Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films Kihwan Kim, Gregory M Hanket, Trang Huynh, and William N Shafarman Citation: Journal of Applied Physics 111, 083710 (2012); doi: 10.1063/1.4704390 View online: http://dx.doi.org/10.1063/1.4704390 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/111/8?ver=pdfcov Published by the AIP Publishing Advertisement: [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 JOURNAL OF APPLIED PHYSICS 111, 083710 (2012) Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films Kihwan Kim, Gregory M Hanket, Trang Huynh, and William N Shafarmana) Institute of Energy Conversion, University of Delaware, Newark, Delaware 19716, USA (Received March 2012; accepted 14 March 2012; published online 19 April 2012) Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in H2Se or H2S In this work, CIGSS films with homogenous Ga distribution and good adhesion were formed using a three-step reaction involving: (1) selenization in H2Se at 400 C for 60 min, (2) temperature ramp-up to 550 C and annealing in Ar for 20 min, and (3) sulfization in H2S at 550 C for 10 The 1st selenization step led to fine grain microstructure with Ga accumulation near the Mo back contact, primarily in a Cu9(In1ÀxGax)4 phase The 2nd Ar anneal step produces significant grain growth with homogenous through-film Ga distribution and the formation of an InSe binary compound near the Mo back contact The 3rd sulfization step did not result in any additional change in Ga distribution or film microstructure but a small S incorporation near the CIGSS film surface and complete reaction of InSe to form CIGSS were observed The three-step process facilitates good control of the film properties by separating different effects of the reaction process and a film growth model is proposed Finally, CIGSS solar cells with the three-step reaction were C 2012 fabricated and devices with efficiency ¼ 14.2% and VOC ¼ 599 mV were obtained V American Institute of Physics [http://dx.doi.org/10.1063/1.4704390] I INTRODUCTION The growing demand for low cost Cu(In,Ga)Se2 (CIGS) or Cu(In,Ga)(Se,S)2 (CIGSS) solar cells has led to increased interest in the reaction of metal precursors in Se- and/or S-containing atmospheres which may have advantages for large-scale manufacturing.1 With precursor reaction in H2Se and H2S, 17.2% efficiency has been demonstrated on 30  30 cm2 submodules and large scale manufacturing is underway.2,3 While a number of options have been demonstrated for deposition of the precursor films, and for the reaction, a common problem is that the reacted CIGS films have Ga accumulation near back contact which results in lower bandgap near the front of the film and, consequently, lower than expected VOC.4–6 Another issue with precursor reaction is the poor quality of the Mo/CIGSS interface which may yield poor adhesion This narrows the process window and causes yield issues with large area processing The adhesion issues have been ascribed to stress built-up in the CIGSS film due to the volume expansion from the metal precursor to CIGSS (Ref 7) or to the formation of a MoSe2 layer on the Mo surface It has been suggested that the MoSe2 orientation may determine adhesion, and the c-axis should be oriented perpendicular to the Mo surface.8,9 Hanket et al.10 characterized the reaction pathways to form Cu(InGa)Se2 or Cu(InGa)S2 films from metal precursors using H2Se and H2S, respectively According to their study, there are preferential and/or fast reactions of In with Se, and Ga with S, that yield an unfavorable Ga grading through the CIGSS films formed by the sequential process of selenization/sulfization The Ga and In could be homogea) Electronic mail: wns@udel.edu 0021-8979/2012/111(8)/083710/8/$30.00 nized but slow interdiffusion required annealing at 600 C for h.5 Other studies have successfully demonstrated a Ga homogenization by adjusting the controlling reaction,11–14 but often with poor adhesion Another approach by Kim et al.15 introduced a three-step reaction process which included selenization at 400 C, annealing at 550 C and, finally selenization at 500 C They realized partial Ga homogenization with improved adhesion but the obtained VOC $ 500 mV was low relative to that expected for the bandgap estimated by the Ga alloying This was attributed to Ga depletion near the surface which apparently occurred during the 3rd step selenization at 500 C In this work, we report a three-step process for reaction of Cu-Ga-In precursor films that gives good control of adhesion and through-film composition with enhanced grain size and produces high VOC in solar cells made from these films The process includes an initial 400 C reaction in H2Se followed by an anneal step in Ar at 550 C and finally a reaction in H2S at 550 C Material characterizations for each step have been carried out, and we describe the film formation process in terms of grain growth and Ga distribution Thin film solar cells were fabricated with the CIGSS absorber layer and the devices exhibited efficiency >14% with VOC % 600 mV II EXPERIMENTAL Mo (700 nm) and metal precursors were sputter deposited from In and Cu0.8Ga0.2 targets onto 100  100 soda lime glass substrates The sputtering system was configured with a rotating (5 rpm) platen so that 350 alternating layers of In and Cu0.8Ga0.2 were deposited corresponding to 700 nm which is sufficient to form a fully reacted CIGSS film with thickness 1.6 $ 1.8 lm 111, 083710-1 C 2012 American Institute of Physics V [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-2 Kim et al The metal precursors were reacted and annealed in a 200 diameter quartz tube with heating jacket which enables temperature up to 600 C and a push-pull rod that enables samples to be inserted or removed from the hot reaction zone.10 Before reaction, the reactor was evacuated to  10À6 Torr to remove moisture and impurities Then, Ar gas was introduced in the reactor and the pressure was allowed to reach atmosphere Ar flow was maintained through the entire reaction with a nominal turnover time of about Prior to heating, samples were held in a room temperature end of tube For the reaction process, the reactor hot zone was first heated to 400 C in flowing H2Se(0.35%)/O2(0.0035%)/ Ar(balance) Then, the samples were pushed in the hot zone and the 1st step selenization was carried out for 60 After the 1st step, the samples were pulled out of the hot zone and the temperature was increased to 550 C in flowing Ar When the temperature stabilized, the sample was moved into the hot zone again and the 2nd step Ar annealing was performed for 20 Finally, with the samples remaining in the hot zone, the 3rd step sulfization was carried out in flowing H2S(0.35%)/O2(0.0035%)/Ar(balance) at 550 C for 10 After the sulfization, the samples were pulled back to the end of the tube and cooled down to T < 100 C in 60 minutes The role of O2 in both selenization and sulfization is believed to form a thin oxide layer on the top of the precursor during the initial reaction to prevent excessive agglomeration10 and to react in the gas phase with H2Se and H2S to form elemental Se and S.16 Solar cells were fabricated with a conventional Mo/ CIGSS/CdS/i-ZnO/ITO/Ni-Al structure A 50 nm thick CdS layer was deposited on the CIGSS absorber layer by chemical bath deposition (CBD) and i-ZnO (50 nm)/ITO (150 nm) layers with sheet resistance 30 X/sq were deposited by RF magnetron sputtering Finally, a grid of Ni (50 nm)/Al (3000 nm) was deposited by e-beam evaporation Cells were delineated by mechanical scribing with area 0.47 cm2 Film characterization included scanning electron microscopy (SEM) images with EDS analysis using an Amray model 1810 microscope with an Oxford Instruments PentaFETV 6900 EDS detector All EDS measurements were taken with a 20 kV accelerating voltage, so the measurement probes approximately lm or half of a lm thick CIGSS film For improved image resolution, some SEM images were acquired using a JEOL JSM-7400 F Asymmetric (2h scan) X-ray diffraction (XRD) analysis was performed to characterize phases in the metal precursors and reacted films using a Rigaku D/Max 2500 With incident angles from 0.5 to 8 , XRD patterns sensitive to sampling depth were acquired Broad spectrum XRD patterns were obtained with a step size of 0.05 and scan speed of 0.5 /min, and fine XRD line profiles were taken with a step size of 0.02 and a scan speed of 0.05 /min Auger electron spectroscopy (AES) measurements for composition depth profiling were conducted using Physical Electronics 660 Scanning Auger Microprobe Current-voltage (J-V) characteristics including total area conversion efficiency were measured under AM 1.5 illumination at 25 C and quantum efficiency (QE) was performed after solar cell fabrication R J Appl Phys 111, 083710 (2012) FIG Plan-view SEM images of a Cu-In-Ga metal precursor III RESULTS A Cu-In-Ga metal precursor SEM images of as-sputtered precursor in Fig show nodules distributed on a smooth background Although the metal precursor was prepared by repetition of In and Cu-Ga layers, it seems significant agglomeration occurs and a smooth layered structure is not obtained Table I shows wide-area and spot-EDS measurements taken from Fig The wide area average EDS measurements give composition ratios Cu/(In ỵ Ga) ẳ 0.82 and Ga/(In ỵ Ga) ẳ 0.19 Spot-EDS measurements of the “nodules” and the smooth “background” show significant differences in composition The nodules are In-rich, while the smooth background is Cu-rich and Ga-rich compared to the average composition XRD patterns taken with incident beam angles of 0.5 and 4 are shown in Fig Since both Cu and Ga solubilities in In are low,17 almost pure In and intermetallics such as Cu3Ga, Cu4In, and Cu9(In1 À xGax)4 are observed.10,18–22 The value of x in the Cu9(In1ÀxGax)4 phase is determined to be $0.4 by applying Vegard’s law.23 Comparison of the different incident angles shows that the surface is relatively In-rich and the bulk film contains the Cu-rich intermetallic phases Therefore, wide-area EDS measurements lead to overestimation of the relative In composition Even though the measured average Cu/(In ỵ Ga) ratio of the metal precursor is 0.82, the reacted film is expected to yield higher Cu/(In ỵ Ga) B Film formation by three-step reaction Fig shows cross sectional SEM images of reacted films after each step The 1st step (H2Se at 400 C for 60 min) leads to fine microstructure and flat voids between TABLE I Compositional values of wide-area and spot-EDS measurements taken from Fig EDS measurement Location Cu (at %) In (at %) Ga (at %) Mo (at %) Cu/ (In ỵ Ga) Ga/ (In þ Ga) Average Nodule Background 38.4 35.6 41.2 37.9 43.6 25.2 8.7 8.1 9.7 15.1 12.6 23.0 0.82 0.69 1.18 0.19 0.16 0.28 [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-3 Kim et al J Appl Phys 111, 083710 (2012) FIG Asymmetric XRD patterns of a Cu-In-Ga metal precursor XRD patterns were taken from incident angles of 0.5 (black) and 4 (red) Symbols of n, , ~, and ! indicate In, Cu3Ga, Cu4In, Cu9(In1Àx,Gax)4, respectively the CIGS thin film and Mo back contact as shown in Fig 3(a) The bottom $500 nm of the film shows finer structure However, after the 2nd step (Ar anneal at 550 C for 20 min), the film exhibits a significant grain growth with an agglomeration of voids as shown in Fig 3(b) Even though there are still the voids in the CIGS film, the contact area between the Mo and CIGS is effectively increased and thus might lead to improvement of film adhesion Finally, after the 3rd step (H2S at 550 C for 10 min), it appears that the films not undergo any significant change as shown in Fig 3(c) Plan-view SEM images of the reacted films from both front and back sides along with associated EDS data are shown in Fig The images from the films back sides were taken after delamination from the Mo/SLG using the technique described elsewhere.10 The front surfaces of the reacted films show a similar trend to the cross sectional observations in Fig The transition between the 1st step and the 2nd step causes a significant change in the morphol- FIG Cross-sectional SEM images of reacted CIGS or CIGSS films after (a) 1st step, (b) 2nd step, and (c) 3rd step FIG Plan-view images of the front and backside of reacted CIGS or CIGSS films after each step with associated EDS data: (a) front side and (b) back side of film after 1st step, (c) front side and (d) back side of film after 2nd step, and (e) front side and (f) back side of film after 3rd step ogy, while the transition between the 2nd step and the 3rd step does not The EDS analysis reveals that the ratio of Ga/ (In ỵ Ga) also changes considerably from the 1st step to the 2nd step After H2Se reaction, the Ga content is much higher at the back half of the film consistent with the commonly observed accumulation of Ga near the Mo contact.10,12,15 However, front and back EDS measurements gave comparable Ga/(In ỵ Ga) ratios as shown in Figs 4(c) and 4(d) so the Ar anneal caused a significant Ga diffusion through the film The H2S reaction step did not cause a significant change in composition except S incorporation to the film surface (Figs 4(e) and 4(f)) Fig shows the (112) peaks of XRD spectra taken from the film surface with incident angles of (a) 0.5 and (b) 8 corresponding to sampling depths of 110 nm and 1.8 lm, respectively.24 The (112) peak positions are summarized in Table II The results from the XRD analyses are in good agreement with the data obtained by the SEM/EDS analyses The (112) peak position after the 1st step is comparable to that of pure CuInSe2 consistent with a lack of Ga incorporation into the chalcopyrite phase This shifts due to Ga homogenization after the 2nd step and the peak position 2h ¼ 26.82 (8 incident angle) corresponds to Ga/(In ỵ Ga) ẳ 0.19 when considering Vegards law and the JCPDS database.23,25,26 After the 3rd step, a broad second peak at 2h ¼ 27.70 is observed only for the scan with 0.5 incident angle and this is attributed to S diffusion at the front of the film The (112) peaks taken from the film back side are shown in Fig and their positions are listed in Table II The surface of the film back side after the 1st step [Fig 6(a)] shows the [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-4 Kim et al J Appl Phys 111, 083710 (2012) FIG Asymmetric XRD patterns of the front side of reacted CIGS or CIGSS films after each step with the incident beam angles of (a) 0.5 and (b) 8 Sampling depths with 0.5 and 8 are $110 nm and 1.8 lm, respectively (112) peak at 26.86 with shoulder peaks at 27.68 and 28.04 which correspond to CuGaSe2 (CGS) and CuGa3Se5, respectively.27,28 After the 2nd step, the shoulder peaks are absent and the difference in the (112) peak positions between the back and front surface measurements is significantly narrowed It seems that at 550 C, there is sufficient energy for the selenized film to be recrystallized with the Ga homogenization No noticeable change was observed from the film back side after the 3rd step C Residual intermetallics and binary compounds SEM images of the Mo surface after delamination are shown in Fig with the associated EDS data in Table III Spot EDS on a nodule in Fig 7(a) reveals Cu and Ga while TABLE II (112) peak position with each step and different incident beam angles Film side Incident angle (x) 1st step 2nd step 3rd step Front side Back side 0.5 0.5 8 26.68 26.84 26.86 26.62 26.82 26.84 26.86 26.90 26.96 26.64 26.82 26.82 FIG Asymmetric XRD patterns of the back side of reacted CIGS or CIGSS films after each step with incident beam angles of (a) 0.5 and (b) 8 only Mo was found from the background Most of these nodules (except residual CIGS particles) were not present on the Mo surface after the 2nd step as shown in Fig 7(b) EDS data show only Mo in this case After the 3rd step, some Se and a roughened Mo surface were observed as shown in Fig 7(c) and the EDS data Fig shows XRD patterns of the Mo surface as observed in Fig taken with the incident beam angle ¼ 0.5 for greatest surface sensitivity The XRD pattern after the 1st step shows a prominent peak at 44.05 Considering the EDS data from the “nodule,” the peak was identified as due to the Cu9(In1ÀxGax)4 intermetallic compound with x $ 0.95.10,21–23 However, no peak from an intermetallic compound is detected after the 2nd step, in accord with the SEM observation It is noteworthy that a MoSe2 phase was found on the Mo surface only after the 3rd step29,30 showing that Se from the film bulk diffused to the Mo to form the MoSe2 during the 3rd step sulfization Finally, an InSe binary compound was also observed in the CIGS film after the 2nd step as shown in Fig 9.31 Peaks from the InSe phase were more intense from the back side of the film than from the front side It seems that the Cu9(In1ÀxGax)4 intermetallic compound reacted with the CIGS under Se- or S- starved condition, and as a result, some CIGS decomposed to form the InSe binary However, it was fully eliminated during the 3rd step [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-5 Kim et al J Appl Phys 111, 083710 (2012) FIG Asymmetric XRD patterns of the surface of Mo back contact after each step: (a) 1st step, (b) 2nd step, and (c) 3rd step All the patterns were taken with 0.5 incident angle corresponding to a sampling depth of $110 nm Symbols of n, , and ~ indicate CIGS, Cu9(In1ÀxGax)4, and MoSe2, respectively and an incomplete reaction degrade the device performance The cell with CIGS film after the 2nd step is fully shunted Presumably, the shunting could be caused by defects formed at the same time as the InSe such as Se-vacancies associated with Se liberation6,32 and/or Cu2Se, even though it was not identified by XRD The solar cell after the 3rd step has an efficiency of 14.2% (without AR coating) Since the Ga homogenization through-film is realized and there is no intermediate product such as InSe, Voc ¼ 599 mV was obtained with high fill factor of 73.5% QE measurements are shown in Fig 11 An estimated bandgap of the CIGSS determined by the inflection point at long wavelength33 gives 1.09 eV CIGSS films finished with sulfization can have a bias dependent collection due to formation of a conduction band barrier with excess S at the surface.12,34,35 This can be assessed by bias dependent QE.36 The ratio QE(0 V)/QE(À1V) shown in Fig 11 is flat indicating negligible bias dependence and, with the high FF, shows that there is no loss due to such a collection barrier FIG Plan-view images of the surface of Mo back contact after delamination of the CIGS film after (a) 1st step, (b) 2nd step, and (c) 3rd step D Device results Solar cells were fabricated from the reacted films and their J-V curves with device parameters are shown in Fig 10 and Table IV With the absorber after the 1st step, the J-V behavior is very leaky and we believe that poor crystallinity TABLE III Compositional values of wide-area and spot-EDS measurements taken from Fig EDS measurement location Cu (at %) In (at %) Ga (at %) Se (at %) Mo (at %) After 1st step: Nodule After 1st step: Background After 2nd step: Background After 3rd step: Background 10.6 < 0.5 — — — — — — < 0.5 — — < 0.5 — — 4.8 84.0 99.4 $100 95.2 FIG Asymmetric XRD patterns of the back side of delaminated CIGS films after (a) 2nd step and (b) 3rd step The InSe phase observed after the 2nd step was completely removed by the 3rd step [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-6 Kim et al J Appl Phys 111, 083710 (2012) FIG 10 Light J-V curves of devices with the reacted CIGS or CIGSS films after (a) 1st step, (b) 2nd step, and (c) 3rd step IV DISCUSSION The most prominent aspect of the three-step reaction is the Ar anneal step which creates through-film Ga homogenization with marked grain growth AES depth profiles after each step, shown in Fig 12, describe the film evolution The depth profile taken from the 1st step indicates Ga accumulation and high concentrations of Cu and Ga at the film/Mo interface consistent with a Cu-Ga intermetallic accumulation A considerable Ga redistribution is realized by the 2nd step and sulfur incorporation is observed with the 3rd step only in the surface region A film-growth model based on the results above is depicted in Fig 13 and related explanations are described below FIG 11 (a) Quantum efficiency of fully processed CIGSS solar cell The estimated bandgap from the quantum efficiency is 1.09 eV The ratio of QE(À1 V)/QE(0 V) (b) is close to indicating little voltage dependent collection B 2nd step: Ar anneal Recrystallization with Ga homogenization is the most prominent effect of the 2nd step as residual Cu-Ga intermetallics are incorporated into the CIGS bulk While the recrystallization and Ga homogenization occur concurrently, it is unclear whether they have a direct connection We previously suggested that the residual Cu9Ga4 intermetallics on the Mo back contact appear to provide a rapid diffusion route A 1st step: Selenization Some previous studies have reported that a reaction preference between In-Se and Ga-Se is likely to be the main cause for formation of Ga-poor CIGS with residual Cu-Ga intermetallics at the Mo interface In the present study, the same phenomena have been observed This step is believed to be the most crucial aspect of the three-step reaction as the extent of the selenization directly affects the Ga homogenization, recrystallization, sulfur incorporation, and adhesion in the following steps The aims of this step are to form a fine crystalline structure with remaining Cu and Ga not reacted with Se as a means to increase surface/interface energy and enable Ga homogenization in the subsequent process This can be understood more clearly with respect to the subsequent steps TABLE IV Summary of device parameters for CIGSS films with each reaction step Film (absorber) condition Voc (V) Jsc (mA/cm2) Fill Factor (%) Efficiency (%) After 1st step After 2nd step After 3rd step 0.252 0.035 0.599 13.8 16.0 32.2 25.9 0.2 73.5 0.9 0.1 14.2 FIG 12 Compositional depth profiles determined by AES of the reacted CIGS (or CIGSS) films after (a) 1st step, (b) 2nd step, and (c) 3rd step The Ga profiles (blue) are shown on the magnified scale of the right axis Ga homogenization occurred after the 2nd step and S (violet) incorporation is limited to the CIGSS film surface after the 3rd step [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 083710-7 Kim et al J Appl Phys 111, 083710 (2012) C 3rd step: Sulfization The main features of this step are sulfur incorporation near the surface and removal of InSe in the film bulk The AES analysis reveals that S is only incorporated near the surface Nevertheless, the InSe binary near the Mo contact is completely removed by the sulfization and MoSe2 is formed on the Mo surface Thus, it appears that sulfization enhances Se indiffusion Accordingly, the degree of S incorporation depends on the amount of InSe in the film and is again controlled by the extent of selenization in the 1st step More InSe causes greater Se induffusion and hence more available sites for S incorporation near the surface It has been shown that too much S incorporation causes poor device performance so the S incorporation needs to be controlled.12,35 V CONCLUSIONS FIG 13 Growth model for the three-step reaction (a) 1st step: due to preferential reaction of In-Se, the surface region contains CIGS with little Ga and the back side has higher Ga content with the Cu9Ga4 intermetallic and finer microstructure; (b) 2nd step: recrystallization and Ga homogenization occur to reduce surface energy as the Cu9Ga4 is incorporated into the CIGS and InSe is formed; (c) 3rd step: S incorporation occurs near the film surface and stimulates Se in-diffusion toward the Mo back contact leading to consumption of InSe and formation of MoSe2 for Ga ions since they are present as a mixture of liquid and solid phase above 485 C.14 It is noteworthy that the recrystallization with the Ga homogenization strongly correlates with the extent of selenization For instance, a film selenized at 450 C for 60 in the 1st step (i.e., a more fully selenized film with a larger grain size and little residual Cu9Ga4 phase remaining10,12–14) did not yield the recrystallization with Ga homogenization in the 2nd step It suggests that the extent of selenization should be controlled to maintain sufficient Cu9Ga4 and a fine microstructure which should result in an enough high interface and/or surface energy to induce the recrystallization during the 2nd Ar anneal step The sensitivity of Ga homogenization and adhesion to the reaction conditions has been discussed in our previous studies.13,14 Formation of the InSe binary may be attributed to a reaction of the residual Cu9Ga4 intermetallic with the CIGS film As Cu and Ga diffuse, they react with Se ions from preexisted CIGS at the backside of the film InSe is formed as the CIGS decomposes since the In-Se bond is weaker than the Ga-Se bond.37 The quantity of InSe also depends on the amount of Cu9Ga4 and therefore on the degree of selenization, and it was shown previously that more InSe could be obtained with shorter selenization time at 400 C.15 Meanwhile, it is not confirmed whether Cu-Se binary compounds are formed as might be expected Even if present in the films, their amount would be small and they were not detected by XRD or Raman spectroscopy In this work, we have described a three-step H2Se/Ar/ H2S reaction of Cu-Ga-In metal precursors to form CIGSS thin films and characterized material properties of the films and solar cells fabricated from them after each step It was shown that this process enables good control of critical film properties including through-film compositional distribution and formation of large grain structure In addition, greater understanding has been gained of mechanisms that cause void formation and poor adhesion at the Mo/Cu(InGa) (SeS)2 interface which commonly occur in films formed by precursor reaction processes Ga accumulation at the back of the reacted film, near the Mo back contact, is caused by the relative stability of the Cu9Ga4 intermetallic phase A critical aspect of the process is that the H2Se reaction time and/or temperature is restricted, so that a two phase film with Cu9Ga4 and CuInSe2 is formed after the first step Then annealing at 550 C in Ar provides sufficient energy to drive interdiffusion of Ga and In and recrystallization to form films with grain size >1 lm The interdiffusion does not require reaction with S, as previously proposed Voids at the back of the fully reacted CIGSS film have comparable size and density as Cu9Ga4 nodules remaining after the first reaction step suggesting that the voids are caused by the agglomeration of the intermetallic phase During the Ar anneal step the film has insufficient Se to fully form Cu(InGa)Se2 and consequently an InSe phase is formed The primary role of the third-step H2S reaction is to complete the reaction and eliminated the InSe The formation of MoSe2 which may cause poor adhesion only occurs after the film has sufficient chalcogen, Se ỵ S, to fully form CIGSS In the three-step process, this occurs only during the H2S reaction Thus controlling the time of this step can be used to minimize the MoSe2 formation Good device performance with high VOC consistent with the Ga intermixing are achieved once all secondary phases are eliminated in the final reaction step ACKNOWLEDGMENTS The authors gratefully acknowledge the 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(2001) 35 U P Singh, W N Shafarman, and R W Birkmire, Sol Energy Mater Sol Cells 90, 623 (2006) 36 S S Hegedus and W N Shafarman, Prog Photovolt: Res Appl 12, 155 (2004) 37 Y Seki, H Watanabe, and J Matsui, J Appl Phys 49, 822 (1978) [This article is copyrighted as indicated in the abstract Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 210.98.0.103 On: Tue, 22 Oct 2013 00:38:48 ...JOURNAL OF APPLIED PHYSICS 111, 083710 (2012) Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films Kihwan Kim,... Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/ or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in... this work, we report a three-step process for reaction of Cu-Ga-In precursor films that gives good control of adhesion and through-film composition with enhanced grain size and produces high VOC