Rapid Fabrication of Cu(In,Ga)Se$_{2}$ Thin Films by the Two-Step Selenization Process

$Page 1: Rapid Fabrication of Cu(In,Ga)Se$_{2}$ Thin Films by the Two-Step Selenization Process$
476 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 3, NO. 1, JANUARY 2013

Rapid Fabrication of Cu(In,Ga)Se2 Thin Films by theTwo-Step Selenization Process

Shogo Ishizuka, Lorelle M. Mansfield, Clay DeHart, Marty Scott, Bobby To, Matthew R. Young, Brian Egaas,and Rommel Noufi

Abstract—Two-step processes currently used for the industrialCu(In,Ga)Se2 (CIGS) module production require a long processtime of several hours for the CIGS absorber formation. In thispaper, we are studying the reaction pathway to rapid selenizationof stacked metal precursors in elemental Se vapor. The objectiveis to understand the reaction kinetics to find the best precursorstructure and the optimal selenization conditions to form high-quality CIGS films with proper Ga depth profiles. In addition tostacked metal precursors, the effect of the use of Se-containingprecursors was also examined. As expected, the stacking order ofthe metal precursors influences the properties of the resulting CIGSabsorbers. The Cu amount deposited for the precursor formationcritically affected the final film and cell properties, as well. We alsofound that the formation of CIGS films with large grain sizes andflat Ga depth profiles was possible even for [Cu]/([In] + [Ga]) <1 conditions with the use of particular precursor structures andselenization conditions. The results suggest that the selenizationreaction pathway can be dictated with the precursor structure,and further improvements are expected by controlling reactionkinetics with precursor structure modification.

Index Terms—Copper indium gallium diselenide, Cu(In, Ga)Se2(CIGS), photovoltaic cells, selenization, thin films.

I. INTRODUCTION

COPPER indium gallium diselenide (CIGS)—Cu(In,Ga)Se2 and related multinary compounds—has

emerged as one of the most promising thin-film technologiesfor solar cells and modules. Among a wide variety of CIGSabsorber growth methods, two-step selenization processes haveoften been used for industrial CIGS module production [1], [2].Recently, two-step processes have been recognized not onlyas a technique suitable for large-scale module production, butalso one of the methods with the most potential to yield high

Manuscript received May 21, 2012; revised August 27, 2012 and Septem-ber 21, 2012; accepted September 27, 2012. Date of publication November 12,2012; date of current version December 19, 2012. This work was supportedby the U.S. Department of Energy SunShot Program’s Funding of the NationalRenewable Energy Laboratory Core Science and Technology Activities, andFoundational Program for Advancing Conversion Efficiency.

S. Ishizuka is with the National Renewable Energy Laboratory, Golden,CO 80401 USA, and also with the National Institute of Advanced IndustrialScience and Technology, Tsukuba, Ibaraki 305-8568, Japan (e-mail: [email protected]).

L. M. Mansfield, C. DeHart, M. Scott, B. To, M. R. Young, B. Egaas,and R. Noufi are with the National Renewable Energy Laboratory, Golden,CO 80401 USA (e-mail: [email protected]; [email protected];[email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JPHOTOV.2012.2222868

cell and module efficiencies comparable with coevaporationtechniques [3], [4]. Nonetheless, the long time required forselenization in the second step and batch processing can bea barrier for cost reduction. Further, the problem of the Gasegregation toward the back of the CIGS absorber, which resultsin the inhomogeneity of films, has often been discussed [5]–[8],and occasionally, the addition of sulfur has been attempted nearthe surface region to obtain high open-circuit voltage Voc [9].Reducing the process time by using the best selenizationreaction pathway is expected to bring increased productionyield regardless of batch processing, as well as the extendedapplication of two-step processes for various types of CIGSmodule production techniques such as in-line and roll-to-rollprocesses. To date, most CIGS films and cells presented inthe literature that use a short selenization process time ofless than 1 h have exhibited rough surface morphologies andmarginal cell efficiencies compared with coevaporated orlong-time selenized films and cells [7], [10], [11]. In this paper,we are studying the reaction pathway to rapid selenizationof stacked metal precursors using elemental Se vapor. Thestudy focuses on finding the best precursor structure and theoptimal selenization conditions to form high-quality CIGSfilms in a short process time without using H2Se gas or addingsulfur. Our ultimate goal is to demonstrate the fabricationof high-efficiency (>20%) solar cells by the rapid two-stepselenization with a short reaction time (<10 min), which is anindustrially applicable fast and high-efficiency CIGS processtechnique.

II. EXPERIMENTAL

CIGS films were grown on Mo-coated soda-lime glass sub-strates in a vacuum chamber. In the first step, the single-metalprecursors Cu, In, and Ga were deposited by thermal evapora-tion without intentional substrate heating. The actual substratetemperature during precursor formation was typically < 30 ˚C.Various stacking orders, such as Cu/In/Ga or In/Ga/Cu, wereexamined for the precursor structure. The targeted [Cu]/([In] +[Ga]) (hereafter abbreviated to Cu/III) composition ratio wasvaried around unity, and the [Ga]/([In] + [Ga]) (hereafter ab-breviated to Ga/III) composition ratio was fixed at 0.39 for theprecursor formation. In addition to the metal stacks, the effect ofthe use of Se-containing precursors, which were coevaporatedwith Se at a low temperature around 200 ˚C, was also examined.In the second step, a selenization process was successively car-ried out at high temperatures, above 550 ˚C, for 30 or 20 minusing thermally evaporated elemental Se in the identical vacuumchamber used for the precursor formation. The Se flux used for

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ISHIZUKA et al.: RAPID FABRICATION OF CU(IN,GA)SE2 THIN FILMS BY THE TWO-STEP SELENIZATION PROCESS 477

Fig. 1. SEM images of the surface of CIGS films grown with stacked metalprecursors. Insert figures show precursor structures used for the film formation.

the selenization process was in the range of 25–45 A/s. Afterthe selenization process, CIGS film quality was evaluated bydetermining the film composition, crystalline phase, elementaldepth distribution profile, morphology, and the efficiency of so-lar cells. CIGS film compositions (atomic composition ratios)were determined by X-ray fluorescence (XRF) measurements.A contamination of stacked elemental metal precursors with Sewas negligible so that it was less than the limit of detectionby XRF measurements. CIGS solar cells were fabricated usinga conventional device structure, i.e., chemical-bath-depositedCdS buffer layers, sputter-deposited ZnO/ZnO:Al transparentconductive oxide films, and Ni/Al grid electrodes with the totalcell area of 0.42 cm2 . Solar cell parameters were determined byin-house measurements under the standard conditions of AM1.5, 100 mW/cm2 , and 25 ˚C.

III. RESULTS AND DISCUSSION

A. Stacked Metal Precursors

Fig. 1(a)–(f) shows surface scanning electron microscopy(SEM) images of CIGS films grown with six precursor structuresunder the Cu/III < 1 condition of about 0.9. The selenizationcondition used for the formation of these films was identical,with the process time of 30 min. Rough surface morphologieswith many crevices and a trace of island structure are shown inFig. 1(a)–(d), whereas the CIGS films grown with the precursorsthat have elemental Cu in the first layer of the stacking orderexhibited relatively smoother surfaces, as shown in Fig. 1(e)and (f). We have observed that the trace of island structure

Fig. 2. (a) XRD patterns of the #1 Ga/In/Cu and #5 Cu/Ga/In stacked metalprecursors before selenization. (b) Surface SEM-EDX mapping images of ele-mental Cu, In, and Ga distribution for these precursors. PDF Card #00-002-1253,#00-004-0836, #00-005-0642, and #00-005-0601 were referenced.

originates from the formation of Ga and In metal particlesformed in the particular precursor structures, as shown inFig. 2(b). In addition, we have observed an extremely high Gaconcentration in and around the islands on the CIGS surface withXRF measurements. XRD patterns and energy dispersive X-rayspectroscopy (EDX) elemental mappings for stacked metal pre-cursors #1 and #5 before selenization shown in Fig. 2(a) and(b) indicate that the stacking order strongly affects the metallicphase and morphology of precursors, and the Cu in the first layerof the stacking order was found to be effective to prevent fromthe In and Ga aggregation.

Cross-sectional SEM images and secondary ion mass spec-trometry (SIMS) depth profiles for the CIGS film grown with aCu/In/Ga precursor [see #6 inset in Fig. 1(f)] are displayed inFig. 3(a) and (b), respectively. Fig. 3(c) and (d) shows SEM andSIMS results obtained from the CIGS film grown with the sameprecursor structure Cu/In/Ga, but this film was grown under aCu/III > 1 condition using a thicker Cu layer in the precur-sor. The Cu/III composition ratio after selenization was 1.38.Fig. 3(a) and (b) revealed that the CIGS film shown in Fig. 1(f)was inhomogeneous at depth, in contrast with the homogeneous-looking surface. It should be noted here that the Ga accumulationin the back of the film was observed, as is often observed anddiscussed in the literature [5]–[8], even though the elementalGa layer was deposited on the top of the metal stack. On theother hand, the CIGS film grown under a Cu/III > 1 condi-tion showed a relatively flat Ga profile and larger grain sizes,although the formation of voids was conspicuous in the backof the film. These results are attributed to the difference ofthe Gibbs free energy of formation for Cu–Se, In–Se, and Ga–Se compound phases, which affects the reaction pathway dur-ing selenization. As generally known, In–Se compounds havelower formation energies than Ga–Se compounds [12]. Further,


Fig. 3. Cross-sectional SEM images and SIMS depth profiles for the CIGSfilms grown using a #6 Cu/In/Ga precursor structure. (a) and (b) were obtainedfrom the same CIGS film shown in Fig. 1(f), grown under a Cu/III < 1 condition,whereas (c) and (d) show the results obtained from the film grown under aCu/III > 1 condition. Approximate Cu/III composition ratios determined byXRF measurements are indicated in the SEM images.

Cu–Se compounds such as CuSe and Cu2Se have lower forma-tion energies than In–Se compounds [12]. Therefore, Cu reactswith Se easily and enhances the elemental migration duringselenization due to the formation of Cu–Se liquid phases. Con-sequently, Cu moves to the reacting film surface and Ga migratesaway from the surface to the back. Thus, CIGS films grown un-der a Cu-rich condition exhibited larger grain sizes and flat Gadepth profiles, although many voids were formed in the backwhere the elemental Cu was originally present in the precursorstructure. A similar trend in grain size and Ga profile varia-tion with the Cu/III composition ratio was observed from CIGSfilms grown with the other precursor structures, except for the#5 Cu/Ga/In precursor.

The CIGS film grown with a #5 Cu/Ga/In precursor, which isshown in Fig. 1(e), exhibited a relatively smooth surface mor-phology similar to the film grown with a #6 Cu/In/Ga precursor.Fig. 4(a) and (b) displays a cross-sectional SEM image andSIMS results observed from the film shown in Fig. 1(e). TheSEM and SIMS results obtained from CIGS films grown withthe same precursor Cu/Ga/In structure, but under a Cu/III > 1condition, are shown in Fig. 4(c) and (d). It should be notedhere that the extremely Cu-poor composition of Cu/III ∼ 0.75was used to prepare the Cu/Ga/In precursor to obtain the finalcomposition of Cu/III ∼ 0.9 after selenization. The CIGS filmsgrown with a Cu/Ga/In precursor often show the trend to beIn-poor—in other words, Cu- and Ga-rich more than the tar-geted composition used for the precursor formation, due to theformation of volatile Se-poor In-Se phases during selenization.Thus, the CIGS film shown in Fig. 4(a) and (b) has the com-

Fig. 4. Cross-sectional SEM images and SIMS depth profiles for the CIGSfilms grown using a #5 Cu/Ga/In precursor structure. (a) and (b) were obtainedfrom the same CIGS film shown in Fig. 1(e).

position ratios of Cu/III ∼ 0.95 and Ga/III ∼ 0.54, althoughthe targeted composition ratios used for the precursor formationwere Cu/III ∼ 0.75 and Ga/III ∼0.39. On the other hand, thecomposition deviations seen in the CIGS films grown with theother five precursor structures were slight; therefore, the CIGSfilms remained near the targeted compositions used for theseprecursor formations of Cu/III ∼ 0.9 and Ga/III ∼ 0.39. As inthe case of the film grown with a #6 Cu/In/Ga precursor, the for-mation of voids was suppressed by decreasing the Cu/III ratio,as shown in Fig. 4(a) and (c). In contrast with the results shownin Fig. 3(a) and (b), however, the CIGS film grown with the #5Cu/Ga/In precursor exhibited a flat Ga profile and large grainsizes, although the film was grown under a Cu/III < 1 condition.This result is attributable to the presence of liquid In–Se phasesthat appear in Se-poor conditions, as discussed later. From theseresults, it is suggested that a precursor structure, which has ele-mental Cu in the first layer and In in the last layer of the stackingorder, is effective to avoid the formation of island structure andto obtain large grains and a flat Ga depth profile. Homogeneousfilms are formed due to the presence of liquid In–Se phases atthe initial stage of the selenization process. The #5 precursoris one of the promising structures that can dictate the properreaction pathway to demonstrate such film properties in a shortprocess time.

Solar cell devices fabricated using the CIGS absorber grownwith a #5 Cu/Ga/In precursor shown in Fig. 1(e) demonstratedrelatively high cell efficiencies of about 14%. CIGS solar cellsmade with a #4 In/Cu/Ga precursor also demonstrated similarlyhigh efficiencies in spite of the rough surface morphology, asshown in Fig. 1(d); the other precursors, including #6 Cu/In/Ga,demonstrated relatively low efficiencies around 12%, as shownin Table I. Fig. 5(a) shows X-ray diffraction (XRD) patterns for


TABLE IPV PARAMETERS DEMONSTRATED FROM CIGS FILMS IN FIG. 1

the CIGS films shown in Fig. 1(a)–(f). The presence of (In,Ga)–Se secondary phases and CuInSe2 (CIS)–CuGaSe2 (CGS) phaseseparations in these films was observed. In the next step, there-fore, we attempted a modification of the film growth conditionto enhance the selenization reaction and to eliminate secondaryphases. Fig. 5(b) shows XRD patterns for the CIGS films grownusing the modified growth condition. Here, a reduced precursorthickness was used to obtain 1.5–2-μm-thick CIGS films thatwere about half of the original CIGS film thickness presentedin Figs. 1–3 and 4(a). Further, the Se flux rate was increased1.6 times higher, and the substrate temperature used during theselenization process was decreased 50 ˚C lower than the orig-inal condition, to reduce the effect of Se re-evaporation fromthe growing CIGS surface. Although the CIS-CGS phase sep-aration still remains, secondary phases consisting of nonchal-copyrite structures were clearly eliminated in Fig. 5(b). Here,the targeted Cu/III and Ga/III composition ratios were 0.9 and0.39, respectively, for all films. The presence of sufficient Seflux could suppress the formation of volatile In–Se phases, andconsequently, the composition deviation from the target wasvery slight, even for the CIGS film using the #5 Cu/Ga/In pre-cursor. Again, the CIGS absorbers made from the #4 In/Cu/Gaand #5 Cu/Ga/In precursors demonstrated higher cell efficien-cies than the other four precursors, with high fill factors (FFs)of over 0.7. Although a slight decrease of the absorption in thelong wavelength region was observed due to the thinner CIGSabsorber thickness, the improved FF strongly affected enhanc-ing cell efficiencies. Notably, cell efficiencies over 15% weredemonstrated from the CIGS cells fabricated with a #5 Cu/Ga/Inprecursor. Thus, the next section focuses on CIGS films and cellsfabricated with the #5 Cu/Ga/In precursor structure.

Fig. 6(a)–(h) shows variations in the surface morphology,grain size, and SIMS depth profiles as a function of the Cu/IIIcomposition ratio in CIGS films grown with #5 Cu/Ga/In precur-sors using the modified growth condition mentioned previously.In contrast with the large grain sizes and flat Ga profile of theCIGS films that were grown under an insufficient Se flux con-dition manifested by the composition deviation attributable tothe formation of volatile Se-poor In–Se phases, as shown inFig. 4(a) and (b), the CIGS films grown under Cu/III < 1 con-ditions exhibited small grain sizes with a steep Ga gradient, asshown in Fig. 6(a), (b), (e), and (f). It is generally known that theSe-poor In–Se phases have a low melting point around 500 ˚C,whereas Se-rich In–Se phases have high melting points, such as890 ˚C for In2Se3 [13]. The large grain sizes shown in Fig. 4(a)

are, therefore, attributable to the enhanced elemental migrationdue to the presence of liquid In–Se phases, which could worklike the liquid Cu–Se phases presented in Cu-rich growth condi-tions. Note that the melting points of Cu–Se phases elevate withdecreasing the amount of Se [13]. Thus, the smaller amount ofCu is also expected to help liquid In–Se phases to enhance theelemental migration under Se-poor conditions.

Fig. 7 shows variations of the XRD spectra for the CIGS filmsshown in Fig. 6(a)–(d). The CIGS films grown under Cu/III >1 conditions exhibited a single-phase-like chalcopyrite struc-ture without the CIS-CGS phase separation, although invisibleCu–Se secondary phases may exist taking the composition ra-tios into account. Fig. 8(a) and (b) shows current–voltage andexternal quantum efficiency curves for the CIGS solar cells fab-ricated using these CIGS absorbers. As shown in Fig. 8(b), theCIGS cells grown under Cu/III < 1 conditions have a nominallysmaller bandgap energy than that of the CIGS cells grown un-der Cu/III > 1 conditions due to the presence of the Ga gradientstructure and the CIS-CGS phase separation in the CIGS ab-sorber, even though the total Ga/III composition of these filmsis almost identical. Nonetheless, the Voc of the Cu/III < 1 cellswere larger than the Cu/III > 1 cells, as shown in Fig. 8(a).The Cu/III < 1 cells also showed higher short-circuit currentdensities Jsc than the Cu/III > 1 cells. The Voc and Jsc deterio-rations observed in the Cu/III > 1 cells are chiefly attributable tothe presence of conductive Cu–Se secondary phases. Althoughcyanide-etching treatments were performed for all CIGS filmsto remove the Cu–Se phases from the film surface before CdSbuffer layer deposition, Cu–Se phases present in the film maystill remain and cause such deteriorations. In fact, the cell ef-ficiencies demonstrated from the Cu/III > 1 cells were lessthan 10% with relatively low FF values of about 0.6. On theother hand, the Cu/III < 1 cells, specifically, the Cu/III ∼ 0.86cell, have demonstrated a relatively high cell efficiency of 15.9%(16.2% with an antireflection (AR) coating) with Voc of 0.584 V,Jsc of 36.1 mA/cm2 (36.8 mA/cm2 with an AR coating), and FFof 0.752. Because of the relatively low Voc due to the presence ofa steep Ga gradient concomitant with a CIS-CGS phase separa-tion, this cell efficiency is still low when compared with coevap-oration or long-time selenization processed CIGS cells; but itis remarkably high among rapid two-step processed CIGS cellsreported thus far [5]–[7], [10], [11]. In particular, the high FF ofover 0.75—demonstrated despite relatively rough surface mor-phology, small grain sizes, and a CIS-CGS phase separation—isworth mentioning, and further improvements are expected withfurther precursor structure and selenization condition modifica-tions based on the #5 stacking order.

B. Se-Containing Precursors

Se-containing precursors are often used to enhance the sel-enization reaction [6], [10], [11], [14]. Here, the effect of usingSe-containing precursor structures was examined. In the firststep, elemental Cu, In, Ga, and Se were coevaporated to formthe precursor at a relatively low substrate temperature of around200 ˚C. The [Se]/[metal] ratio used for the Se-containing pre-cursor was about 3. In the second step, a selenization process


Fig. 5. (a) XRD patterns of the CIGS films shown in Fig. 1(a)–(f). (b) XRD patterns of the CIGS films grown using a modified condition with thinner precursorthicknesses, an increased Se flux and a lower substrate temperature during the selenization process. The metal/Se composition ratio was almost unity for the allfilms. PDF Card #01-070-6426, #01-075-0104, #01-070-2541, #01-071-0250, #01-071-0375, #01-076-0975, and #01-076-2310 were referenced.

Fig. 6. (a)–(d) Surface and cross-sectional SEM images of the CIGS films grown with #5 Cu/Ga/In precursors with various Cu/III composition ratios aroundunity. (e)–(h) SIMS depth profiles for the films.


Fig. 7. XRD patterns of the CIGS films shown in Fig. 6(a)–(d).

Fig. 8. (a) Current–voltage curves and (b) external quantum efficiency curvesmeasured from the solar cells fabricated using the CIGS absorbers shown inFig. 6(a)–(d).

Fig. 9. XRD pattern of the CIGS film grown using a Se-containing precursorwith the composition ratios of Cu/III ∼ 0.89 and Ga/III ∼ 0.30.

was performed at high temperature for 20 min in a Se vaporwith the Se flux of 45 A/s. Figs. 9 and 10 show XRD and SIMSresults obtained from the CIGS film grown with the compositionratios of Cu/III ∼ 0.89 and Ga/III ∼ 0.30. Although the CIGSfilm was grown under the Cu/III < 1 condition, a single-phaseXRD pattern with a flat Ga profile was observed. The flat Gaprofile is attributable to the presence of Se in the precursor, inwhich the migration of metallic elements to react with the Sethat came from the surface should be suppressed. This film alsoshowed no void formation in the back of the film. These resultssuggest that the use of Se-containing precursor structures is ef-fective to obtain a single-phase CIGS structure and to controlthe elemental immobilization during the selenization process incontrast with the remarkable Ga migration to the back that isobserved in simply stacked metal precursor structures. We haveobserved that such Se-containing precursors were also effectiveto demonstrate relatively high cell efficiencies close to 15% (Voc∼ 0.624 V, Jsc ∼ 31.7 mA/cm2 , and FF ∼ 0.748). Thus, the

Fig. 10. Elemental SIMS depth profiles observed from the same CIGS filmshown in Fig. 9.

combination of stacked metals with Se-containing structures tocontrol reaction kinetics is expected to demonstrate the precisecontrollability of Ga profiles and lead to further improvementsand enhancements in film quality and cell efficiencies.

IV. SUMMARY

Rapid two-step selenization growth of CIGS absorbers wasstudied to understand the reaction kinetics in order to find thebest precursor structure and the optimal selenization conditionsto develop an industrially applicable, fast, and high-efficiencyCIGS film growth technique. The stacking order of the metalprecursors and the presence of Se in precursors were found toinfluence the properties of the resulting CIGS absorbers andcould control the film morphology and homogeneity. CIGS ab-sorbers made with #4 In/Cu/Ga and #5 Cu/Ga/In precursorsreproducibly demonstrated relatively high cell efficiencies eventhough these stacking orders are extremely different from eachother. Although the underlying kinetics in #4 precursor struc-ture should be further investigated, #5 precursor was found tobe one of the best structures to dictate proper reaction pathways,in which the formation of island structure is avoidable and Se-poor liquid In–Se phases are available to enhance grain growthregardless of Cu-poor conditions. This result suggests that thecontrol of CIGS film properties and further improvements insolar cell device performance are possible by modifying theprecursor structure based on these stacking orders to dictateproper selenization reaction pathways even for a short seleniza-tion process time.

ACKNOWLEDGMENT

The authors would like to thank M. A. Contreras, I. Repins,and K. Ramanathan at NREL for fruitful discussions and con-tinuous encouragement.

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Authors’ photographs and biographies not available at the time of publication.

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Rapid Fabrication of Cu(In,Ga)Se$_{2}$ Thin Films by the Two-Step Selenization Process