indium sulfide and relatives in the world of photovoltaics

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Solar Energy 83 (2009) 363–371

Indium sulfide and relatives in the world of photovoltaics

N. Barreau*

Institut des Materiaux Jean Rouxel (IMN)-UMR 6502, Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3, France

Received 30 June 2008; received in revised form 25 August 2008; accepted 26 August 2008Available online 17 September 2008

Communicated by: Associate Editor Takhir M Razykov

Abstract

Cu(In,Ga)Se2-based solar cells buffered with indium sulfide grown by numerous techniques have reached efficiencies comparable tothose achieved by standard devices buffered with (CBD)CdS. The present paper firstly recalls some of the properties of the indium sulfidesingle crystal and points out the disagreements concerning the thin films properties inventoried in the literature. Secondly, the influence ofthe presence of some ‘‘foreign elements” within the indium sulfide on its properties is presented. It is shown that these ‘‘foreign elements”,even at low concentration levels, are possibly at the origin of the thin films properties deviations compared to the single crystal. Theimpact of these contaminants on the solar cells performance is finally discussed.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Indium sulfide; Buffer layer; Solar cell

1. Introduction

The substitution of cadmium sulfide buffer layers byalternative materials is among the challenges of theCu(In,Ga)(S,Se)2 thin films based solar cells communitysince the end of 1990s (Siebentritt, 2004; Hariskos et al.,2005). A general conclusion of the investigations duringthe last decade suggests that the most relevant materialsare zinc based (i.e. Zn(O,S) (Ennaoui et al., 2006; Platzer-Bjorkman et al., 2003) and (Zn,Mg)O (Torndahl et al.,2007) and indium sulfide derivatives (Kessler et al., 1993;Gall et al., 2007; Naghavi et al., 2003; Allsop et al., 2005;Strohm et al., 2005; Hariskos et al., 1996). However, if thezinc based material properties required in order to reachoptimal performance with standard Cu(In,Ga)Se2 absorberappear similar whatever the deposition technique used, the

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doi:10.1016/j.solener.2008.08.008

* Tel.: +33 2 51 12 55 26; fax: +33 2 51 12 55 28.E-mail address: [email protected]

situation is not as clear in the case of indium sulfide. Highefficiency cells can indeed be achieved with indium sulfidebuffer layers grown by almost all of the standard depositiontechniques, i.e. physical vapour deposition (PVD), chemicalvapour deposition (CVD) and chemical bath deposition(CBD), although the films grown by each technique havespecific crystalline, electrical and optical properties. It there-fore appears difficult to define the indium sulfide film prop-erties in view of their use as buffer layer. This lack ofknowledge definitely hinders the industrial implementationof indium sulfide buffer layers.

The goal of this paper is to present a state of the art ofindium sulfide buffer layers. However, in order to try toconceptualize the cell operation with such buffers, firstlythe structural, electrical and optical properties of bulkindium sulfide will be recalled and disagreements concern-ing the thin films properties inventoried in the literaturewill be pointed out. Secondly, the influence of isovalentsubstitution and insertion/substitution mechanisms willbe presented. With the help of this information, the perfor-mance of the devices buffered with indium sulfide willfinally be discussed.

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364 N. Barreau / Solar Energy 83 (2009) 363–371

2. The indium sesquisulfide single crystal

The indium sulfide In2S3 can crystallize in one of threestructures, a, b, and c (Rehwald and Harbeke, 1965). Thephases usually obtained are a and b; the phase c beingachieved at high temperature (T > 750 �C) (Kundra andAli, 1976). Both a and b phases can be described as spinel-like structures (Rooymans, 1959; Steigmann et al., 1965),i.e. [Al2]MgO4, where Al occupies all of the octahedral sitesand Mg the tetrahedral sites. However, contrary to the caseof a normal spinel structure, one third of the tetrahedral sitesremains empty, which leads to the quasi-quaternary com-pound formula: [In2]Oh[In2/3h1/3]TdS4 (h is the vacant sitesand Td and Oh represent the tetrahedral and octahedralsites, respectively). The cationic vacancies in tetrahedralsites can either be statistically distributed or ordered alongthe lattice c-axis. In the case of the a phase, the vacancy dis-tribution is statistical and the structure is cubic with the lat-tice parameter ca = 1.0774 nm. For the b phase, due to thevacancies ordering the structure is described as a quadraticsuper cell consisting of three spinel blocks stacked up alongthe c-axis, which leads to the lattice parameter valuecb = 3 � ca = 3.2322 nm; the parameter ab then corre-sponds to ab = aa/

ffiffiffi

2p

= 0.7619 nm (space group I41/amd)(King, 1962). Such a lattice description explains well thereason why the b-In2S3 is commonly formulated[In6]Oh[In2h]TdS12. A representation of the lattice of b-In2S3 is shown in Fig. 1.

The optical bandgap of b-In2S3 single crystals isreported to be direct with a value of 2.0 eV at room temper-ature (Rehwald and Harbeke, 1965; Kambas et al., 1985;Garlick et al., 1963). The electrical properties of this b

Fig. 1. Representation of the b-In2S3 crystalline lattice.

phase have been accurately studied by Rehwald andHarbeke (1965). They have observed it to have an n-typeelectrical conductivity, however, the conductivity stronglydepends on the sulphur concentration; a lack of sulphurcompared to the In2S3 stoichiometry increases the electrondensity, i.e. increases the n-type character.

3. Thin film indium sulfide

Indium sulfide (In2S3) thin films have been successfullysynthesized by numerous techniques based on thermalevaporation (Barreau et al., 2001; El Shazly et al., 1998;George et al., 1988; Guillen et al., 2004; Yoosuf andJayaraj, 2005; Herrero and Ortega, 1988), RF sputtering(Hariskos et al., 2004), atomic layer deposition (ALD)(Naghavi et al., 2004; Asikainen et al., 1994), metal organicchemical vapour deposition (MOCVD) (O’Brien et al.,1998; Bessergenev et al., 1996), spray ions layer gas reac-tion (ILGAR) (Allsop et al., 2006), spray pyrolysis (Kimand Kim, 1986; Ernits et al., 2007), spin coating (Yasakiet al., 1999) and chemical bath deposition (CBD) (Kessleret al., 1993; Bayon et al., 1999; Kaufmann et al., 2000; Yos-hida et al., 1997; Yamaguchi et al., 2003; Lokhande et al.,1999; Hariskos et al., 1994).

The crystalline properties of the films depend stronglyon their growth technique. The grain size varies from ‘‘aslarge as the film thickness” in the case of evaporation(Barreau et al., 2000) to a few nanometers when the filmsare grown by CBD (Bayon et al., 1999; Kaufmann et al.,2000; Yoshida et al., 1997). It must be noted that all ofthe techniques leading to well crystallized films do notrequire substrate/process temperatures higher than 150–200 �C. A typical X-ray diffraction pattern of a co-evapo-rated film on a Mo-coated glass substrate at 200 �C isshown in Fig. 2. When the grains show preferential orien-tation, the family of b phase (10 3) planes is usually parallelto the substrate (Bessergenev et al., 1996).

The composition of the films also depends on the growthprocess. A general observation is that when the deposition

Fig. 2. X-ray diffraction pattern of a co-evaporated In2S3 film on amolybdenum-coated glass substrate.

N. Barreau / Solar Energy 83 (2009) 363–371 365

technique requires the use of chemical precursors, it is verycommon to find residual precursor elements within thefilms. Typically, chlorine is frequently detected in the filmsgrown by ALD (with InCl3 precursor) (Asikainen et al.,1994), ILGAR (Allsop et al., 2006) and spray pyrolysis(Bouguila et al., 1997). In the case of CBD, Bayon andHerrero (2000) as well as Hariskos et al. (1996) concludedthat the layers are of In(S,OH) compound, the S/OH ratiodecreasing as the bath pH is increased (Bayon and Herrero,2001). In contrast to these Refs., Yoshida et al. (1997)claim that their CBD films are pure sulfides, i.e. exemptof hydroxide, although the pH is higher than in Ref.(Bayon and Herrero, 2001). The techniques leading to theleast contaminated films appear to be PVD processes suchas evaporation and RF sputtering.

The optical properties of the indium sulfide filmsstrongly vary between the various studies. The bandgapvalues reported in the literature extend from 2.0 eV up to3.7 eV. Fig. 3 depicts the bandgap measured on filmsgrown by different techniques. The bandgap values of thePVD grown films, between 2.0 eV and 2.3 eV, are the clos-est to the single crystal. It should be noted that these band-gaps were determined assuming a direct allowed transition.In the case of the ALD process, the bandgap nature itself isnot consensual. Contrary to Asikainen et al. (1994) andNaghavi et al. (2004), an indirect bandgap was assumedby Sterner et al. (2005) as well as Allsop et al. (2006) forthe ILGAR process. Such different assumptions may con-tribute to the large differences between reported values,i.e. from 2.10 eV (Sterner et al., 2005) to 2.85 eV (Naghaviet al., 2004). Although the films were grown from similarprecursors, Naghavi et al. (2004) observed a bandgap wid-ening when increasing the deposition substrate temperaturewhereas, Sterner et al. (2005) did not note such an effect.The most extended range of reported bandgap values is

Fig. 3. Plot of the indium sulfide films bandgap values reported in theliterature for different growth techniques (1: Hariskos et al. (2004); 2:Herrero et al. (1988); 3: George et al. (1988); 4: Barreau et al. (2002); 5:Sterner et al. (2005); 6: Asikainen et al. (1994); 7: Naghavi et al. (2004); 8:Allsop et al. (2006); 9: Kim and Kim (1986); 10: Yasaki et al. (1999); 11:Kaufmann et al. (2000); 12: Bayon and co-workers (2000); 13: Yoshidaet al. (1997); 14: Lokhande et al. (1999)).

found for the CBD grown layers (see Fig. 3). The strongimpact of the bath composition and temperature on thecomposition and crystallinity necessary also impacts theoptical properties. Yoshida et al. (1997) observed a sulphurover-stoichiometry and suggest that, similar to the case ofsprayed films (Kim and Kim, 1986), this increases thegap. On the other hand, (Bayon and co-workers (2000))observed that the bandgap widens as the pH increases,which is when the S/OH decreases. Nevertheless, if indeedboth Yoshida et al. (1997) and (Bayon and co-workers(2000)) suggest that the bandgap widening could be partlyimputable to composition effects, they also agree that thesmall grain size (<10 nm) generated by the CBD processmay induce quantization effects. Yasaki et al. (1999) havesimilar conclusions, they observed that the bandgap of spincoated films widens as the grain size decreases. In contrastto these results, Kaufmann et al. (2000) reported a 2.2 eVbandgap although the crystallites are not larger than 2–4 nm and the films contained hydroxides.

Despite such optical behaviours, all of the films arereported to be n-type. Nevertheless, a wide range of electri-cal conductivity is encountered; literature relates valuesfrom 10�1 (Bessergenev et al., 1996) to 107 X cm (Asikainenet al., 1994) depending on the deposition process but alsoon the history of the sample as in the case of the singlecrystal.

This presentation of the thin films that can genericallybe called indium sulfide shows that their properties canbe very different to those of the indium sulfide crystal.However, all of the indium sulfide thin films, with largeenough grain size (i.e. regardless quantization effects)should present characteristics close to those of the singlecrystal. Up until now, a remaining question is the originof such deviations from the expected material properties.The following section is devoted to the potential causesfor the origin of these deviations.

4. Indium sulfide derivatives

Recent investigations dealing with the impact of theintroduction of foreign atoms within the indium sulfidehave initiated the know-how concerning the true status ofindium sulfide. Herein, the influence of partial substitutionby an isovalent chalcogen is presented as a first example.Secondly, the properties of compounds formed throughinsertion/substitution mechanisms are presented.

Within the indium sulfide crystalline matrix, indiumand/or sulphur atoms can be substituted by other elements,referenced to as third elements in this paper. The presenceof third element can lead to drastic changes in film proper-ties. In order to illustrate this, the example of In2S3�3xO3x

compounds is here considered. By means of a two stepsprocess, i.e. annealing at 200 �C of indium and sulphurevaporated at low substrate temperature, it has been shownthat partial substitution of sulphur by oxygen within theindium sulfide matrix can be obtained (Barreau et al.,2002). The In2S3�3xO3x films thus grown remain single


phase up to x = 0.15 and the lattice parameters of the spi-nel-like structure decreases as x increases (Barreau et al.,2002). Values of x larger than 0.15 result in multi-phasefilms; peaks assigned to both indium oxide (hexagonal lat-tice) and indium sulfide can be observed on the X-ray dif-fraction diagrams. The optical properties of single phasefilms have been observed to strongly depend on x; theFig. 4a shows that the bandgap, considered to be direct,linearly increases from 2.1 eV to 2.9 eV when x varies from0 to 0.14 (Barreau et al., 2002). One may note thatalthough the grain size of the films decreases with increas-ing x, it is observed not to be small enough in order to leadto a quantization effect. One should be aware that even a S/In ratio of 1.4 in the films can induce an absolute bandgapwidening of 0.3–0.4 eV (see Fig. 4a). The electronic bandstructure of In2S3�3xO3x compounds has been calculated(Robles et al., 2005) and the results suggest that the band-gap widens under lattice compression. Moreover, accord-ing to X-ray photoelectron spectroscopy measurements(Barreau et al., 2003), the substitution of sulphur by oxy-

Fig. 4. (a) In2S3-3xO3x thin film bandgap value versus x and (b)[In16]Oh[In5.33�x/3Mxh2.67�2x/3]TdS32 (M = Na,Cu) thin film bandgapvalue versus x.

Fig. 5. Representation of the band structure evolution with x of (a)In2S3�3xO3x and (b) (Cu1�xNax)In5S8.

gen mainly affects the conduction band minimum position,i.e. the electron affinity decreases as x increases. The bandevolution expected from these results is illustrated in theFig. 5a.

In addition to the above described anionic substitution(O for S), another possibility for third element inclusioninto the indium sulfide matrix is occupation of cationicvacancy illustrated in the previously justified formula[In6]Oh[In2h]TdS12. It has been observed that in the pres-ence of an excess of sulphur compared to the In2S3 stoichi-ometry, sodium can be introduced within the indiumsulfide matrix (Barreau et al., 2002). Transmission electronmicroscopy studies lead to the conclusion that the structurethus achieved could be described as

½In16�Oh½In5:23�x=3Nax�2:67�2x=3�TdS32

ðx � 4;� is the vacanciesÞ ð1Þ

which assumes that two thirds of the introduced sodiumatoms effectively fill cationic vacancies, the other thirdbeing substituted to indium in order to keep electron neu-trality. This compound formation therefore results of aninsertion/substitution mechanism that is

½In3þ�Td þ 2� ½��Td ! 3½Naþ�Td: ð2Þ

The above formulation (Eq. (1)) clearly shows that theindium to sulphur ratio is raised from 1.5 to 1.6 when thesodium addition reaches the upper value x = 4. Also

Fig. 6. (a) Absorption spectra of (Cu1�xNax)In5S8 powders when x = 0,0.25, 0.5, 0.75 and 1 and (b) XPS valence band spectra of (Cu1�xNax)In5S8

powders when x = 0, 0.5 and 1.


according to this formulation, the maximum sodium con-tent (x = 4) corresponds to NaIn5S8 compound (at. Na/In = 0.2). Powder of such a composition has been achievedat long term (7 days) and high temperature (700 �C) reac-tion conditions, whereas under thin film form, the maxi-mum amount of sodium that could be introduced (at200 �C, t < 1 h) globally corresponds to a sodium toindium ratio of 0.12 (Barreau et al., 2002). When highersodium amounts are provided during the synthesis, theexcess migrates to the film surface and forms sodium car-bonates when air exposed. No NaInS2 phases could beobserved in the thin films, as they were for the powders.Such insertion/substitution mechanisms have also beenobserved in the case of copper addition, leading to the com-pounds with a [In16]Oh[In5.33�x/3Cuxh2.67�2x/3]TdS32 formu-lation (x 6 4) Barreau et al., 2004. The lattice parametersdecrease in case of the sodium based compounds, whilethey increase when the third atom is copper. In contrastto the case of sodium, the limit composition CuIn5S8 (i.e.x = 4) has been achieved both in thin film (at 200 �C)and powder forms, moreover, a copper excess comparedto the CuIn5S8 stoichiometry leads to the formation ofCuInS2 even for thin films grown at 200 �C. Such an obser-vation suggests that the copper based phase is more favour-ably formed than the sodium based one.

The optical properties of these compounds are influ-enced by the nature and the amount of the introduced thirdelement (i.e. sodium or copper) (Barreau et al., 2002;2004). The Fig. 4b plots the evolution of the bandgap(assumed to be direct) versus x in both cases. It clearlyappears that the bandgap for the sodium containing thinfilms linearly increases with increasing x, whereas it linearlydecreases in the case of the copper containing compound.Such an optical behaviour constitutes another illustrationthat the optical properties of indium sulfide thin films canbe changed by the introduction of a third atom withinthe indium sulfide structure.

In order to further investigate these materials properties,powders with (Cu1�xNax)In5S8 composition have beensynthesized (Barreau et al., 2006). Accurate XRD studieshave confirmed that the crystalline structure of the sam-ples, whatever x, is the same; it derivates from the defectspinel In2S3 in which all of the vacancies are filled by Cuand/or Na and indium is partly substituted according tothe mechanism described in Eq. (2). Although the bandgapof these powders was not accurately determined from opti-cal measurements, it clearly appears to increase as x

increases (see Fig. 6a). Restricting to the extreme composi-tions x = 0 (Na-free) and x = 1 (Cu-free), one can observethat the bandgap value of CuIn5S8 phase measured fromthe powder is very close to that deduced from the thinfilms; 1.5 eV for the powder against 1.6 eV for the thin film.On the other hand, although the gaps measured fromsodium containing thin films (with x < 4) were wider thanthat of the powder (x = 4), both investigations consensu-ally concluded that the introduction of sodium provideswider bandgap materials. Regarding the whole x range,

the evolution appears almost linear as long as the samplescontain copper; a clear onset is however observed for thecopper free compound gap. In order to further explore thisissue, (Cu1�xNax)In5S8 powders with composition corre-sponding to x = 0, 0.5 and 1 have been investigated withthe help of XPS (without air exposure) (Lafond et al.,2007). The valence band spectra acquired from these sam-ples (see Fig. 6b) has allowed the evaluation of the energydifference between the Fermi-level and the valence bandmaximum for each sample. Combining these values withthe bandgap, it has been derived the band structure evolu-tion with x (see Fig. 5b). According to this representation,as long as samples contain copper (i.e. x < 1) the bandgapbroadening with increasing x appears to mainly affect thevalence band maximum energy position. On the otherhand, for the pure sodium containing material (i.e. x = 1)both bands are shifted. Such behaviour suggests that theorigins of the copper and the sodium impacts on the elec-tronic band structure are different. In a first approach,the electronegativity of copper is slightly higher than thatof indium, which means that the impact of the introductionof copper within the indium sulfide matrix is mainly due to

Fig. 7. Plot of the best efficiencies reached by CIGSe-based solar cellsbuffered with In2S3 deposited by different techniques and efficienciesachieved by reference cells buffered with standard (CBD)CdS (1: Hariskoset al. (1996); 2: Naghavi et al. (2003); 3: Allsop et al. (2005); Strohm et al.(2005); 5: Gall et al. (2007); 6: Harisko et al. (2004)).

Fig. 8. SIMS profiles of sodium (left) and copper (right) throughoutCIGSe/(ALD)In2S3 structures This figure is extracted from Spiering et al.,2004.


the presence of the copper orbitals and thus may explainthat only the valence band maximum is affected. On theother hand, the electronegativity of sodium is much lowerthan that of indium, which increases the ionic characterof the bonds and therefore affects both the valence andconduction bands. The precise determination of the(Cu1�xNax)In5S8 crystalline structure has also allowedthe calculation of their electronic structure (Lafond et al.,2007). The first observation is that the theoretical resultscorroborate the band structure evolution expected fromthe experimental bandgap measurements, i.e. the bandgapincreases with increasing x. Secondly, the bandgap of thematerials containing copper (x < 1) is indirect, whereas itis direct when only sodium is contained (x = 1, NaIn5S8).This latter information demonstrates that even the natureof the so-called indium sulfide is influenced by the thirdatom introduced.

The presence of third elements also impacts the electricalproperties of the films. Unfortunately, accurate investiga-tions have not been reported so far, however, what couldbe deduced from our first measurements is that all of thesecompounds remain n-type. These investigations also sug-gest that the electrical conductivity of the films increasesby one or two decades in the case of sodium containingor oxygen containing compounds, respectively. On theother hand, the comparison of the electrical conductivityof In2S3 and CuIn5S8 films show the material containingcopper to be ten thousand times more resistive, 106 against102 X cm.

All of these studies show that for In2S3, the indium andsulphur elements can be substituted by isovalent atoms, thecrystalline matrix remaining unchanged at least for lowconcentrations. Moreover, the specific lacunar crystallinespinel-like structure of the indium sulfide makes heterova-lent element insertion/substitution mechanisms possible. Ithas been shown that the presence of these third elements,even in very small amounts, possibly at the detection limitof routinely used analyses techniques, may induce impor-tant changes in the films properties.

5. The indium sulfide buffer layer

In the present paper, the attention will only be focussedon devices based on Cu(In1�xGax)Se2 (0.2 < x < 0.4)absorbers (CIGSe). The resulting solar cells, buffered withindium sulfide, have demonstrated high conversion efficien-cies. The Fig. 7 summarizes the highest efficiencies reportedin the literature as well as the performance of associatedreference cells buffered with (CBD)CdS. As shown in thisfigure, indium sulfide buffer layers grown by numeroustechniques can lead to high efficiency cells, while each tech-nique confers specific films properties.

Common particularities can nevertheless be extractedfrom all of the efficient processes. With the exception ofthe CBD process, a general trend is that the best device per-formance is reached when the substrate temperature duringthe buffer layer deposition is about 200 �C; note that for the

ILGAR process, the reported 250 �C degrees correspond tothe sample holder temperature and the upper surface of thesubstrate is likely to be cooler due to the effect of the carriergas (Allsop et al., 2006). For lower substrate temperatures,the lower cell efficiencies then achieved can be improved byair annealing at 200 �C for long periods (30–60 min) com-pared to standard (CBD)CdS-based cells annealing. Onthe other hand, for higher substrate temperatures, typicallyabove 230 �C, the devices are definitely hindered andannealing has no beneficial effect. Such a general trend ofthe impact of provided energy during the buffer depositionon the cells is commonly interpreted as an interface forma-tion influence. The CIGSe/indium sulfide interface hastherefore been investigated for most of the high efficiencydevices. It has been observed that in the case of In2S3 bufferlayers grown by ALD (Spiering et al., 2004), ILGAR (Baret al., 2007), (co-)evaporation (Barreau et al., 2003) and RFsputtering (Hariskos et al., 2004), copper atoms diffuse


from the CIGSe into the In2S3 leading to a copper depletedabsorber surface (see Fig. 8). On the other hand, no obvi-ous diffusion of gallium or sulphur/selenium interdiffusionhas been observed, the extinction of the gallium signal istherefore used to position the buffer/absorber interface indepth profiles. Sodium has also been detected within theindium sulfide buffer layers, however, such diffusion hasbeen observed to be competitive with the copper diffusion(Barreau et al., 2004), i.e. the presence of high sodiumamounts at the absorber surface prior the In2S3 growthminimizes copper diffusion. The copper diffusion is more-over mainly dependent of the buffer layer growth tempera-ture, the higher the deposition temperature, the higher isthe diffusion. Although no direct proof has been provided,it is probable that the out diffusion of copper fromthe CIGSe leads to the formation of a copper-poorCu(In1�xGax)3Se5 phase at the CIGSe surface. Consideringthese observations, one may assume that the compoundformed within the buffer layer close to the interface belongsto the [In16]Oh[In5.33�x/3Axh2.67�2x/3]TdS32 (A: Cu,Na)family. Abou-Ras et al., 2005a, b have further investigatedthe CIGSe/(ALD, evaporated)In2S3 interface by means ofHR-TEM. They have particularly studied absorber/bufferlayer structures formed at different temperatures and theeffect of device post-fabrication annealing. They havepointed out that when the indium sulfide is grown on theCIGSe at temperatures around 200 �C, there exists a clearorientation relationship between the (112) planes of theCIGSe and the (103) planes of the buffer layer, which isrequired for a defectless interface. For higher depositiontemperature, they observed the existence of a third phaseat the CIGSe/In2S3 interface which they identified asCuIn5S8 (Abou-Ras et al., 2005b). This latter result per-fectly agrees with the assumption that a strong copper dif-fusion, enhanced by high deposition temperature, shouldlead at some point to the limit copper concentration(x = 4) corresponding to the CuIn5S8 phase.

6. Discussion

Many In2S3 deposition techniques, leading to very dif-ferent material properties, allow the achievement of highefficiency cells. It thus seems that the indium sulfide prop-erties are, more than in any other CIGSe based device, lessimportant than the interface it forms with the CIGSe. Theremaining question is which interface is expected to givethe higher performance. In the following one vision ofthe system is presented, however all of the others are notto be disregarded.

The ideal absorber/buffer layer interface should avoidcarrier recombination. With this aim, at least two condi-tions are recommended (Klenk, 2001): (i) the Fermi-levelis close to the conduction band at the interface (invertedsurface), which can be achieved by a highly doped bufferlayer or appropriated interface charges; (ii) there is norecombination barrier reduction, meaning there is no inter-face cliff.

The best cells are achieved in the conditions providingthe highest interface crystalline quality, i.e. good latticematching and orientation relationship. Nevertheless, suchconditions are not necessary in order to reach high effi-ciency since they are not satisfied in the case of buffersgrown from compound evaporation at low substrate tem-peratures (14.8% efficiency) (Strohm et al., 2005). Further-more, Abou-Ras et al. (2005b) have observed that for highsubstrate deposition temperature (low cell performance),most probably inducing interface CuIn5S8 phase forma-tion, there exists such an orientation relationship betweenthe CIGSe and the CuIn5S8. The high crystalline interfacequality is therefore not the only important parameteralthough one can easily believe that it minimizes carrierrecombination.

The fact that interfacial CuIn5S8 phase formation harmsthe device performance can be a starting point for the dis-cussion. Although the properties of this material are stillunder investigation, recent qualitative results are helpful.From the optical point of view, its indirect bandgap valueis reported to be around 1.5 eV, which is still wider than theabsorber layer, but narrower than the isostructural indiumsulfide. Electronic structure calculations show that thebandgap narrowing from one compound to the other ismainly due do to the presence of the Cu3d orbitals at thetop of the valence band, the lower conduction band statesremaining unchanged. One can thus deduce that the elec-tron affinity of the [In16]Oh[In5.33�x/3Cuxh2.67�2x/3]TdS32

material remains similar whatever x in the range [0;4].The In2S3 electron affinity has been estimated, with the helpof indirect XPS methods, to be 4.7 eV (Barreau et al.,2003). This evaluation is corroborated by the resultsreported by Schulmeyer et al. (2004), who concluded thatthe conduction band offset at the evaporated CuGaSe2/In2S3 interface is 0.56 eV. Neglecting interface dipole effectsand assuming the CuGaSe2 electron affinity equals 4.1 eV,the indium sulfide or the compound containing copper isalso about 4.7 eV. Consequently, regarding the band align-ment at the CIGSe/In2S3 interface, the conduction bandcliff of 0.4 eV can explain the poor performance of cellsbased on CIGSe/CuIn5S8 junction. However, such a con-clusion appears in perfect contradiction with the high effi-ciency cells realized with differently grown indium sulfidebuffer layers. Nevertheless, according to the results exposedin Sections 3 and 4, one can believe that during the bufferlayer deposition, indium sulfide contamination inherentto the process may occur and drastically change its proper-ties. Basically, at least two In2S3 property evolutions canhelp to interpret the high cells performance: (i) the interfaceconduction band cliff reduction, which can be realized byall of the elements inducing electron affinity decrease; (ii)the increase of the electron density (i.e. improve n-typecharacter) which according to Klenk (2001) should mini-mize the negative effect of the conduction band cliff.

In Section 4, it has been recalled that the substitution ofsulphur by oxygen within indium sulfide results in thedecrease of the materials electron affinity. Assuming that


such a phenomenon also occurs when copper is addedwithin the crystalline matrix, oxygen thus appears to be agood candidate to decrease the interface conduction bandcliff. It has recently been observed that the presence of oxy-gen within the buffer layer improves the cells Voc (Gallet al., 2007). A similar effect can not be excluded in the caseof ALD grown buffer layers since the indium precursorcontains oxygen; Spiering et al. (2004) actually did detectthe presence of oxygen within the ALD grown buffer layer(see Fig. 7) . The introduction of sodium can also decreasethe electron affinity, but this case is more complicated sinceit also accumulates at the CIGSe sub-surface proportion-ally to the out diffusing copper from the CIGSe.

Regarding the effect of third atoms on the electrical con-ductivity, as mentioned above, the introduction of oxygenand sodium may improve the n-type character but onlyslightly compared to the opposite effect of the copper intro-duction. It can thus appear improbable that oxygen and/orsodium can compensate the n-type loss due to the copperdiffusion, and even less probable that it can minimize theeffects of the conduction band cliff.

Other films particularity may have positive effects. Forinstance the effect of sulphur under-stoichiometry com-pared to the In2S3 seems to be associated to higher cellsperformance. Such a conclusion has been drawn in the caseof both sputtered (Hariskos et al., 2004) and evaporated(Strohm et al., 2005) thin films although the hinderedparameters are not consensual. The lack of sulphur may(i) as in the case of the single crystal improve the n-typecharacter and/or (ii) enhance the oxygen introduction,especially after air annealing. Both of these phenomenaare expected to have positive effects on the cells perfor-mance. The most unknown influence so far remains theeventual effect of the presence of chlorine within the films.

From the optical point of view, most of the high efficiencycells buffered with indium sulfide show a current gain in theshort wavelength range compared to the reference cells (i.e.with (CBD)CdS buffer) (Naghavi et al., 2003; Allsop et al.,2005; Hariskos et al., 1996; Kaufmann et al., 2000). Thisimprovement is attributed either to a wider bandgap bufferlayer (Naghavi et al., 2003; Hariskos et al., 1996) or to thefact that its bandgap is indirect (Allsop et al., 2005) andhence induces less photon absorption. Both of these justifica-tions appear receivable. However, this current gain is fre-quently annihilated by the lower carrier collection in thewider wavelength region (Kaufmann et al., 2000; Naghaviet al., 2003; Hariskos et al., 1996). Such a loss could beimputable to a too lightly doped buffer layer, which may alsopartly explain the frequently lower Voc reached by the cellsbuffered with indium sulfide compared to those buffered withthe standard (CBD)CdS.

7. Conclusions

The characteristics of indium sulfide single crystal havebeen well described and are similar from one report toanother. Thin films of this material have been synthesized

by a broad panel of techniques, leading as for many othermaterials to specific properties. However, in the case ofindium sulfide, these characteristics can be extremely differ-ent to those of the single crystal. Such specificities are mostcommonly attributed to the thin film form itself and not toeventual bulk material properties. The properties of theindium sulfide are strongly dependent on the eventual pres-ence of third atoms in its crystalline matrix, meaning thateven a small amount of contaminant can induce broadchanges in the optical and electrical properties of thesefilms. These considerations are very important regardingthe following issues. Fundamentally, one can believe thatthe indium sulfide, due to both its intrinsic properties andto the interface it forms with the CIGSe absorber, is notwell adapted as buffer for �1.2 eV CIGSe absorber-baseddevices. Nevertheless, the experimental device results havedemonstrated the opposite conclusion. Therefore, eitherall of the assumptions concerning the In2S3 and CIGSe/In2S3 interface are incorrect, or the efficient indium sulfidebuffers are not pure In2S3. On such a basis, the correlationof the solar cells performance with the material purity, thusdeviations of the properties from those of pure In2S3 singlecrystals, clearly shows that the lower the process inducedIn2S3 contamination/under-stoichiometry, the lower thecells performance. One may therefore believe that high per-formance cells require the introduction of an adequatethird atom. However, with the aim of an industrial imple-mentation, the adapted third atom should definitely not bea process noise but a perfectly controlled process parame-ter. This latter condition imposes to progress in the under-standing of the impact of impurities in the indium sulfidesystem.

Acknowledgements

The author thanks John Kessler for its fruitful help andacknowledges all of the members of the University of Nan-tes involved in the investigations dealing with the indiumsulfide derivatives.

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