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Spatial Grain Growth and Composition Evolution during Sulfurizing Metastable Wurtzite Cu2ZnSnS4 Nanocrystal-Based Coatings Xu Liu,†,§ Jialiang Huang,†,§ Fangzhou Zhou,† Fangyang Liu,† John A. Stride,‡ and Xiaojing Hao*,† †

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ‡ School of Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: A drawback of nanocrystal-based processing, that leads to the notoriously poor crystallinity of pure-sulfide Cu2ZnSnS4 absorber, was recently reported to be effectively overcome by the annealing of thin films made from the ink of metastable wurtzite Cu2ZnSnS4 nanocrystals in a combined S and SnS atmosphere. However, the formation pathway from nanometer-sized crystals in the wurtzite phase to micrometersized grains with the kesterite phase during this process still lacks in-depth study. In this work, the spatial grain growth and composition evolution during the sulfurization of wurtzite nanocrystal coatings are systematically investigated by classifying samples into temperature and time series. In the process of heating up, the reversible migration of Cu and Zn species contributes to a continuous growth of kesterite Cu2ZnSnS4 grains on the surface. At higher temperature, a fast phase-transition growth from wurtzite Cu2ZnSnS4 nanoparticles to kesterite grains is also directly observed in the region away from the surface. After reaching 580 °C, the thin film experiences impressive decomposition and reorganization changes as a function of time, which cause the formation of an absorber with good crystallinity and homogeneous compositional distribution. The solar cell device, fabricated by employing this pure-sulfide Cu2ZnSnS4 absorber from wurtzite nanocrystal-based coatings, demonstrated an energy conversion efficiency of 6.0% in the absence of an antireflection coating.

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INTRODUCTION

Recently, we overcame the grain-growth challenge of CZTS coatings deposited from CZTS nanocrystal inks and demonstrated the corresponding CZTS solar cells to have over 4.8% efficiency by using S atmosphere, postdepositing a NaF layer, and tuning the cation ratios in the precursor solution.2 More recently, we further reduced the thickness of the fine-grained layer and effectively enhanced the crystalline quality of the absorber layer by modifying the sulfurization atmosphere and dissolving NaCl into the inks.6 Tuning the amount of Na is necessary for CZTS absorbers made from wurtzite nanoparticles, yet it is insufficient to obtain solar cell devices with better performances. The introduction of SnS powder along with the S pellets in the sulfurization treatment offers an alloying leverage to improve the quality of absorber. Using this approach, we have optimized the compositional distribution and microstructure of the final absorber. Improved performance of CZTS solar cells with the highest efficiency of 6.0% was hence obtained.6 These trials on using the wurtzite CZTS nanocrystal-based precursor coatings demonstrated a puresulfide CZTS device performance level that is comparable to

Among the family of Cu-based direct band gap semiconductor materials, Cu2ZnSnS4 (CZTS) is a potential candidate for an eco-friendly sustainable light absorbing material to be used in the thin film photovoltaic field due to its high absorption coefficient, cheap constituents, optimal band gap, and decent alloying property with related materials.1−3 Despite the high efficiencies achieved from vacuum-based deposition technologies, lower production costs coupled with high throughput are increasingly driving the research into nonvacuum deposition processing.2−6 One promising approach is to directly sulfurize the substrates coated with the thin film deposited from CZTS nanocrystal-based inks. However, poor crystallinity, numerous detrimental boundaries, and potential impurities from the synthesis solvent within the sintered absorber materials still remain as drawbacks, impeding the development of nanocrystal processing.7 Moreover, forming a homogeneous CZTS absorber layer with compact large grains by sulfurizing kesterite CZTS nanocrystals has been proven to be difficult.2,8,9 In contrast, partial conversion into large grains is much easier to be achieved by an anion-exchange reaction based on the selenization processing of kesterite CZTS nanocrystals at high temperatures.5,10,11 © 2017 American Chemical Society

Received: October 27, 2016 Revised: February 21, 2017 Published: February 22, 2017 2110

DOI: 10.1021/acs.chemmater.6b04603 Chem. Mater. 2017, 29, 2110−2121

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Chemistry of Materials

Figure 1. Plan view SEM images of CZTS thin films annealed at (a) 350 °C in air, (b) 450 °C in RTP, (c) 490 °C in RTP, (d) 530 °C in RTP, and (e) 580 °C in RTP. All scale bars are 1 μm. (f) Corresponding XRD patterns of thin films annealed at various conditions (●, Cu2−XS; ⧫, kesterite CZTS). (g) Corresponding Raman spectra of thin films annealed at various conditions with 514 nm excitation.

those achieved from electrodeposition, thermal evaporation, and/or sputtering routes.12−15 However, despite the initial breakthrough in the application of wurtzite nanocrystals, the reaction pathway from a coating made of wurtzite nanoparticles to a thin film consisting of large grain kesterite still lacks clear clarification. Moreover, an in-depth understanding of the mechanisms taking place during the transformation course is also essential to further adjust the synthesis parameters, such as optimizing the annealing conditions, in order to deliver improved device performance.

Within this framework, we herein present a systematic study of the spatial growth of course of grains and the evolution of compositions during the high temperature treatment of the CZTS precursor coatings. We observed that, during the treatment of sulfurization, both microstructure and composition of the thin films experience significant changes at different processing stages. To clearly investigate the formation pathway, samples are classified into two categories: (1) temperature series (the heating up stage before reaching the given sulfurization temperature, 580 °C); (2) time series (the holding 2111


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450 °C in RTP, 490 °C in RTP, 530 °C in RTP, and 580 °C in RTP, respectively. For the as-deposited CZTS precursor coating baked at 350 °C in air (Figure 1a), a well-defined smooth and compact surface morphology was obtained, which is the prerequisite of the investigation of formation pathway in this work. For the sample sulfurized at 450 °C (Figure 1b), large faceted crystals with sizes of up to 1 μm emerge on top of the nanocrystal floor layer. However, these micrometer-sized crystals completely vanish when the temperature increases to 490 °C, as shown in Figure 1c. In contrast, numerous small grains emerge with the average size less than 100 nm. As the temperature rises to 530 °C, the average size of grains on the surface gradually increases to several hundred nanometers (Figure 1d). Finally, when the sulfurization temperature reaches 580 °C, a densely continuous structure consisting of many microsized grains is observed, as shown in Figure 1e. The SEMEDS results show that the metal stoichiometry ratios of Cu/Sn and Zn/Sn are about (1) 1.45 and 1.06 for the sample baked at 350 °C in air, (2) 1.53 and 1.05 for the sample sulfurized at 450 °C, (3) 1.50 and 1.05 for the sample sulfurized at 490 °C, (4) 1.50 and 1.05 for the sample sulfurized at 530 °C, and (5) 1.80 and 1.16 for the sample sulfurized at 580 °C, respectively. Usually, the increase in Cu/Sn and Zn/Sn ratios in the sulfurized CZTS materials demonstrates that there is Sn loss occurring during the sulfurization process.2,18 Therefore, compared to the little change of Cu/Sn and Zn/Sn ratios for the sample annealed at temperatures below 530 °C, the sample sulfurized at 580 °C shows greater loss of Sn. Note that the ratios of Cu/Sn and Zn/Sn calculated by SEM-EDS only correspond to the overall film and not within local regions, such as individual grains. The presence of binary phases significantly influences the overall stoichiometry ratios of sulfurized or selenized thin films, especially for the samples fabricated by offstoichiometric precursor ratios.19 Our previous work confirmed this result by comparing TEM-EDS and SEM-EDS data.6 Figure 1f,g shows the corresponding XRD patterns and Raman spectra, respectively. For the baked precursor coating, the XRD pattern shows the same characteristic peaks as those of asprepared nanocrystals (Figure SI1a), suggesting that the baking conditions are not sufficient to induce phase transition. Moreover, the strong Raman peak at 334 cm−1 matches well with the reported Raman A mode of wurtzite CZTS materials.17 No peaks belonging to other phases were observed in the XRD pattern and Raman spectrum, indicating the purity of the as-baked sample. In contrast, for the sample sulfurized at 450 °C, besides the strongest peak of wurtzite CZTS materials (334 cm−1), another strong peak was at about 471 cm−1 belonging to the Cu2−XS phase. This can be explained by the crystallographic similarity of the CZTS and binary phases. Note that, due to the limited penetration depth of the 514 nm laser (less than 170 nm), this Cu2−XS may only form near the surface area. It is highly possible that a portion of the large faceted crystals shown in Figure 1b is this Cu2−XS material. As the temperature increases to 490 °C, new XRD peaks marked with “●”and “⧫” were observed. Interestingly, the peaks marked with “●” can be ascribed to another Cu2−XS material (JCPDS No. 36-0380) rather than the Cu2−XS (JCPDS No. 33-0491) obtained at 450 °C. The diffraction peak around 33.0° (marked with “⧫”) corresponds to the characteristic (200) plane of the kesterite CZTS structure, suggesting the beginning of phase transition from wurtzite to kesterite within the thin film.20 Moreover, the corresponding Raman peak at 471 cm−1 nearly disappears, indicating the Cu2−XS obtained at 450 °C drastically

time/annealing after reaching the given sulfurization temperature, 580 °C). In the process of heating up, Cu and Zn migrate to the surface region at 450 °C, leading to a locally Cu-rich and Zn-rich environment, despite a Cu-poor overall composition for the overall thin film. Therefore, a direct and fast formation of grained Cu2−XS and ZnS on top of the wurtzite nanocrystal layer is observed. With increased annealing temperature, Cu and Zn then quickly diffuse back to the remaining particle layer. Cu2−XS and ZnS grains completely vanish at 490 °C. In contrast, a thin layer consisting of small Cu-poor kesterite CZTS grains forms on top of the wurtzite nanocrystal layer and continuously grows as a function of temperature. Compared to the changes of the surface, the remaining region shows a relatively slower formation of kesterite CZTS large grains, starting from wurtzite nanoparticles. During the holding time period at a given annealing temperature (580 °C), the large grained CZTS top layer continues to grow with sulfurization time. In contrast, the CZTS grains in the bottom layer undergo a significant decomposition. Specifically, increasing amounts of Sn are continuously lost from the bottom layer, leading to Cuand Zn-containing binary materials, as well as the observation of unique MoSX crystalline nanoplates. Finally, after sufficient sulfurization treatment, the large grained CZTS top layer coalesces with the remaining CZTS, Cu2−XS, and partial ZnS materials within the porous bottom layer, contributing to the formation of the absorber layer with good crystallinity and homogeneous compositional distribution. The solar cell device, fabricated by employing this pure-sulfide CZTS absorber from wurtzite nanocrystal-based coatings, demonstrated an energy conversion efficiency of 6.0% in the absence of an antireflection coating.

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RESULTS AND DISCUSSION To investigate the grain-growth pathway from nanometer-sized crystals to micrometer-sized grains, we first examined the synthesis of wurtzite CZTS nanoparticles to gain as-prepared samples with the desired phase and crystallinity. Details of the synthetic procedure are provided in the Experimental Section, following our previous reports.6 In brief, wurtzite CZTS nanocrystals are prepared by injecting 1-DDT and t-DDT into a mixture of Cu(OAc)2, Zn(OAc)2, and Sn(OAc)2 dissolved in a TOA-ODE solution at 140 °C, followed by the reaction at 250 °C for 1 h. Structural characterization of the CZTS nanocrystals was performed with XRD and TEM (Figure SI1). The characteristic XRD peaks of wurtzite CZTS from the planes, such as (100), (002), (101), (101), (102), (110), and (103), are visible in the pattern, matching well with previously reported values.16,17 TEM images, as shown in Figure SI1b, further present the well-defined particle size distribution and highly crystalline nature of the as-synthesized CZTS. The inset in Figure SI1b is a typical HRTEM image, indicating the interplanar distance of 0.33 nm from the (100) plane of wurtzite CZTS. Next, we prepared the CZTS nanocrystal inks which we spincoated onto Mo-coated soda lime glasses (SLG) and baked at 350 °C in air. This process was repeated several times in order to reach the desired film thickness and then followed by postsulfurization treatment inside a rapid thermal processing (RTP) furnace. Fabrication details are described in the Experimental Section. In order to study the reaction kinetics during the heating up process, samples were removed as the temperature reached the desired values. Figure 1a−e shows plan view SEM images of samples annealed at 350 °C in air, 2112


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Figure 2. Cross-sectional bright field images and corresponding selected area electron diffraction (SAED) patterns obtained near the region marked with circles in the samples annealed at (a) 350 °C in air, (b) 450 °C in RTP, (c) 490 °C in RTP, (d) 530 °C in RTP, and (e) 580 °C in RTP. 2113


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Figure 3. HAADF STEM images and EDS scans taken along the corresponding line scans of CZTS thin films annealed at (a) 350 °C in air, (b) 450 °C in RTP, (c) 490 °C in RTP, (d) 530 °C in RTP, and (e) 580 °C in RTP. (b-1, b-2) The magnified HAADF STEM image and corresponding EDS scans of the squared area in (b). (c-1) The magnified HAADF STEM image and corresponding EDS scans of the squared area in (c).

decreases. The shoulder near the main 334 cm−1 peak implies the formation of kesterite CZTS as well. The XRD pattern of the sample sulfurized at 530 °C shows the intensity of the peak at about 28.5° becomes the strongest one and that the relative intensity of the wurtzite CZTS peaks decrease, suggesting an increase of kesterite CZTS material. Moreover, the corresponding Raman results show that the strongest peak already redshifts to 338 cm−1 (Raman A mode of kesterite CZTS) from 334 cm−1, indicating the grained layer shown in Figure 1d is nearly all the kesterite structure in the range of penetration depth of the 514 nm laser. Finally, kesterite CZTS dominates the XRD pattern after the sulfurization temperature rises up to 580 °C, and only kesterite CZTS peaks were observed in the corresponding Raman spectra. Figure 2 displays cross-sectional bright field TEM images of samples annealed at different temperatures and their corresponding selected area electron diffraction (SAED) of the regions marked with circles. The TEM image of the baked

precursor coating shown in Figure 2a matches well with the smooth surface morphology from the plan view SEM image in Figure 1a. The laminated structure of the precursor thin film is clearly observed, showing that the thickness of each coating is about 90−100 nm. Figure 2a-I is the SAED pattern obtained from the marked region (circle 1). The blurred concentric rings in the SAED pattern suggest the polycrystalline characteristic of the tiny crystals with poor crystallinity in the precursor. All these crystals have the wurtzite CZTS structure, confirmed by indexing. Figure 2b is the bright field TEM image of the sample sulfurized at 450 °C, agreeing with the morphology shown in Figure SI1b. Disconnected faceted large crystals with various sizes form on top of the laminated floor layer. Figure 2b-I is the SAED pattern of one crystal marked by circle 1, demonstrating its fully crystallized nature. Moreover, indexing of the SAED pattern reveals that this big faceted crystal is made of Cu2−XS material. In contrast, the SAED pattern of the neighboring grain marked with circle 2 shown in Figure 2b-II consists of ZnS. 2114


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Figure 4. Plan view SEM images of CZTS thin films annealed at 580 °C for (a) 1 min, (b) 2 min, (c) 4 min, and (d) 10 min. All scale bars are 1 μm. (e) Corresponding XRD patterns of thin films annealed at 580 °C for different times. (f) Corresponding Raman spectra of thin films annealed at 580 °C for different times with 514 nm excitation.

However, the laminated bottom layer still has concentric rings and remains in the wurtzite CZTS phase (see Figure SI2). Figure 2c is the bright field TEM image of the sample sulfurized at 490 °C, confirming the replacement of large crystals by numerous small grains, as shown in Figure 1c. However, these small grains only exist on the surface of the thin film rather than emanating from the back contact. Most regions of the thin film are still composed of nanoparticles. Figure 2b-I,b-II shows the corresponding SAED patterns of the regions marked by circles 1 and 2, respectively. The pattern of the top thin layer

demonstrates the formation of the kesterite CZTS phase, despite the particle floor layer mainly retaining the wurtzite nature. As the temperature increases to 530 °C, we observe that the sulfurized sample remains a bilayered structure similar to that observed at 490 °C but with a thicker top layer as shown in Figure 2d. Note that the laminated feature of the particle floor layer completely disappears, implying the preliminary coalescence of the bottom crystals at 530 °C. Figure 2d-I,d-II displays SAED patterns of the regions marked with circles 1 and 2, respectively. Compared to the pattern of samples treated 2115


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Figure 5. Cross-sectional bright field TEM images of the samples sulfurized at 580 °C for (a) 1 min, (b) 2 min, (c) 4 min, and (d) 10 min. (c-1) The magnified TEM image of the squared area in (c). (d-1) The magnified TEM image of the squared area in (d).

at lower temperatures, indexing of the floor layer shows the clear formation of a kesterite CZTS phase in the bottom layer. Finally, the sample sulfurized at 580 °C exhibits considerable crystallographic differences to samples annealed at relatively lower temperatures, as shown in Figure 2e. In addition to the further increase in the thickness of the top layer, the average lateral size of individual grains significantly grows up to 1 μm, agreeing with the morphology observed from the plan view SEM image shown in Figure 1e. Moreover, for the floor layer, grains with sizes up to hundreds of nanometers occupy most regions, rather than the nanoparticles or small grains observed at relatively lower temperatures. There are many voids forming at the interface between the top and bottom grained layers. Compared to other samples annealed at lower temperatures, a MoSX layer with thickness of about 200 nm forms near the back contact. The corresponding SAED patterns shown in Figure 2eI,e-II confirm that kesterite CZTS already dominates the thin film in both the top grained layer and the bottom porous layer. To study the compositional evolution during the heating step of sulfurization, we used TEM-EDS scans to show the local compositional profile of samples obtained at different annealing temperatures. Figure 3a shows a high angle annular dark field (HAADF) image and TEM-EDS scan taken along the yellow line of the wurtzite CZTS precursor coating. Obviously, Cu, Zn, and Sn remain quite homogeneous through the precursor coating baked at 350 °C in air, suggesting the baking process does not disturb the compositional distribution. Figure 3b shows the HAADF image and TEM-EDS scan taken along the yellow line of the sulfurized sample at 450 °C. Impressively, compared to the homogeneous compositional distribution of the precursor coating, Cu and Zn migrate toward the surface

region for this sample. Figure 3b-1,b-2 shows the magnified image of the square area marked in Figure 3b. According to TEM-EDS results taken along the red line in Figure 3b-1 and the blue line in Figure 3b-2, we directly confirm the existence of Cu2−XS and ZnS binary phases on the surface of thin film. Figure 3c shows the HAADF image of the sample sulfurized at 490 °C and the corresponding TEM-EDS scan taken along the yellow line. Interestingly, Cu and Zn diffuse back to the remaining region, contributing to the disappearance of Cu2−XS and ZnS. For the dominant nanoparticle floor layer, all cations present homogeneous compositional distribution. To further reveal the local compositional profile of the top thin layer, we have magnified the square area marked in Figure 3c. Figure 3c-1 shows that the signal intensity of Cu, in the top thin layer, is unexpectedly lower than that in the particle floor, suggesting a much Cu-poorer environment of the top CZTS layer. Figure 3d shows the HAADF image and TEM-EDS scan taken along the yellow line of the sulfurized sample at 530 °C. The compositional profile is similar to the one observed in the sample sulfurized at 490 °C, including the Cu-poor phenomenon of the grained top CZTS layer (see Figure SI3). As the temperature rises to the final 580 °C, the signal intensity of Zn slightly increases in the region near the back contact, accompanied by the appearance of a MoSX layer, shown in Figure 3e. The The EDS mapping results of samples annealed at different temperatures (Figure SI4) further confirm the findings from TEM-EDS line scans. These EDS data match well with the above results from SEM, XRD, Raman, TEM, and SAED analysis. To study the reaction kinetics at 580 °C, samples were taken out after 1, 2, 4, and 10 min, respectively. Figure 4a−d shows 2116


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Figure 6. HAADF STEM images and EDS scans taken along the corresponding line scans of CZTS thin films annealed at 580 °C for (a) 1 min, (b) 2 min, (c) 4 min, and (d) 10 min. All scale bars are 500 nm.

plan view SEM images of samples sulfurized at 580 °C for different holding/annealing durations and displays densely packed large grains with sizes of up to a few micrometers. No noticeable holes or valleys are observed on the surface. In addition, the absorbers show slow surface morphology evolution of large grains with increasing annealing time. According to corresponding SEM-EDS results, the metal stoichiometry ratios of Cu/Sn and Zn/Sn are about (1) 1.75 and 1.07 after 1 min, (2) 1.73 and 1.05 after 2 min, (3) 1.83 and 1.21 after 4 min, and (4) 1.91 and 1.34 after 10 min, respectively. The increasing Cu/Sn and Zn/Sn ratios indicate greater Sn loss occurring for the samples annealed for 4 and 10 min. Figure 4f shows the corresponding XRD patterns of samples with different sulfurization holding times. No characteristic XRD peaks belonging to wurtzite CZTS structure are observed across all samples. Besides the strong peaks of kesterite CZTS, the peaks marked with “●” can be also ascribed to Cu2−XS material (JCPDS No. 36-0380) observed in the above samples sulfurized at temperatures above 490 °C. Interestingly, the relative intensity of XRD peaks belonging to this Cu2−XS encounters several changes in the range of sulfurization time, implying a changing quantity. Specifically, compared to that of the sample annealed for 1 min, the relative XRD intensity of Cu2−XS first reduces at 2 min, then turns stronger at 4 min, and finally vanishes at 10 min. The peak at 27° marked with “∗” is caused by the stacking faults in a CZTS crystal.5 Elongating the sulfurization time effectively inhibits the formation of this reflection. Figure 4g shows the Raman spectra of samples with different sulfurization holding times. Only kesterite CZTS peaks are observed for all samples in the range of penetration depth of the 514 nm laser. Figure 5 shows the cross-sectional bright field TEM images of the samples sulfurized at 580 °C for 1, 2, 4, and 10 min, respectively. Clearly, the thickness of the MoSX layer increases with annealing time. Compared to the sample annealed at 580 °C for 0 min shown in Figure 2e, the sample with a holding time of 1 min (Figure 5a) has a thicker grained top layer and better crystallized bottom grains. However, the grains in the

bottom layer are smaller than the ones in the top layer, despite its average size has already reached a few hundred nanometers. As the holding time increases to 2 min, the thickness of the large grained top layer is further improved. However, an unexpected decomposition reaction is observed in the bottom layer, leading to the replacement of grains by small particles, as shown in Figure 5b. This decomposition is aggravated with the increase of sulfurization time. More voids are observed in the bottom layer of the sample annealed for 4 min (see Figure 5c). By studying the result of the magnified square area in panel c, we observe the clear formation of unique crystalline nanoplates embedded in the space region. Interestingly, the interlayer distance (0.62 nm) of these nanoplates corresponds to the (002) plane of MoSX.21 In contrast, the thickness of the large grained top layer shows no obvious variation. Finally, the porous bottom layer observed in the sample annealed for 4 min is compressed into a compact thin layer with MoSX nanoplates and other amorphous materials (Figure 5d,d-1). A compact and well-crystallized absorber consisting of only large grains was obtained. These are prerequisites for a good device performance. To study the compositional evolution during the given holding time at 580 °C, the TEM-EDS is used to show the local compositional profile of samples sulfurized for various durations. Figure 6a shows the HAADF image and TEMEDS scan taken along the yellow line of the sample sulfurized for 1 min. Compared to the homogeneous compositional profile of the top large grained layer, cations start to present obvious fluctuation in the bottom grained layer, especially in the region near the back contact. Specifically, within the bottom layer, the signal intensity of Cu is locally stronger in some regions and the signal intensity of Zn shows an increasingly stronger tendency toward the back contact. In contrast, the Sn signal drops significantly at the interface between the bottom layer and the back contact, suggesting the loss of Sn in these regions. These results are mainly caused by the detrimental reaction between the CZTS materials and the standard Mo back contact.22 Figure 6b shows the HAADF image and TEM2117


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Figure 7. Schematic illustration of grain growth and compositional evolution during sulfurizing metastable wurtzite CZTS nanocrystal-based coating.

These EDS data match well with the above results analyzed from SEM, XRD, Raman, TEM, and SAED. Figure 7 shows a schematic illustration of spatial grain growth and compositional evolution during the sulfurization of a metastable wurtzite CZTS nanocrystal-based coating. Only the surface region of the precursor layer is crystallized at low sulfurization temperature, with the emergence of binary sulfides due to the migration of Cu and Zn elements toward the surface (Figure 7a,b). The formation of binary phase, especially Cu2−XS, could be expected to always exist as liquid. The presence of liquid Cu2−XSe, alkali metals, and molten chalcogenide has been confirmed by previous studies, and the corresponding mechanisms regarding assisting grain growth in chalcogenide films were proposed as well.7,24−27 Recently, Cu2−XS was reported to be facilely obtained on the surface by element diffusion during sulfurizing the Cu−Zn−Sn metallic precursor at low temperature, because of its more favorable Gibbs free energy of formation.18,28 Moreover, a stable Cu− Zn−S ternary compound cannot be synthesized due to the low solubility of ZnS with Cu2−XS.18 Therefore, Cu2−XS and ZnS are segregated from each other. In addition, the reactivity order of the cations in an individual wurtzite CZTS nanocrystal was found to increase in the order of Sn (least reactive) < Zn < Cu (most reactive).29 In this work, the precursor coating composed of close-packed wurtzite nanocrystals may follow similar rules as well. The dense structure of the precursor thin film may facilitate the diffusion of Cu and Zn through numerous boundaries between nanoparticles and then cause the formation of binary sulfides. With increasing sulfurization temperature, Cu and Zn move back to the wurtzite CZTS nanocrystal-based floor layer, agreeing with the characterization on the disappearance of faceted binary crystals but a thin layer

EDS scan taken along the yellow line of the sample sulfurized for 2 min. Note that the compositional distribution of Cu becomes uniform through the thin film including the top and the bottom layers, despite the structure of the bottom layer transferring from large grains with voids into smaller but even more porous grains. With longer holding duration (4 min) at 580 °C, more evident compositional evolution is observed (see Figure 6c). Besides the compositional fluctuation within the porous bottom layer, the signal intensities of all cations composing the CZTS absorber show lower levels than the ones within the large grained top layer, which mainly results from the fact that the bottom layer becomes even more porous and the overall mass density has significantly decreased. Additionally, the decrease in the Sn signal is more dramatic than other elements such as Cu and Zn, which cannot be fully explained by the mass density change. The presence of Sn within the bottom layer can be barely detected beyond the region near the top layer where there are still considerable signal intensities of Cu and Zn. This Sn loss can be explained by the formation of volatile SnSX material during thermal processing.23 Besides, Cu and Zn may also diffuse into the large grained top CZTS layer due to the low vapor pressure of Cu2−XS and ZnS. Figure 6d shows HAADF image and TEM-EDS scan taken along the yellow line of the sulfurized sample. As has been reported,6 the thin layer between the compositionally homogeneous CZTS absorber and the MoSX layer is C-rich. Hardly any Zn and Sn signal can be observed in this thin layer. The EDS mapping results of samples with different annealing duration (Figure SI5) confirm the analysis from TEM-EDS line scans, such as the presence of Cu2−XS and ZnS binary phases for different samples as well as the Sn loss during the thermal processing. 2118


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redistribution and reorganization due to the sulfurization treatment. In the heating stage, grains of Cu2−XS and ZnS first form on top of the wurtzite CZTS nanocrystal-based floor due to mass compositional diffusion toward the surface region. Along with the formation of a thin layer composed of small kesterite CZTS grains, Cu and Zn are observed to move back into the nanocrystal floor. As the sulfurization continues, the thin CZTS layer on the top continuously grows into a compact large grained layer by transport of material from the region away from the surface. In contrast, the bottom region undergoes a distinct formation pathway. Wurtzite CZTS nanocrystals transform into kesterite CZTS nuclei by a fast and direct solid-state phase transition. Then, the kesterite nuclei in the bottom layer grow into large grains by incorporation of neighboring nanocrystals. However, the kesterite CZTS grains in the bottom are not preserved during sulfurization due to insufficient S pressure and the instability of Sn (IV). Volatile Sn leaves from the bottom layer as SnS vapor, causing the extremely porous microstructure having voids and the formation of Cu2−XS and ZnS binary phases. Finally, after sufficient sulfurization, the absorber layer with good crystallinity and homogeneous composition forms by coalescence of the large grains of the upper CZTS layer and different grains remaining in the bottom layer. The solar cell with efficiency of 6% confirms its potential in the field of photovoltaics. The insight inspired by this work will enhance our understanding of the formation pathway of the sulfurization of wurtzite nanocrystals and assist to further boost cell efficiency in the near future.

consisting of small kesterite CZTS grains instead (Figure 7b,c). At 490 °C, a continuous layer is formed by small kesterite CZTS grains at the surface with grain size below 100 nm, while most of the thin film underneath remains in the wurtzite CZTS structure, indicating no occurrence of the grain growth induced by the phase transition. Therefore, the nucleation of kesterite CZTS observed on the surface is more likely attributed to the reaction of different metal-containing compounds under Curich conditions, which is well-known to be favorable for the grain growth of CZTS-based and CIGS-based materials.30 Similar observations have been reported elsewhere.18,28,31 As annealing temperature further increases, the grained top thin layer grows by the diffusion of elements from the nanocrystal bottom layer. Moreover, the nuclei of kesterite CZTS structure start to form within the nanocrystal floor due to the unstable nature of wurtzite CZTS phase at high temperature. As reported, under an atmosphere of only inert gas, the grain growth of kesterite CZTS from wurtzite CZTS nanoparticles is driven by a fast and direct solid-state phase transition due to the low energy barrier between these two structures.1 The kesterite CZTS crystal seed quickly grows into grains by incorporation of neighboring nanoparticles.1 In this work, after the sulfurization temperature reaches 580 °C, clearly large grains with sizes up to several hundred nanometers are obtained (Figure 7c,d), agreeing with previous results. At fixed sulfurization temperature, the crystalline quality of grains within the bottom layer initially improves and then gradually decreases, decomposing into smaller grains (Figure 7d,f). The unstable nature of Sn (IV) and atmosphere of low S partial pressure are regarded as two main driving forces for the decomposition of CZTS materials annealed at high temperature.23 The high vapor pressure of SnS further worsens this decomposition reaction by making it irreversible.23 In this work, the compact and thick top layer may inhibit the pathway of S and Sn out of the bottom layers, thus leading to continuous Sn loss starting from the region near the back contact. As more Sn element leaves the bottom layer as SnS vapor, it interferes with the formation of porous microstructure and results in an increasing extent of nonvolatile Cu2−XS and ZnS binary phases (Figure 7g). Finally, after sufficient sulfurization treatment, the large grained CZTS layer on the top coalesces with different materials (including CZTS and most binary phases) that remain in the bottom layer. The highly porous structure in the bottom of the film is compressed into a thin layer that consists of amorphous C and MoSX nanoplates. The absorber layer with good crystallinity and homogeneous composition is therefore obtained (Figure 7h). We further fabricated the corresponding CZTS solar cells by using the final CZTS absorber to confirm its potential application in the field of photovoltaics. The performance of a typical CZTS device without antireflection coating is shown in Figure SI6, demonstrating open circuit voltage (VOC) of 556.35 mV, short circuit current density (JSC) of 18.94 mA cm−2, fill factor (FF) of 56.7%, and PCE of 6.0% under AM 1.5 illumination. To the best of our knowledge, the PCE has the highest reported value for pure-sulfide CZTS thin film solar cells from sulfurizing quaternary CZTS nanocrystals.

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EXPERIMENTAL SECTION

Chemicals and Materials. Cu(OAc)2·H2O (copper(II) acetate monohydrate, >98%), Zn(OAc)2·2H2O (zinc(II) acetate dehydrate, >99%), Sn(OAc)2 (tin(II) acetate, 99%), TOPO (trioctylphosphine oxide, 99%), ODE (1-octadecene, 90% tech), 1-DDT (1-dodecanethiol, 98%), t-DDT (tert-dodecylmercaptan, 98.5%), TOA (trioctylamine, 98%), NaCl (sodium chloride >99.7%), and DMSO (dimethyl sulfoxide, >99.7%) were purchased from Aldrich. All chemicals were used as received without any further purification. Synthesis of Wurtzite CZTS Nanocrystals. The experimental details regarding the nanocrystal synthesis can be found in our previous work (see refs 2 and 6). In a representative synthesis, Cu(OAc)2·H2O, Zn(OAc)2·2H2O, and Sn(OAc)2 were dissolved into a mixture of TOA and ODE and then evacuated at room temperature for 1 h. The Cu/Zn/Sn ratio in the metal precursors was offstoichiometric (1.4/1.05/1.00). This precursor solution was then heated to 140 °C and held at 140 °C for 10 min under a N2 atm, where a mixture of 1-DDT and t-DDT was injected into the flask. The reaction was allowed to progress at 250 °C for 1 h with continuous stirring. The as-prepared nanocrystals were then washed several times by 2-propanol and centrifuged to yield a dark centrifuged precipitate. Preparation of CZTS Thin Films. The experiment details regarding the ink preparation can be found in our previous work (see ref 6). The NaCl salt is dissolved into DMSO to form 0.015 M NaCl-DMSO solution, and the wurtzite CZTS nanocrystals are subsequently dispersed to form inks for the sequence experiment. The precursor thin films are deposited by spinning precursor inks on Mocoated soda lime glass (SLG), followed by annealing on a hot plate at 350 °C for 3 min in air. This coating step is repeated until achieving the desired thickness. In our previous paper (see ref 2c), the precursor films are then transferred into a RTP furnace and annealed at 580 °C in a combined sulfur and SnS atmosphere (evaporation of 0.2 g of solid sulfur pellets and 0.05 g of solid SnS powder) for 10 min. Before starting the heating up process, the RTP chamber is first vacuumed and then purged with N2 gas. All samples adopt a heating rate of 2.6 °C/s and cool down naturally. To study the reaction kinetics during

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CONCLUSION In summary, we show the spatial grain growth and evolution in composition during sulfurizing metastable wurtzite CZTS nanocrystal-based coatings in the study of temperature- and time-dependent annealing. Cations experience significant 2119



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the heating process, the samples are taken out when the temperature reaches 450, 490, 530, and 580 °C. To study the reaction kinetics at fixed temperature (580 °C), the samples are taken out after 1, 2, 4, and 10 min, respectively. The time, when the sample is taken out from the RTP furnace after the temperature reaches 580 °C, is designated as 0 min. Solar Cell Device Fabrication. The sulfurized films as described above were first submerged in DI water for a few minutes and then immediately put into NH4OH/CdSO4 solution for chemical bath deposition (CBD) of 50−60 nm CdS as detailed elsewhere.2 A 100 nm i-ZnO layer and a 200 nm ITO layer were then sequentially sputtered on top of the CdS layer.6 The final devices were scribed into small areas of about 0.1 cm2, with a small dap of silver paint as the front contact. Characterization. X-ray diffraction (XRD) measurement was performed by the PANalytical’s Empyrean thin film Xpert materials research diffraction system at a voltage of 45 kV and a current of 40 mA with Cu Kα radiation (λ = 15.4 Å). Raman measurement was conducted by an inVia Renishaw spectrometer coupled with a microscope at 514 nm and room temperature. The scanning electron microscopy (SEM) images and the corresponding elemental analysis were taken by using the FEI Nova NanoSEM system with a Bruker Silicon Drift Energy Dispersive X-ray microanalysis system. The microstructure and elemental distribution in each layer of the film were measured by a JEOL JEM-ARM200F (200 kV) aberrationcorrected scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray spectroscopy (EDS) system. The device performance was obtained by light current−voltage (I−V) measurements under AM1.5 (1000 W m−2) irradiation and external quantum efficiency (EQE) measurements (QEX10 spectral response system from PV measurements, Inc.).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04603. Additional characterization, including SAED pattern, TEM images, EDS line-scan, EDS mapping images, image of furnace, and performance of different devices (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xu Liu: 0000-0003-4849-8533 Author Contributions §

X.L. and J.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under the Australian Government through the Australian Renewable Energy Agency (grant number 1-USO028) and Australian Research Council’s Discovery funding scheme (grant number DE160101100). Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. The authors acknowledge the use of the facilities and the scientific and technical assistance of the Electron Microscope Unit (EMU) in UNSW for TEM analysis and Electron Microscopy Centre in the University of Wollongong for TEM/EDS analysis. 2120


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