The Effect of Ga Content on the Selenization of Co-evaporated CuGa/In Films and their Photovoltaic Performance

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1 The Effect of Ga Content on the Selenization of Co-evaporated CuGa/In Films and their Photovoltaic Performance Christopher P. Muzzillo 1,2, Lorelle M. Mansfield 2, Clay DeHart 2, Karen Bowers 2, Robert C. Reedy 2, Bobby To 2, Rommel Noufi 3, Kannan Ramanathan 2, and Timothy J. Anderson 1 1 University of Florida, Gainesville, FL 32601, USA 2 National Renewable Energy Laboratory, Golden, CO 80401, USA 3 Retired, rommel.noufi@gmail.com Abstract Thin CuGa/In films with varying composition were deposited by co-evaporation and then selenized in situ with evaporated selenium. This growth process was interrupted at various stages to study the selenization behavior of metal precursors by GIXRD, SIMS, XRF, SEM, and EPMA. Precursor phase constitution and morphology were found to be similar to well-studied sputtered precursors. The phase evolution during selenization was also found to be similar to sputtered precursors, with greater Ga/(Ga+In) compositions requiring longer selenization time to completely form the chalcopyrite phase. Solar cells were fabricated with absorbers of varying composition and characterized by JV measurements. Relatively high Ga contents could be reached before photovoltaic performance degraded significantly. Champion power conversion efficiencies of 14.5, 14.4, and 12.2% were achieved with Ga/(Ga+In) ~ 30, 50, and 70%, respectively. Index Terms CIGS, co-evaporation, Cu(In,Ga)Se 2, gallium content, selenization, wide band gap. I. INTRODUCTION Solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) absorber films formed by chalcogenization of metal precursors have demonstrated the crucial combination of economic feasibility and record-breaking power conversion efficiency [1]. Metal precursor films are typically deposited by sequentially sputtering from Cu Ga and In targets, which has historically restricted precursors to low Ga content, as low melting temperature alloys are unsuitable for sputtering. Electrochemical co-deposition has also been used, but independent control of Cu/(Ga+In) (Cu/III) and Ga/(Ga+In) (Ga/III) has proven difficult [2]. Heretofore, very little work has been reported on selenization of Cu-Ga-In precursors with Ga/III > 30%. Nevertheless, increasing Ga/III is an attractive route to increase open-circuit voltage (V OC ) to reduce resistive losses in modules. Other benefits include decreased temperature coefficient, reduced In material cost, and the potential for a more efficient wide band gap top cell in a tandem device, which is currently being pursued industrially [3]. For these reasons, a study of selenization behavior at different Ga/III is warranted. A previous study on atmospheric pressure H 2 Se selenization (90 min at 410 C) of sputtered and evaporated Cu/Ga/In precursors with Ga/III of 0 to 100% in 25% increments reported the 50% composition to have the highest power conversion efficiency (13.1%) [4]. A subsequent study by the same group detailed the typically observed segregation of Inrich and Ga-rich phases both in precursors and selenized films, and the related slower selenization kinetics of Ga relative to In [5]. Another study made use of a Cu 34 Ga 66 sputtering target to increase Ga/III [6]. However, standard selenization resulted in poor adhesion, and so only results from a more complicated deposition scheme were reported. A more recent study on atmospheric pressure (C 2 H 5 ) 2 Se selenization of co-evaporated CuIn/CuGa/Cu precursors achieved single-phase films and reported low-temperature photoluminescence spectra for ten Ga/III compositions [7]. Vacuum selenization of evaporated In/Ga/Cu/In precursors on glass revealed increased band gaps with increasing Ga/III [8]. To the authors knowledge, this is all the published work on the effect of Ga content on selenization mechanisms of Cu-Ga-In films with Ga/III > 30% and their photovoltaic (PV) performance. The potential value of selenized CIGS with high Ga content and lack of literature on the topic make it an attractive area of research. II. EXPERIMENTAL Back contacts of 0.8 µm Mo were DC sputtered onto sodalime glass (SLG) substrates. Co-evaporation of 0.6 µm CuGa/In bilayer precursors was then performed with electron impact emission spectroscopy (EIES) control. Precursors contained Se contamination of at.-%, as measured by electron probe microanalysis (EPMA) at a 20 kv accelerating voltage. Precursors were studied by grazing incidence X-ray diffraction (GIXRD), and were vacuum annealed to 200 and 400 C to assist in peak identification. Secondary ion mass spectrometry (SIMS), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) were also used to characterize precursor films. In situ selenization of precursors was performed by ramping substrate temperature at 100 C/min, until 200 C was reached, at which point they were exposed to Se vapor at a flux that would give a Se growth rate of 4 nm/s on an unheated substrate. The initial stages of selenization were studied by turning off the heater at substrate temperatures of 400 and 585 C (see Fig. 1). Otherwise, the ramp continued /14/$ IEEE 1649

2 Fig. 1. Substrate temperature versus time (actual data) of a 200 C precursor anneal (blue), 400 C selenization (green), 585 C selenization (yellow), and complete growth (red). Selenium exposure occurred in filled areas. Fig. 2. Symmetric XRD patterns for metal precursor films with Ga/III ~ 30 (top red line), 50 (middle black line), and 70% (bottom blue line). γ refers to γ-cu 9 (Ga,In) 4. W peak is from W impurity in the Cu radiation source. to 600 C, followed by a 20 min temperature soak, and then cooling at 20 C/min. In all cases the Se flux was turned off when the substrate had cooled to 300 C. The molar amount of Se required to fully form the chalcopyrite would be equivalent to ~0.8 µm Se deposited on a cool substrate, while the 400 C, 585 C, and complete selenizations supplied equivalent Se thicknesses of 1.0, 2.5, and 9.8 µm, respectively. The 585 C selenization films were also peeled, and the Mo/CIGS interface was characterized with GIXRD. An additional precursor with Ga/III = 100% was prepared, selenized up to 400 C, and studied with XRD. Chemical bath deposition of nm CdS was then performed, followed by RF sputtering of 60 nm intrinsic ZnO, 120 nm ZnO:Al (2 wt.-% Al 2 O 3 target), and evaporated 50 nm Ni and 3 µm Al top contact grids. More than 260 SLG/Mo/Cu(In,Ga)Se 2 /CdS/i- Fig. 3. Plan view and cross-sectional SEM micrographs of SLG/Mo/CuGa/In precursors with Ga/III ~ 30 (top), 50 (middle), and 70% (bottom). ZnO/ZnO:Al/Ni/Al solar cells (0.42 cm 2 each) were isolated by photolithography, and current density-voltage (JV) measurements were performed on a temperature-controlled stage at 25 C. A solar simulator with a 1,000 W xenon arc lamp was used to illuminate devices with 100 mw/cm 2 AM1.5 after calibrating the intensity with a standard solar cell. A. Precursors III. RESULTS Metal precursor films were found to mostly contain In and γ-cu 9 (Ga,In) 4 ( γ ; see Fig. 2). As expected, the ratio of γ to In increases with increasing Ga/III. The γ peaks also shifted to greater 2θ with increasing Ga/III, which could be due to more compressive strain, decreased Cu content (i.e. more V Cu ), or increased Ga/III, although the final explanation is considered to be the most plausible one. The Ga/III ~ 30% film also contained a small amount of metastable CuIn, which was confirmed by the growth of that peak after a 200 C vacuum anneal, and the disappearance of that peak after a 400 C vacuum anneal (not shown; CuIn decomposition has been reported to occur around 300 C [9]). The Ga/III ~ 70% film /14/$ IEEE 1650

3 Fig. 5. Symmetric XRD patterns for films selenized up to 400 C with Ga/III ~ 30 (top red line), 50 (middle black line), and 70% (bottom blue line). γ refers to γ-cu 9 (Ga,In) 4. β and W peaks are from Cu K β and W impurity in the Cu radiation source. Fig. 4. SIMS profiles of precursors with Ga/III ~ 30 (top), 50 (middle), and 70% (bottom). CuCs + (red C ), GaCs + (blue G ), InCs + (green I ), and MoCs + (black M ) ion signals are shown in addition to Na (orange). Data is raw and panels have the same scale. contained a substantial amount of CuGa 2, which showed a precipitous increase in crystallinity and/or phase amount after the 200 and 400 C anneals (not shown), in agreement with previous reports [4], [10] [11]. SEM and SIMS (Fig. 3 and 4) confirmed the segregation of a smooth, homogenous, Cuand Ga-rich film at the Mo interface, with micrometer-sized In islands on top. The size and coverage of In islands decreased with increasing Ga/III (Fig. 3). This micrometer-scale roughness difference caused a change in apparent SIMS thickness, though precursor molar amounts were equal. B. 400 C Selenization After the very brief selenization up to 400 C, XRD scans showed almost complete disappearance of metal phases (Fig. 5). Due to peak overlap, the exact amount of γ remaining in the films is difficult to determine. The dominant phase was CuInSe 2, and increasing Ga/III had less CuInSe 2, as inferred by peak areas. Ga/III ~ 30% still contained a small amount of unreacted InSe, although no GaSe was observed. GIXRD (not shown) also helped confirm the existence of a Cu-deficient ternary compound peak, nominally referred to as CuIn 3 Se 5. Its TABLE I COMPOSITIONS OF PRECURSOR, 400 AND 585 C SELENIZATION, AND CHAMPION FILMS, AS MEASURED BY XRF Nominal Ga/III (%) Prec. 400 C selen. 585 C selen. Champ. Cu/III (%) 82.3 ± ± ± 2.1 Ga/III (%) 31.4 ± ± ± 1.7 Cu/III (%) 66.7 ± ± ± 3.1 Ga/III (%) 24.8 ± ± ± 2.4 Se/M (%) 66.5 ± ± ± 1.3 Cu/III (%) 71.9 ± ± ± 4.1 Ga/III (%) 26.0 ± ± ± 3.2 Se/M (%) 104 ± ± ± 3 Cu/III (%) 84.4 ± ± ± 1.7 Ga/III (%) 29.7 ± ± ± 1.4 Se/M (%) 102 ± ± ± 2 appearance is expected to be a result of that film s more Cupoor composition, and not its Ga composition (see Table I). The Ga/III ~ 50% film had intermixed (Ga 0.53 In 0.47 )Se in almost equal proportion to CuInSe 2, while Ga/III ~ 70% had substantial amounts of segregated GaSe and InSe. It is not yet clear why moderate Ga content (50%) favored initial Ga and In intermixing, while lower and higher Ga contents (30 and 70%) did not. This is an interesting outcome of this study and deserves further inquiry. SIMS profiles for samples of each Ga content can be found in Fig. 6. The Se composition falls more quickly with depth into the film as Ga/III is increased, which suggests slower selenization kinetics. This is consistent with the XRD /14/$ IEEE 1651

4 Fig. 7. Symmetric XRD patterns for films selenized up to 585 C with Ga/III ~ 30 (top red line), 50 (middle black line), and 70% (bottom blue line). β and W peaks are from Cu K β and W impurity in the Cu radiation source. Fig. 6. SIMS profiles of 400 C selenized films with Ga/III ~ 30 (top), 50 (middle), and 70% (bottom). CuCs + (red C ), GaCs + (blue G ), InCs + (green I ), SeCs + (purple S ), and MoCs + (black M ) ion signals are shown in addition to Na (orange). Data is raw and each panel has the same scale. findings. The Cu profile for Ga/III ~ 30% is relatively flat, while those of Ga/III ~ 50 and 70% have peaks near the surface and near the Mo interface. GIXRD (not shown) suggests that these peaks correspond to CuInSe 2 at the surface, and unreacted γ at the Mo interface. The drop in Cu content could be associated with (Ga,In)Se. The In profiles show similar trends to the Se profiles, while the Ga profiles are all sharply graded. The difference in Ga profiles is probably due to the thicker Cu- and Ga-rich films in precursors with increasing Ga/III (see Fig. 3). The Na profile, like Cu, is relatively flat in Ga/III ~ 30%, and shows more pronounced peaks at the surface and rear interface for Ga/III ~ 50 and 70%. For the 400 C selenized films, bulk Se/(Cu+Ga+In) (Se/M) composition by XRF (Table I) decreased with increasing Ga/III. The amount of metal and Se supplied the films was estimated from EIES and quartz crystal data. Combined with XRF compositions, the percentage of supplied Se which was incorporated into the film (sticking coefficient or utilization fraction) was estimated to be 49, 39, and 35% for Ga/III ~ 30, 50, and 70%, respectively. All of these trends are consistent with slower selenization kinetics with increasing Ga/III. An additional Ga/III = 100% precursor was selenized up to 400 C and characterized by XRD to determine if the presence of GaSe in films with Ga/III ~ 50 and 70% was related to Ga content or processing. This partially selenized film contained Cu 2 Se, CuGaSe 2, GaSe, CuGa 2, and possibly γ, in order of predominance. Due to peak overlap, no conclusions could be drawn regarding the presence or amount of γ. C. 585 C Selenization A longer partial selenization up to 585 C resulted in almost complete conversion to the chalcopyrite phase, as evidenced by XRD (Fig. 7). The films also reached stoichiometric Se incorporation (Se/M ~ 100%; Table I). GIXRD showed all Ga/III contents to exhibit the typically observed segregation of In-rich and Ga-rich CIGS at the surface and back interface, respectively. Symmetric XRD revealed three trends with increasing Ga/III: more GaSe, greater Ga-rich to In-rich chalcopyrite phase ratio, and greater Ga/III contents in both the Ga- and In-rich chalcopyrite phases. In an effort to detect residual intermetallic phases, the CIGS films were peeled. All samples had good adhesion, and typically peeled at the SLG/Mo interface. GIXRD performed on the peeled and exposed surfaces at the Mo/CIGS interface did not reveal any γ phase, though peak overlap disallows a conclusive statement regarding its absence. D. Complete Growth Solar cells were fabricated with absorbers that were more fully selenized (20 min at 600 C), and PV parameters for the best devices with each Ga content are summarized in Table II /14/$ IEEE 1652

5 TABLE II PHOTOVOLTAIC PARAMETERS OF EACH CHAMPION Nominal Ga/III (%) η (%) V OC (mv) J SC (ma/cm 2 ) FF (%) Though devices were made with absorbers of many different compositions, the Cu/III of all champions was effectively the same, ~85% (Table I). The anticipated trend of increasing V OC and decreasing short-circuit current density (J SC ) was observed with increasing Ga/III, while fill factors (FF) were all similar. The champion efficiencies were effectively the same for Ga/III ~ 30 and 50%, while worse performance was obtained for Ga/III ~ 70%. The only previous report of device results for selenized absorbers with varying Ga/III also found enhanced performance for Ga/III ~ 50% (η = 10.4, 13.1, and 6.4% for Ga/III = 25, 50, and 75%, respectively, where the final result required an extra anneal step [4]). The present study and [4] both used simple processes temperature soaks with Se exposure. The main difference was that here CuGa/In precursors were co-evaporated and reacted with Se vapor in vacuum for 20 min at 600 C, while [4] sputtered and evaporated Cu/Ga/In and reacted with H 2 Se at atmospheric pressure for 90 min at 450 C. Co-evaporated Cu-Ga-In precursor films have previously been found to selenize substantially faster than sputtered precursors [12]. IV. DISCUSSION Characterization of the co-evaporated precursors by XRD, SEM, and SIMS all showed results similar to what has previously been observed in sputtered CuGa/In precursors. Indeed, Cu-Ga-In thin films segregate into this bilayer structure whether they are sequentially sputtered from Cu 8 Ga 2 and In targets [13], the same repeated in 350 exposures [14], sputtered from Cu-Ga-In ternary targets [15], co-sputtered from dual targets [16], co-evaporated CuGa/CuIn bilayers [13], evaporated Cu/Ga/In layers repeated 4 times, or the same repeated 8 times [13]. These deposition processes all appear to result in roughly similar precursor films, which likely dictates the diffusion-limited selenization reaction mechanism. However, the aspect ratio, size, and coverage of In islands as well as the precise nature of the Cu-Ga film s grain boundaries could dominate selenization rates and ultimate absorber quality. Changing the precursor deposition process could therefore change economic feasibility, or cost per peak Watt of power produced. Further, more precise measurements will be needed to characterize the influence of deposition process on grain structure. XRD on 400 and 585 C selenizations suggest that the formation of InSe, CuInSe 2, GaSe, and CuGaSe 2 occurs regardless of Ga/III. The following reactions are proposed, in order of kinetic favorability: In+ Se InSe (1) In+ γ - Cu Ga ( In, Cu) + γ -Cu Ga (2a) ( In, Cu) + InSe+ Se In+ CuInSe (2b) γ -Cu Ga + Se γ -Cu Ga + GaSe (3) γ -Cu Ga + GaSe+ Se γ -Cu Ga + CuGaSe (4) Here (In,Cu) represents a nearly pure liquid or solid In phase with Cu dissolved. The stoichiometric coefficients of the γ phase were chosen for simplicity, their actual values are unknown. The extent of Ga and In intermixing is also unknown, and therefore omitted. These reactions are in good agreement with previous observations. InSe formation has been observed by high temperature XRD (HTXRD) during the selenization of precursor stacks of 4 and 8 Cu/Ga/In [13]. Previously, GaSe was not observed by HTXRD during selenization of co-evaporated Cu-Ga (Ga/III = 100%) precursors [10]. On the other hand, GaSe was reported in condensed Se cap selenization up to 450 and 550 C of electrodeposited precursors with Ga/III = 100% [11]. That study, like the present one, did not observe GaSe in partially selenized precursors with Ga/III ~ 30%. As the 400 C selenized precursor with Ga/III = 100% contained GaSe in this study as well, it is likely that observation of GaSe was a result of higher Ga/III contents. The method used here and by [11] of interrupting selenization, cooling, and then characterizing yielded different results from the HTXRD technique of [10]. Possible reasons for this departure are the slower temperature ramp used in HTXRD (20 C/min versus 100 and 480 C/min [11]), the low Se partial pressure during the initial stages of HTXRD, the continued Se exposure during cooling which is avoided with HTXRD, or the low signal-to-noise ratio of 8 2θ/min HTXRD scans (which may have obscured Ga-Se compound peaks). The final reason is included because no Ga-Se compounds were observed at all in [10], and it is perhaps unlikely that a film with more Ga than Cu would form copper selenides and not form gallium selenide products in parallel. Assuming almost all of the precursors In is segregated, the underlying Cu-Ga films with Ga/III ~ 30, 50, and 70% have Cu 74 Ga 26, Cu 63 Ga 37, and Cu 55 Ga 45 compositions. The Cu-Ga phase diagram consists of ζ + γ 1 -Cu 70 Ga 30, γ 2 -Cu 63 Ga 37, and γ 3 -Cu 58 Ga 42 + CuGa 2 at those compositions, respectively [17]. The γ compound accommodates compositional variations with V Cu [17]. Therefore, substantially reducing the γ phase s Cu content could facilitate intragrain and/or intergrain diffusion. On the other hand, films with increased Ga/III also have slower chemical reaction rates (since In + Se reactions are faster than Ga + Se reactions [5]). The reduced selenization /14/$ IEEE 1653

6 rates observed for increasing Ga/III in the present study were likely dominated by the difference in In + Se relative to Ga + Se rates. A more precise technique would be needed to determine the effect of diffusivity changes with γ composition. The results of the present study indicate that for co-evaporated precursors, changing Ga/III does not change the selenization mechanism, but does change the selenization rate, which is attributed only to substituting In with Ga. The drop in efficiency for the Ga/III ~ 70% champion was small compared to the drop in efficiency for the same overall composition in ungraded co-evaporated absorbers (η = 14.9, 15.0, 13.1, and 10.1% for Ga/III = 30, 43, 58, and 69% [18]). The difference in performance deterioration with bulk composition is likely due to the front surface PV-active region of selenized devices having a lower Ga/III than the bulk film. Nevertheless, this indicates that the selenized high Ga/III material had relatively good electronic properties, despite using a simple, brief selenization. An investigation of the devices optoelectronic properties is underway, which may reveal routes for engineering better selenization processes for precursors with greater Ga/III content. V. CONCLUSIONS The present results suggest that selenization occurs in films with Ga/III ~ 30, 50, and 70% by similar mechanisms, with the main difference being that the latter are slower to fully form the chalcopyrite phase. This kinetic difference is in good agreement with previous literature [5]. The apparent absence of unreacted γ after the 585 C selenization indicates that although characterization suggests co-evaporated precursors are very similar to sputtered precursors, the former selenize faster, in agreement with [12]. A finer-grained structure in coevaporated precursors is hypothesized to allow more diffusion, although further study is needed. Finally, solar cells with η of 14.5 and 14.4% were achieved for Ga/III ~ 30 and 50%, respectively. These findings suggest that the selenization of precursors with Ga/III greater than 30% merits far more attention than it has so far received. ACKNOWLEDGEMENT The authors would like to thank Stephen Glynn and Carolyn Beal for their assistance with experiments, and acknowledge the financial assistance of the Department of Energy under FPACE contract DE-FOA REFERENCES [1] Press release, Solar Frontier sets thin-film PV world record with 20.9% CIS cell, Tokyo, [2] M. Ganchev, J. Kois, M. Kaelin, S. Bereznev, E. Tzvetkova, O. Volobujeva, N. Stratieva, and A. 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