Liquid Flux–Assisted Mechanism for Modest Temperature Synthesis of Large‐Grain BaZrS3 and BaHfS3 Chalcogenide Perovskites

Chalcogenide perovskites are promising semiconductor materials with attractive optoelectronic properties and appreciable stability, making them enticing candidates for photovoltaics and related electronic applications. Traditional synthesis methods for these materials have long suffered from high‐temperature requirements of 800–1000 °C. However, the recently developed solution processing route provides a way to circumvent this. By utilizing barium thiolate and ZrH2, this method is capable of synthesizing BaZrS3 perovskite at modest temperatures (500–600 °C), generating crystalline domains on the order of hundreds of nanometers in size. Herein, a systematic study of this solution processing route is done to gain a mechanistic understanding of the process and to supplement the development of device quality fabrication methodologies. A barium polysulfide liquid flux is identified as playing a key role in the rapid synthesis of large‐grain BaZrS3 perovskite at modest temperatures. Additionally, this mechanism is successfully extended to the related BaHfS3 perovskite. The reported findings identify viable precursors, key temperature regimes, and reaction conditions that are likely to enable the large‐grain chalcogenide perovskite growth, essential toward the formation of device‐quality thin films.


Introduction
Chalcogenide perovskites are an emerging class of semiconductor materials with attractive optoelectronic properties, such as a high absorption coefficient (%10 5 cm À1 ), [1,2] tunable bandgap, [2,3] and high-dielectric constant. [4] They are superior to their halide perovskite counterparts in terms of stability, [5] and are composed of earth-abundant and nontoxic constituents. [6,7] Altogether, this makes them attractive candidates for absorber materials in photovoltaics. However, a major hurdle in the application of chalcogenide perovskites is the extreme temperatures used in traditional synthesis techniques. Initial solid-state syntheses were conducted at temperatures in the range of 800-1000°C. [8][9][10][11][12] Even vacuum processing techniques often used temperatures above 900°C. [13] This creates difficulties in identifying possible substrates and contact layers for semiconductor devices that can withstand such high temperatures.
An understanding of traditional chalcogenide perovskite synthesis suggests that these high temperatures may not be an absolute necessity, depending on the design of the synthesis methodology. Avoiding highly stable precursors and situations where long-range solid-state diffusion is required may negate the need for excessively high temperatures to form the chalcogenide perovskites. Subsequently, a focused effort by the research community has enabled a reduction in synthesis temperatures. Comparotto et al. altered their vacuum processing route to avoid oxide impurities and to include heat treatment under a sulfur-containing atmosphere. As a result, they obtained crystalline BaZrS 3 at temperatures as low as 600°C. [14,15] Nanoparticles of BaZrS 3 have also been synthesized at temperatures less than 350°C, providing evidence that these materials can be solution-processed. [16,17] But it remains to be seen if these nanoparticles can be processed into large-grain thin films. Our group recently developed a direct-tofilm solution-processed route for BaMS 3 (M = Ti, Zr, Hf ) materials that utilizes heat treatment in a sulfur-containing atmosphere at modest temperatures from 500 to 575°C. [18] For the case of BaZrS 3 , large grains on the order of hundreds of nanometers were obtained, which is desirable for applications in thin-film photovoltaics.
Large grain sizes achieved at modest temperatures and short reaction times are key features of liquid flux-assisted synthesis, which is prevalent in chalcogenide photovoltaic materials like Cu(In,Ga)(S,Se) 2 and Cu 2 ZnSn(S,Se) 4  Chalcogenide perovskites are promising semiconductor materials with attractive optoelectronic properties and appreciable stability, making them enticing candidates for photovoltaics and related electronic applications. Traditional synthesis methods for these materials have long suffered from hightemperature requirements of 800-1000°C. However, the recently developed solution processing route provides a way to circumvent this. By utilizing barium thiolate and ZrH 2 , this method is capable of synthesizing BaZrS 3 perovskite at modest temperatures (500-600°C), generating crystalline domains on the order of hundreds of nanometers in size. Herein, a systematic study of this solution processing route is done to gain a mechanistic understanding of the process and to supplement the development of device quality fabrication methodologies. A barium polysulfide liquid flux is identified as playing a key role in the rapid synthesis of large-grain BaZrS 3 perovskite at modest temperatures. Additionally, this mechanism is successfully extended to the related BaHfS 3 perovskite. The reported findings identify viable precursors, key temperature regimes, and reaction conditions that are likely to enable the large-grain chalcogenide perovskite growth, essential toward the formation of devicequality thin films.
selenium-containing atmosphere, liquid selenium condenses on the film and acts as a fluxing agent. The selenium liquid dissolves the precursor film and enables large-grain Cu 2 ZnSnSe 4 . [19] A similar mechanism was also proposed for the conversion of nanocrystalline Cu(In,Ga)S 2 into large-grain Cu(In,Ga) (S,Se) 2 . [20] The liquid flux enables accelerated mass transport such that when nucleation occurs, large grain sizes can be achieved. In light of this, the previously discussed processes that have achieved large grains of BaZrS 3 at temperatures below 600°C may also be relying upon a liquid flux.
The hypothesis of BaS 3 as a liquid flux for the synthesis of BaZrS 3 was first proposed by the Scragg group but has not been experimentally validated. [14,15] Freund et al. attempted to use a BaS 3 liquid phase on top of a Zr substrate to fabricate BaZrS 3 but were unsuccessful in forming the perovskite. [21] Identifying a viable liquid flux could ease the synthesis of chalcogenide perovskites by overcoming diffusion limitations. This in turn can guide researchers toward identifying favorable synthesis conditions to achieve coarse-grain thin films. Hence, in this work, we have systematically studied the growth mechanism of BaZrS 3 through our earlier reported hybrid direct-to-film solution-processed route using molecular and nanoparticle precursors. The formation and retention of a BaS x (x > 3) liquid phase have been identified as the driving force for the enhanced grain growth observed. Excess sulfur has been found to be critical to accessing this liquid phase. Additionally, ZrS 3 has also been identified as a viable precursor for the synthesis of BaZrS 3 .

Liquid-Assisted Grain Growth Mechanism
Liquid fluxing agents have long played an important role in bulk material growth, enabling control over crystal size and morphology while bringing down the growth temperature. The liquid medium dissolves and promotes chemical interactions between the precursors and assists in crystallization at temperatures well below the typical melting point of the constituents. [22,23] But these liquid fluxing agents can also be utilized for material synthesis and grain growth in thin-film formation. Liquid phases provided through alkali metals, [24] molten chalcogenides, [20,25] and additional dopants (Sb 2 S 3 , CuSbS 2 , NaSb 5 S 8 ) [26] have achieved enhanced grain growth in several related chalcogenide materials. A liquid fluxing agent could similarly be a route to achieving large grain sizes for chalcogenide perovskites. In this work, we seek to determine if a liquid flux is responsible for the considerable grain growth observed in our earlier reported hybrid direct-to-film solution-processed route and if so, to identify the liquid flux.
Based on our observations, a three-stage growth mechanism involving a liquid flux has been identified for this solutiondeposition approach as illustrated in Figure 1a. The individual steps in this mechanism are 1) the formation of binary polysulfides, 2) a diffusion-limited growth regime for BaZrS 3 , and 3) a liquid flux-assisted grain growth regime for BaZrS 3 . This proposed mechanism is expanded upon in the following text.

Formation of Binary Polysulfides
Initially, a precursor film containing solution-deposited BaS and ZrH 2 is sealed in an evacuated ampule with sulfur and heated in a tube furnace. Upon heating, sulfur volatilizes and provides a sulfur-containing atmosphere in the ampule. This sulfur can then react with the precursor film. During the heat-up, the BaS and ZrH 2 in the precursor film are converted into their respective polysulfides. This conversion is confirmed by the observation of BaS 3 in the 375°C quenched sample and ZrS 3 in the 500°C quenched sample X-ray diffraction (XRD) patterns as shown in Figure 1b. Raman spectroscopy also confirmed the presence of both BaS 3 and ZrS 3 (Figure 1c). The ZrO 2 peaks identified in the Raman spectra of the quenched samples could be due to the oxidation of ZrH 2 in the films upon exposure to air for characterization. The formation of binary polysulfides is an important observation as it has been previously thought that the formation of ZrS 3 could prevent further reaction to produce the ternary perovskite. [27] However, this finding suggests that ZrS 3 is a viable precursor for BaZrS 3 synthesis.

Diffusion Limited Growth of BaZrS 3
We further observed that the reaction between the polysulfides starts to form the perovskite at temperatures as low as 500°C. This shows that the synthesis of BaZrS 3 at modest temperatures is thermodynamically feasible. However, our attempts to drive the reaction toward completion at these low temperatures were unsuccessful, even for extended heating periods of 288 h as shown in Figure 2a. An increase in the perovskite phase relative to BaS 3 and ZrS 3 can be identified by comparing the intensity of the respective XRD peaks, but the binary polysulfide phases persist. This shows that the perovskite formation is slow at this temperature, likely due to diffusion limitations in the solid-solid interactions of the polysulfide reactants. The additional peak around 22.2°can be matched with S 8 , indicating a sulfur-rich environment existing in the films.

Liquid Flux-Assisted Growth of BaZrS 3
A rapid shift in the formation mechanism from the earlier discussed diffusion-limited regime to an accelerated growth regime is noticed at temperatures above 525°C. Comparatively largegrain perovskite (apparent grain sizes on the order of hundreds of nanometers) with appreciable phase purity is achieved for heating periods of less than 15 min. This accelerated growth is consistent with the emergence of a liquid phase, overcoming mass transport limitations and assisting in nucleation and grain growth. These observations show that the liquid flux forms at around 525°C. To further support the proposed presence of liquid flux, SEM images were taken to look for changes in morphology (Figure 2b,c). At 500°C, the film shows significant cracking leading to large voids between the material. However, in the sample sulfurized at 525°C, the material moved into these voids, suggesting a liquid flux was able to flow into the cracks.

Identification of the Liquid Flux
Subtleties of our experimental observations suggest that the liquid flux is related to barium and sulfur but is not BaS 3 . If a pure BaS precursor thin film (with no ZrH 2 ) is used in this process, a randomly oriented BaS 3 is produced when BaS is heated in the presence of sulfur at lower temperatures as shown in Figure 3. However, at sulfurization temperatures above 525°C, the film is observed to contain a highly oriented BaS 3 . We interpret this observation as the randomly oriented BaS 3 going through a liquid phase at higher temperatures and recrystallizing in an oriented fashion. This liquid phase emerges between 500 and 525°C, whereas BaS 3 is reported to melt at 554°C. [28] Additionally, when BaS 3 and ZrS 3 powders were sulfurized at 575°C in an ampule without sulfur, a significant amount of secondary phases were observed, including BaS 2 as seen in Figure 4. This indicates that BaS 3 itself is not stable at 575°C and decomposes without a sulfur-containing atmosphere.
A deeper look at the barium-sulfur phase diagram shown in Figure S4, Supporting Information, can allow an understanding of the experimental observations. [29] Indeed, a liquid phase is    www.advancedsciencenews.com www.advenergysustres.com sulfurization, an excess of sulfur is present, which can drive the formation of the sulfur-rich barium polysulfide liquid phase. This BaS x (x > 3) liquid is reported to exist at temperatures as low as 520°C, in line with our proposed liquid flux onset temperature. Additionally, as this liquid cools down, it would revert to BaS 3 and sulfur, which explains why BaS 3 is obtained when sulfurizing a pure BaS thin-film sample (with no ZrH 2 ) as in Figure 3.

Mechanism Validation with Accelerated Reactions of Binary Sulfides
Typically, solid-state reactions to make chalcogenide perovskites from the sulfide binaries have been limited to high temperatures and extended periods. The reaction between BaS and ZrS 2 has been done at 800°C for 15 h to form crystalline BaZrS 3 . [2] However, our proposed mechanism suggests that adding sulfur to this reaction should create a BaS x liquid flux and allow the reaction to take place at lower temperatures and shorter periods. When BaS and ZrS 2 powders are combined in an ampule without sulfur and heated at 575°C for 48 h, XRD shows that the binary metal sulfides are retained. However, when excess sulfur is added to the ampule such that the Ba:Zr:S mole ratio is 1:1:9, the ternary perovskite BaZrS 3 is obtained under the same reaction conditions within 1 h (Figure 5a). This result supports our hypothesized liquid flux growth mechanism. Similar reactions between BaS and ZrS 3 powders in the presence of excess sulfur led to the formation of BaZrS 3 (Figure 5b). This further supports the idea of ZrS 3 being a viable precursor. Interestingly, reactions involving ZrS 3 took around 12 h to achieve appreciable phase purity of the ternary material. This might be due to the crystalline domain size differences between the commercially bought ZrS 2 and the laboratory-synthesized ZrS 3 . However, the kinetic differences between ZrS 2 and ZrS 3 as possible precursors need to be studied further.

Effect of the Sulfur Vapor Pressure
Sopiha et al. suggested that there might be a narrow window of sulfur partial pressure within which the formation of BaZrS 3 is favored. In contrast, high partial pressures of sulfur could cause BaZrS 3 to be unstable and decompose to BaS 3 and ZrS 3 . [7] In our case, increasing sulfur vapor pressure inside the ampule up to 0.74 bar showed an increasing trend in the crystallinity of BaZrS 3 without appreciable formation of ZrS 3 . This can be seen with the decreasing full width at half maximum (FWHM) for 25.2°peak with increasing sulfur pressure (described in Supporting Information) in thin-film samples, shown in Figure 6. There may still be some higher sulfur pressure, beyond the range explored in this study, that would favor the formation of the binary polysulfides. In that case, the upper limit for the partial pressure of sulfur at which BaZrS 3 is still favorable must be above 0.74 bar.

Application in the Related Barium Perovskites
Since the proposed liquid flux mechanism is based on the interactions between barium and sulfur, it is reasonable to predict that this mechanism is applicable to other barium and sulfurcontaining chalcogenide perovskites. BaHfS 3 is another chalcogenide perovskite of interest. With a slightly higher bandgap of around 2.1-2.2 eV, BaHfS 3 may still be viable as a top absorber in a tandem solar cell, but could also be interesting for indoor photovoltaic, light-emitting diode, or water-splitting applications. Additionally, sharper photoluminescence has been obtained for BaHfS 3 compared to BaZrS 3 . [18] As BaHfS 3 also contains barium and sulfur, a liquid BaS x flux should also be applicable to its  synthesis. While early reports on the synthesis of BaHfS 3 using binary sulfides utilized a temperature of 1100°C for 48 h, [11] we observed that adding excess sulfur to powder precursors BaS and HfS 2 such that the Ba:Hf:S mole ratio is 1:1:9 brought the reaction conditions down to 575°C and 12 h (Figure 7a). This confirms that BaS x can be an accessible liquid fluxing agent for barium-and sulfur-containing materials beyond BaZrS 3 . Additional quenching studies on thin films show that BaHfS 3 exhibits a slightly different growth mechanism from that of BaZrS 3 . Sulfurization of thin films containing BaS and HfH 2 leads to the conversion of the BaS into the barium polysulfide (BaS x ) liquid phase as evidenced by the presence of BaS 3 in the XRD patterns of the quenched samples at lower temperatures. However, the sulfurization of HfH 2 seems to be slower than ZrH 2 . This leads to the persistence of HfH 2 until temperatures of 575°C while showing the formation of HfS 2 as in Figure 7b. The formation of the ternary material begins at temperatures as low as 500°C. For the Ba-Hf-S material system, distinguishing between the Ruddlesden-Popper phases and the distorted perovskite phase is difficult since most of the XRD peaks are overlapping, and no definitive Raman peaks are available in the literature. However, compared to historical methods for synthesizing BaHfS 3 , the temperatures used here are relatively low, suggesting that the liquid BaS x flux is still playing a significant role.

Conclusion
Here, we have demonstrated the existence of an accessible liquid flux in the synthesis of BaZrS 3 , which can overcome the diffusional limitations suffered in existing chalcogenide perovskite synthesis routes. The liquid flux has been identified as a sulfurrich barium polysulfide phase (BaS x where x > 3) existing at temperatures above 525°C under a sulfur atmosphere. The impact of this liquid phase has been confirmed through accelerated reactions of binary sulfides, and its generality has been extended to the related BaHfS 3 perovskite. Moreover, ZrS 3 and HfS 2 have been identified as potential precursors for the BaZrS 3 and BaHfS 3 synthesis, respectively, contrary to previous thought that they may be unsuitable as precursors. Together, these results show that the chalcogenide perovskites BaZrS 3 and BaHfS 3 can be synthesized at modest temperatures and achieve significant grain growth using a liquid fluxing agent.  This work then lays the groundwork for future translation to high-quality thin-film production in a device-compatible fabrication scheme.
Thin-Film Preparation: All sample preparation and handling were done in the nitrogen atmosphere of a glove box or the argon atmosphere of a Schlenk line owing to the oxophilic nature of the precursors. The samples were fabricated based on the solution processing route reported previously. [18] In brief, the precursor inks were prepared by combining Cp* 2 Ba, ZrH 2 , and MePT at a 1:1:10 mol ratio in excess BA and vortex mixed. The ink was then drop-cast on Al 2 O 3 -coated Eagle XG glass substrates, annealed at 95°C for 2 min, then ramped to 420°C, and kept at that temperature for 25 min. Following the hotplate annealing, the samples contained a mixture of BaS and ZrH 2 . The samples were then loaded into an evacuated ampule with sulfur and sealed at pressures below 200 mtorr, followed by sulfurization in a tube furnace. The samples were heated at an average rate of 80°C min À1 until the set point was reached. They were held at that temperature for 5 min, followed by rapid quenching under argon flow at an average cooling rate of 20°C min À1 . Samples were quenched at various stages of this heat treatment step to identify the different intermediate phases that the sample goes through as it is heated and to observe the evolution of BaZrS 3 grains. The identified phases are listed in Table 1. Further details about the sulfurization process are discussed in Supporting Information.
Powder Preparation: BaS 3 powder was synthesized by sulfurizing commercially bought BaS powder with excess sulfur (three times the stochiometric requirement). It was sealed in an ampule and heated at 400°C for 17 h. Similarly, ZrS 3 powder was synthesized by sulfurizing commercially bought ZrS 3 powder with excess sulfur (three times the stochiometric requirement). It was sealed in an ampule and heated at 575°C for 35 h. The XRD data for the synthesized BaS 3 and ZrS 3 are shown in Figure S3, Supporting Information. All powder reactions were carried out with commercially obtained or laboratory-synthesized precursors without subsequent grinding or mechanical treatment. For both thin-film samples and powder reactions, the sulfur vapor pressure inside the ampule was kept below 0.9 bar to prevent over-pressurization. The sulfur vapor pressure calculations followed are shown in Supporting Information.
Characterization: XRD data were collected using Rigaku SmartLab diffractometer under a Cu Kα (λ = 1.5406 Å) source operated at 40 kV/44 mA in parallel beam mode with a k-beta filter. Raman spectrums were collected using a Thermo Scientific DXR Raman Microscope with an excitation laser of 633 nm wavelength. Scanning electron microscopy (SEM) images were collected using an FEI Nova SEM with an operating voltage of 5 kV and a working distance of 5 mm.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.