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Author’s Accepted Manuscript Void and secondary phase formation mechanisms of CZTSSe using Sn/Cu/Zn/Mo stacked elemental precursors Se-Yun Kim, Dae-Ho Son, Young-Ill Kim, SeungHyun Kim, Sammi Kim, Kwangseok Ahn, ShiJoon Sung, Dae-Kue Hwang, Kee-Jeong Yang, JinKyu Kang, Dae-Hwan Kim
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S2211-2855(19)30176-4 https://doi.org/10.1016/j.nanoen.2019.02.063 NANOEN3507
To appear in: Nano Energy Received date: 14 December 2018 Revised date: 18 February 2019 Accepted date: 23 February 2019 Cite this article as: Se-Yun Kim, Dae-Ho Son, Young-Ill Kim, Seung-Hyun Kim, Sammi Kim, Kwangseok Ahn, Shi-Joon Sung, Dae-Kue Hwang, KeeJeong Yang, Jin-Kyu Kang and Dae-Hwan Kim, Void and secondary phase formation mechanisms of CZTSSe using Sn/Cu/Zn/Mo stacked elemental precursors, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.02.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Void and secondary phase formation mechanisms of CZTSSe using Sn/Cu/Zn/Mo stacked elemental precursors Se-Yun Kim*, Dae-Ho Son*, Young-Ill Kim, Seung-Hyun Kim, Sammi Kim, Kwangseok Ahn, ShiJoon Sung, Dae-Kue Hwang, Kee-Jeong Yang, Jin-Kyu Kang, Dae-Hwan Kim Convergence Research Center for Solar Energy, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea KEYWORDS : CZTSSe, Metal precursor, Two-step process, Void, Secondary phase, Formation mechanism
Author Contributions *These authors contributed equally to this work. Corresponding Author Dae-Hwan Kim : [email protected] Jin-Kyu Kang : [email protected]
ABSTRACT In recent years, Cu2ZnSn(S1-xSex)4 (CZTSSe) prepared by a two-step process using metal precursors has been reported to exhibit a relatively high power conversion efficiency, and a high efficiency of 12.5% by two-step process contained via sputtering method was recently confirmed
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by our group. In this study, we proposed formation mechanisms for the CZTSSe double layer, voids and ZnSSe layer, which were observed in the CZTSSe using metal precursor. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens such as S and Se, almost all Zn is consumed to form the ZnSSe layer; as a result, large voids are produced first under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe layer via migration of the Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Because of the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, a CZTSSe double layer can be formed, and ZnSSe can exist between these CZTSSe layers. Finally, we propose a method based on the formation mechanism to control the voids and secondary phases, which affect the fill factor and output current. Graphical abstract: The new formation mechanisms of the voids, ZnSSe layer and CZTSSe double layer that were observed in the sister sample of 12.5% of CZTSSe cell.
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Keywords: CZTSSe; Metal precursor; Two-step process; Void; Secondary phase; Formation mechanism
1. Introduction To achieve the goals set out in the Paris Agreement, the electricity output of photovoltaic (PV) plants should be much higher than current levels. Si PVs are likely not the only solution for increasing electricity production. Because Si cells do not effectively collect sunlight, an alternative highly crystalline thin-film layer is also required to achieve high efficiency [1,2]. In addition, the non-flexible material properties of Si PVs cause problems that make them difficult to apply in various fields involved with expanding PV plants. As an alternative, thin-film PV technology with advanced material properties such as a high light absorption coefficient has been developed, and this technology enables solar modules to be integrated with buildings and wearable devices. Thin-film technology that can realize efficiencies as high as that of Si PVs through the use of materials such as CdTe and CIGS already exists. However, these materials have limitations due to the toxicity of Cd and Te and the scarcity of In, Ga, and Te. As another alternative, organo-metal hybrid halide perovskites (CH3NH3PbI3) have garnered an immense amount of research interest due to a high efficiency of 23.3% [3]. However, the issues raised by the toxicity of lead [4] and the long-term stability of the material under humidity and light remain open questions [5,6]. Hence, the development of a low-cost PV technology with non-toxic and earth-abundant materials is still needed. Among the emerging PVs, kesterite Cu2SnZnS4 (CZTS), Cu2SnZnSe4 (CZTSe), and Cu2ZnSn(S1-xSex)4 (CZTSSe) thin-film solar cells are considered promising for
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large-area module production using earth-abundant and non-toxic elements [1,7]. These market trends for PV, and the status of alternative PV technologies being developed have led researchers to devote much attention to kesterite PVs, although the record efficiency of these devices has not been improved since 2014. Thus far, the highest reported power conversion efficiency for a CZTSSe cell is 12.6%, which was achieved with CZTSS layer prepared via a two-step process using hydrazine solution [8]. Hydrazine-solution-based processes using various organic solvents have disadvantages in terms of the risk of explosion, wastewater treatment requirements and incompatibility with large-area processes. Thus, numerous attempts have been made to replace the hydrazine solution process with a compound precursor or a metal precursor; however, no methods have led to a device with an efficiency greater than that achieved with a CZTSSe layer prepared via the two-step hydrazine process [9]. Thus far, a CZTSSe double layer, a ZnSSe layer between these CZTSSe layers and large voids at the Mo rear contact side have been commonly observed in the microstructures of the CZTSSe light-absorber layer formed by a two-step process using metal precursors [10-16]; in contrast, hydrazine based CZTSSe showed relatively small amounts of voids and secondary phases, and CZTSSe absorbers prepared from compound precursor containing S or Se have shown relatively void-free interfaces and secondary phases that are not easily observed by simple scanning electron microscopy (SEM) [17]. Likewise, the CZTSSe double layer, ZnSSe and voids were always observed in CZTSSe absorver layer from our process for the high-efficiency. The presence of voids and secondary phases could increase the series resistance (Rs) [18] and reduce the shunt resistance (Rsh) [19-21]. Thus, the voids and secondary phases should be controlled.
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The CZTSSe formation mechanism has been proposed based on the results of in situ X-ray diffraction (XRD) and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM-EDS) mapping [11,12,22-29]. However, these data are not sufficient to clarify the origin of the CZTSSe double layer, ZnSSe secondary phase, and voids commonly observed in CZTSSe absorbers. So far, two formation mechanism models for voids and secondary phases at the Mo-back contact side are described as follows. First, the decomposition reaction of CZTS(e) or the formation of SnS(e) is expected to be the cause of void formation [30,31]. The ease of the reduction of Sn(IV) and the favorability of MoS(e)2 formation can induce phase separation by removing S(e) from the CZTS(e) at the Moback contact side, as shown in Eq. (1), where volatile Sn(II)S(e) is formed by the reduction of Sn(IV)S(e)2 [31]: 2Cu2ZnSnS(e)4 + Mo → 2Cu2S(e) + 2ZnS(e) + 2SnS(e) + MoS(e)2
Eq. (1)
A high S(e) partial pressure has been reported to reduce void formation by inhibiting the formation of volatile SnS(e); when the partial pressure is high, the SnS(e)2 and Sn2S(e)3 phases, which are hardly volatilized, are stabilized [32]. Second, the Kirkendall effect between the Cu metal and Cu2S(e) causes void formation [33]. The inclination toward Cu vacancy migration occurs because of the difference between the diffusion fluxes of Cu and S(e), and the voids in the Cu metal side are formed by the accumulation of Cu vacancies; Scragg predicted that the Cu in a Cu-Sn alloy, in particular, would participate in the Kirkendall effect [33]. Each model has an important meaning; however, these simplified models present difficulties in fully understanding the actual void formation mechanism during the sulfo-selenization process
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without corresponding experimental evidence. Consequently, attempts to suppress void formation based on each model often appear to have limited effectiveness [34]. In this study, we report on the formation mechanisms of the double layer, voids, and ZnSSe layer in the CZTSSe which was shown high efficiency of 12.5% certified by the Korea Institute of Energy Research (KIER). Detailed formation mechanisms are reasonably proposed based on phase evolution tracking at various temperatures by means of TEM analysis. The samples were collected at various temperatures during sulfo-selenization, which is the same process used for fabricating the highest-efficiency cells. Metal precursors with a stacked structure (Sn/Cu/Zn) were deposited onto a Mo-coated soda-lime glass substrate by DC sputtering and were sulfoselenized in an rapid thermal processing (RTP) chamber. H2S gas and Se shots were used as chalcogen sources. The samples collected at meaningful times and annealing temperatures were analyzed mainly by STEM.
2. Experimental section 2.1 Device fabrication The structure of the CZTSSe solar cell consisted of a soda-lime glass (SLG) substrate, a 600 nmthick Mo layer as a back contact layer, an approximately 1.8 μm-thick CZTSSe absorber layer deposited via sputtering and annealed under Se and H2S gas (diluted with 90 vol% Ar), a 50 nmthick CdS buffer layer deposited via chemical bath deposition, a 50 nm-thick intrinsic ZnO layer deposited via sputtering, a 300 nm-thick Al-doped ZnO (AZO) layer deposited via sputtering, a 10 nm-thick Ni and 2 μm-thick Al grid deposited via e-beam evaporation, and a 100 nm-thick MgF2 layer deposited via e-beam evaporation.
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The Mo layer was deposited onto the SLG substrate via DC magnetron sputtering using a Mo target with a purity of 99.99%. The metal precursors for the CZTSSe absorber layer were deposited onto the Mo layer using 99.99% pure Sn, Cu, and Zn sputtering targets with a stacking order of Sn (275 nm)/Cu (160 nm)/Zn (188 nm)/Mo. For the sulfo-selenization process, all the samples were placed in a sample box that consisted of a Se boat made of quartz, a sample holder made of SiC-coated graphite, and a quartz cover plate. Before annealing began, Se (approximately 250 mg), H2S (diluted with 90 vol% Ar, 250 sccm, approximately 8 min) and Ar (2000 sccm, approximately 8 min) gas were supplied to the RTP chamber. To prevent severe decomposition of the CZTSSe, the annealing processes were conducted in a RTP chamber at a pressure slightly below 1 atm. The sample was heated from room temperature to 300 C over 560 s and then maintained at 300 C for 900 s. Subsequently, the sample was heated from 300 C to 480 C over 1800 s and then maintained at 480 C for 600 s. Se was purchased from SigmaAldrich and used without purification. The remaining layers in the device, as previously described, were subsequently added using the aforementioned procedures.
2.2 Device & Material characterization Current– voltage characteristics were measured under a simulated air mass spectrum of 1.5 global (AM 1.5 G) and 100 mW/cm2 (1 sun) illumination at 25℃ using a solar simulator (WACOM, WXS-155S-L2_Class-AAA) in Korea Institute of Energy Research (KIER) for certification. The EQE values were measured under a AM 1.5 G and 100 mW/cm2 illumination at 25℃ using SOMA Optcis (S-9230) in KIER. Surface and cross-sectional images of the absorber layers were obtained by field-emission scanning electron microscopy (FE-SEM, Hitachi Co., model S-4800). The samples for STEM
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measurement were prepared with a dual-beam focused ion beam (FIB) system (Hitachi Co., model NB5000). The STEM-EDS measurements were performed using a QUANTAX-200 (Bruker Co.) to analyze the compositions of the absorber layers. The etched samples were prepared with a multi-functional FIB system (FEI Co., model Helios NanoLab G3UC).
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3. Results and discussion Fig. 1 shows the current–voltage (J–V) characteristics of the highest efficiency CZTSSe device along with the external quantum efficiency (EQE) and statistical properties of device performance parameters of the certified CZTSSe devices. The performance of all devices was certified by the Korea institute of energy research under standard test conditions. The area of the certified devices is more than twice as large as the previous result [10], and the cell designated area including the top contact grid was defined by mechanical scribing to be around 0.5 cm2. The certified highest efficiency device exhibits an efficiency of 12.5 %, open circuit voltage (VOC) of 0.516 V, fill factor (FF) of 66.89 % and short-circuit current (JSC) of 36.13 mA/cm2 in Fig. 1(a). The EQE spectrum of this device is high in the visible range and then decreases in the longerwavelength region, as shown in Fig. 1(b). The bandgap (Eg) of the CZTSSe thin film was calculated by the dEQE / dλ method, and the value was 1.09 eV. VOC deficit defined as Eg/q − VOC, where q is the elementary charge. The device shows a Voc deficit of 0.574 V, which is one of the lowest VOC deficits in our devices. The statistical certified 14 devices results are presented in Fig. 1(c). A large number of certified devices exceeded the conversion efficiency of more than 12 % and showed an average JSC of around 36 mA/cm2, a VOC of 0.518 V, and an FF of 65%. The area of the statistically processed devices is not all the same due to mechanical scribing, and has an area of about 0.48 cm2 to 0.52 cm2. The detailed results of the electrical properties of device will be discussed in a future manuscript. In this paper, we report the formation mechanism of CZTS double layer, void and ZnSSe layer. Fig. 2 (a) and (b) shows the FE-SEM and STEM images of the sister sample of the CZTSSe cell with a power conversion efficiency of 12.5%. The CZTSSe layer consists of a double layer in which the upper layer is dense and void-free, whereas the lower layer consists of voids (empty
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volumes) and CZTSSe (filled volume). The relatively large-grained ZnSSe secondary phase layer lies between the upper CZTSSe and the lower CZTSSe. Zn-, Cu- and Cu-Sn-SSe secondary phases are observed in the MoSSe layers. The thickness of the MoSSe layer on the Mo electrode is approximately 500 nm. To observe the overall vertical microstructure, 200 cross-sectional images of a 20 μm × 4 μm area were obtained using an FIB slice technique, as shown in Fig. S1 and in the supplemental PowerPoint file (S1.pptx) containing an animated image. To observe the distribution of the voids, the sister sample was etched with a Ga-ion beam using FIB, as shown in Fig. 2(c); the etched area was 30 μm × 20 μm; the change in the microstructure as a function of the etching time is shown in Fig. S2 and the supplemental PowerPoint files (S2(a).pptx and S2(b).pptx) containing animated images. The two types (large and small) of voids were observed in the lower part of the CZTSSe layer. A high-magnification image is shown in Fig 1(d), and corresponding EDS maps of Cu, Zn, Sn, S, Se and Mo are shown in Fig. S3. In the CZTSSe double layer, unevenly distributed voids and a ZnSSe secondary phase layer were observed in all areas. As evident from the high efficiency, sulfo-selenization using sputtered pure-metal precursors is a very useful process due to its many advantages, which include a low processing temperature, low target cost, and compatibility with commercialization. If the CZTSSe double layer, voids and ZnSSe layers are effectively controlled, the efficiency of CZTSSe solar cells can be increased further. To control the voids and secondary phases in the CZTSSe layer, their formation mechanisms should be understood. Thus, the phase transition behavior from the metal precursors to CZTSSe was investigated at multiple points on our optimized temperature profile line for sulfo-selenization, which is shown in Fig. 3(a).
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Fig. 3(a) shows the temperature profile of our two-step process, the inset images in Fig. 3(a) show the photographs of samples withdrawn at specific points during annealing, and Fig. 3(b)(h) shows the corresponding vertical STEM images of each point. Although the results from these samples do not fully represent the in situ process, the formation mechanisms of the CZTSSe double layer, ZnS and voids during the sulfo-selenization process can be deduced. Fig. 3(b) shows the cross-sectional STEM images of the as-deposited metal precursors. Fig. 3(c) shows the STEM image of the sample cooled immediately after reaching 300 C. Fig. 3(d) shows the cooled sample after annealing at 300 C for 10 min. Fig. 3(e) shows the sample cooled as soon as it reached 400 C. Dramatic changes in the microstructure are observed in Fig. 3(e); thus, STEM analysis was conducted at different locations in the same sample. We assumed that the temperature inside this sample was lower than that outside the sample. Fig. 3(f) shows the sample that was cooled as soon as it reached 440 C. Fig. 3(g) shows the sample that was cooled as soon as it reached 480 C. Each sample in Fig. 3(b)–(g) was investigated in detail by FE-SEM and STEM-EDS mapping, as shown in Figs. 4-9; Fig. 2(h) was already shown in Fig. 1. The microstructure and composition of the Sn/Cu/Zn/Mo metal stack prepared by DC magnetron sputtering at room temperature were investigated by FE-SEM and STEM, as shown in Fig. 4. The top view and the cross-section view (Fig. 4(a)) confirm that a continuous film of Sn is not formed due to island shape deposition, as shown in Fig. 4(a). This formation is thought to be due to the poor wetting property of Sn; Cu-Zn or/and Cu are expected to be partially exposed to the surface with Sn. From the STEM-EDS mapping images in Fig. 4(c) and (d), although the three metal layers were deposited in the order Zn, Cu, Sn at room temperature, alloys with unevenly distributed compositions were formed. For example, the Cu-Zn alloy (γ-Cu5Zn8, Cu:Zn:Sn = 37:60:3) formed as the lower layer with a columnar structure, the Cu-Sn alloy (η′-Cu6Sn5,
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Cu:Zn:Sn = 46:1:53) and Sn (Cu:Zn:Sn = 1:0:99) formed as the upper layer, which was not continuous, and a Cu residue (Cu:Zn:Sn = 94:1:5) was located on the Cu-Zn alloy layer, as clearly shown in Fig. 4(d); the phases were assigned and predicted based on the XRD data (not shown in this manuscript) and binary phase diagram of the Cu-Sn and Cu-Zn system [35,36], and the composition ratios were from the STEM-EDS data. The alloy could be synthesized at room temperature by sputtering, presumably due to the transport of high energy by the sputtered atoms during deposition. Fig. 5(a) and (b) shows the FE-SEM and STEM images of the sample obtained by cooling as soon as it reached 300 C. The STEM-EDS mapping images of this sample are shown in Fig. 5(c), and its EDS line scan is shown in Fig. S4. After the sample was annealed at 300 C, the layered precursor structure changed to a mixed structure of Cu-Zn and Cu-Sn or Sn alternately distributed in the transverse (lateral) direction. The Cu-Zn alloy (β′-CuZn, Cu:Zn:Sn= 53:41:6) and small amount of Cu-Sn alloy (η-Cu6Sn5, Cu:Zn:Sn= 57:6:37) were mixed with liquid Sn (Cu:Zn:Sn= 12:2:86). The composition ratio of the Cu-Zn alloy is similar to β′-CuZn in the phase diagram at 300 C [36]. Additionally, the phases of liquid Sn (Cu:Zn:Sn = 12:2:86) and η-Cu6Sn5 can form in the area consisting of η′Cu6Sn5 and Sn phases at approximately 227C, as shown in the Cu-Sn phase diagram [35]. The β′-CuZn was assumed to form mainly due to Zn extraction from the γ-Cu5Zn8 phase through preferential reaction of Zn and S in the H2S-Ar gas atmosphere and dezincification. A S-rich ZnSSe thin layer was observed on the surface of the mixed-state metal grains. From the tendencies of preferential reactions (Sn-Se < Sn-Zn < Cu-Sn < Cu-Se < Cu-Zn < Zn-Se) [29], Zn from the Cu-Zn alloy reacted with S(e), resulting in a ZnSSe shell due to the preferential reaction of Zn with S (or Se) over Cu with S (or Se), but Cu was still bound in the form of Cu-Zn alloy. In the
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Sn composition map in Fig. 5(c), the Sn, which was deposited last, moved downward as a liquid phase; the melting point of Sn is approximately 231 C. Physical contraction was expected to occur because of the movement of the liquid Sn, and the contracted microstructure of the grain is observed in the top view of the FE-SEM images shown in Fig. 5(a). Thus far, this rearrangement of Sn has been described as a phenomenon in which the density of Sn is higher than that of the other alloys or metals, thus the liquid Sn sinks below the substrate due to gravity [28,33]. However, in our process, the Sn moves in the direction of the Mo interface regardless of the direction of the precursor thin film, as shown in Fig. S5. These results suggest that the rearrangement of Sn is not related to density but rather to the wettability between liquid metal and solid metal.
During pre-annealing at 300 C for 15 min, the thickness of the dense ZnSSe shell increased, large voids formed near the Mo-back contact, and the non-uniformity of the Sn composition distribution increased. A relatively thick ZnSSe layer formed along the morphology of the precursor, as shown in the third image in Fig. 6(a). Fig. 6(b) shows a wide-range cross-sectional STEM image, and the corresponding STEM-EDS maps are shown in Fig. 6(c) and (d). Cu-Zn (α-Cu2Zn, Cu:Zn:Sn = 60:33:7), the first type of Cu-Sn (ε-Cu3Sn, Cu:Zn:Sn = 60:8:32) alloy and the second type of Cu-Sn (η-Cu6Sn5, Cu:Zn:Sn:Mo = 21:6:55:18) alloy in the metal state were under the dense ZnSSe shell, as shown in Fig. S6; the effect of Mo will be investigated in future work. Based on the tendencies of preferential reactions (Sn-Se < Sn-Zn < Cu-Sn < Cu-Se < CuZn < Zn-Se) [29], the Zn in the β′-CuZn phase was consumed by forming the ZnSSe shell, and αCu2Zn phase can be formed at this reaction stage [36]; thus, some of Cu that was in the Cu-Zn
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alloy was released into the Cu-Sn alloy. Consequently, the solid-state ε-Cu3Sn and η-Cu6Sn5 phases, which were formed in the relatively Cu-rich region in the Cu-Sn phase diagram, can form [35]. The Cu-Zn is located near the ZnSSe, and the two types of Cu-Sn are located close to the Mo-back contact, as shown in Fig. 6. The presence of Cu-Zn and Cu-Sn alloys under the dense ZnSSe shell is likely due to the preferential sulfo-selenization reaction of Zn. The growth of the ZnSSe shell by preferential reaction of Zn-SSe causes void formation due to the consumption of Zn from the Cu-Zn alloy. Additionally, voids can be further grown due to the rearrangement of the metal alloys by liquid Sn. Interestingly, the uneven distribution of the Sn was observed in the STEM-EDS maps of Fig. 6(c) and (d), and the compositional and microstructural non-uniformity of the CZTSSe absorber is assumed to originate from the uneven distribution of the liquid Sn. Cu is expected to gradually move from the Cu-Zn alloy to the CuSn alloy, thus the areas with η-Cu6Sn5 and liquid Sn phases can be changed to a new composition with ε-Cu3Sn and η-Cu6Sn5 according to the Cu-Sn phase diagram [35]. So, the amount of liquid Sn is expected to gradually decrease. The distribution and portions of voids were observed by etching the ZnSSe shells, as shown in Fig. S7; the distributions of the Cu-Sn and Cu-Zn alloys were observed. As previously mentioned, these voids are expected to be generated by dezincification from the Cu-Zn alloy and by the formation of ZnSSe. This dezincification phenomenon and void formation were also observed when the same metal precursors were annealed under an Ar-only atmosphere, as shown in Fig. S8. As a result, void formation by dezincification can be regarded as an inevitable phenomenon that is commonly observed when metal precursors are used in the two-step process. Fig. 7 shows the dramatic changes in microstructure and composition during heating up to 400 C after pre-annealing. Fig. 7(a) shows the cross-section image of the sample. The sample can be
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divided into an α region under the ZnSSe shell filled with Sn-Cu alloys, such as ε-Cu3Sn (Cu:Zn:Sn= 78:1:21) and η-Cu6Sn5 (Cu:Zn:Sn:Mo=25:6:51:18), and a β region emptied under the ZnSSe shell; the STEM-EDS line scan is shown in Fig. S9. The content of Zn (less than 6%) was negligible in the Sn-Cu alloy present under ZnSSe. Thus, almost all Zn in the α-Cu2Zn was consumed to form the ZnSSe shell through preferential reaction, enabling formation of the Curich Cu-Sn alloy. Hence, the ratio of Cu and Sn can be expected to be 2:1 and the formation of solid-state ε-Cu3Sn and η-Cu6Sn5 phases can be expected at temperatures less than the peritectic reaction (η-Cu6Sn5 → ε-Cu3Sn + Liquid Sn ) temperature [35]; above the temperature of the peritectic reaction point, the liquid Sn containing 13.3% Cu can appear again from the η-Cu6Sn5 phase (liquid Sn containing a small amount of Cu, hereafter referred to as "liquid Sn" for convenience). The amount of voids under the ZnSSe shell increased due to the continuous preferential reaction of Zn-SSe during heating, as shown in Fig. 7(b) and Fig.S10 . It is expected that the formation of large voids is related with the agglomeration of Cu-Sn alloy containing liquid phase when ZnSSe layer is formed. Based on the result about unevenly distribution of the small voids initially found in the sample after 300℃ heat treatment for 15min, as shown in Fig. 6 and Fig.S10, it was expected that the Cu-Sn alloys containing the liquid phase were agglomerated while Zn was consumed; according to the Cu-Sn phase diagram, there are sections where liquid Sn co-exists with solid state alloy while the sample temperature reaches at 400℃. Therefore, it is believed that Cu-Sn alloy agglomeration by liquid phase contributes to formation of large voids through ripening growth mechanism. After the ZnSSe shell was etched by FIB, the portions of the α and β regions were investigated; the α region is where the Cu-Sn alloy exists under ZnSSe, and the β region is where the voids
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exist under ZnSSe. The distribution of the Cu-Sn alloy is shown in Fig. S10; the Cu-Sn alloy existed in the pocket with ZnSSe walls. As color grading was observed on the surface of the sample obtained at 400 C (Fig. 7), mass transfer of Cu and Sn was expected to actively occur. Because a slight temperature difference along the sample was expected due to the poor thermal conductivity of the soda-lime glass, STEM-EDS mapping was performed at different positions to observe the changes in the narrow temperature region where dramatic changes occur. Since liquid Sn is not observed in the central part of the sample, the temperature in this region was lower than the peritectic (ε-Cu3Sn + Liquid Sn → η-Cu6Sn5) point (approximately 408 C) [35]; thus, solid-state ε-Cu3Sn and η-Cu6Sn5 phases can exist. By contrast, the edge of the sample was expected to have experienced a higher temperature than the peritectic point because liquid Sn was observed; thus ε-Cu3Sn and liquid Sn phases formed. As a result, in addition to the void formation due to Zn consumption, additional void formation due to mass transfer of Cu and Sn was observed, as shown in Fig. 7(d)-(f). Interestingly, Cu2Se and SnSe formed on the ZnSSe as the main reaction components of the upper CZTSSe; each STEM-EDS line scan is shown in Fig. S9. Cu and Sn are expected to reach the surface through the ZnSSe layer and react with Se. The actual position the of ZnSSe layer was not substantially changed; however, ZnSSe was not the uppermost layer but the middle layer due to the formation of Cu2Se and SnSe on ZnSSe at temperatures greater than 400 C. Rapid grain growth of ZnSSe occurred at approximately 400 °C, as shown in Fig. S11. In addition, it was confirmed that relatively stronger signals of Cu and Sn are observed at the grain boundary of Zn-SSe layer than at grain interior, as shown in Fig. S11. Thus, the mass transfer of Cu and Sn was expected to occurs along the grain boundaries of ZnSSe. As a result of the mass
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transfer, Cu2Se is mainly distributed on the ZnSSe at position β, where voids exist under the ZnSSe, and SnSe is mainly distributed on ZnSSe at the position α, where Cu-Sn alloy exists under ZnSSe, as shown in Fig. 7(d) and (e) and Fig. S12. Quantitative differences between the Cu2Se and SnSe phases exist locally; however, SnSe and Cu2Se cover all ZnSSe surfaces; thus, the Cu and Sn components are expected to be dispersed through the grain boundaries of ZnSSe. This phenomenon is presumably due to the capillary forces between the liquid and solid phases and is thought to stem from the liquid phase, such as liquid Sn. The liquid Sn (Cu:Zn:Sn:Mo:S = 9:0:86:2:2) can remain under the ZnSSe layer at temperatures above the peritectic reaction point [35], as shown in Fig. 7(f) and Fig. S13; thus, the mass transfer of the Cu and Sn was expected to occur along the grain boundaries by capillary force. The purity of the liquid Sn at the peritectic reaction point was approximately 86.7% [35]. CZTSSe is well known to be formed by the reaction in Eq. (2) [12] or Eq. (3) [11,22,26]: Cu2S(e) + SnS(e) + ZnS(e) + (1/2)S(e)2(gas) → Cu2ZnSnS(e)4
Eq. (2)
Cu2SnS(e)3 + ZnS(e) → Cu2ZnSnS(e)4
Eq. (3)
Interestingly, the reaction in Eq. (2) can be expected at lower temperatures, as shown in Fig. S14, which shows a high-magnification version of the images in Fig. 7(e); in Fig. S14, the Cu2Se, SnSe, ZnSSe and Cu-Zn-Sn-SSe phases are observed without the Cu-Sn-SSe phase; thus, the reaction in Eq. (2) is expected. However, the main reaction mechanism for forming the top CZTSSe in the CZTSSe double layer was expected to be Eq. (3) because the Cu-Sn-Se phase on the ZnSSe shell was observed in the wide area between the Cu-Se and Sn-Se phases, as shown in Fig. 7(f) and Fig. S13. The reaction path of the lower CZTSSe was also expected to proceed according to Eq. (3) We speculate that the pocket position where the liquid Sn exists is the position where the lower
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CZTSSe is formed. Thus, the Cu component that moved to the upper part moved back with the Se as the Cu-Se liquid phase to the lower part through the ZnSSe grain boundaries, reacted with liquid Sn to form the Cu-Sn-Se phase, and then finally reacted with the ZnSSe shell to form the lower CZTSSe through the reaction in Eq. (3); the liquid Cu-Se phase can be formed in the CuSe phase diagram [37], and the liquid phase of Cu-Sn-Se can be formed at 400 C in the Cu2Se3Sn and Cu2SnSe3-Se phase diagrams [38,39]. According to the results in Fig. 7, voids are formed first by Zn consumption due to the formation of the ZnSSe shell. The mass transfer of Cu and Sn from below the ZnSSe shell to above the ZnSSe shell leads to additional void formation under the ZnSSe. These results are important evidence related to the formation mechanisms of the CZTSSe double layer and ZnSSe layer between the CZTSSe layers and voids. Fig. 8(a) shows the STEM-EDS maps of the sample cooled from 440 C. The CZTSSe double layer, ZnSSe layer and voids were observed in this sample. The CTSe phase was also found near the surface of the upper CZTSSe layer. The CTSSe phase in this sample was observed as Cu2SnSe3-x (Cu:Sn:Se:S = 39:20:39:2), as shown in Fig. 8(b). The thickness of the ZnSSe layer decreased due to the reaction between the CTSSe and ZnSSe. An additional CZTSSe phase might be formed by additional reactions between the CTSSe and Zn components. The thickness of the MoSSe was approximately 50 nm. The compositional variation between each grain, as shown in Fig. 8(c), was expected. Fig. 9(a) shows the STEM-EDS maps of the sample cooled after reaching 480C . The CZTSSe double layer, ZnSSe layer and voids were observed in this sample without the CTSSe phase near the surface. The thickness of the MoSSe layer was approximately 120 nm. Cross-sectional STEM-EDS line scans are shown in Fig. 9(b) and (c). The compositional variation between the
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etched grains was not significant. At this stage, most of the double layer, ZnSSe, and voids are formed. When the annealing time was increased, the distribution of each element was more uniform, and grain growth of the CZTS and an increase in the MoSSe thickness were observed as shown in Fig. 1. Fig. 10 shows a schematic of the formation mechanisms of the CZTSSe double layer, the ZnSSe layer between the CZTSSe layers and the voids in the lower CZTSSe layer. Alloy type metal precursors were prepared by sputtering at room temperature, as shown in Fig. 10(a). The melting of the Sn, rearrangement of the metal alloys and formation of the ZnSSe shell by dezincification began to occur during the heating process, as shown in Fig. 10(b). Due to persistent dezincification and the preferential reaction of Zn-SSe, almost all Zn is consumed to form the ZnSSe layer; as a result, voids are produced under the ZnSSe layer, as shown in Fig. 10(c) and (d). The thickness of the ZnSSe layer increases until the Zn from the Cu-Zn alloy is consumed and decreases as the ZnSSe is consumed as CZTSSe synthesis begins. The Cu-Sn alloy under the ZnSSe layer migrates onto the ZnSSe layer through the grain boundaries of the ZnSSe layer, and secondary phases such as Cu-SSe, Sn-SSe and Cu-Sn-SSe are formed on the ZnSSe layer, as shown in Fig. 10(e)-(g). At this time, additional voids are formed under the ZnSSe layer due to the mass transfer of Cu and Sn. The void formation due to the mass transfer of Cu and Sn is likely related to the Kirkendall effect [33]. The Sn-SSe and Cu-SSe on the ZnSSe form Cu-SnSSe and react with the Zn-SSe layer to form the upper CZTSSe as shown in Fig. 10(h); based on the reaction in Eq. (3), the position of CZTSSe is likely formed between the CTSe and ZnSSe, as shown in Fig. 10(h). Additional grain growth of the CZTSSe is expected during annealing at 480C, as shown in Fig. 10(i) and (j).
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4. Conclusion In this study, the formation mechanisms of the double layer, voids and ZnSSe layer were proposed. Two mechanisms have been developed to explain void formation. In the first mechanisms, large voids are formed by the dezincification and preferential reaction of Zn-SSe from the Sn/Cu/Zn/Mo metal precursors; the use of Zn-SSe precursor layer might inhibit the formation of voids from the dezincification and preferential reaction of Zn. In the second mechanism, additional small voids are formed by the mass transfer of Cu and Sn from under the ZnSSe layer to above the ZnSSe layer. Consequently, the use of a Sn-Cu alloy/ZnSSe/Mo precursor stack might inhibit both large voids corresponding to dezincification and additional tiny voids corresponding to the mass transfer of Cu and Sn. We are conducting experiments using the aforementioned structures and will report the results in the near future. The CZTSSe double layer and the ZnSSe layer between the CZTSSe layers were formed due to the formation of a ZnSSe intermediate layer by dezincification and the mass transfer of Cu and Sn through the ZnSSe layer. The results of this study suggest that the CZTSSe double layer, ZnSSe layer, and void formation, which are typical problems encountered when using metal precursors, can be controlled by changing the design of the precursors. Compared with the void/secondary phase free CZTSSe (12.6%) synthesized through the hydrazine process, the CZTSSe synthesized with metal precursors had an efficiency of 12.5% even though its microstructure had many voids and secondary phases. These results suggest that defect management at the interface, grain boundaries and bulk might be a key factor rather than void and secondary phase control. Thus, we are in the process of identifying the defect status of our cells at the interface, grain boundaries, and bulk. Nevertheless, the effects of the CZTSSe double layer, ZnSSe layer, and voids on the FF and output current cannot be ignored. For future
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work, we will continue to study controlling the secondary phases and voids while maintaining the superb quality of the large-grained CZTSSe upper layer near the front junction shown in this study.
Fig. 1. Photovoltaic device properties for the 12.5% efficienct solar cell. (a) Certified JV curve and (b) EQE curve for the highest efficiency CZTSSe device. (c) Statistical plot of devic e performance parameters of the certified CZTSSe devices.
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Fig. 2. (a) Large-area FE-SEM image of a CZTSSe cross-section prepared by FIB etching. (b) STEM-EDS maps. (c) FE-SEM image of the voids and lower part of the CZTSSe surface and (d) high-magnification FE-SEM image of the area in the dotted square in Fig. 2 (c) after an additional etch.
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Fig. 3. (a) Schematic of the temperature profiles. Dotted lines are used to indicate cooling but do not represent the actual cooling profiles. Cross-sectional STEM images of the (b) pre-annealed Sn/Cu/Zn/Mo metal precursors, (c) sample cooled after reaching 300 C, (d) sample cooled after annealing at 300 C for 15 min, (e) sample cooled after reaching 400 C (e1, e2, and e3 are the results of analyses at the location indicated in the inset image (e) in Fig. 3 (a)), (f) sample cooled after reaching 440 C, and (g) sample cooled after reaching 480 C.
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Fig. 4. (a) Surface and cross-sectional FE-SEM images of the Sn/Cu/Zn/Mo metal precursors. The image of the metal precursors is shown as the inset image in Fig. 4 (a). (b) Wide-range cross-sectional STEM image and (c)-(d) STEM-EDS maps of the metal precursors.
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Fig. 5. (a) Surface FE-SEM images of the sample cooled after reaching 300 C. The image of the sample is shown as the inset image in Fig. 5 (a). (b) Wide-range cross-sectional STEM image and (c) STEM-EDS maps of the sample.
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Fig. 6. (a) Surface and cross-sectional FE-SEM images of the sample cooled after annealing at 300 C for 15 min. The image of the sample is shown as the inset image in Fig. 6 (a). (b) Widerange cross-sectional STEM image and (c)-(d) STEM-EDS maps of the sample.
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Fig. 7. (a) Cross-sectional FE-SEM images of the sample cooled after reaching 400 C. (b) ZnSSe etched-surface image and corresponding EDS maps (tilted view), (c)-(f) surface FE-SEM images and cross-sectional STEM-EDS maps at different locations in the sample as the inset images. The actual temperature of the (f) location is expected to have been higher than that of the (c) location.
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Fig. 8. (a) Cross-sectional STEM-EDS maps (b) and (c) compositional line scans of the sample cooled after reaching 440 C.
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Fig. 9. (a) Cross-sectional STEM-EDS maps and (b) and (c) compositional line scans of the sample cooled after reaching 480 C.
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Fig. 10. Schematic showing the formation of the CZTSSe double layer, the ZnSSe layer between the CZTSSe layers and voids in the Mo-back contact side. Schematic of the (a) metal precursors, (b) sample cooled after reaching 300 C , (c) sample cooled after annealing at 300 C for 15 min, (d)-(g) sample cooled after reaching 400 C , (h) sample cooled after reaching 440 C , (i) sample cooled after reaching 480 C, and (j) final CZTSSe absorber sample cooled after annealing at 480 C for 10 min. The 1st and 2nd Cu-Sn alloys are expected to be ε-Cu3Sn and ηCu6Sn5, respectively.
ACKNOWLEDGMENT This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980), the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT, KOREA (2016M1A2A2936781), and the DGIST R & D Programs of the Ministry of Science, ICT & Future Planning of Korea (19-BD-05). We thank Mr. Cheon and Mr. Eun in Center for Core Research Facilities (CCRF) for the STEM measurements. REFERENCES [1] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, S. Guaha, Sol. Energy Mater. Sol. Cells. 95 (2011) 1421-1436.
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[20] R. B. V. Chalapathy, G. S. Jung, B. T. Ahn, Sol. Energy Mater. Sol. Cells. 95 (2011) 32163221. [21] S. Zhuk, A. Kushwaha, T. K. S. Wong, S. Masudy-Panah, A. Smirnov, G. K. Dalapati, Sol. Energy Mater. Sol. Cells. 171 (2017) 239-252. [22] R. Schurr, A. Hölzing, S. Jost, R. Hock, T. Voβ, J. Schulze, A. Kirbs, A. Ennaoui, M. LuxSteiner, A. Weber, I. Kötschau, H. W. Schock, Thin Solid Films 517 (2009) 2465-2468. [23] D. M. Berg, A. Crossay, J. Guillot, V. Izquierdo-Roca, A. Pérez-Rodriguez, S. Ahmed, H. Deligianni, S. Siebentritt, P. J. Dale, Thin Solid Films 573 (2014) 148-158. [24] H. Yoo, R. A. Wibowo, G. Manoharan, R. Lechner, S. Jost, A. Verger, J. Palm, R. Hock, Thin Solid Films 582 (2015) 245-248. [25] R. A. Wibowo, S. A. Moeckel, H. Yoo, C. Hetzner, A. Hoelzing, P. Wellmann, R. Hock, Mater. Chem. Phys. 142 (2013) 311-317. [26] H. Yoo, R. A. Wibowo, A. Hölzing, R. Lechner, J. Palm, S. Jost, M. Gowtham, F. Sorin, B. Louis, R. Hock, Thin Solid Films 535 (2013) 73-77. [27] C.-Y. Su, C. Y. Chiu, J.-M. Ting, Sci. Rep. 5 (2015) 9291. [28] W. Li, J. Chen, C. Yan, X. Hao, J. Alloys Compd. 632 (2015) 178-184. [29] H. Yoo, “Reaction mechanism between Cu-Zn-Sn-Se components for the formation of Cu2ZnSnSe4 fil”, Doctoral thesis, University of Erlangen-Nurnberg, 2016. [30] J. J. Scragg, J. T. Wätjen, M. Edoff, T. Ericson, T. Kubart, C. A Platzer-Björkman, J. Am. Chem. Soc. 134 (2012) 19330-19333. [31] J. J. Scragg T. Kubart, J. T. Wätjen, T. Ericson, M. K. Linnarsson, C. Platzer-Björkman, Chem. Mater. 25 (2013) 3162-3171. [32] J. J. Scragg, T. Ericson, T. Kubart, M. Edoff, C. Platzer-Björkman, Chem. Mater. 23 (2011) 4625-4633. [33] J. J. Scragg, “Copper Zinc Thin Sulfide Thin Films for Photovoltaics: synthesis and Characterisation by Electrochemical Methods”, Doctoral thesis, University of Bath, UK, 2011. [34] K. Kaur, N. Kumar, M. Kumar, J. Mater. Chem. A. 5 (2017) 3069-3090. [35] S. Fürtauer, D. Li, D. Cupid, H. Flandorfer, Intermetallics, 34 (2013) 142-147. [36] Copper Development Association (CDA), Equilibrium Diagrams of Copper Alloys, Copper Development Association, London, UK, 1963. [37] V. M. Glazov, A. S. Pashinkin, V. A. Fedorov, Inorg. Mater. 36 (2000) 641-652. [38] L. I. Berger, E. G. Kotina, Y. V. Oboznenko, A. E. Obodovskaya, Inorg. Mater. 9 (1973) 201-202.
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[39] G. Effenberg, S. Ilyenko, Springer, ISBN 978-3-540-32589-5 (2006) 361-373.
Se-Yun Kim is a researcher in Convergence Research Center for Solar Energy at Daegu Gyeongbuk Institute of Science and Technology (DGIST). He received his Ph.D. in School of Materials Science and Engineering (Electronic Materials Science and Engineering), Kyungpook National University, South Korea in 2015. His research interests include thin film growth, p-type transparent oxide semiconductor, halide perovskite, and Cu-based chalcogenide for thin film solar cell.
Dae-Ho Son received the B.S. degree in the physics from Hanyang University, Seoul Korea in 2006, and the M.S. degrees in Nano Science & Technology at University of Seoul, Seoul, Korea, in 2008. He is presently a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. His current research interests include photovoltaic electronics, thin film growth, chalcogenide photovoltaics and thin film devices
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Young-Ill Kim received his M.S. degree from Department of Nano Science & Technology, University of Seoul, Korea in 2012. Now he is a researcher in DGIST, developing the vacuum process for thin film solar cells.
Seung-Hyun Kim received his M.S. degree from Department of Physics, Yeungnam University, Korea in 2016. Now He is a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. His current research interests include thin film growth, chalcogenide photovoltaics, and analysis of photovoltaic devices.
Sammi Kim received her M.S. degree in chemical engineering from Yeungnam University, Korea in 2014. She worked engineer in the PosLX project team at RIST from 2014 to 2017. She joined DGIST in 2017 and currently she is a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. She current research interests include thin film growth, chalcogenide photovoltaics and flexible thin film devices.
Kwangseok Ahn is a Ph.D. candidate and study about OFETs (Organic Field Effect Transistors) at the Department of Physics at Soongsil University. He is joined Convergence Research Center
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for Solar Energy, DGIST in 2017. His research interests focus on analysis of the electrical, optical properties in chalcogenide photovoltaics.
Shi-Joon Sung is a principal researcher in Convergence Research Center for Solar Energy at Daegu Gyeongbuk Institute of Science and Technology (DGIST). He received his Ph.D. degree in Chemical & Biomolecular Engineering from the Korean Advanced Institute of Science and Technology (KAIST) in 2004. He has worked in the Samsung Electronics until 2007 as a senior research engineer. His research interests are inorganic compound semiconductor thin films and their applications to various energy and electronic device technology.
Dae-Kue Hwang received his Ph.D. degree from Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), South Korea in 2008. He worked as postdoctoral fellow at Northwestern University, Illinois from 2008 to 2010. He joined DGIST in 2010 and currently he is a principal researcher at the DGIST. His research interests include light emitting diode, oxide thin film transistors, quantum dot device and thin film solar cells. Details can be found at https:// www.researchgate.net/profile/Dae-Kue_Hwang.
Kee-Jeong Yang received his M.S. degree from Department of Chemical Engineering, POSTECH and Ph.D. degree from Department of Chemical Engineering, Kwangwoon University. He worked at Samsung electro-Mechanics research center from 2001 to 2003, and at LG Display LCD from 2004 to 2005. He joined DGIST in 2005. He has worked flexible,
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transparent, and micro display-related research. His current research focuses on development of chalcogenide-based photovoltaics on SLG and various substrates. Details can be found at http://www.dgist.ac.kr. Also you can contact to [email protected].
Jin-Kyu Kang received the B.S. in engineering chemistry from Seoul National University in 1991, and M.S and Ph.D. degrees in chemical engineering from POSTECH in 1993 and 2000. He joined Samsung Electronics, and led the research about low temperature poly-Si TFT- LCD. He has been a principal researcher at the DGIST, Korea since 2005. His research is focused developing and characterizing for thin film solar cells (CIGS, CZTS, DSSC). He was honored by the Ministry of Trade, Industry and Energy in 2015 and the Ministry of Science and ICT in 2018 for his works. He holds more than 150 patents and has published more than 90 papers.
Dae-Hwan Kim received his Ph.D. degree from Department of Chemical Engineering, POSTECH, KOREA in 2002. From 2002, he worked at Samsung Electronics' Semiconductor R&D Business and invented the lean-free MESH capacitor process for 80nm or less scale DRAM. He joined DGIST in 2005 and currently he is a director of Convergence Research Center for Solar Energy, DGIST. His research interests include oxide thin film transistors and thin film solar cells (CIGS, CZTS and SbS). Details can be found at https://www.researchgate.net/profile/Dae-Hwan_Kim.
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Highlights The new formation mechanisms of the voids, ZnSSe layer and CZTSSe double layer that were observed in the sister sample of 12.5% of CZTSSe cell. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens, such as S and Se, almost all the Zn is preferentially consumed to form the ZnSSe layer; large voids are first produced under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe shell by migration of Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Due to the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, the CZTSSe double layer is formed and ZnSSe exists between the CZTSSe double layers.
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PII: DOI: Reference:
www.elsevier.com/locate/nanoenergy
S2211-2855(19)30176-4 https://doi.org/10.1016/j.nanoen.2019.02.063 NANOEN3507
To appear in: Nano Energy Received date: 14 December 2018 Revised date: 18 February 2019 Accepted date: 23 February 2019 Cite this article as: Se-Yun Kim, Dae-Ho Son, Young-Ill Kim, Seung-Hyun Kim, Sammi Kim, Kwangseok Ahn, Shi-Joon Sung, Dae-Kue Hwang, KeeJeong Yang, Jin-Kyu Kang and Dae-Hwan Kim, Void and secondary phase formation mechanisms of CZTSSe using Sn/Cu/Zn/Mo stacked elemental precursors, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.02.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Void and secondary phase formation mechanisms of CZTSSe using Sn/Cu/Zn/Mo stacked elemental precursors Se-Yun Kim*, Dae-Ho Son*, Young-Ill Kim, Seung-Hyun Kim, Sammi Kim, Kwangseok Ahn, ShiJoon Sung, Dae-Kue Hwang, Kee-Jeong Yang, Jin-Kyu Kang, Dae-Hwan Kim Convergence Research Center for Solar Energy, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea KEYWORDS : CZTSSe, Metal precursor, Two-step process, Void, Secondary phase, Formation mechanism
Author Contributions *These authors contributed equally to this work. Corresponding Author Dae-Hwan Kim : [email protected] Jin-Kyu Kang : [email protected]
ABSTRACT In recent years, Cu2ZnSn(S1-xSex)4 (CZTSSe) prepared by a two-step process using metal precursors has been reported to exhibit a relatively high power conversion efficiency, and a high efficiency of 12.5% by two-step process contained via sputtering method was recently confirmed
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by our group. In this study, we proposed formation mechanisms for the CZTSSe double layer, voids and ZnSSe layer, which were observed in the CZTSSe using metal precursor. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens such as S and Se, almost all Zn is consumed to form the ZnSSe layer; as a result, large voids are produced first under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe layer via migration of the Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Because of the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, a CZTSSe double layer can be formed, and ZnSSe can exist between these CZTSSe layers. Finally, we propose a method based on the formation mechanism to control the voids and secondary phases, which affect the fill factor and output current. Graphical abstract: The new formation mechanisms of the voids, ZnSSe layer and CZTSSe double layer that were observed in the sister sample of 12.5% of CZTSSe cell.
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Keywords: CZTSSe; Metal precursor; Two-step process; Void; Secondary phase; Formation mechanism
1. Introduction To achieve the goals set out in the Paris Agreement, the electricity output of photovoltaic (PV) plants should be much higher than current levels. Si PVs are likely not the only solution for increasing electricity production. Because Si cells do not effectively collect sunlight, an alternative highly crystalline thin-film layer is also required to achieve high efficiency [1,2]. In addition, the non-flexible material properties of Si PVs cause problems that make them difficult to apply in various fields involved with expanding PV plants. As an alternative, thin-film PV technology with advanced material properties such as a high light absorption coefficient has been developed, and this technology enables solar modules to be integrated with buildings and wearable devices. Thin-film technology that can realize efficiencies as high as that of Si PVs through the use of materials such as CdTe and CIGS already exists. However, these materials have limitations due to the toxicity of Cd and Te and the scarcity of In, Ga, and Te. As another alternative, organo-metal hybrid halide perovskites (CH3NH3PbI3) have garnered an immense amount of research interest due to a high efficiency of 23.3% [3]. However, the issues raised by the toxicity of lead [4] and the long-term stability of the material under humidity and light remain open questions [5,6]. Hence, the development of a low-cost PV technology with non-toxic and earth-abundant materials is still needed. Among the emerging PVs, kesterite Cu2SnZnS4 (CZTS), Cu2SnZnSe4 (CZTSe), and Cu2ZnSn(S1-xSex)4 (CZTSSe) thin-film solar cells are considered promising for
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large-area module production using earth-abundant and non-toxic elements [1,7]. These market trends for PV, and the status of alternative PV technologies being developed have led researchers to devote much attention to kesterite PVs, although the record efficiency of these devices has not been improved since 2014. Thus far, the highest reported power conversion efficiency for a CZTSSe cell is 12.6%, which was achieved with CZTSS layer prepared via a two-step process using hydrazine solution [8]. Hydrazine-solution-based processes using various organic solvents have disadvantages in terms of the risk of explosion, wastewater treatment requirements and incompatibility with large-area processes. Thus, numerous attempts have been made to replace the hydrazine solution process with a compound precursor or a metal precursor; however, no methods have led to a device with an efficiency greater than that achieved with a CZTSSe layer prepared via the two-step hydrazine process [9]. Thus far, a CZTSSe double layer, a ZnSSe layer between these CZTSSe layers and large voids at the Mo rear contact side have been commonly observed in the microstructures of the CZTSSe light-absorber layer formed by a two-step process using metal precursors [10-16]; in contrast, hydrazine based CZTSSe showed relatively small amounts of voids and secondary phases, and CZTSSe absorbers prepared from compound precursor containing S or Se have shown relatively void-free interfaces and secondary phases that are not easily observed by simple scanning electron microscopy (SEM) [17]. Likewise, the CZTSSe double layer, ZnSSe and voids were always observed in CZTSSe absorver layer from our process for the high-efficiency. The presence of voids and secondary phases could increase the series resistance (Rs) [18] and reduce the shunt resistance (Rsh) [19-21]. Thus, the voids and secondary phases should be controlled.
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The CZTSSe formation mechanism has been proposed based on the results of in situ X-ray diffraction (XRD) and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM-EDS) mapping [11,12,22-29]. However, these data are not sufficient to clarify the origin of the CZTSSe double layer, ZnSSe secondary phase, and voids commonly observed in CZTSSe absorbers. So far, two formation mechanism models for voids and secondary phases at the Mo-back contact side are described as follows. First, the decomposition reaction of CZTS(e) or the formation of SnS(e) is expected to be the cause of void formation [30,31]. The ease of the reduction of Sn(IV) and the favorability of MoS(e)2 formation can induce phase separation by removing S(e) from the CZTS(e) at the Moback contact side, as shown in Eq. (1), where volatile Sn(II)S(e) is formed by the reduction of Sn(IV)S(e)2 [31]: 2Cu2ZnSnS(e)4 + Mo → 2Cu2S(e) + 2ZnS(e) + 2SnS(e) + MoS(e)2
Eq. (1)
A high S(e) partial pressure has been reported to reduce void formation by inhibiting the formation of volatile SnS(e); when the partial pressure is high, the SnS(e)2 and Sn2S(e)3 phases, which are hardly volatilized, are stabilized [32]. Second, the Kirkendall effect between the Cu metal and Cu2S(e) causes void formation [33]. The inclination toward Cu vacancy migration occurs because of the difference between the diffusion fluxes of Cu and S(e), and the voids in the Cu metal side are formed by the accumulation of Cu vacancies; Scragg predicted that the Cu in a Cu-Sn alloy, in particular, would participate in the Kirkendall effect [33]. Each model has an important meaning; however, these simplified models present difficulties in fully understanding the actual void formation mechanism during the sulfo-selenization process
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without corresponding experimental evidence. Consequently, attempts to suppress void formation based on each model often appear to have limited effectiveness [34]. In this study, we report on the formation mechanisms of the double layer, voids, and ZnSSe layer in the CZTSSe which was shown high efficiency of 12.5% certified by the Korea Institute of Energy Research (KIER). Detailed formation mechanisms are reasonably proposed based on phase evolution tracking at various temperatures by means of TEM analysis. The samples were collected at various temperatures during sulfo-selenization, which is the same process used for fabricating the highest-efficiency cells. Metal precursors with a stacked structure (Sn/Cu/Zn) were deposited onto a Mo-coated soda-lime glass substrate by DC sputtering and were sulfoselenized in an rapid thermal processing (RTP) chamber. H2S gas and Se shots were used as chalcogen sources. The samples collected at meaningful times and annealing temperatures were analyzed mainly by STEM.
2. Experimental section 2.1 Device fabrication The structure of the CZTSSe solar cell consisted of a soda-lime glass (SLG) substrate, a 600 nmthick Mo layer as a back contact layer, an approximately 1.8 μm-thick CZTSSe absorber layer deposited via sputtering and annealed under Se and H2S gas (diluted with 90 vol% Ar), a 50 nmthick CdS buffer layer deposited via chemical bath deposition, a 50 nm-thick intrinsic ZnO layer deposited via sputtering, a 300 nm-thick Al-doped ZnO (AZO) layer deposited via sputtering, a 10 nm-thick Ni and 2 μm-thick Al grid deposited via e-beam evaporation, and a 100 nm-thick MgF2 layer deposited via e-beam evaporation.
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The Mo layer was deposited onto the SLG substrate via DC magnetron sputtering using a Mo target with a purity of 99.99%. The metal precursors for the CZTSSe absorber layer were deposited onto the Mo layer using 99.99% pure Sn, Cu, and Zn sputtering targets with a stacking order of Sn (275 nm)/Cu (160 nm)/Zn (188 nm)/Mo. For the sulfo-selenization process, all the samples were placed in a sample box that consisted of a Se boat made of quartz, a sample holder made of SiC-coated graphite, and a quartz cover plate. Before annealing began, Se (approximately 250 mg), H2S (diluted with 90 vol% Ar, 250 sccm, approximately 8 min) and Ar (2000 sccm, approximately 8 min) gas were supplied to the RTP chamber. To prevent severe decomposition of the CZTSSe, the annealing processes were conducted in a RTP chamber at a pressure slightly below 1 atm. The sample was heated from room temperature to 300 C over 560 s and then maintained at 300 C for 900 s. Subsequently, the sample was heated from 300 C to 480 C over 1800 s and then maintained at 480 C for 600 s. Se was purchased from SigmaAldrich and used without purification. The remaining layers in the device, as previously described, were subsequently added using the aforementioned procedures.
2.2 Device & Material characterization Current– voltage characteristics were measured under a simulated air mass spectrum of 1.5 global (AM 1.5 G) and 100 mW/cm2 (1 sun) illumination at 25℃ using a solar simulator (WACOM, WXS-155S-L2_Class-AAA) in Korea Institute of Energy Research (KIER) for certification. The EQE values were measured under a AM 1.5 G and 100 mW/cm2 illumination at 25℃ using SOMA Optcis (S-9230) in KIER. Surface and cross-sectional images of the absorber layers were obtained by field-emission scanning electron microscopy (FE-SEM, Hitachi Co., model S-4800). The samples for STEM
7
measurement were prepared with a dual-beam focused ion beam (FIB) system (Hitachi Co., model NB5000). The STEM-EDS measurements were performed using a QUANTAX-200 (Bruker Co.) to analyze the compositions of the absorber layers. The etched samples were prepared with a multi-functional FIB system (FEI Co., model Helios NanoLab G3UC).
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3. Results and discussion Fig. 1 shows the current–voltage (J–V) characteristics of the highest efficiency CZTSSe device along with the external quantum efficiency (EQE) and statistical properties of device performance parameters of the certified CZTSSe devices. The performance of all devices was certified by the Korea institute of energy research under standard test conditions. The area of the certified devices is more than twice as large as the previous result [10], and the cell designated area including the top contact grid was defined by mechanical scribing to be around 0.5 cm2. The certified highest efficiency device exhibits an efficiency of 12.5 %, open circuit voltage (VOC) of 0.516 V, fill factor (FF) of 66.89 % and short-circuit current (JSC) of 36.13 mA/cm2 in Fig. 1(a). The EQE spectrum of this device is high in the visible range and then decreases in the longerwavelength region, as shown in Fig. 1(b). The bandgap (Eg) of the CZTSSe thin film was calculated by the dEQE / dλ method, and the value was 1.09 eV. VOC deficit defined as Eg/q − VOC, where q is the elementary charge. The device shows a Voc deficit of 0.574 V, which is one of the lowest VOC deficits in our devices. The statistical certified 14 devices results are presented in Fig. 1(c). A large number of certified devices exceeded the conversion efficiency of more than 12 % and showed an average JSC of around 36 mA/cm2, a VOC of 0.518 V, and an FF of 65%. The area of the statistically processed devices is not all the same due to mechanical scribing, and has an area of about 0.48 cm2 to 0.52 cm2. The detailed results of the electrical properties of device will be discussed in a future manuscript. In this paper, we report the formation mechanism of CZTS double layer, void and ZnSSe layer. Fig. 2 (a) and (b) shows the FE-SEM and STEM images of the sister sample of the CZTSSe cell with a power conversion efficiency of 12.5%. The CZTSSe layer consists of a double layer in which the upper layer is dense and void-free, whereas the lower layer consists of voids (empty
9
volumes) and CZTSSe (filled volume). The relatively large-grained ZnSSe secondary phase layer lies between the upper CZTSSe and the lower CZTSSe. Zn-, Cu- and Cu-Sn-SSe secondary phases are observed in the MoSSe layers. The thickness of the MoSSe layer on the Mo electrode is approximately 500 nm. To observe the overall vertical microstructure, 200 cross-sectional images of a 20 μm × 4 μm area were obtained using an FIB slice technique, as shown in Fig. S1 and in the supplemental PowerPoint file (S1.pptx) containing an animated image. To observe the distribution of the voids, the sister sample was etched with a Ga-ion beam using FIB, as shown in Fig. 2(c); the etched area was 30 μm × 20 μm; the change in the microstructure as a function of the etching time is shown in Fig. S2 and the supplemental PowerPoint files (S2(a).pptx and S2(b).pptx) containing animated images. The two types (large and small) of voids were observed in the lower part of the CZTSSe layer. A high-magnification image is shown in Fig 1(d), and corresponding EDS maps of Cu, Zn, Sn, S, Se and Mo are shown in Fig. S3. In the CZTSSe double layer, unevenly distributed voids and a ZnSSe secondary phase layer were observed in all areas. As evident from the high efficiency, sulfo-selenization using sputtered pure-metal precursors is a very useful process due to its many advantages, which include a low processing temperature, low target cost, and compatibility with commercialization. If the CZTSSe double layer, voids and ZnSSe layers are effectively controlled, the efficiency of CZTSSe solar cells can be increased further. To control the voids and secondary phases in the CZTSSe layer, their formation mechanisms should be understood. Thus, the phase transition behavior from the metal precursors to CZTSSe was investigated at multiple points on our optimized temperature profile line for sulfo-selenization, which is shown in Fig. 3(a).
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Fig. 3(a) shows the temperature profile of our two-step process, the inset images in Fig. 3(a) show the photographs of samples withdrawn at specific points during annealing, and Fig. 3(b)(h) shows the corresponding vertical STEM images of each point. Although the results from these samples do not fully represent the in situ process, the formation mechanisms of the CZTSSe double layer, ZnS and voids during the sulfo-selenization process can be deduced. Fig. 3(b) shows the cross-sectional STEM images of the as-deposited metal precursors. Fig. 3(c) shows the STEM image of the sample cooled immediately after reaching 300 C. Fig. 3(d) shows the cooled sample after annealing at 300 C for 10 min. Fig. 3(e) shows the sample cooled as soon as it reached 400 C. Dramatic changes in the microstructure are observed in Fig. 3(e); thus, STEM analysis was conducted at different locations in the same sample. We assumed that the temperature inside this sample was lower than that outside the sample. Fig. 3(f) shows the sample that was cooled as soon as it reached 440 C. Fig. 3(g) shows the sample that was cooled as soon as it reached 480 C. Each sample in Fig. 3(b)–(g) was investigated in detail by FE-SEM and STEM-EDS mapping, as shown in Figs. 4-9; Fig. 2(h) was already shown in Fig. 1. The microstructure and composition of the Sn/Cu/Zn/Mo metal stack prepared by DC magnetron sputtering at room temperature were investigated by FE-SEM and STEM, as shown in Fig. 4. The top view and the cross-section view (Fig. 4(a)) confirm that a continuous film of Sn is not formed due to island shape deposition, as shown in Fig. 4(a). This formation is thought to be due to the poor wetting property of Sn; Cu-Zn or/and Cu are expected to be partially exposed to the surface with Sn. From the STEM-EDS mapping images in Fig. 4(c) and (d), although the three metal layers were deposited in the order Zn, Cu, Sn at room temperature, alloys with unevenly distributed compositions were formed. For example, the Cu-Zn alloy (γ-Cu5Zn8, Cu:Zn:Sn = 37:60:3) formed as the lower layer with a columnar structure, the Cu-Sn alloy (η′-Cu6Sn5,
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Cu:Zn:Sn = 46:1:53) and Sn (Cu:Zn:Sn = 1:0:99) formed as the upper layer, which was not continuous, and a Cu residue (Cu:Zn:Sn = 94:1:5) was located on the Cu-Zn alloy layer, as clearly shown in Fig. 4(d); the phases were assigned and predicted based on the XRD data (not shown in this manuscript) and binary phase diagram of the Cu-Sn and Cu-Zn system [35,36], and the composition ratios were from the STEM-EDS data. The alloy could be synthesized at room temperature by sputtering, presumably due to the transport of high energy by the sputtered atoms during deposition. Fig. 5(a) and (b) shows the FE-SEM and STEM images of the sample obtained by cooling as soon as it reached 300 C. The STEM-EDS mapping images of this sample are shown in Fig. 5(c), and its EDS line scan is shown in Fig. S4. After the sample was annealed at 300 C, the layered precursor structure changed to a mixed structure of Cu-Zn and Cu-Sn or Sn alternately distributed in the transverse (lateral) direction. The Cu-Zn alloy (β′-CuZn, Cu:Zn:Sn= 53:41:6) and small amount of Cu-Sn alloy (η-Cu6Sn5, Cu:Zn:Sn= 57:6:37) were mixed with liquid Sn (Cu:Zn:Sn= 12:2:86). The composition ratio of the Cu-Zn alloy is similar to β′-CuZn in the phase diagram at 300 C [36]. Additionally, the phases of liquid Sn (Cu:Zn:Sn = 12:2:86) and η-Cu6Sn5 can form in the area consisting of η′Cu6Sn5 and Sn phases at approximately 227C, as shown in the Cu-Sn phase diagram [35]. The β′-CuZn was assumed to form mainly due to Zn extraction from the γ-Cu5Zn8 phase through preferential reaction of Zn and S in the H2S-Ar gas atmosphere and dezincification. A S-rich ZnSSe thin layer was observed on the surface of the mixed-state metal grains. From the tendencies of preferential reactions (Sn-Se < Sn-Zn < Cu-Sn < Cu-Se < Cu-Zn < Zn-Se) [29], Zn from the Cu-Zn alloy reacted with S(e), resulting in a ZnSSe shell due to the preferential reaction of Zn with S (or Se) over Cu with S (or Se), but Cu was still bound in the form of Cu-Zn alloy. In the
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Sn composition map in Fig. 5(c), the Sn, which was deposited last, moved downward as a liquid phase; the melting point of Sn is approximately 231 C. Physical contraction was expected to occur because of the movement of the liquid Sn, and the contracted microstructure of the grain is observed in the top view of the FE-SEM images shown in Fig. 5(a). Thus far, this rearrangement of Sn has been described as a phenomenon in which the density of Sn is higher than that of the other alloys or metals, thus the liquid Sn sinks below the substrate due to gravity [28,33]. However, in our process, the Sn moves in the direction of the Mo interface regardless of the direction of the precursor thin film, as shown in Fig. S5. These results suggest that the rearrangement of Sn is not related to density but rather to the wettability between liquid metal and solid metal.
During pre-annealing at 300 C for 15 min, the thickness of the dense ZnSSe shell increased, large voids formed near the Mo-back contact, and the non-uniformity of the Sn composition distribution increased. A relatively thick ZnSSe layer formed along the morphology of the precursor, as shown in the third image in Fig. 6(a). Fig. 6(b) shows a wide-range cross-sectional STEM image, and the corresponding STEM-EDS maps are shown in Fig. 6(c) and (d). Cu-Zn (α-Cu2Zn, Cu:Zn:Sn = 60:33:7), the first type of Cu-Sn (ε-Cu3Sn, Cu:Zn:Sn = 60:8:32) alloy and the second type of Cu-Sn (η-Cu6Sn5, Cu:Zn:Sn:Mo = 21:6:55:18) alloy in the metal state were under the dense ZnSSe shell, as shown in Fig. S6; the effect of Mo will be investigated in future work. Based on the tendencies of preferential reactions (Sn-Se < Sn-Zn < Cu-Sn < Cu-Se < CuZn < Zn-Se) [29], the Zn in the β′-CuZn phase was consumed by forming the ZnSSe shell, and αCu2Zn phase can be formed at this reaction stage [36]; thus, some of Cu that was in the Cu-Zn
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alloy was released into the Cu-Sn alloy. Consequently, the solid-state ε-Cu3Sn and η-Cu6Sn5 phases, which were formed in the relatively Cu-rich region in the Cu-Sn phase diagram, can form [35]. The Cu-Zn is located near the ZnSSe, and the two types of Cu-Sn are located close to the Mo-back contact, as shown in Fig. 6. The presence of Cu-Zn and Cu-Sn alloys under the dense ZnSSe shell is likely due to the preferential sulfo-selenization reaction of Zn. The growth of the ZnSSe shell by preferential reaction of Zn-SSe causes void formation due to the consumption of Zn from the Cu-Zn alloy. Additionally, voids can be further grown due to the rearrangement of the metal alloys by liquid Sn. Interestingly, the uneven distribution of the Sn was observed in the STEM-EDS maps of Fig. 6(c) and (d), and the compositional and microstructural non-uniformity of the CZTSSe absorber is assumed to originate from the uneven distribution of the liquid Sn. Cu is expected to gradually move from the Cu-Zn alloy to the CuSn alloy, thus the areas with η-Cu6Sn5 and liquid Sn phases can be changed to a new composition with ε-Cu3Sn and η-Cu6Sn5 according to the Cu-Sn phase diagram [35]. So, the amount of liquid Sn is expected to gradually decrease. The distribution and portions of voids were observed by etching the ZnSSe shells, as shown in Fig. S7; the distributions of the Cu-Sn and Cu-Zn alloys were observed. As previously mentioned, these voids are expected to be generated by dezincification from the Cu-Zn alloy and by the formation of ZnSSe. This dezincification phenomenon and void formation were also observed when the same metal precursors were annealed under an Ar-only atmosphere, as shown in Fig. S8. As a result, void formation by dezincification can be regarded as an inevitable phenomenon that is commonly observed when metal precursors are used in the two-step process. Fig. 7 shows the dramatic changes in microstructure and composition during heating up to 400 C after pre-annealing. Fig. 7(a) shows the cross-section image of the sample. The sample can be
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divided into an α region under the ZnSSe shell filled with Sn-Cu alloys, such as ε-Cu3Sn (Cu:Zn:Sn= 78:1:21) and η-Cu6Sn5 (Cu:Zn:Sn:Mo=25:6:51:18), and a β region emptied under the ZnSSe shell; the STEM-EDS line scan is shown in Fig. S9. The content of Zn (less than 6%) was negligible in the Sn-Cu alloy present under ZnSSe. Thus, almost all Zn in the α-Cu2Zn was consumed to form the ZnSSe shell through preferential reaction, enabling formation of the Curich Cu-Sn alloy. Hence, the ratio of Cu and Sn can be expected to be 2:1 and the formation of solid-state ε-Cu3Sn and η-Cu6Sn5 phases can be expected at temperatures less than the peritectic reaction (η-Cu6Sn5 → ε-Cu3Sn + Liquid Sn ) temperature [35]; above the temperature of the peritectic reaction point, the liquid Sn containing 13.3% Cu can appear again from the η-Cu6Sn5 phase (liquid Sn containing a small amount of Cu, hereafter referred to as "liquid Sn" for convenience). The amount of voids under the ZnSSe shell increased due to the continuous preferential reaction of Zn-SSe during heating, as shown in Fig. 7(b) and Fig.S10 . It is expected that the formation of large voids is related with the agglomeration of Cu-Sn alloy containing liquid phase when ZnSSe layer is formed. Based on the result about unevenly distribution of the small voids initially found in the sample after 300℃ heat treatment for 15min, as shown in Fig. 6 and Fig.S10, it was expected that the Cu-Sn alloys containing the liquid phase were agglomerated while Zn was consumed; according to the Cu-Sn phase diagram, there are sections where liquid Sn co-exists with solid state alloy while the sample temperature reaches at 400℃. Therefore, it is believed that Cu-Sn alloy agglomeration by liquid phase contributes to formation of large voids through ripening growth mechanism. After the ZnSSe shell was etched by FIB, the portions of the α and β regions were investigated; the α region is where the Cu-Sn alloy exists under ZnSSe, and the β region is where the voids
15
exist under ZnSSe. The distribution of the Cu-Sn alloy is shown in Fig. S10; the Cu-Sn alloy existed in the pocket with ZnSSe walls. As color grading was observed on the surface of the sample obtained at 400 C (Fig. 7), mass transfer of Cu and Sn was expected to actively occur. Because a slight temperature difference along the sample was expected due to the poor thermal conductivity of the soda-lime glass, STEM-EDS mapping was performed at different positions to observe the changes in the narrow temperature region where dramatic changes occur. Since liquid Sn is not observed in the central part of the sample, the temperature in this region was lower than the peritectic (ε-Cu3Sn + Liquid Sn → η-Cu6Sn5) point (approximately 408 C) [35]; thus, solid-state ε-Cu3Sn and η-Cu6Sn5 phases can exist. By contrast, the edge of the sample was expected to have experienced a higher temperature than the peritectic point because liquid Sn was observed; thus ε-Cu3Sn and liquid Sn phases formed. As a result, in addition to the void formation due to Zn consumption, additional void formation due to mass transfer of Cu and Sn was observed, as shown in Fig. 7(d)-(f). Interestingly, Cu2Se and SnSe formed on the ZnSSe as the main reaction components of the upper CZTSSe; each STEM-EDS line scan is shown in Fig. S9. Cu and Sn are expected to reach the surface through the ZnSSe layer and react with Se. The actual position the of ZnSSe layer was not substantially changed; however, ZnSSe was not the uppermost layer but the middle layer due to the formation of Cu2Se and SnSe on ZnSSe at temperatures greater than 400 C. Rapid grain growth of ZnSSe occurred at approximately 400 °C, as shown in Fig. S11. In addition, it was confirmed that relatively stronger signals of Cu and Sn are observed at the grain boundary of Zn-SSe layer than at grain interior, as shown in Fig. S11. Thus, the mass transfer of Cu and Sn was expected to occurs along the grain boundaries of ZnSSe. As a result of the mass
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transfer, Cu2Se is mainly distributed on the ZnSSe at position β, where voids exist under the ZnSSe, and SnSe is mainly distributed on ZnSSe at the position α, where Cu-Sn alloy exists under ZnSSe, as shown in Fig. 7(d) and (e) and Fig. S12. Quantitative differences between the Cu2Se and SnSe phases exist locally; however, SnSe and Cu2Se cover all ZnSSe surfaces; thus, the Cu and Sn components are expected to be dispersed through the grain boundaries of ZnSSe. This phenomenon is presumably due to the capillary forces between the liquid and solid phases and is thought to stem from the liquid phase, such as liquid Sn. The liquid Sn (Cu:Zn:Sn:Mo:S = 9:0:86:2:2) can remain under the ZnSSe layer at temperatures above the peritectic reaction point [35], as shown in Fig. 7(f) and Fig. S13; thus, the mass transfer of the Cu and Sn was expected to occur along the grain boundaries by capillary force. The purity of the liquid Sn at the peritectic reaction point was approximately 86.7% [35]. CZTSSe is well known to be formed by the reaction in Eq. (2) [12] or Eq. (3) [11,22,26]: Cu2S(e) + SnS(e) + ZnS(e) + (1/2)S(e)2(gas) → Cu2ZnSnS(e)4
Eq. (2)
Cu2SnS(e)3 + ZnS(e) → Cu2ZnSnS(e)4
Eq. (3)
Interestingly, the reaction in Eq. (2) can be expected at lower temperatures, as shown in Fig. S14, which shows a high-magnification version of the images in Fig. 7(e); in Fig. S14, the Cu2Se, SnSe, ZnSSe and Cu-Zn-Sn-SSe phases are observed without the Cu-Sn-SSe phase; thus, the reaction in Eq. (2) is expected. However, the main reaction mechanism for forming the top CZTSSe in the CZTSSe double layer was expected to be Eq. (3) because the Cu-Sn-Se phase on the ZnSSe shell was observed in the wide area between the Cu-Se and Sn-Se phases, as shown in Fig. 7(f) and Fig. S13. The reaction path of the lower CZTSSe was also expected to proceed according to Eq. (3) We speculate that the pocket position where the liquid Sn exists is the position where the lower
17
CZTSSe is formed. Thus, the Cu component that moved to the upper part moved back with the Se as the Cu-Se liquid phase to the lower part through the ZnSSe grain boundaries, reacted with liquid Sn to form the Cu-Sn-Se phase, and then finally reacted with the ZnSSe shell to form the lower CZTSSe through the reaction in Eq. (3); the liquid Cu-Se phase can be formed in the CuSe phase diagram [37], and the liquid phase of Cu-Sn-Se can be formed at 400 C in the Cu2Se3Sn and Cu2SnSe3-Se phase diagrams [38,39]. According to the results in Fig. 7, voids are formed first by Zn consumption due to the formation of the ZnSSe shell. The mass transfer of Cu and Sn from below the ZnSSe shell to above the ZnSSe shell leads to additional void formation under the ZnSSe. These results are important evidence related to the formation mechanisms of the CZTSSe double layer and ZnSSe layer between the CZTSSe layers and voids. Fig. 8(a) shows the STEM-EDS maps of the sample cooled from 440 C. The CZTSSe double layer, ZnSSe layer and voids were observed in this sample. The CTSe phase was also found near the surface of the upper CZTSSe layer. The CTSSe phase in this sample was observed as Cu2SnSe3-x (Cu:Sn:Se:S = 39:20:39:2), as shown in Fig. 8(b). The thickness of the ZnSSe layer decreased due to the reaction between the CTSSe and ZnSSe. An additional CZTSSe phase might be formed by additional reactions between the CTSSe and Zn components. The thickness of the MoSSe was approximately 50 nm. The compositional variation between each grain, as shown in Fig. 8(c), was expected. Fig. 9(a) shows the STEM-EDS maps of the sample cooled after reaching 480C . The CZTSSe double layer, ZnSSe layer and voids were observed in this sample without the CTSSe phase near the surface. The thickness of the MoSSe layer was approximately 120 nm. Cross-sectional STEM-EDS line scans are shown in Fig. 9(b) and (c). The compositional variation between the
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etched grains was not significant. At this stage, most of the double layer, ZnSSe, and voids are formed. When the annealing time was increased, the distribution of each element was more uniform, and grain growth of the CZTS and an increase in the MoSSe thickness were observed as shown in Fig. 1. Fig. 10 shows a schematic of the formation mechanisms of the CZTSSe double layer, the ZnSSe layer between the CZTSSe layers and the voids in the lower CZTSSe layer. Alloy type metal precursors were prepared by sputtering at room temperature, as shown in Fig. 10(a). The melting of the Sn, rearrangement of the metal alloys and formation of the ZnSSe shell by dezincification began to occur during the heating process, as shown in Fig. 10(b). Due to persistent dezincification and the preferential reaction of Zn-SSe, almost all Zn is consumed to form the ZnSSe layer; as a result, voids are produced under the ZnSSe layer, as shown in Fig. 10(c) and (d). The thickness of the ZnSSe layer increases until the Zn from the Cu-Zn alloy is consumed and decreases as the ZnSSe is consumed as CZTSSe synthesis begins. The Cu-Sn alloy under the ZnSSe layer migrates onto the ZnSSe layer through the grain boundaries of the ZnSSe layer, and secondary phases such as Cu-SSe, Sn-SSe and Cu-Sn-SSe are formed on the ZnSSe layer, as shown in Fig. 10(e)-(g). At this time, additional voids are formed under the ZnSSe layer due to the mass transfer of Cu and Sn. The void formation due to the mass transfer of Cu and Sn is likely related to the Kirkendall effect [33]. The Sn-SSe and Cu-SSe on the ZnSSe form Cu-SnSSe and react with the Zn-SSe layer to form the upper CZTSSe as shown in Fig. 10(h); based on the reaction in Eq. (3), the position of CZTSSe is likely formed between the CTSe and ZnSSe, as shown in Fig. 10(h). Additional grain growth of the CZTSSe is expected during annealing at 480C, as shown in Fig. 10(i) and (j).
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4. Conclusion In this study, the formation mechanisms of the double layer, voids and ZnSSe layer were proposed. Two mechanisms have been developed to explain void formation. In the first mechanisms, large voids are formed by the dezincification and preferential reaction of Zn-SSe from the Sn/Cu/Zn/Mo metal precursors; the use of Zn-SSe precursor layer might inhibit the formation of voids from the dezincification and preferential reaction of Zn. In the second mechanism, additional small voids are formed by the mass transfer of Cu and Sn from under the ZnSSe layer to above the ZnSSe layer. Consequently, the use of a Sn-Cu alloy/ZnSSe/Mo precursor stack might inhibit both large voids corresponding to dezincification and additional tiny voids corresponding to the mass transfer of Cu and Sn. We are conducting experiments using the aforementioned structures and will report the results in the near future. The CZTSSe double layer and the ZnSSe layer between the CZTSSe layers were formed due to the formation of a ZnSSe intermediate layer by dezincification and the mass transfer of Cu and Sn through the ZnSSe layer. The results of this study suggest that the CZTSSe double layer, ZnSSe layer, and void formation, which are typical problems encountered when using metal precursors, can be controlled by changing the design of the precursors. Compared with the void/secondary phase free CZTSSe (12.6%) synthesized through the hydrazine process, the CZTSSe synthesized with metal precursors had an efficiency of 12.5% even though its microstructure had many voids and secondary phases. These results suggest that defect management at the interface, grain boundaries and bulk might be a key factor rather than void and secondary phase control. Thus, we are in the process of identifying the defect status of our cells at the interface, grain boundaries, and bulk. Nevertheless, the effects of the CZTSSe double layer, ZnSSe layer, and voids on the FF and output current cannot be ignored. For future
20
work, we will continue to study controlling the secondary phases and voids while maintaining the superb quality of the large-grained CZTSSe upper layer near the front junction shown in this study.
Fig. 1. Photovoltaic device properties for the 12.5% efficienct solar cell. (a) Certified JV curve and (b) EQE curve for the highest efficiency CZTSSe device. (c) Statistical plot of devic e performance parameters of the certified CZTSSe devices.
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Fig. 2. (a) Large-area FE-SEM image of a CZTSSe cross-section prepared by FIB etching. (b) STEM-EDS maps. (c) FE-SEM image of the voids and lower part of the CZTSSe surface and (d) high-magnification FE-SEM image of the area in the dotted square in Fig. 2 (c) after an additional etch.
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Fig. 3. (a) Schematic of the temperature profiles. Dotted lines are used to indicate cooling but do not represent the actual cooling profiles. Cross-sectional STEM images of the (b) pre-annealed Sn/Cu/Zn/Mo metal precursors, (c) sample cooled after reaching 300 C, (d) sample cooled after annealing at 300 C for 15 min, (e) sample cooled after reaching 400 C (e1, e2, and e3 are the results of analyses at the location indicated in the inset image (e) in Fig. 3 (a)), (f) sample cooled after reaching 440 C, and (g) sample cooled after reaching 480 C.
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Fig. 4. (a) Surface and cross-sectional FE-SEM images of the Sn/Cu/Zn/Mo metal precursors. The image of the metal precursors is shown as the inset image in Fig. 4 (a). (b) Wide-range cross-sectional STEM image and (c)-(d) STEM-EDS maps of the metal precursors.
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Fig. 5. (a) Surface FE-SEM images of the sample cooled after reaching 300 C. The image of the sample is shown as the inset image in Fig. 5 (a). (b) Wide-range cross-sectional STEM image and (c) STEM-EDS maps of the sample.
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Fig. 6. (a) Surface and cross-sectional FE-SEM images of the sample cooled after annealing at 300 C for 15 min. The image of the sample is shown as the inset image in Fig. 6 (a). (b) Widerange cross-sectional STEM image and (c)-(d) STEM-EDS maps of the sample.
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Fig. 7. (a) Cross-sectional FE-SEM images of the sample cooled after reaching 400 C. (b) ZnSSe etched-surface image and corresponding EDS maps (tilted view), (c)-(f) surface FE-SEM images and cross-sectional STEM-EDS maps at different locations in the sample as the inset images. The actual temperature of the (f) location is expected to have been higher than that of the (c) location.
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Fig. 8. (a) Cross-sectional STEM-EDS maps (b) and (c) compositional line scans of the sample cooled after reaching 440 C.
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Fig. 9. (a) Cross-sectional STEM-EDS maps and (b) and (c) compositional line scans of the sample cooled after reaching 480 C.
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Fig. 10. Schematic showing the formation of the CZTSSe double layer, the ZnSSe layer between the CZTSSe layers and voids in the Mo-back contact side. Schematic of the (a) metal precursors, (b) sample cooled after reaching 300 C , (c) sample cooled after annealing at 300 C for 15 min, (d)-(g) sample cooled after reaching 400 C , (h) sample cooled after reaching 440 C , (i) sample cooled after reaching 480 C, and (j) final CZTSSe absorber sample cooled after annealing at 480 C for 10 min. The 1st and 2nd Cu-Sn alloys are expected to be ε-Cu3Sn and ηCu6Sn5, respectively.
ACKNOWLEDGMENT This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980), the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT, KOREA (2016M1A2A2936781), and the DGIST R & D Programs of the Ministry of Science, ICT & Future Planning of Korea (19-BD-05). We thank Mr. Cheon and Mr. Eun in Center for Core Research Facilities (CCRF) for the STEM measurements. REFERENCES [1] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, S. Guaha, Sol. Energy Mater. Sol. Cells. 95 (2011) 1421-1436.
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[2] T. D. Lee, A. U. Ebong, Renewable Sustainable Energy Rev. 70 (2017) 1286-1297. [3] Best Research-Cell Efficiencies chart in the US Department of Energy’s National Renewable Energy Laboratory. <
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[39] G. Effenberg, S. Ilyenko, Springer, ISBN 978-3-540-32589-5 (2006) 361-373.
Se-Yun Kim is a researcher in Convergence Research Center for Solar Energy at Daegu Gyeongbuk Institute of Science and Technology (DGIST). He received his Ph.D. in School of Materials Science and Engineering (Electronic Materials Science and Engineering), Kyungpook National University, South Korea in 2015. His research interests include thin film growth, p-type transparent oxide semiconductor, halide perovskite, and Cu-based chalcogenide for thin film solar cell.
Dae-Ho Son received the B.S. degree in the physics from Hanyang University, Seoul Korea in 2006, and the M.S. degrees in Nano Science & Technology at University of Seoul, Seoul, Korea, in 2008. He is presently a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. His current research interests include photovoltaic electronics, thin film growth, chalcogenide photovoltaics and thin film devices
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Young-Ill Kim received his M.S. degree from Department of Nano Science & Technology, University of Seoul, Korea in 2012. Now he is a researcher in DGIST, developing the vacuum process for thin film solar cells.
Seung-Hyun Kim received his M.S. degree from Department of Physics, Yeungnam University, Korea in 2016. Now He is a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. His current research interests include thin film growth, chalcogenide photovoltaics, and analysis of photovoltaic devices.
Sammi Kim received her M.S. degree in chemical engineering from Yeungnam University, Korea in 2014. She worked engineer in the PosLX project team at RIST from 2014 to 2017. She joined DGIST in 2017 and currently she is a researcher in the Convergence Research Center for Solar Energy at DGIST, Korea. She current research interests include thin film growth, chalcogenide photovoltaics and flexible thin film devices.
Kwangseok Ahn is a Ph.D. candidate and study about OFETs (Organic Field Effect Transistors) at the Department of Physics at Soongsil University. He is joined Convergence Research Center
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for Solar Energy, DGIST in 2017. His research interests focus on analysis of the electrical, optical properties in chalcogenide photovoltaics.
Shi-Joon Sung is a principal researcher in Convergence Research Center for Solar Energy at Daegu Gyeongbuk Institute of Science and Technology (DGIST). He received his Ph.D. degree in Chemical & Biomolecular Engineering from the Korean Advanced Institute of Science and Technology (KAIST) in 2004. He has worked in the Samsung Electronics until 2007 as a senior research engineer. His research interests are inorganic compound semiconductor thin films and their applications to various energy and electronic device technology.
Dae-Kue Hwang received his Ph.D. degree from Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), South Korea in 2008. He worked as postdoctoral fellow at Northwestern University, Illinois from 2008 to 2010. He joined DGIST in 2010 and currently he is a principal researcher at the DGIST. His research interests include light emitting diode, oxide thin film transistors, quantum dot device and thin film solar cells. Details can be found at https:// www.researchgate.net/profile/Dae-Kue_Hwang.
Kee-Jeong Yang received his M.S. degree from Department of Chemical Engineering, POSTECH and Ph.D. degree from Department of Chemical Engineering, Kwangwoon University. He worked at Samsung electro-Mechanics research center from 2001 to 2003, and at LG Display LCD from 2004 to 2005. He joined DGIST in 2005. He has worked flexible,
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transparent, and micro display-related research. His current research focuses on development of chalcogenide-based photovoltaics on SLG and various substrates. Details can be found at http://www.dgist.ac.kr. Also you can contact to [email protected].
Jin-Kyu Kang received the B.S. in engineering chemistry from Seoul National University in 1991, and M.S and Ph.D. degrees in chemical engineering from POSTECH in 1993 and 2000. He joined Samsung Electronics, and led the research about low temperature poly-Si TFT- LCD. He has been a principal researcher at the DGIST, Korea since 2005. His research is focused developing and characterizing for thin film solar cells (CIGS, CZTS, DSSC). He was honored by the Ministry of Trade, Industry and Energy in 2015 and the Ministry of Science and ICT in 2018 for his works. He holds more than 150 patents and has published more than 90 papers.
Dae-Hwan Kim received his Ph.D. degree from Department of Chemical Engineering, POSTECH, KOREA in 2002. From 2002, he worked at Samsung Electronics' Semiconductor R&D Business and invented the lean-free MESH capacitor process for 80nm or less scale DRAM. He joined DGIST in 2005 and currently he is a director of Convergence Research Center for Solar Energy, DGIST. His research interests include oxide thin film transistors and thin film solar cells (CIGS, CZTS and SbS). Details can be found at https://www.researchgate.net/profile/Dae-Hwan_Kim.
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Highlights The new formation mechanisms of the voids, ZnSSe layer and CZTSSe double layer that were observed in the sister sample of 12.5% of CZTSSe cell. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens, such as S and Se, almost all the Zn is preferentially consumed to form the ZnSSe layer; large voids are first produced under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe shell by migration of Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Due to the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, the CZTSSe double layer is formed and ZnSSe exists between the CZTSSe double layers.
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