Cigs

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Solar Energy 85 (2011) 2666–2678

Progress in electrodeposited absorber layer for CuIn(1�x)GaxSe2

(CIGS) solar cells

Viswanathan S. Saji, Ik-Ho Choi, Chi-Woo Lee ⇑

Department of Advanced Materials Chemistry, Korea University, Jochiwon, Choongnam 339-700, Republic of Korea

Received 1 July 2011; received in revised form 26 July 2011; accepted 5 August 2011Available online 7 September 2011

Communicated by: Associate Editor Takhir Razykov

Abstract

Thin film solar cells with chalcopyrite CuInSe2/Cu(InGa)Se2 (CIS/CIGS) absorber layers have attracted significant research interestas an important light-to-electricity converter with widespread commercialization prospects. When compared to the ternary CIS, the qua-ternary CIGS has more desirable optical band gap and has been found to be the most efficient among all the CIS-based derivatives. Amidvarious fabrication methods available for the absorber layer, electrodeposition may be the most effective alternative to the expensive vac-uum based techniques. This paper reviewed the developments in the area of electrodeposition for the fabrication of the CIGS absorberlayer. The difficulties in incorporating the optimum amount of Ga in the film and the likely mechanism behind the deposition were high-lighted. The role of deposition parameters was discussed along with the phase and microstructure variation of an as-electrodepositedCIGS layer from a typical acid bath. Related novel strategies such as individual In, Ga and their binary alloy deposition for applicationsin CIGS solar cells were briefed.� 2011 Elsevier Ltd. All rights reserved.

Keywords: Electrodeposition; Absorber layer; Cu(In(1�x)Gax)Se2 (CIGS) solar cell

1. Introduction

1.1. CIGS thin film solar cells

Direct conversion of sunlight into electricity (photovol-taics) may be the most powerful and potential alternativeto fossil fuels. In a photovoltaic solar cell, absorbed lightexcites dormant electrons in an inorganic semiconductor(absorber) and the energized electrons are subsequentlycaptured in a typical pn junction (Chopra et al., 2004;Kemell et al., 2005). Despite the fact that significantfraction of the total solar cell market for use in terrestrialapplications is constituted by Si wafer solar cells (Green,2009), the market for thin film solar cells is booming andis expected to grow significantly in the coming years

0038-092X/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2011.08.003

⇑ Corresponding author. Tel.: +82 41 860 1333; fax: +82 41 867 6823.E-mail address: [email protected] (C.-W. Lee).

(Razykov et al., 2011). Thin film solar cells have the keyadvantage of dimensionality and offer a wide variety ofchoices in terms of device design and fabrication. Theymake use of semiconductors with direct band gap.

Among the various thin film absorbers, the main lowcost materials of current interest are amorphous Si, CdTeand CuInSe2 and its alloys with Ga and/or S. Thoughamorphous Si:H solar cells have many advantages suchas non-toxicity, abundant resources and low temperature/low cost processing, the cell efficiency is comparativelylow (�10%). They suffer from light induced degradationleading to long term stability issues (Chopra et al., 2004).CdTe and CIS-based cells are newer additions to the thinfilm family. The maximum reported efficiency of CdTe cellis �17% (Morales-Acevedo, 2006). However, the extremetoxicity of Cd and the environmental regulations makethem less attractive. Chalcopyrite CuInSe2/Cu(In(1�x)/Gax)Se2 (CIS/CIGS) cells are conceivably the most

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Fig. 2. Band gap energy vs. lattice parameter of various semiconductors(Jaffe and Zunger, 1984).

V.S. Saji et al. / Solar Energy 85 (2011) 2666–2678 2667

promising material in this category with a maximumreported laboratory scale cell efficiency of �20% (Repinset al., 2008; Jager-Waldau, 2011). Fig. 1 depicts the chalco-pyrite crystal structure. They have very high absorptioncoefficient (�1 � 105/cm) (Horig et al., 1978; Tuttle et al.,1988; Alonso et al., 2002; Han et al., 2004). Efficienciesabove 17% were reported on flexible metal substrates(Kessler and Rudmann, 2004; Khelifi et al., 2010). The effi-ciency achieved through flexible polymer substrates wasless than 15% (Niki et al., 2010). The highest module effi-ciency reported is �13% (Karg, 2001; Kushiya, 2003).CIS-based cells have demonstrated the excellent long termstability with high radiation resistance and impurity toler-ance (Guillemoles et al., 2000), which was rationalized interms of the large non-stoichiometric range (Guillemoles,2002). One key issue of concern is the availability of lesscommon elements. Efforts are currently underway toenhance the efficiency to the theoretical maximum(�30%) as well as to develop cheaper deposition strategiesfor the absorber layer (Bhattacharya and Rajeshwar, 1986;Bhattacharya et al., 1998a; Ludwig et al., 2010).

The band gap of CIS is relatively low (1.04 eV); slightlybelow the range of optimum conversion efficiency of solarcells (1.4–1.5 eV) (Loferski, 1956). The band gap can besuitably adjusted to a desired value by alloying with Ga,Al and/or S. The ternary compound CuInS2 has a directband gap of 1.53 eV. However relatively rapid diffusionof metals and impurity species occur in this phase even atlow temperatures (Siebentritt, 2002). Ga substitution forIn is probably the best option for increasing the bandgap (in-between 1.04 eV of CuInSe2 and 1.68 eV of CuG-aSe2) to a desired value and to achieve an enhanced currentconversion efficiency (Fig. 2) (Ramanathan et al., 2003;Kodigala, 2010). Among the different CIS-based quater-nary compounds investigated (Cu(InGa)Se2, CuIn(SeS)2,Cu(InAl)Se2, Cu(InGa)S2, etc.), CIGS has been found to

Fig. 1. Chalcopyrite crystal structure (a = b = 5.78 A, c = 11.62 A).

be the most efficient one till date (Kodigala, 2010; Jager-Waldau, 2011).

Theoretically, to achieve the optimal band gap andhigher cell efficiency, addition of �60–70% Ga is needed(Birkmire and Eser, 1997; Wei et al., 1998). However, whenthe Ga content was increased to this level, the cell efficiencywas inferior (Shafarman et al., 1996). The phenomenonwas explained in terms of the film inhomogeneity, leadingto band gap fluctuations (Contreras et al., 1996; Werneret al., 2005; Gutay and Bauer, 2007). Photoluminescencemeasurements indicated that a higher Ga content in thefilm led to larger inhomogeneities (Gutay and Bauer,2007). Ludwig et al. (2010) conducted the computer simu-lation studies on the spatial distribution of In and Ga andshowed that In-rich CIGS exhibited a higher homogeneitythan Ga-rich CIGS. Some groups reported that Ga prefer-entially enriched in the Mo back contact side (Repins et al.,2008). Khelifi et al. (2010) suggested that Ga enrichmenthappened at both edges of the absorber. The spatial varia-tion of the Ga content within the CIGS layer could be uti-lized to optimize the CIGS absorber band-gap profile andhence to improve the cell efficiency (Morales-Acevedo,2010).

Mo sputtered on a silica-glass substrate is usually theback contact in CIGS configuration (Fig. 3). The preferred

Window layer

Buffer layer

Absorber layer

Back contact

SubstrateSoda-lime glass

CIGS (1-2 µm)

Mo (0.5-1 µm)

CdS (30-50 nm)

ZnO (150 nm)

Fig. 3. Schematic layer sequence of a standard CIGS solar cell.

2668 V.S. Saji et al. / Solar Energy 85 (2011) 2666–2678

window is ZnO deposited by means of sputtering or chem-ical vapor deposition. A thin CdS acts as the buffer layerwhich protects the absorber layer against the chemicalreactions and the mechanical damage. The CdS buffer layerhelps in widening the depletion layers at the interfaceallowing a higher open circuit voltage (Voc). It also helpsin optimizing the band alignment of the device (Schockand Noufi, 2000; Bhattacharya and Ramanathan, 2004).The device structure of the maximum efficiency cellreported consisted of soda-lime glass as substrate, sput-tered Mo back contact, three stage co-evaporated CIGSlayer, chemical bath deposited CdS, sputtered resistive/conductive ZnO bi-layer, e-beam-evaporated Ni/Al grids,antireflective MgF2 coating, and photolithographic deviceisolation (Repins et al., 2008).

The efficiency of a CIGS device can be limited by opti-cal, collection and recombination losses (Siebentritt,2011). In spite of having very high short current density(Jsc), the main reason for the low CIGS cell efficiency inmany of the reported works is associated with the low fillfactor (FF) values. According to the reporters of the max-imum efficiency cell, the enhancement in efficiency wasmainly due to a small increment in FF, which was attrib-uted to the decreased recombination (Repins et al., 2008).Tracu and Rau discovered that the electron recombinationin CIGS varied with the film stoichiometry (Turcu andRau, 2003). The deposition of CdS or other films on CIGSwas found to reduce the surface recombination (Keyeset al., 2003; Repins et al., 2006). Shirakata et al. (2009)has recently shown that no remarkable change in the car-rier-recombination dynamics occurred due to the deposi-tion of CdS on CIGS, implying that the buffer layer didnot affect the recombination kinetics significantly. Time-resolved photoluminescence measurements indicated thatfor a high quality CIGS film, the surface and grain bound-ary recombination were relatively low (less than �103 cm/s)(Metzger et al., 2009). By ion milling experiments, Minem-oto et al. (2011) showed that there was no significant differ-ence in the number of effective recombination centersbetween the grain boundaries and the crystal grains.Serhan et al. (2011) addressed the defect properties as afunction of Ga content in the film and suggested that theheterointerface recombination mechanisms were predomi-nant in CIGS. Works were done to understand the electrontransport and the recombination in the absorber layer andthe CIGS/CdS interface (Contreras and Noufi, 1997;Chaure et al., 2005; Bayhan and Kavasoglu, 2006).

The techniques for the deposition of the chalcopyritelayer can be either vacuum or solution based methods(Savadogo, 1998; Kaelin et al., 2004; Delahoy et al.,2004; Venkatachalam et al., 2009). The most widelyemployed and successful vacuum based approach is ‘co-evaporation’, in which all the four constituent elementswere simultaneously delivered to the substrate and the filmis formed in a single step. The deposition can be performedby sequential evaporation. In spite of the fact that the cellefficiencies obtained from CIGS layer fabricated by the

vacuum processes were higher, the cheap and convenientapproaches magnetize significant research interest as apotential alternative. Among the different solution basedmethods, the most intensely investigated and striking strat-egy may be the electrodeposition. The method has manyadvantages such as low instrumental and materials cost,high deposition speed, efficient utilization of raw materialsand feasibility in making large area films (Bhattacharyaand Rajeshwar, 1986; Bhattacharya, 2010).

1.2. Electrodeposition of CIGS

Electrodeposition, the electroreduction (and deposition)of metal ions provided from salt-containing electrolytesolutions over a conducting surface, occurs under the influ-ence of an electric field and includes a number of phase for-mation phenomena. Metal ions are converted to metal anddeposited on the cathode surface when the applied poten-tial exceeds the standard reduction potential of the ions(Galsstone, 1960). When compared to the electrodepositionof a single element, the deposition of multielement com-pounds is more complex as the different elements have dif-ferent redox potentials and reduction kinetics. For adesired stoichiometry attainment, the deposition becomesmore complicated. Solubility of different salts in a commonsolvent poses another problem (Dini, 1992; Lee et al., 2010;Saji et al., 2011).

While many reports are available on the electrodepos-ited CIS films, the reported studies on the electrodepositionof CIGS are limited. The reason may originate from thedifficulties in controlling the electrochemistry of the fourspecies of wide-ranging potentials. The standard reductionpotential values of Se4+/Se, Cu2+/Cu, In3+/In and Ga3+/Ga are +0.740, +0.342, �0.338 and �0.523 V vs. SHErespectively. The significantly differing potentials oftencause the preferential deposition of single element (Calixtoet al., 2006). Due to the more negative reduction potential,the incorporation of the desired Ga level into the filmbecomes difficult. Avoiding co-deposition of the oxidesand other secondary phases and dealing with the instabilityof In3+ and Ga3+ ions at neutral and alkaline pH add fur-ther concerns.

CIGS layer can be electrodeposited in one step or by atwo stage approach. In a simultaneous (one step) deposi-tion, the concentration and pH of the electrolyte ought tobe adjusted such that the electrode potentials of all the indi-vidual elements may come closer to each other. The twostage strategy can be employed via stacked layer structuredeposition of pure elements or of combinations with binaryor even ternary films, followed by a selenization treatment(Zank et al., 1996; Friedfeld et al., 1999). In order to sim-plify the thin film fabrication and to reduce the harmfulemissions, one step deposition is often preferred. The pre-vailing factors in the co-deposition of such compounds atany given temperature and current density are (i) the elec-trode potentials of individual ions in the electrolyte, (ii)cathodic polarization caused by the difference in deposition


potentials, (iii) relative ion concentrations in the electro-lyte, (iv) the solution potential that is the dissolving ten-dency of the deposited compound, and (v) the hydrogenoverpotential on the deposited cathode surface. Eventhough the direction of effect of each variable can be pre-dicted, it is difficult to evaluate quantitatively the magni-tude of such changes (Budevski et al., 2000).

All the as-deposited films so far are poorly crystallineand may consist of Cu–Se second phases, which necessitatea post-annealing treatment. The high temperature post-annealing helps to make compact, crystalline and crackfree films with uniform grain size. The post-annealing isperformed in a Se-rich environment to maintain the filmstoichiometry. The film morphology and the resultingconversion efficiency depend on the relative ratio of thecomponent elements in the fabricated layer. TheCu/(In + Ga) and Ga/(In + Ga) ratios are critical in deter-mining the cell performance (Bhattacharya et al., 1998a;Ludwig et al., 2010). Some well written reviews are avail-able on the electrodeposited CIS/CIGS solar cells (Lincotet al., 2004; Kaelin et al., 2004; Lincot, 2005; Hibberdet al., 2010). In this work, we review the topic with up-todate information from the literature with the emphasis onthe basic electrochemical parameters that control CIGSdeposition.

2. Efficiency of electrodeposited CIGS solar cells

In earlier works, CIGS solar cells fabricated from elec-trodeposited absorber layers yielded very low efficiency.As a solution, a limited vacuum based physical vapor depo-sition (PVD at �500 �C) was utilized to adjust the stoichi-ometry of the as-electrodeposited films. The as-depositedfilm was poor in Ga and rich in Cu and Se (Cu2.00In0.71-

Ga0.06Se2.00), which was adjusted (Cu0.92In0.76Ga0.24Se2.00)by the PVD step. In addition to the adjustment of compo-sition, the high operating temperature was found helpfulin film crystallization. The film was exposed to Se vaporduring the cool down time. The absorber layer in a cellyielded an overall cell efficiency (g) of 12.3%, with

Jsc = 30.60 mA/cm2, Voc = 0.54 V and FF = 66.93%(Bhattacharya et al., 1997). Addition of In and Ga by thePVD and the selenization at high temperature was founddecisive. An electrodeposited CIS layer which was subjectedto PVD of In, Ga and Se yielded 10% efficiency cell(Jsc = 26.6 mA/cm2, Voc = 0.55 V and FF = 67.9%)(Calixto et al., 1998; Sebastian et al., 1998). The as-electro-deposited CIS film exhibited better crystallinity than theCIGS film which was attributed to the incorporation ofGa in In sites (Sebastian et al., 1999). In a subsequent work,the group reported a 14.1% efficiency cell (Jsc = 29.03 mA/cm2, Voc = 0.656 V and FF = 74.1%) employing one stepelectrodeposited and PVD adjusted CIGS film (Bhattach-arya et al., 1998b). Subsequently, a 15.4% efficiency cellwas reported with the same sequence (Bhattacharya et al.,1999, 2000a). Composition of the as-deposited film(CuIn0.32Ga0.01Se0.93) was adjusted (CuIn0.72Ga0.47Se2.05)

by about 3000 A Ga and 7200 A In by the PVD step. In asucceeding work, the group achieved a near stoichiometricfilm employing a buffered bath. The as-deposited film wasfound to be almost stoichiometric or slightly Cu-rich andthereby reduced the PVD step to only a minimal amountof In (Jsc = 35.41 mA/cm2, Voc = 0.413 V, FF = 64% andg = 9.4%) (Bhattacharya and Fernandez, 2003). Theauthors attributed the low efficiency to the micro-cracksobserved in the film and not to the reduced PVD step. Alarge area cell with 9.87% efficiency was reported (Hermannet al., 2001).

Efficient CIGS solar cells prepared directly from nearstoichiometric precursor films electrodeposited withoutthe PVD process were reported by several research groups.Lincot et al. (2004) reported an 11.3% efficiency cell(Jsc = 23.2 mA/cm2, Voc = 0.77 V, FF = 63.4%). Guimardet al. (2003) recorded a cell efficiency of 10.2%(Jsc = 23.2 mA/cm2, Voc = 0.74 V and FF = 59.6%). A4.8% superstrate cell was reported by employing a largearea absorber film (Kampmann et al., 2000). Ganchevet al. (2006) reported a 4.35% efficiency cell(Jsc = 31.7 mA/cm2, Voc = 300 mV, FF = 45.6%) utilizinga thiocynate assisted bath. Selenization/crystallization wasperformed in a quartz tube furnace at 560 �C under Se pres-sure of 10 mbar. Dale et al. (2007) reported a cell efficiencyof 4.5%. Efficiencies above 10% were achieved on flexiblemetal substrates (Kampmann et al., 2003; Lincot et al.,2004). Record efficiencies in this category are close to 14%at the cell level and about 11% at the module level (Hibberdet al., 2010).

The enhancement in current conversion efficiencyobtained for the electrodeposited cells as a result of thePVD step was attributed to better film stoichiometryand the resulting band gap. It can be seen that the cellsconstructed with the PVD adjusted CIGS layer alwaysshowed higher FF values. The PVD step was beneficialin avoiding micro cracks and defects in the film. How-ever, this method would not be preferable in terms ofconvenience and cost factor. Current research indicatesthat sooner or later novel electrodeposition approachesyielding crystalline absorber layers with high order ofhomogeneity in both composition and morphology maybe expected to be achieved.

3. Role of deposition parameters

The electrodeposition of high quality CIGS film requiresthe strict control of chemical composition and pH of theplating bath as well as the deposition potential. In the con-ventional plating, the chemicals such as supporting electro-lytes, complexing ligands, stabilizing agents, and wettingagent are added to the bath besides the precursor salts, toenhance the deposition (Dini, 1992). Process conditions likebath temperature, agitation, etc. can significantly affect thefilm composition and quality. The stagnant bath used byBhattacharya et al. (1997) was 0.016 M Cu(NO3)2�6H2O,0.080 M InCl3, 0.024 M H2SeO3, and 0.080 M Ga(NO3)3


(pH = 1.6). The bath used by the group to achieve 14.1%and 15.4% efficiency cell was 0.009 M CuC12, 0.027 MInCl3, 0.040 M GaCl3, 0.009 M H2SeO3, and 0.280 M LiCl.Addition of the supporting electrolyte (LiCl) was foundhelpful to incorporate more Ga in the precursor and toimprove the film morphology (Bhattacharya et al.,1998b). Attempts were made to optimize the CIGS electro-lyte bath by the addition of complexing agents and bufferingagents for good quality films. With the selection of appro-priate complexing agents such as thiocynate and citrate,Cu, In and Ga ions could be solubilized at high pH regimeand their reduction potentials could be brought down closerto that of Se reduction potential (Dini, 1992; Ganchev et al.,2006). For example, citrate ions present a complexing effecton Cu2+ and H2SeO3 (Lai et al., 2009). A pH 3 buffer solu-tion (a mixture of sulfamic acid and potassium biphthalate)was employed to enhance the bath stability and to avoidmetal oxides or hydroxides precipitation. The pH bufferpermits the absorption of the free OH� and H+ ions thatexist in the deposition bath and the as-deposited precursorswere found to be stoichiometric or slightly Cu-rich (Bhat-tacharya and Fernandez, 2003). In spite of those attempts,there is no clear proof on the precise role of such additivesand in fact the usage of such organic additives may compli-cate the deposition. Novel efforts in making CIGS films atnear neutral pH values, which can lead to good quality filmsby avoiding the hydrogen reduction kinetics, are required.

Most of the reported works suggest that a depositionpotential in the range of �0.6 to �0.9 V vs. SCE is goodto achieve nearly stoichiometric film (Matsuoka et al.,1994; Kang et al., 2009). Potential below �1 V is not pref-erable due to the hydrogen charging. The film compositionchanges significantly even for small changes in the deposi-tion potential. At higher potentials (��0.1 V vs. SCE), thefilm formed constitutes of Cu–Se (50:50 at.%) phases. Culevels showed a significant decrease at �0.2 V and a furtherdecrease for the film grown at �0.4 V, dropping to �25at.% (Budevski et al., 2000). At higher cathodic voltages,incorporation of In and Ga increases with resulting reduc-tion of Se and Cu (Fig. 4A).

By suitably adjusting the bath parameters, it is possibleto make films with different grain morphologies. In conven-tional plating, the deposited films can have different struc-ture based on their grain morphology such as columnar,fibrous, fine grained/very fine grained and banded. Theusual film morphology of electrodeposited CIGS is foundto be columnar (Xia et al., 2008). Such columnar structuresare characteristic of deposits from the acidic solutions ofsimple ions operated at elevated temperature or low cur-rent density. Deposits of this type usually exhibit lowerstrength and hardness than other structures but high ductil-ity (Dini, 1992). CIGS films having columnar structurewith large grains (�1 lm) are found attractive as theycan enhance the conductivity. It should be noted that thegrain boundaries in CIGS were suggested not to have animmoderate electronic activity in the recent review (Rauet al., 2009), that low recombination activity at grain

boundaries was claimed from the evidence for a reduceddensity of deep level defects by scanning tunneling spec-troscopy (Monig et al., 2010), and that Na was observedto be mainly segregated at the grain boundaries when itwas available during the CIGS coevaporation (Cadelet al., 2010). A more recent report based on current imag-ing tunneling spectroscopy maps described that distinctgranular inhomogeneities were observed, that grains ofmetallic as well as semiconducting characteristics werefound, and that the average surface band gap of 1.4 eVwas estimated compared to the bulk band gap of 1.2 eV(Monig et al., 2011). The parallel findings remain to beexplored with electrochemically synthesized CIGS samples.

The percentage composition of the precursor chemicalsin the deposition bath affects the film morphology and filmcomposition. Fernandez and Bhattacharya suggested thatat low CuCl2 concentration, formation of micro cracks inthe film were observed. On the other hand, when the con-centration of CuCl2 was increased, the morphology of thefilm was smooth and free of micro cracks (Fernandezaand Bhattacharya, 2005). Many groups have reported thatfilm is compact and homogeneous when the amount ofCu2+ is more in the bath (Xia et al., 2008, 2006). A similareffect occurred when there was an increment of InCl3 andGaCl3 concentration in the electrolytic bath. Higher con-centrations of selenous acid caused micro-cracks in the film(Fernandeza and Bhattacharya, 2005). It has been reportedthat a concentration ratio of (Se4+)/(Cu2+) � 1.75 wasrequired for maintaining the suitable deposit levels of Seand Cu in the film (Calixto et al., 2006). When theconcentration of GaCl3 in the electrolytic bath was high,the Cu/(In + Ga) ratio increased and the Ga/(In + Ga)ratio decreased (Fig. 4B).

Figs. 5 and 6 represent the phase and microstructurevariation of an as-electrodeposited CIGS layer from atypical acid bath (pH = 1.5) having different Cu/Se content(this work). In a typical deposition, the bath used was1 mM CuC12, 1 mM InCl3, 3 mM H2SeO3, 5 mM Ga2-(SO4)3 and 0.5 M LiCl. The deposition was performed at�0.9 V vs. Ag/AgCl for 1 h under stagnant condition. Atlower Cu concentration, the film formed was found tohave non-homogeneous appearance with pore-like areas(Fig. 5A). When the Cu concentration was increased(Fig. 5B), uniform film was formed with the typical surfacemorphology. On the other hand, when the Cu concentra-tion exceeded certain limit, uniform film formation wasnot observed (Fig. 5C). However, the correspondingXRD curves revealed CIGS amorphous peaks irrespectiveof the Cu concentration. In a similar line, Fig. 6 shows thesurface morphology of films deposited from a bath differ-ing in Se concentration. This contrasting effect in the filmappearance was not observed for a similar variation of Inand Ga concentrations in the bath (not shown). This canbe due to the fact that the initiation step in the depositionis the formation of Cu–Se phases (see Section 5).

In a typical deposition, the competition between growthand nucleation decides the granularity of the deposit. The

Fig. 4. Ga variation profiles in electrodeposited films; (A) according to the applied potential vs. Ag/AgCl on a glass/TCO/n-CdS substrate (Chaure et al.,2005), Copyright Elsevier 2005; (B) according to the Ga concentration in the deposition bath at �1 V vs. pseudo-Pt on Mo glass (Fernandeza andBhattacharya, 2005), Copyright Elsevier 2005; (C) depth profile showing Ga distribution in electrodeposited films �1 V vs. pseudo-Pt on Mo glass(Bhattacharya et al., 2000b), Copyright Elsevier 2000.


higher the nucleation rate during deposition, the finer thecrystal grains of the deposit. The initial stages in the depo-sition are strongly dependent on the atomic structure andsurface inhomogeneities of the substrate (Budevski et al.,2000). A thorough substrate cleaning before deposition isimportant to get good film adhesion. The film thicknessis generally a direct function of the time of deposition.

4. Challenges in depositing stoichiometric film

The major difficulty in electrodeposition of stoichiome-tric CIGS film lies in the incorporation of optimum Gain the film. Optimization of both Ga content and Ga distri-bution in the film is essential to enhance the cell efficiency.AES depth profile analysis of the electrodeposited layer byBhattacharya et al. (2000b) revealed a non-uniform distri-bution of Ga in the film (Fig. 4C), which was suggestednot to be helpful for the electron collection mechanism.Some studies reported that the absorber layer having more

Ga on the back facilitates electron collection (Bhattacharyaet al., 2000b; Ganchev et al., 2006). Such accumulation ofGa, especially near the back contact is commonly observedin selenized chalcopyrite films. Lincot et al. (2004) in theirreview provided a detailed account on the difficulties in Gaincorporation in the film. More recent studies showed thatthe parameters such as electrolyte agitation ccould influ-ence the Ga deposition. According to Ganchev et al.(2006) the Ga content was found to decrease with the rateof electrolyte agitation increased. Kois et al. (2008) haveshown that the Ga content in a CuInGa film can beadjusted by 2–30 at.% from changing the stirring rate,deposition time and solution composition. When the elec-trolyte was agitated, the reduction rate of both Cu andIn increased, while the Ga was deposited at a constant rate.An alteration in Ga distribution and an accumulation atthe back contact was observed after selenization of the film.Kang et al. (2010) showed that the deposition from Se-poorprecursors resulted in the increased Ga incorporation in the

20

A B

C

Fig. 5. Variation of microstructure of as-electrodeposited CIGS films at room temperature (24 ± 2 �C) from various Cu:In:Se:Ga (mM) baths; havingdifferent Cu concentration. (A) 0.25:1:3:5, (B) 1:1:3:5, (C) 3:1:3:5. The corresponding XRD plots of the as electrodeposited films were also provided (thiswork).

pores A B

C

Fig. 6. Variation of microstructure of as-electrodeposited CIGS films at room temperature from various Cu:In:Se:Ga (mM) baths; having different Seconcentration. (A) 2:2:1:5 (B) 2:2:4:5, (C) 2:2:10:5 (this work).


film, suggesting that there existed an inverse relationbetween Ga incorporation in the film and the percentageof Se in the deposition bath. In a more recent work, Liuet al. (2011a) have shown that sodium sulfamate in the bath

as a complexing agent had a direct correlation with the Gacontent in the film. With an increase in the concentration ofsodium sulfamate in the bath, (Cu + Se)/(In + Ga) ratiodecreased, and the film composition transformed from


Cu-rich to Cu-poor. A very high concentration of Ga in thebath was not helpful to enhance the Ga incorporation (Fuet al., 2009a). It has been shown that Raman spectroscopycould be employed in finding the extent of Ga incorpora-tion in the film. Peak positions were found blue-shifted lin-early with increasing (Ga)/(In + Ga) ratio (Choi, 2011).

Another difficulty lies in the crystallization step of theelectrodeposited films. Vapor pressure of Se is low and Sevolatizes from the film at high temperatures. The high tem-perature annealing (at 450–600 �C) is usually performed ina Se atmosphere (such as H2Se gas, Se powder, and(C2H5)Se), in a tubular furnace in flowing Ar atmosphere(Dirnstorfer et al., 2000; Engelmann et al., 2001; Dhereet al., 2005). H2Se is often preferred over elemental Se asthe compositional change and loss of the material duringthe selenization is a major problem with the later. Somegroups have employed a two-zone furnace with the temper-ature of the substrate at �500 �C and that of the Se sourceat �250 �C (Ganchev et al., 2006). Rapid thermal processerwas employed in the selenization process.

The selenization step is environmentally unfriendly dueto the toxicity of Se. In a recent work, Bamiduro et al. usedSe free Ar atmosphere and a lower temperature (300 �C)for annealing CIGS film. The authors produced the platelettype grains having the uniformly distributed dense islands(Bamiduro et al., 2011). However, the efficiency was notstudied in a cell. Annealing CIGS film at 250 �C underthe ambient laboratory conditions was found to result inimproved crystal quality (Lee et al., 2010). Quite a few sim-ilar investigations were reported on CIS absorbers andfound that the electronic properties of the crystallized films(in Se free atmospheres) were not sufficient for solar cellapplications (Guillen and Herrero, 1994; Lincot et al.,2004). Developing more convenient and novel seleniza-tion/crystallization treatments is highly demanding forwidespread commercialization of CIGS solar cells.

5. Mechanism of deposition

When compared to CIS, the CIGS electrodepositionmechanism is little investigated. Studies proved that theearly stages of the CIS film growth were controlled byCu-rich phases. The initially formed Cu2Se phases proceedas a precursor for Se(IV) reduction to Se(0) leading to theformation of CuSe2 phases. Se(IV) reduction on the previ-ously formed Cu2Se phase is more feasible than the directdeposition of Se on Mo at room temperature. It has beenreported that the deposition of Se(0) did not occur sepa-rately but only when the binary Cu–Se phases have beendeposited (Mishra and Rajeshwar, 1989; Chassaing et al.,2008). The Cu–Se phase formed was shown to provideactive sites for the In incorporation (Roussel et al., 2008;Chassaing et al., 2009). Thouin et al. suggested that thestoichiometry of the CIS film was controlled by the ratiobetween Se and Cu fluxes on the electrode which in turncontrolled the insertion of In by the Kroger’s mechanism(Panicker et al., 1978) between excess Se in the copper

selenide film and In ions (Thouin and Vedel, 1995). Calixtoet al. (2005) suggested that the In incorporation involved achemical reaction of In(III) species with the H2Se formedby reduction of the previously formed Cu–Se phases.

It is expected that the electrochemical mechanism ofCIGS deposition may not be very different from that ofCIS. The Ga incorporation might be occurring in a similarway to that of In. It is suggested that the insertion of Gainto the growing films proceeds by reaction with H2Se(and/or Se) to form Ga2Se3. A surface-induced reductionfor Ga by initially deposited copper selenides to formCuGaSe2 may not occur, dissimilar to the behavior of In(Li et al., 2010). Lai et al. (2009) reported an underpoten-tial deposition mechanism for In and Ga, which involvedIn3+ and Ga3+ reduction by a surface-induced effect fromCu3Se2 and/or reaction with H2Se.

It is usually difficult to find individual Ga and In reduc-tion potentials in CIGS. Cyclic voltammetry coupled withmicrostructural studies suggested that the rapid increasein the cathodic current observed at �0.3 V vs. SCE wasattributed to the onset of reduction of In3+ and Ga3+.The peaks at �0.35 and �0.45 V were suggested to corre-spond to the reduction of In3+ and Ga3+, respectively(Lai et al., 2009). Chassaing et al. (2008) have shown thatIn insertion started beyond a potential threshold close to�0.6 V and the In/Cu ratio increased towards more nega-tive potentials. According to Thouin et al. (1994) In2Se3

is co-deposited with CIS, under some experimental condi-tions, for instance at very negative deposition potentialsand with Se(IV) flux at the electrode higher than twicethe Cu(II) flux. In2Se3 can form a pseudobinary systemwith Cu2Se, with the general formula (Cu2Se)(1�x)(In2Se3)x,which for x = 0.5 becomes CuInSe2 (Folmer et al., 1985).Souilah et al. (2010) investigated the Cu2Se–In2Se3–Ga2Se3

system and reconfirmed that the chalcopyrite structure wasvery flexible and could adapt itself to the substitution of Gafor In as well as to a wide range of copper vacancy content.

All these studies support the fact that formation of ini-tial Cu–Se phases is obligatory to enable the incorporationof In and Ga in the film. A significant fraction of secondphases may be present which disappears in high tempera-ture annealing. A satisfactory explanation on Ga incorpo-ration is still missing, which is critical in this direction.

6. Related strategies

In and Ga electrodeposition methods demand specialattention than other commonly plated metals such as Cu.The high negative deposition potential of Ga causes hydro-gen charging in addition to Ga plating, which causes thecomposition inhomogeneity and pinholes in the film. Asan alternative to the acidic aqueous bath for CIGS deposi-tion, ionic liquid (e.g. choline chloride/urea eutecticmixture) based systems were investigated by a few. Theyhave advantages such as wide electrochemical window,high thermal stability, very low vapor pressures, lowtoxicity, and good solubility of organic and inorganic


materials (Shivagan et al., 2007; Lau et al., 2011). Someresearchers employed the modified pulse current electrode-position technique as an alternative to conventional dcelectrodeposition of CIGS (Fu et al., 2009b; Liu et al.,2011b). The pulse plating which uses a modulated currentwaveform may result in better leveling of the deposit andminimize the porosity, the contamination, etc. The methodoffers more process controllable parameters, which can beadjusted independently and can withstand much higherinstantaneous current densities, resulting in more uniquecomposition and microstructure. The morphology of thefilm can be improved by adjusting the duty cycle. Howeversuch attempts have so far not resulted in a higher efficiencycell.

Some studies explored In, Ga and their binary alloysdeposition for applications in CIS/CIGS solar cells. Itwas shown that deposition of In on Mo/Cu begins from�0.70 and goes up to �1.0 V vs. SCE. Deposition wasunder mixed control at �0.70 V and under diffusion con-trol in-between �0.70 V and �0.80 V (Cao and West,2002; Valderrama et al., 2008). The deposition of Inadheres to a three dimensional growth under instantaneousnucleation limited by diffusion. In the first minute of depo-sition a compact layer of Cu–In alloy was formed on whichthe In film was grown. After 30 min deposition, large crys-tallites of In was formed and as the potential was increasedagglomerates were formed, resulting in compact and mas-sive surfaces (Valderrama et al., 2008). In a similar line,Huang et al. (2011) suggested that In deposition on Cuwent through two stages; the formation of a continuoussmooth layer followed by the formation of the islands. ACu layer was plated on the PVD Cu seed prior to the Indeposition. Fast inter-diffusion was observed between theelectroplated Cu and In and a CuIn2 alloy phase formedduring the In deposition at room temperature. The islandgrowth was believed to be delayed until a threshold valueof In content was reached at the surface. Steichen et al.

Electrodepositionof CIGS

Challenges

Advantages

• Nt• E• H• L

• Precise control of film stoichiometry (opdistribution)• Difficulties in the selenization/crystalliza• Poor understanding on the mechanism of

Required strategies

• Precise optimization and formulation of• Novel deposition strategies• Novel selenization/crystallization techniq• Studies on individual In and Ga depositio• Studies on band gap grading and efficien• Synergism in electrodeposited films with

Fig. 7. Major challenges a

(2011) demonstrated that Ga could be electrodepositedfrom Reline without hydrogen evolution interference withhigh plating efficiency on Mo. They have also described anew low cost synthetic route for the preparation of CuG-aSe2 thin films. Annealing of the one step electrodepositedCu–Ga films in a selenium atmosphere allowed the forma-tion of high quality CuGaSe2 absorber layers. Wang et al.(2010) patented a novel method to deposit the uniform,defect free and smooth Ga film employing a monohydroxylalcohol and water based bath.

Some novel multi-step deposition strategies werereported. Bhattacharya reported fabrication of CIGSabsorber layers using a two-step electrodeposition process;Cu-rich CIGS was electrodeposited first, followed by In–Se(Bhattacharya, 2010). The deposited Cu layer facilitatedthe electrodeposition of subsequent In, Ga, and Se layers.In another interesting approach, a sequential alloy deposi-tion was employed for co-deposition of In–Se and Ga–Sefilms with high repeatability and controllable In/Se andGa/Se molar ratios by a two-stage process (Aksu et al.,2009).

A pre-heat treatment can have significant impact on amulti-step electrodeposition. Lee et al. (2011) has shownthat adherence of a CuInS2 film to the Mo substrate whichwas formed by sulfurization of Cu/In bilayer film wasstrongly dependent on the additional pre-heating condition(110 �C) subjected to the bilayer.

One dimensional semiconductor nanostructures areattractive as they can enhance the current conversion effi-ciency by allowing the charge carriers to diffuse radiallyfor a shorter distance toward the junction. Inguanta et al.(2010) reported some preliminary work concerning thedeposition of CIGS nanowires using AAO templates.Kwak et al. (2010) successfully deposited CIGS films oversingle-crystalline CdS nanowires that were grown on thefluorine-doped tin oxide/soda-lime glass substrate, with aseries of voltage loading experiments. The one dimensional

earing current conversion efficiency o that of vacuum deposited filmsconomic and convenientigh deposition speedarge area films

timization of Ga content and Ga

tion step Ga incorporation

standards on bath parameters

uesn mechanismcy optimizationother solution based approaches

nd required strategies.


single crystalline CdS nanowires were expected to enhancethe carrier collection efficiency. The CIGS/CdS nanowiresolar cells showed higher conversion efficiencies when com-pared to equivalent CIGS solar cells containing chemicallydeposited CdS thin film, though the highest achieved labo-ratory scale efficiency was only 6.18%.

7. Conclusions and outlook

Electrodeposited CIGS solar cells present an impressivealternative to the vacuum deposited cells as a low cost solarcell technology. Though the current conversion efficienciesreported for the electrodeposited CIGS cells are lower thanthe vacuum deposited cells, it is highly expected that withsuitable modifications this simple manufacturing processwill turn out to be the best in the near future. The majorchallenges of electrodeposition method and the requiredstrategies to solve them are outlined in Fig. 7. The develop-ment of novel plating methods that can yield highly crystal-lized CIGS films at the room temperature can result inunexpected results. This in effect may help to avoid thecomplicated and unfriendly selenization step. Effectivedoping of the CIGS may be beneficial to achieve crystalli-zation and thereby reduce the processing temperature.Multiple one-step deposition combined with low tempera-ture heat treatments can be helpful. Strict optimization isrequired in the relative ratio of the elements in the deposi-tion bath for the desirable film formation. Precise organiza-tion of deposition parameters can control grain geometry,film composition, and elemental distribution. By suitableoptimization, the pulse deposition can yield better films.Effective combination of electrodeposition with other solu-tion based approaches such as spray coating needs to beexplored. Similarly, CIGS solar cells based on one dimen-sional nanostructures can yield better result. More studiesare required to understand the mechanism of deposition,especially on Ga incorporation. Studies on electrodeposit-ion of individual and binary alloys of In and Ga will behelpful in this direction. More studies on band gap grading,interface properties and loss mechanisms in electrodepos-ited CIGS cells are requisites. The long term stability ofelectrodeposited CIGS solar cells needs to be comparedwith that of the vacuum counterparts.

Acknowledgement

This work was financially supported by NationalResearch Foundation of Korea (2010-0029164).

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