Controlled growth of SbSI thin films from amorphous Sb 2 S 3 for low-temperature solution processed chalcohalide solar cells

We report a simple solution processing method for fabricating low-temperature SbSI solar cells. The method consists of two steps: the formation of amorphous Sb2S3 and its transformation to SbSI. A pure SbSI phase with a high crystallinity was obtained at a low temperature of 200 °C. In addition, the SbSI morphology was controlled by tuning the input ratio of SbCl3:thiourea and a dense film was obtained at a ratio of 1:1.3. A planar SbSI solar cell thus-fabricated exhibits a short-circuit current density of 5.45 mA cm−2, an open-circuit voltage of 0.548 V, and a fill factor of 0.31, corresponding to a power conversion efficiency of 0.93% under a 100 mW cm−2 illumination condition.

3][4][5][6][7] The PCE now reaches 23.2%, very recently achieved by the group of Seo. 7 In addition to high photovoltaic performance, these perovskites have distinct advantages such as low-cost processing, light weight, flexibility, and scalability. 5,6,8Therefore, Pb-based perovskites are now considered to be the most promising substitute for the silicon widely used in the solar market.However, several critical challenges to commercializing perovskite solar cells remain, including longterm stability, interfacial problems, and toxicity. 5,8These challenges have prompted many researchers to focus on finding new high-performance photovoltaic materials.
Brandt et al. reported that the high performance of the Pb-perovskite solar cell derives from the specific 6s 2 electronic configuration of Pb 2+ , which makes Pb-perovskite a defect-tolerant semiconductor. 9Based on that study, they 9 and Ganose et al. 10 proposed materials containing post-transition metals with ns 2 electronic configurations, such as In + , Sn 2+ , Ge 3+ , Sb 3+ , and Bi 3+ .0][11][12][13] In addition, the high performance (close to an 8% PCE) achieved in solar cells based on Sb chalcogenides (Sb 2 S 3 11 and Sb 2 Se 3 14 ) makes them more attractive as alternative materials.Recently, several Sb-based materials such as MA 3 Sb 2 X 9 (X = halogen), 15 Cs 2 InSbX 6 , 16 SbSI, 17 and MASbSI 2 18 have been proposed as potential photovoltaic materials.Among these, SbSI, which belongs to the group of V-VI-VII chalcohalides, may be a good candidate because of four interesting features.First, SbSI has a similar band structure to that of Pb-perovskite, 9 offering the possibility of achieving similar high performance.Most recently, Seok group successfully implemented SbSI in sensitized solar cells for the first time and demonstrated this material's potential by achieving an ∼3% PCE. 17 Second, SbSI is a typical photoactive ferroelectric material, a so-called photoferroic material. 19This characteristic can be expected to improve the performance of the solar cell because of the ferroelectric contribution. 19,20Third, SbSI has properties that are suitable for solar cell applications, such as the low effective mass of its charge carriers, its high absorption coefficient, and its bandgap (E g ) close to 1.8 eV. 9,21,22Finally, SbSI can be applied in various capacities, including as a photodetector, 22,23 photocatalyst, 24 humidity sensor, 25 thermoelectric device, 26 and self-powered optoelectronic device, 27 in addition to its use in solar cells.This broad range of applications can be further extended to include room-temperature radiation detection and p-type transparent conduction by substituting Bi for Sb. 21,28This last characteristic could facilitate the design of multifunctional devices based on V-VI-VII chalcohalides.
SbSI has been fabricated by a variety of methods, including hydrothermal 29 and sonochemical techniques, 25 chemical vapor deposition, 29 and Sb 2 S 3 -based two-step processing. 17,22Among these methods, two-step processing is a particular effective approach because of its simplicity and lowtemperature fabrication.In this approach, the SbSI is fabricated by forming Sb 2 S 3 (step I), which is then converted into SbSI by reacting with SbI 3 (step II), as described in the chemical reaction (1), 17,22 Sb 2 S 3 + SbI 3 → 3SbSI. ( In previous studies, Sb 2 S 3 was first fabricated by chemical bath deposition (CBD) in step I 17,22 and was then converted into SbSI by either SbI 3 evaporation 22 or SbI 3 solution processing 17 in step II.
Because both previous studies were based on CBD-synthesized Sb 2 S 3 films, the inherent limitations of the CBD process such as formation of impurity phases and difficult control of the Sb/S ratio 12 ultimately limit the ability to control the growth of the final SbSI thin film.In addition, these two-step processing methods were time-consuming because step II actually required multiple steps.
Recently, we reported a simple solution processing approach based on a SbCl 3 -thiourea (TU) molecular solution to overcome the limitations of the CBD method. 12,13Using the proposed approach not only could we obtain pure Sb 2 S 3 but also we could control the Sb/S ratio of the Sb 2 S 3 . 12Therefore, we expected that this approach would enable control over the growth of SbSI thin films.
Herein, we report a simple two-step solution processing method based on using the SbCl 3 -TU solution method to form amorphous Sb 2 S 3 (am-Sb 2 S 3 ) in step I and applying SbI 3 to am-Sb 2 S 3 in step II to obtain the final SbSI thin films.We also use highly concentrated solutions to simplify each step to eliminate the need for multiple sub-steps at each step.As a result, not only can we control the structure and morphology of the final SbSI film but also we can obtain a dense SbSI thin film with a pure SbSI phase at a low temperature of 200 • C. Furthermore, we successfully apply the thin SbSI films to planar SbSI solar cells.This work is the first demonstration of the controlled growth of SbSI thin films and their application in planar solar cells.
Figure 1 shows the experimental procedure executed to fabricate the SbSI thin films in two steps: the formation of am-Sb 2 S 3 (step I) and its conversion to SbSI (step II).First, an am-Sb 2 S 3 film with a controlled SbCl 3 :TU ratio is deposited on a TiO 2 blocking layer (TiO 2 -BL)/F-doped SnO 2 (FTO) substrate by spin coating SbCl 3 -TU solutions and annealing at 150 • C. 12,13 The SbCl 3 -TU solution is synthesized by mixing SbCl 3 and TU in N,N-dimethylformamide (DMF) at the desired molar ratio.The SbI 3 solution [dissolved in N-methyl-2-pyrrolidone (NMP) solvent] is then spin-coated and annealed for conversion into the SbSI film.
To confirm the formation of crystalline SbSI by the proposed method, we examined X-ray diffraction (XRD) patterns and UV-visible absorption spectra, as shown in Figs.2(a) and 2(b), respectively.For this analysis, the samples were prepared on TiO 2 -BL/FTO substrates under the same conditions with a fixed SbCl 3 :TU ratio of 1:2 except for the various annealing temperatures applied in step II.The sample not subjected to step II (denoted as am-Sb 2 S 3 in Fig. 2) shows only two types of XRD peaks, denoted as T and F, derived from the underlying TiO 2 -BL/FTO substrates.This sample's E g and color are 2.02 eV and orange, respectively, as shown in Fig. 2(b), revealing the amorphous phase Sb 2 S 3 12,30 formed after step I.In the XRD pattern of the sample annealed at 100 • C, weak and broad peaks, different from those of the substrate, are observed.At the higher annealing temperature of 150 • C, all peaks except the substrate peaks shift to higher angles [indicated by the red arrows in Fig. 2(a) inset], compared to those of the 100 • C sample.The positions of the shifted peaks correspond to those of the SbSI phase [denoted as red diamonds in Fig. 2(a)].The full width half maximum of the peaks also narrows.Further increasing the annealing temperature to 200 • C enhances the peak intensity without changing the peak positions.These temperature increases change E g and color to 1.96 eV and purple, respectively.These two characteristics are consistent with those of SbSI, 17,22 confirming the SbSI formation at 150 and 200 • C.These results suggest that small amounts of metastable and intermediate phases form at 100 • C, and that a stable SbSI phase forms at higher temperatures of 150 and 200 • C.However, the Sb 2 S 3 phase appears instead of the SbSI phase at higher temperatures of >200 • C [Figs.2(a) and S1].Furthermore, the color and E g become dark brown and 1.7 eV, respectively [Fig.2][13] The disappearance of the SbSI phase observed under these high temperature conditions may be attributed to a severe shortage of SbI 3 source for transformation to SbSI, as revealed in Fig. S2 and Table S1.In addition, the SbSI is thermally instable at high temperatures of ≥250 • C due to the property that SbSI easily decomposes into Sb 2 S 3 and SbI 3 at such high temperatures. 31,32Therefore, the Sb 2 S 3 phase can be preferentially formed at temperatures of 250 and 300 • C. Pure phase SbSI cannot be obtained from crystalline Sb 2 S 3 films even if the SbI 3 step (step II) is performed, as shown in Fig. S3.These results cumulatively indicate that the pure-phase SbSI can be formed under low-temperature conditions below 250 • C following the proposed method.
In addition to investigating the effects of the annealing temperature in step II, we controlled the SbCl 3 :TU ratio of the solution in step I and investigated its effects on morphology, structure, and absorption properties, as shown in Fig. 3.These samples were fabricated on TiO 2 -BL/FTO substrates with various SbCl 3 :TU ratios of 1:1.5-3.5.The annealing temperature for step II was fixed as 200 • C to promote pure SbSI formation.As shown in Figs.3(a) and S4, the fabricated films contain islands of am-Sb 2 S 3 , the widths of which increase from tens to hundreds of nanometers as their number decreases with increasing TU ratios.The SbSI films obtained from such am-Sb 2 S 3 films show different morphologies depending on the ratio, as shown in Figs.As the TU ratio increases, the film becomes dense, and a continuous film with a 300-nm thickness is obtained with a ratio of 1:3.0.Further TU increases make the film surface uneven, as indicated by the red line in Fig. 3(c).Figure 3(d) shows the XRD patterns of the SbSI films fabricated with different SbCl 3 :TU ratios.To obtain quantitative data on the crystallinity of the SbSI phase, we summed the intensities of the three SbSI peaks in Fig. 3(d) for each sample and plotted the sum as a function of the ratio, as shown in Fig. 3(e).The crystallinity of the SbSI thin films progressively increases until the ratio of 1:3.0 and then weakens as the TU ratio increases further.The absorption spectra [Fig.3(f)] follow a trend similar to that of XRD patterns.These results indicate that a dense film has a higher crystallinity and absorption.In addition, the sample having the 1:3 solution ratio exhibits the best photovoltaic performance (Fig. S5), suggesting that the high crystallinity and uniform morphology are significant for attaining high performance.
The formation of dense SbSI films under high TU conditions may be explained by three factors.First, the amount of TU can be an important variable in determining the morphology of SbSI films.The sulfur becomes more strongly involved in the formation of metal-TU complexes under excess TU conditions. 33This leads to a sufficient supply of sulfur and compensation of sulfur loss in the SbSI formation, facilitating a dense film formation.However, under an excessive TU condition (e.g., the 1:3.5 ratio), the C, H, and N-containing residues generated from extra TU not involved in the SbSI formation may cause the surface to become uneven.By contrast, under a low TU condition (e.g., 1:1.5), the SbSI may be only partially formed because of easy sulfur loss at each step and insufficient sulfur for the SbSI formation, resulting in the formation of nanorod-shaped films or porous films with many voids.For example, SbSI nanorods have been reported to form preferentially from a sulfur-deficient Sb 2 S 3 film synthesized by CBD. 22 second factor promoting the film density is the am-Sb 2 S 3 island shape, which may help to form a continuous thin film.To elucidate how these islands contribute to the formation of the dense film, we investigated the morphological changes as a function of the reaction time during the early stages of step II to track the conversion process into the SbSI dense film, as shown in Fig. 4. We used the SbCl 3 -TU solution (1:3 ratio) to fabricate the SbSI films, which are annealed for different reaction times during step II, for the analysis.For the sample without annealing (0 s), a very smooth surface with submicron islands is observed.Because the size and shape of these islands are very similar to those of the amorphous Sb 2 S 3 shown in Fig. 3 Based on the transformation behaviors during the early stages of step II, it can be deduced that the SbSI starts to nucleate from the Sb 2 S 3 islands and aggregates into the micron dots, which continuously grow until the SbSI covers the entire surface.As a result, a compact, dense SbSI film is formed when the reaction is completed, as shown in Fig. 3.These results demonstrate that island-shaped Sb 2 S 3 plays a major role in the formation of a dense film.To confirm the effect of the shape of Sb 2 S 3 on the film morphology, we fabricated two types of am-Sb 2 S 3 films with different morphologies and converted them into SbSI by performing step II, as shown in Fig. S6.The SbSI films fabricated from these differently shaped Sb 2 S 3 films exhibit discontinuities.In addition, the size of the islands should be considered because a compact film cannot be formed from relatively small islands (Fig. 3).Therefore, the shape of size of the am-Sb 2 S 3 islands could be critical to the formation of a dense SbSI thin film.
Third, the solvent used for the SbI 3 dissolution may play an important role in determining the SbSI morphology.Figure S7(a) shows photographs of three SbSI films, fabricated with three SbI 3 solvents: DMF, NMP (used in this study), and dimethyl sulfoxide (DMSO).The samples fabricated with NMP and DMSO exhibit mirror-like surfaces, implying that these samples have a uniform and smooth surface and confirming the dense and uniform morphology of the SbSI films.By contrast, the DMF sample presents a rough surface, indicating that this SbSI film has an uneven morphology.
To confirm the solvent effects on surface morphology, we further characterized the root-mean-square (rms) surface roughness of three samples using an atomic force microscope (AFM), as shown in Fig. S7(b).The DMF sample has an rms value of 175.4 nm, which is much greater than those of NMP (rms: 9.9 nm) and DMSO (rms: 10.7 nm).This result supports our conclusion that the uniformity depends on the SbI 3 solvent.The solvent-induced differences in the surfaces may be simply attributed to differences in the vapor pressures of the solvents used: NMP and DMSO have very low vapor pressures of 0.3 and 0.6 mmHg at 25 • C, respectively, much lower than that of DMF (3.7 mmHg).Because solvents with low vapor pressures enable the formation of uniform nucleation sites and crystallize slowly because of their low volatility, 34 uniform and dense SbSI films can be readily formed with NMP and DMSO solvents.
The three factors discussed above are closely linked and affect each other.For example, the TU amount (controlled by the SbCl 3 :TU ratio) determines the morphology of the am-Sb 2 S 3 that serve as the base for the SbSI formation, as shown in Fig. 3. Therefore, these three factors should be considered together for the controlled growth of SbSI films although further study is required to elucidate the formation mechanism.
The SbSI is easy to grow into one-dimensional rods or urchin-like structures, [22][23][24][25]32 even from a continuous thin film. 22 herefore, to apply these discontinuous SbSI films into planar optoelectronic devices, an insulating polymer such as poly(methyl methacrylate) 22,27 should be integrated into such films to minimize leakage currents caused by the presence of inevitable voids in the SbSI layer.By contrast, our method enables the fabrication of SbSI planar optoelectronic devices without the need for additional insulating polymers because of the dense SbSI thin films thus formed.
To prove that our method is practically applicable in planar optoelectronic devices, we applied it to the fabrication of planar SbSI solar cells.For this application, we chose the n-i-p device stack, widely used in next-generation solar cells. 2 In the fabricated device, the planar SbSI, obtained under the optimized conditions (Figs. 2 and 3), was deposited on the substrates.TiO 2 and poly(3-hexylthiophene) (P3HT) were used as electron and hole transporting layers, respectively, to promote efficient charge transfer, as illustrated in Fig. 5(a).We first measured current density (J)-voltage (V) curves of the device by varying scan directions at a fixed sweep delay time of 50 ms, as shown in the inset of Fig. 5(b).Under the reverse scan (RS) mode, the device yields a short-circuit current density (J SC ) of 5.45 mA cm −2 , an open-circuit voltage (V OC ) of 0.548 V, and a fill factor (FF) of 0.31, leading to a PCE of 0.93%; under the forward scan (FS) mode, the device exhibits a slightly lower PCE of 0.85% with a J SC of 5.32 mA cm −2 , a V OC of 0.532 V, and a FF of 0.30.In addition to the low hysteresis in the J-V curve in either scan direction, the device shows negligible changes in PCE at different sweep delay times for both scan directions [Fig.5(b)].The steady state photocurrent density measured at a maximum voltage of 0.31 V is ∼3.12 mA cm −2 , corresponding to a stabilized PCE of ∼0.93% [Fig.5(c)].This PCE was well maintained during a month of exposure to ambient air conditions, thus suggesting its good stability (Fig. S8).From these results, the method introduced here can clearly be considered applicable to the fabrication of planar chalcohalide solar cells.In addition to the planar device, this fabrication method can be applied to devices based on the mesoporous TiO 2 (mp-TiO 2 ) (Fig. S9) and thus to various device architectures.However, the performance of the mp-TiO 2 -based device was much worse than that of the planar device (Table S2).
In conclusion, we proposed an advanced two-step solution processing method for the controlled growth of SbSI thin films.The structure and morphology of the SbSI thin films were controlled, and dense SbSI films were obtained at a low annealing temperature of 200 • C from am-Sb 2 S 3 film synthesized with a specific ratio of SbCl 3 :TU = 1:3.The planar SbSI solar cell exhibited a PCE of ∼0.93% with hysteresis-less behaviors in the J-V curve.Therefore, the proposed technique will contribute to the development of methods for fabricating low-temperature chalcohalide solar cells.
See supplementary material for experimental methods and additional data.

FIG. 2 .
FIG. 2. (a) XRD patterns and (b) UV-visible absorption spectra of samples fabricated at different annealing temperatures of step II.In (a), the Sb 2 S 3 and SbSI phases were indexed based on the orthorhombic Sb 2 S 3 structure (ICSD reference code 00-006-0474) and orthorhombic SbSI structure (ICSD reference code 01-074-1195), respectively.Inset images in (a) and (b) show magnified XRD patterns of three samples (100, 150, and 200 • C) in the range from 19.8 • to 21.2 • and photographs of three typical samples, respectively.The values of E g , as shown in (b), were calculated from the absorption edges of each spectrum.

FIG. 3 .
FIG. 3. Effects of SbCl 3 :TU solution ratios in step I: (a) Surface field emission scanning electron microscope (FESEM) images of the am-Sb 2 S 3 thin films fabricated after step I; (b) surface and (c) cross-sectional FESEM images; (d) XRD patterns; (e) calculated SbSI phase intensities; and (f) UV-visible absorption spectra of the SbSI thin films.
FIG. 4. FESEM images of the samples obtained at different reaction time during step II: (a) high-and (b) low-magnification surface images.(c) Typical FESEM surface image around a dot.

FIG. 5 .
FIG. 5. Energy level diagram for the SbSI planar solar cell (a) and its photovoltaic device performances [(b) and (c)].The diagram (a) was constructed based on the optical E g [Fig.2(b)] and two Refs.17 and 20.Graphs of (b) the PCE variation as a function of sweep delay times (10, 20, 50, 100, 200, 500, 800, and 1000 ms) and (c) the steady state PCE and current density J.The inset graph in (b) shows the J-V curve of the device measured at a fixed sweep delay time of 50 ms.
This work was supported by the DGIST R&D Programs of the Ministry of Science and ICT, Republic of Korea (Grants Nos.18-ET-01 and 18-01-HRSS-04).Y.C.C. gratefully acknowledges to Professor Sang Il Seok and Dr. Riming Nie, who belong to UNIST (Republic of Korea), for their valuable comments.