsc = 29.8 mA/cm2,Voc = 677 mV, and FF = 75.6% under a light intensity of100 mW/cm2) (Rumberg et al., 2000). In2S3 powder fromfour different suppliers was evaporated onto both soda limeglass and CIGS absorbers (from both HMI and WurthSolar). All four powders were characterized to study thedifferences in their stoichiometry, purity, crystal size, andcrystal phase. 50-nm thick In2S3 films were grown byPVD (base pressure of chamber 5.0E�5 mbar) with cru-cible temperature set to 720 �C, while the substrate wasnot heated and stayed below 50 �C. Films were character-ized to measure the impact of different powders upon theperformance of the buffer layer and the solar cell. Best solarcell (Mo/CIGS/In2S3/ZnO/Al/Ni) efficiency was achievedfrom powder that exhibited b-In2S3 crystallinity,long-term stability, and no impurities/contamination (forexample, Cl or O), and CIGS absorbers from HMI.Furthermore, multiple depositions were carried-out withthe same crucible filling, extending the range from freshIn2S3 powder to nearly complete evaporation (710 min ofaccumulated deposition time) to measure the stability.No degradation in film or device performance wasobserved. Post-annealing treatment (�35–45 min at200 �C in air) further optimized the performance. With thissingle source approach, evaporation of elemental S wasavoided, which could be extremely beneficial if the tech-nique is applied to industrial-scale production. However,the quality of evaporated In2S3 powder, which may varyafter a certain period of time, is a concern. XPS surfaceanalysis indicated a significant loss of S in the evaporatedIn2S3 powder, changing the stoichiometry after a few runs(Pistor et al., 2009).
The reason for the difference in device performance withco-evaporated and direct-evaporated buffer layers is stillconfusing and not fully investigated. Nevertheless, investi-gation had indicated that the junction quality was
dependent upon the interfacial diffusion mechanism thatoccurs while depositing In2S3 films.
4.3. Ultrasonic Spray Pyrolysis (USP)
USP-deposited In2S3 buffer layers were used to fabricatesolar cells (Mo/CIGS/In2S3/i-ZnO/ZnO:Al), whichachieved the maximum conversion efficiency of 13.4%(Jsc = 33.4 mA/cm2, Voc = 585 mV, and FF = 69% undera light intensity of 1000 mW/cm2) (Buecheler et al., 2009)in 2009, a joint work by LTFP and SFLMTR. USP is alow-cost, non-vacuum, and industrially applicable tech-nique (Fella et al., 2010; Buecheler et al., 2009). It consistedof an ultrasonic atomizer and a droplet transporting sys-tem, which were used to deposit In2S3 films onto a heatedsubstrate. A chamber-based ultrasonic atomizer was usedto 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 thefunnel, creating a closed deposition area above the sub-strate to prevent oxidation. The chamber was vented withN2 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 withrespect 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. Completeevaporation of the solvent above the substrate surfacewas confirmed by SEM. The solution was sprayed for300 s onto the substrate to yield 30 nm thick In2S3 films,which exhibited homogenous layer formation withoutany evidence of droplet formation. It was observed thatIn2S3 buffer layers formed through the reaction betweenInClx fragments and CS2. An increase in growth rate wasobserved with increase in temperature of the substrate.The SEM cross-sectional view of the solar cell revealedno 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, andFF = 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 depositiontechnique that can easily be up-scaled for industrial pro-duction (Aydin et al., 2014). Copper indium sulfide(CuInS2) films were prepared from an aqueous solutioncontaining copper chloride (CuCl2.2H2O), InCl3, andCS(NH2)2, whereas, In2S3 films required InCl3 andCS(NH2)2 as precursors. The spray rate and substrate tem-perature were 20 ml/min and 300 ± 5 �C. Initially, two
140 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146
layers of both CuInS2 and In2S3 were sprayed in two steps(spraying 375 ml of the solution first and 300 ml later fordepositing CuInS2, and similarly, spraying 200 ml of thesolution first and 150 ml later for depositing In2S3) toincrease the thickness of the layers and avoid pinholes.However, the performance of the device improved with asingle layer of CuInS2 and a double layer of In2S3. Thethickness for In2S3 and CuInS2 films were �0.85 lm and1.1 lm, respectively. The thicker In2S3 films prevented Cudiffusion that historically has degraded the performanceof devices (Pistor et al., 2009). The solar cell was kept atthe preparation temperature for 1 h after deposition.X-ray diffraction (XRD) analysis revealed that a Ag layercoating deposited by vacuum evaporation over the surfaceof In2S3 improved the crystallinity of the In2S3 buffer layer.There was no oxygen in the bulk of the solar cell, whichresulted in better collection of photogenerated carriers atthe electrode. The performance of the device was influencedby the surface chemistry of the absorber layer. It was alsoobserved 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 beimplemented to production lines for CIGS modules(Ennaoui, 2000; Lee et al., 2000). The In2S3 buffer layerwas deposited using two different sputtering systems. Thefirst was from a ceramic In2S3 target in argon (Ar) andthe second from a metallic In target in a hydrogen sulfide(H2S)/Ar gas mixture. Power densities were in the rangeof 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 efficienciesup to 12.2% (Jsc = 31.5 mA/cm2, Voc = 665 mV, andFF = 78% under a light intensity of 100 mW/cm2)(Naghavi et al., 2010). Deposition parameters were variedto 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 deviceperformance. The performance was also limited with lowdeposition temperatures, which eventually improved afterannealing 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 IEC61646 standard in damp heat (1000 h, 85 �C, and 85% rel-ative humidity) exhibiting no significant loss in the outputpower and no transient effects, and passing the test withallowed 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 coverageover 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 efficiencyof 13.5% (Jsc = 30.6 mA/cm2, Voc = 604 mV, andFF = 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, theabsorber layer was co-evaporated over ITO-coated glasssubstrates. The deposition temperature in the reactor waskept constant at 160 �C. N2 gas was used as a carrier andpurging gas. Pulse duration was 300 ms for indiumacetyleacetonate (In(acac)3) and 500 ms for N2 purgepulses (flow rate: 700 sccm). In was deposited usingIn(acac)3, which formed a thin In2S3 layer on the substratewhen reacted with H2S gas. ZnO was used as a windowlayer 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 anenergy conversion efficiency of 13.5%, Voc in excess of600 mV, and FF approaching that of referenceCdS-buffered solar cells for In2S3 films with thicknessaround 30 nm. The In2S3 buffer layer was found to havean optical bandgap of 3.25 eV, which was confirmed, asthere 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 usedthe spray-ILGAR method, a sequential and cyclic tech-nique, which enables thin film deposition viaaerosol-assisted chemical vapor deposition (AACVD) Baret al., 2006; Saez-Araoz et al., 2012. In 2012, solar cells(Mo/CIGS/In2S3/ZnO) containing ILGAR-depositedIn2S3 films yielded the maximum energy conversion effi-ciency of 16.1% (Jsc = 35.5 mA/cm2, Voc = 631 mV, andFF = 72% under a light intensity of 100 mW/cm2)(Saez-Araoz et al., 2012) at HMI. An ethanol solution con-taining In(acac)3 or InCl3 was sprayed onto CIGS-coatedsubstrates (DC sputtering of Cu–Ga–In precursors fol-lowed by chalcogenization; from BOSCH Solar CISTechGmbH), which were heated to 250 �C. The generated aero-sols were carried in a N2 gas stream through a narrow glasstube to form an In(Cl, O, OH) precursor film, which wasconverted to In2S3 after reacting with H2S gas. Thesequential-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 byrepeating the cycle multiple times. Interestingly, the chlo-rine (Cl) content in the buffer layers had a significantimpact upon the performance of the device. Depositionparameters 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, FFshowed a decrease with increasing Cl-content. However,
M.A. Mughal et al. / Solar Energy 120 (2015) 131–146 141
Jsc was nearly independent of the Cl content (up to 22 at.%chlorine). At H2S flow rates as low as 77 ml/min, goodworking solar cells were achieved. Lower H2S flow ratesresulted in poor device performance due to the incompletesulfurization reaction of the precursor layer during thespray step, which resulted in formation of In(O,OH,C,S)rather than In2S3. In a few experiments, 1% of water wasadded to the solution and resulted in an increase in deposi-tion rate. The Cl-free buffer layers exhibited a bandgap of2.0 eV, whereas, the layer containing 14 at.% Cl had abandgap of 2.4 eV. Films were expected to potentiallyincrease the absorption in the blue wavelength region. Inaddition, it is also believed that increasing Cl content inbuffer layers also influences Cu diffusion into the In2S3 film,which is beneficial in obtaining optimal performance(Saez-Araoz et al., 2012).
4.8. Electrodeposition (ED)
In 2011, electrodeposited In2S3-buffered solar cells(Mo/CIGS/In2S3/i-ZnO/Zn:Al) reached the maximumenergy conversion efficiency of 10.2% (Jsc = 32 mA/cm2,Voc = 569 mV, and FF = 56% under a light intensity of100 mW/cm2) (Naghavi et al., 2011). This was a significantbreakthrough 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-electrodeelectrochemical cell setup using a saturated mercurous sul-fate (Hg2SO4) electrode as reference electrode, Mo-coatedglass 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 filmswere electrodeposited at 60 �C onto co-evaporated CIGSabsorbers (provided by Wurth Solar) with complete surfacecoverage. i-ZnO/ZnO:Al top-window layers were depositedby radio-frequency (RF) sputtering to complete the solarcell structure. Films deposited at less negative electrodepotential (<�0.9 V/MSE) onto CIGS absorbers weredense, homogenous, and uniform. Increase in potentialand thickness of the buffer layer resulted in a transitionin the morphology from nanocolumn arrays to disorderednanorods. Regardless of the electrode potential, composi-tional analysis revealed that the films contained oxygen.Furthermore, the cells were annealed at 200 �C for10 min and light-soaking was performed at room tempera-ture for 1 h. Deposition potential and thickness had themost significant impact upon the performance of thedevice. However, the device performance was limited bylow FF and Voc. Based upon results obtained atIRDEP/IREM, the energy efficiency can be improved ifthe interface quality between the CIGS and In2S3 layer isimproved, 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 byCBD were employed as a buffer layers in CIGS solar cellsalong 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 of100 mW/cm2) (Hariskos et al., 1996). In2S3 depositionstarted with deposition of In(OH)3 buffer layers grown inan aqueous solution containing InCl3 and CS(NH2)2 at70 �C. In2S3 buffer layers containing oxide/hydroxideformed when CS(NH2)2 was replaced with thioacetamide(C2H5NS). The stoichiometry of both layers was dependentupon the concentration of InCl3 and C2H5NS, depositiontemperature, and time. The solar cell (CIGS/Inx(OH,S)y/In2S3/ZnO) initially yielded 9.5% efficiency, whichwas then increased to 15.7% (active area) after optimizingthe deposition time at 20 min and thickness at 10 nm.Efficiencies up to 9.7% resulted with CBD In2S3 buffer lay-ers over 30 � 30 cm2 solar cell modules. The devicesreached the highest efficiencies after annealing and illumi-nation, however, efficiencies dramatically dropped as FFdecreased when devices were stored in the dark. Furtherstudies on growth mechanisms and nucleation of crystalgrowth were conducted by CIEMAT, revealing degrada-tion of the performance of the device due to the impuritiesfrom absorption of species like oxygen, which reacted withphotogenerated carriers. The presence of acceptor defectsat 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 techniquethat uses metalorganic precursors. In the 1980’s, theMOCVD technique was first used to deposit In2S3 usingsingle source precursors, which failed due to limited pro-cess control and high decomposition temperatures. In2008, MOCVD had its first major success in synthesizingIn2S3 films using the metalorganic precursors, trimethylindium (TMIn) and t-butyl-thiol (tBuSH). 60-nm thickfilms were deposited over CIGS/Mo/glass (co-evaporatedCIGS from ZSW and Wurth Solar) and Mo/glass sub-strates, respectively. The source materials (TMIn andtBuSH) were transported using N2 gas into the processchamber. 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 therecord efficiency of 12.3% (under standard test conditionsof 100 mW/cm2 AM 1.5 illumination and 25 �C) after5 min post-annealing treatment at 200 �C in air (Spieringet al., 2009). However, the reproducibility of the resultswas not investigated due to poor homogeneity of the films
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at low deposition temperatures of 300–325 �C. In order toeliminate or reduce the diffusion between buffer and absor-ber, the growth studies focused on the lowest possible tem-peratures (since former studies have shown Cuincorporation within the buffer layer at high temperatures).No growth was observed on either CIGS or Mo substratesat temperatures below 300 �C, while at 350 �C, the growthof In2S3 on CIGS absorbers started with a thin seed layer,followed by tetrahedral structure formation. This structuredisappeared 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 intothe buffer layer, probably forming Cu–In–S compounds,which disordered the In2S3 structure and its electrical prop-erties. Similarly, sodium (Na) diffusion from the absorberinto the buffer layer was significant at higher temperatures.The efficiency of the device decreased with increasing depo-sition temperature. Quantum efficiency (QE) measurementsrevealed that an enhanced current collection in the bluewavelength region (300–500 nm) was observed for cellswith In2S3 used as a buffer layer compared to referenceCBD-CdS cells. However, the current collection wasslightly below the reference cell for longer wavelengths.Further improvement in the performance of the device willdepend heavily upon the improved homogeneity of theIn2S3 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 improvephotosensitivity, 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 reachingconversion efficiency of 1.85% (Rajeshmon et al., 2013).Diffusion of evaporated metallic In on In2S3 films followedby annealing improved the crystallinity and carrier collec-tion. IRDEP studied the high oxygen content in In2S3 filmsdeposited by plasma enhanced ALD to find correlationbetween the species detected in vapor phase and film prop-erties (Bugot et al., 2015). The stoichiometry and bandgapswere controlled by varying the plasma power.Furthermore, there is also research focusing on fabricatingdevices 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 organicsolar cells from sol–gel derived In2S3 films that exhibitedconversion 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 ondifferent types of substrates. Influence of various processparameters upon the properties of TFSCs was discussed.A comparative assessment of different buffer layersrevealed that the performance of In2S3-buffered TFSCs isvery similar to that of the reference CdS-buffered TFSCs.
Therefore, In2S3 is among the forerunners of buffer layerswith potential to replace hazardous CdS. In2S3 films incor-porating CIS and CIGS absorbers have yielded conversionefficiencies above 9%, with highest being 16.4% by theALD technique.
The study of synthesis of In2S3 films by various deposi-tion techniques revealed that the solar cell efficiency waslinked with the judicious selection of deposition parametersand post-deposition annealing treatments. Composition,structure, morphology, and thickness of the films werehighly 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 annealingtemperature, had a significant favorable impact upon theperformance. However, Cu was a major diffusing element.The presence of these contaminants in In2S3’s crystallinematrix (as third atoms) can induce broad changes in theoptical and electrical properties of the films. Grain size ofIn2S3 films was significantly affected by the substrate typeand film thickness. Lower Voc for In2S3-buffered TFSCcompare to CdS-buffered TFSC could be attributed to bulkrecombination via states generated by intrinsic defectsinduced by a lattice mismatch at the heterojunction inter-face. In addition, the performances of the solar cells werenot only impacted by the buffer/absorber interface recom-bination, but also seem to be impacted by the defects at thebuffer/window interface. We also confirmed that the per-formance of In2S3-buffered solar cells was highly absorberdependent and there is an obvious need to use the windowlayer.
It was observed that in preparation of In2S3 films, thevacuum-based deposition techniques offer slight advantageover solution-based techniques in terms of processing oflarge-area substrates, better uniformity, and high control-lability of the deposition parameters (composition, temper-ature, etc.). In addition, solar cells made fromvacuum-based techniques, with the exception of ILGARexhibited better FF’s. This performance could be attribu-ted to formation of a heterojunction providing highestinterface crystalline quality, i.e. good lattice matching thatoffered minimum charge carrier recombination and suffi-cient carrier transport. Comparatively, solution-basedtechniques were cost effective, however, they haven’t beenable to match the performance of solar cells synthesizedby vacuum-based techniques, which are often associatedwith low deposition rates and need of expensive equipment.The quality and long-term stability of the source materialhad also been a major factor in accomplishing the highestsolar cell performance. Although CBD-CdS is reproducibleand yields good performance, there are drawbacks con-cerning industrial-upscaling from use of the carcinogenthiourea and hazardous cadmium in large amounts. Thespray-ILGAR method yielded an efficiency of 16.1%. Itoffers several advantages: no plugging of a nozzle, smallerdroplet size, a narrower size distribution leading to betterhomogeneity, greater material yield, and easy recovery of
M.A. Mughal et al. / Solar Energy 120 (2015) 131–146 143
the residuals, unlike existing Cd-based CBD technology forbuffer deposition. One significant advantage of usingsolution-based techniques to synthesize In2S3 films was thatthe tetragonal structure is achievable by varying depositionparameters, without the need for annealing. On the otherhand, most vacuum-deposited In2S3 films exhibited amor-phous structures and required post-deposition annealingin order to achieve the same crystalline structure.Moreover, all deposition techniques offered capabilities tocontrol/engineer In2S3’s electrical, optical, thermal, andstructural properties, which helped improve the perfor-mance of solar cells.
In2S3 buffer layers were stable and offered opportunityfor device preparation at high temperatures compared toits counterparts, which could allow formation of tandemjunctions in the future. Solar cells with In2S3 buffer layersexhibited excellent lifetime measurements and long-termstability, further reinforcing In2S3 as a buffer material forCd-free devices. However, with the aim of an industrialimplementation, future studies need to focus on interfaceformation during deposition and post-deposition treatmentin order to avoid defects and interdiffusion. A concertedeffort in optimizing the process parameters would furtherimprove the performance parameters of In2S3-bufferedTFSCs, making possible the widespread adoption of thistechnology.
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Glossary
AGU: Aoyama Gakuin University, Setagaya-ku, Tokyo, JapanAIX: Aixtron (Semiconductor Industry Company), Herzogenrath,
GermanyASC: Angstrom Solar Centre, Uppsala University, Uppsala, SwedenALD: Atomic Layer DepositionALE: Atomic Layer EpitaxyBHT: Beuth University of Applied Sciences, Berlin, GermanyCBD: Chemical Bath DepositionCdTe: Cadmium TellurideCIEMAT: Spanish Research Centre for Energy, Environment, and
Technology, Madrid, SpainCIGS: Copper Indium Gallium SelenideCIS: Copper Indium DisulfideCREST: Center for Renewable Energy Systems and Technology,
Leicestershire, UKCSU: Colorado State University, Fort Collins, CO, USA
146 M.A. Mughal et al. / Solar Energy 120 (2015) 131–146
CSP: Chemical Spray PyrolysisCUSAT: Cochin University of Science and Technology, Kochi, IndiaCVD: Chemical Vapor DepositionCZTS: Copper Zinc Tin SulfurDC-sputtering: Direct Current-SputteringED: ElectrodepositionENSCP: National Chemical Engineering Institute in Paris, France (Ecole
Nationale Superieure De Chimie De Paris)ENSMP: MINES Paris Tech (Ecole Nationale Superieure des Mines de
Paris), FranceETHZ: Swiss Federal Institute of Technology, Zurich, Switzerland
(Eidgenossische Technische Hochschule Zurich)FhG-ISE: Fraunhofer Institute for Solar Energy Systems, Freiburg,
GermanyFTO: Fluorine-doped Tin OxideGmbH: Gesellschaft mit beschrankter HaftungHMI: Hahn-Meitner-Institute, Berlin, GermanyITO: Indium Tin OxideHZB: Helmholtz-Zentrum Berlin for Materials and Energy, Berlin,
GermanyKAIST: Korea Advanced Institute of Science and Technology, Daejeon,
South KoreaILGAR: Ionic Layer Gas Atomic ReactionIPE: Institute for Physical Electronics, Universitat Stuttgart, Stuttgart,
GermanyIRDEP: Institute of Research and Development on Photovoltaic Energy,
Chatou Cedex, FranceIREM: Institute of Research for Energy Materials, Lavoisier Institute,
Versailles Cedex, FranceIU: Iwate University, Morioka, Japan
KIER: Korea Institute of Energy Research, Taejon, South KoreaLTFP: Laboratory for Thin Films and Photovoltaics, Empa, SwitzerlandMOCVD: Metal Organic Chemical Vapor DepositionMOPVE: Metal Organic Physical Vapor EpitaxyMVU: Michigan Virtual University, Lansing, MINREL: National Renewable Energy Laboratory, Golden, CO, USANSC: Nordic Solar Energy, Kista, SwedenPVD: Physical Vapor DepositionSCCM: Standard Cubic Centimeter/MinuteSINGULUS: Technologies AG, Kahl am Main, GermanySFLMTR: Swiss Federal Laboratories for Materials Testing and
Research, Empa, SwitzerlandSSG: Sunshine Solar GroupTFPL: Thin Film Physics Laboratory, Shivaji University, Kolhapur,
IndiaTFSC: Thin Film Solar CellTIT: Tokyo institute of technology, Tokyo, JapanUD: University of Delaware, Newark, DE, USAUO: University of Oldenburg, Oldenburg, GermanyUQ: University of Queensland, Brisbane, AustraliaUR: University of Regensburg, Regensburg, GermanyUB: University of Barcelona, Barcelona, SpainUT: University of Toledo, Toledo, Ohio, USACSU: Colorado State UniversityUSP: Ultrasonic Spray PyrolysisWIS: Weizmann Institute of Science, Rehovot, IsraelWSU: Washington State University, Pullman, WA, USAZSW: Center for Solar Energy and Hydrogen Research, Stuttgart,
Germany (Zentrum fuer Sonnenenergie-und Wasserstoff-Forschung)
