Sb₂S₃ solar cells: functional layer preparation and device performance

Abstract

Photovoltaic power generation, as a rapidly growing new energy source, is a technology that uses a semiconductor to convert light energy into electrical energy. Sb₂S₃ solar cells possess the advantages of simple binary components, abundant resources, nontoxicity, and excellent stability; they have recently attracted extensive investigation interest. Various strategies, such as solution and vacuum deposition, have been used to fabricate functional layers, including Sb₂S₃ photoactive layers, electron transport layers and hole transport layers. Here, we briefly review the preparation methodologies, morphologies, structures, optoelectronic properties and the corresponding solar cell performance of functional layers for both planar heterojunction solar cells and mesoporous sensitized solar cells. Some viewpoints of further improvement of Sb₂S₃ solar cell conversion efficiency from the aspect of material fabrication and modification are given.

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Qian Wang

Qian Wang received her bachelor's degree from Anhui Jianzhu University in 2010 and her master's degree from China University of Mining and Technology in 2013. After that, she worked as a lecturer at Liupanshui Normal University until 2018. She is currently a Ph.D. candidate under the supervision of Prof. Jun Zhu at Hefei University of Technology. Her research interests focus on the design and fabrication of novel solar cells.

Zhu Chen

Zhu Chen received his bachelor's degree from Hefei University in 2018. Currently, he is a master's student at the Hefei University of Technology. His research interests focus on the preparation of perovskite solar cells.

Yuhan Wei

Yuhan Wei received her Ph.D. degree in 2009 from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Then, she worked as a postdoctoral fellow at City University of Hong Kong in 2010. She has been an associate professor of Qingdao University of Science and Technology since 2015. Her current research interests focus on polymer structures and properties, polymer solar cells and Sb2S3 solar cells.

Jun Zhu

Jun Zhu received his B.S. (2000) and Ph.D. (2005) from the University of Science and Technology of China. He joined Hefei Institutes of Physical Science, Chinese Academy of Sciences, where he was promoted to professor in 2015. Now, he is a professor at Hefei University of Technology. His research interests focus on novel thin film solar cells, including Sb2S3 solar cells and perovskite solar cells.

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1. Introduction

Solar cells are an important way to utilize solar energy directly; they convert irradiation energy into electricity. In 1839, Becquerel discovered the photovoltaic effect with light-sensitive materials such as AgCl and AgBr.^1,2 Adams and Day found in 1877 that selenium semiconductors generated current when exposed to sunlight.³ To date, selenium solar cells have achieved a power conversion efficiency (PCE) of 6.5%.⁴ In 1954, Chapin, Fuller and Pearson of Bell Labs developed the first crystalline silicon solar cell, achieving 4.5% PCE.⁵ Since then, solar cells have captured the attention of researchers worldwide. The photovoltaic industry based on crystalline silicon solar cells has been rapidly growing since the 1990s. In 2017, the cumulative solar photovoltaic capacity reached almost 398 GW and generated over 460 TWh, representing around 2% of global power output.⁶

Crystalline silicon solar cells are the first generation solar cells, and the highest PCE of monocrystalline silicon solar cells has approached 26.7%.⁷ Thin film solar cells, including Cu(In, Ga)Se₂ (CIGS), amorphous Si (a-Si) and CdTe solar cells, sometimes called second generation solar cells, have been extensively investigated since the last century. However, the high cost of In and Ga and the toxicity of Cd decrease the commercial competitiveness of CIGS and CdTe solar cells.⁸ Perovskite solar cells, organic polymer solar cells, dye-sensitized solar cells and quantum dot solar cells have the advantages of solution-processing, adjustable chemical composition and absorption spectroscopy. Especially, the highest PCEs of perovskite solar cells and organic polymer solar cells have approached 25.2%⁷ and 16.5%,⁷ respectively, showing high commercialization potential. The research and development history of solar cells has demonstrated the importance of novel semiconductor photovoltaic materials.

In addition to the above light absorption materials, some earth-abundant and environmentally “green” binary materials have attracted the attention of researchers, such as FeS₂,⁹ Cu₂S,¹⁰ Cu₂O,¹¹ SnS¹² and Sb₂(S,Se)₃.^13,14 FeS₂'s band gap of 0.95 eV and absorption coefficient of 10⁵ cm⁻¹ are very attractive. Due to the large absorption coefficient, only a 40 nm thickness of pyrite film is required to absorb 90% of incident light.¹⁵ Cu₂S is stoichiometrically and structurally complex.¹⁶ With a band gap of 1.1 to 1.4 eV,¹⁷ it appears to be an ideal material with low cost and low toxicity. Cu₂O has a direct band gap of 2.0 eV, which is suitable for the top cell in a tandem device.¹⁸ SnS has a bandgap in the range of 1.1 eV and an absorption coefficient of ∼10⁴ to 10⁵ cm⁻¹.^19,20 Sb₂S₃ and Sb₂Se₃ are nearly isomorphous; they crystallize in orthorhombic crystal structures (Pnma(62) space group).²¹ The band gaps of Sb₂S₃ and Sb₂Se₃ are ∼1.7 and ∼1.1 eV, respectively. Sulfur and selenium in Sb₂(S,Se)₃ can form a homogenous alloy which renders the band gap tunable from 1.1 to 1.7 eV, falling into the suitable energy gap requirement predicted by Shockley–Queisser theory.²² In principle, a PCE based on Sb₂(S,Se)₃ can reach ∼32%.²³ However, none of these materials have yet reached the PCE level that is required for industrial applications or is comparable with matured thin film technologies.

Note that Sb₂Se₃ and Sb₂S₃ have similar orthorhombic crystal structures. The band gap of Sb₂Se₃ (∼1.1 eV) is smaller than that of Sb₂S₃; a smaller band gap is beneficial for absorbing light towards longer wavelengths. In 2019, Li et al. reported the highest efficiency to date of 9.2% for Sb₂Se₃-based solar cells.²⁴ On the other hand, Sb₂S₃ has a large bandgap of ∼1.7 eV, it can generate high open circuit voltage (V_oc) in devices and is a promising material for the top cell in Si-based tandem solar cells, with an expected maximum theoretical PCE of over 40%.²⁵ As a single junction solar cell, a maximum PCE of 28.64% was achieved at a band gap energy of 1.7 eV according to the Shockley–Queisser limit.²³ Sb₂S₃ is the main focus of this paper; here, we will emphasize the preparation and development of Sb₂S₃ solar cell functional layers.

Sb₂S₃ has the advantages of abundant resources, nontoxicity, high stability, etc. As a binary semiconductor compound with a single stable phase, no impurity phase will be produced. Moreover, Sb₂S₃ has a bandgap of ∼1.7 eV and a high absorption coefficient (1.8 × 10⁵ cm⁻¹ at 450 nm);²⁶ thus, it is an attractive material for solar cells. The melting point of Sb₂S₃ (∼550 °C) is lower than those of other photovoltaic metal chalcogenides (CIGS 990 °C to 1070 °C),²⁷ enabling relatively low synthesis temperatures;²⁸ this is in line with the requirements of energy conservation and consumption reduction. In 1992, Savadogo and Mandal reported the first Sb₂S₃ photoelectrochemical solar cells;²⁹ later, the first solid planar Sb₂S₃ solar cell with a PCE of 5.19% was reported.³⁰ The first report on Sb₂S₃ mesoporous sensitized solar cells was published by Itzhaik et al.³¹ Both planar and mesoporous Sb₂S₃ solar cells have been extensively investigated recently. Fig. 1 shows the progress of the PCE of Sb₂S₃ solar cells.

Fig. 1 Progress of the PCE of Sb₂S₃ solar cells. Figures in square brackets indicate reference numbers. The efficiency measurement conditions in the inset are not standard test conditions.

From the aspect of their working mechanisms, Sb₂S₃ solar cells can be divided into two categories: mesoporous sensitized solar cells and planar heterojunction solar cells. For sensitized Sb₂S₃ solar cells, which are also sometimes called quantum dot sensitized solar cells, the working mechanism is similar to that of dye sensitized solar cells.^42,43 The Sb₂S₃ nanocrystals deposited on the mesoporous oxide scaffold harvest sunlight first, after which electron–hole pairs are produced in the Sb₂S₃ layer; then, the electrons are injected into the wide band gap oxide and the holes into the liquid electrolyte or solid hole transporting material (HTM). For planar heterojunction Sb₂S₃ solar cells, TiO₂/Sb₂S₃/HTM solar cells with similar n–i–p structures have achieved relatively high PCE.^13,41 Although their structures are similar to those of high-efficiency n–i–p perovskite solar cells⁴⁴ and the working mechanism is also assumed to be similar, it is noteworthy that the defect properties and carrier concentrations of lead halide perovskite and Sb₂S₃ are very different.^45,46 In addition, the planar configuration without an HTM layer has delivered attractive PCE recently,^47,48 indicating that further investigation of the working mechanism is needed.

Compared with other types of solar cells, Sb₂S₃ solar cells are less efficient, and much progress is still necessary before they can be used in industry. Limited review articles focusing on Sb₂S₃ solar cells have been published, and these reviews usually include articles on metal chalcogenide semiconductor planar heterojunctions and sensitized solar cells.^{42,43,49–51} Wang et al.⁸ published a review of Sb₂(S,Se)₃-based solar cells; they presented the synthetic approaches towards Sb₂(S,Se)₃ films, including Sb₂S₃ film, for solar cell construction. Kondrotas et al.⁴⁵ presented a comprehensive review of Sb₂S₃ solar cells, examined their potential as the front subcell for Si-based tandem solar cells, and provided a roadmap for further technology development. In this review, we focus on the preparation and development of Sb₂S₃ solar cell functional layers, including their preparation methdologies, morphological structures, optoelectronic properties and corresponding solar cell performance for both planar heterojunction solar cells and mesoporous sensitized solar cells. Sb₂S₃ films prepared by chemical bath deposition (CBD), spin-coating, vacuum deposition, etc. are first reviewed. Then, the influences of the electron and hole transport materials are examined. Finally, we present some viewpoints toward further PCE improvement from the aspects of material fabrication and device mechanisms.

2. Sb₂S₃ film fabrication, properties and solar cell efficiency

Most Sb₂S₃ solar cells reported to date have been deposited by CBD,^{13,29–33,38,39} spin coating^{27,28,52–55} or thermal evaporation (TE).^56–59 While there have been a few reports of Sb₂S₃ solar cells produced using atomic-layer deposition (ALD),^35,60,61 the in situ hydrothermal approach,⁶² spray pyrolysis,⁶³ sputtering^64,65 and vacuum coating,⁶⁶ novel synthetic methods to produce uniform, dense Sb₂S₃ films with fewer defects are still under investigation.

Solution processes, including CBD and spin coating, are promising preparation technologies due to their low demand of equipment, simple operation and low cost.⁶⁷ The CBD method holds two key advantages, one is that film doping can be easily achieved by adding different concentrations of dopants to the chemical bath, and the other is that during the synthesis process, the solution can readily permeate into the porous structure, where the film can co-deposit.⁴⁵ However, the formation of Sb₂O₃, SbOCl or other side products cannot be avoided in the conventional aqueous phase CBD technique; this can lead to deep traps and recombination of generated charge carriers.³⁵ The conventional CBD technique usually does not afford large-grained compact films;⁴⁵ however, the PCE of Sb₂S₃ solar cells prepared by spin coating is still lower than the best record (7.5%) that was obtained via the CBD method, although the spin-coating-based Sb₂S₃ film had a large grain size.²⁷ Vacuum deposition can achieve compact, high purity and larger-grained films.⁴⁵ However, vacuum deposition methods, including TE, ALD and sputtering, have several drawbacks. In the TE process, if a carbon-containing solvent is used, the annealing process is likely to result in carbon residue in the film. The residual carbon can induce recombination sites in the film. The ALD-based film growth rate is low.⁸

2.1 CBD

CBD is one of the simplest methods to deposit films of metal sulfides, selenides, oxides, etc.; it has a very long history and has been comprehensively reviewed.^68,69 Sb₂S₃ thin films were deposited by CBD from both acidic and alkaline media in the first report,⁷⁰ using Sb₂O₃-EDTA complex and Na₂S₂O₃ as the Sb and S source, respectively. The resulting Sb₂S₃ films showed optical band gaps of 1.82 to 1.97 eV due to their amorphous states without annealing. Savadogo and Mandal⁷¹ later reported a different chemical deposition method involving an aqueous solution of potassium antimonyl tartrate as the Sb³⁺ source, thioacetamide as the source of S²⁻, triethanolamine, ammonia, and silicotungstic acid (STA) as a complexing agent. The as-deposited films were amorphous, and annealing at 300 °C for 1 h in a N₂ atmosphere led to polycrystalline n type films. From the scanning electron microscopy (SEM) images (Fig. 2), the 300 °C annealed film showed substantial granular growth. The observed binding energy peak of the Sb (3d) energy level in X-ray photoelectron spectroscopy (XPS) for the annealed films demonstrated the impurity of a thin layer of Sb₂O₃. Various solar cells were prepared using Sb₂S₃ films in subsequent research; n-Sb₂S₃/0.01 M KI/C photoeletrochemical solar cells with a PCE of 3.9% at 40 mW cm⁻²,²⁹ n-Sb₂S₃/p-Si³⁰ and n-Sb₂S₃/p-Ge⁷² heterojunction solar cells with PCEs of 5.19% and 7.3% under AM 1 illumination, and Pt/n-Sb₂S₃ Schottky barrier solar cells with a PCE of 5.5% under AM 1 illumination³¹ were obtained.

Fig. 2 (a) and (b) Sb₂S₃ film deposited on glass and annealed at 300 °C in a N₂ atmosphere for 1 h before and after STA treatment. Reprinted from Savadogo et al.,⁷¹ with permission.

To avoid the tendency of hydrolysis when using aqueous media in the CBD method, Messina et al.⁷³ dissolved 650 mg SbCl₃ (Aldrich) in 2.5 ml acetone instead of water solution and added 25 ml 1 M Na₂S₂O₃ solution and sufficient water to deposit Sb₂S₃ thin films. Smooth, specularly reflective thin films were obtained at deposition temperatures from −3 °C to 10 °C. However, the fabricated solar cells with the structure FTO/CdS/Sb₂S₃/Ag(paint) showed a low current density of 0.02 mA cm⁻².

By optimizing the processing conditions of CBD, such as deposition time and pre- and post-treatments, Zimmermann et al.⁷⁴ fabricated planar heterojunction ITO/TiO₂/Sb₂S₃/hole transport material(HTM)/Ag solar cells with a PCE exceeding 4%. They noted that fine-tuning of the deposition and post-deposition processes was extremely important for achieving high PCEs above 3%. The best device performance was found for compact Sb₂S₃ layer thicknesses of about 50 to 70 nm with larger crystallites. Although the authors placed the samples tilted face down into the bath to avoid large particles, large crystallites ∼1 μm in size were widely distributed on the film surface, as shown in the SEM image (Fig. 3).

Fig. 3 Scanning electron microscopy image of the Sb₂S₃ surface. Reprinted from Zimmermann et al.,⁷⁴ with permission.

Sb₂S₃ was deposited on nanoporous TiO₂ (Degussa, P25) film by CBD for the first time in 2009 by Itzhaik et al.³¹ The as-deposited Sb₂S₃ layer thickness was estimated to be in the range of 5 to 10 nm from SEM images. The authors stated that X-ray diffraction (XRD) showed the presence of some Sb₂O₃ when Sb₂S₃ was removed hot from the oven and allowed to cool in air. Due to the fact that much poorer performance was obtained if the as-deposited Sb₂S₃ film was cooled under N₂, the authors assumed that the surface oxide Sb₂O₃ acted as a passivation layer. The conversion efficiency reached 3.37% for the FTO/c-TiO₂/mp-TiO₂/In-OH-S/Sb₂S₃/KSCN/CuSCN/Au cell structure.

With increasing deposition time, the thickness of Sb₂S₃ layers deposited onto mesoporous TiO₂ films should increase, which is expected to have a remarkable effect on the solar cell performance. Chang et al.³⁷ investigated the effects of Sb₂S₃ CBD times from 1 to 4 h in FTO/TiO₂/Sb₂S₃/P3HT-sensitized solar cells. The deposited Sb₂S₃ showed a broad distribution of particle sizes or thin layers. Large particles were also observed in the cross-section SEM image of the device, probably due to Sb₂S₃ (Fig. 4a). The absorbance of the Sb₂S₃-sensitized films increased with the Sb₂S₃ deposition time (t) from 1 to 4 h (Fig. 4b). Meanwhile, a thicker Sb₂S₃ layer (t = 4 h) exhibited decreased values of incident-photon-to-current efficiency (IPCE), current density (J_sc) and V_oc compared to the Sb₂S₃ layer obtained at t = 3 h (Fig. 4c). The thick absorber layers may increase the charge carrier recombination before they are injected into the electron and hole conductors.⁷⁵ A device under the deposition time of 3 h demonstrated 5.06% and 5.13% PCE before and after adding the interfacial modifier decylphosphonic acid (DPA).

Fig. 4 (a) Cross-section SEM image of FTO/TiO₂/Sb₂S₃/P3HT. (b) Absorption spectra of P3HT and Sb₂S₃-deposited mp-TiO₂ for various deposition times on the surface of mesoporous TiO₂ layers in a chemical bath. (c) Current density–voltage (J–V) curves for TiO₂/P3HT/Au and TiO₂/Sb₂S₃/P3HT/Au fabricated with different deposition times. Reprinted from Chang et al.,³⁷ with permission.

Gui et al.⁷⁶ also investigated the influence of CBD time on solid Sb₂S₃ sensitized solar cells. They evaluated the grain sizes of the Sb₂S₃ samples from their XRD patterns using Scherrer's equation; there were no significant differences in the films deposited for 1 h, 2 h, or 3 h, all of which contain grains about 19 to 20 nm in size. It is noteworthy that there is usually inaccuracy when employing Scherrer's equation, especially due to the fact that deposited Sb₂S₃ has a broad distribution of particle sizes. In addition, diffraction peaks corresponding to Sb₂O₃ phase with orientations of (111) and (222) were observed in the XRD patterns (Fig. 5a). After the chemical bath deposition, a surface treatment consisting of thermal oxidation and subsequent HCl etching of the Sb₂S₃ layer was introduced to improve the solar cell performance (Fig. 5b). The best PCE of 2.32% was obtained for the 3 h CBD time, and the optimal particle size of the TiO₂ mesoporous film (∼2 μm) was 100 nm.

Fig. 5 (a) X-Ray diffraction patterns of 1 h, 2 h and 3 h-deposited Sb₂S₃ films. (b) Schematic of a surface modification consisting of a 2-step treatment: (i) thermal oxidation, (ii) HCl treatment. Reprinted from Gui et al.,⁷⁶ with permission.

Thioacetamide (TA) was used to sulfurize the surface of Sb₂S₃, decreasing the content of oxide impurities and improving the performance of solid Sb₂S₃-sensitized solar cells, by Choi et al.¹³ The 0.01 g mL⁻¹ TA solutions diluted in DMF were spin coated on DMF-treated FTO/BL-TiO₂/mp-TiO₂/Sb₂S₃ at 3000 rpm for 60 s; then, the samples were annealed at 300 °C. Hydrogen sulfide (H₂S) decomposed from TA reacted with the surface Sb₂O₃ to form Sb₂S₃, and the assumed reactions are:

CH₃CSNH₂ → CH₃CN(g) + H₂S(g)

Sb₂O₃ + 3H₂S(g) → Sb₂S₃ + 3H₂O(g)

TA treatment decreased the amount of Sb₂O₃, as confirmed by the XPS spectra (Fig. 6a and b); the relative atomic ratio of Sb₂O₃/Sb₂S₃ dramatically decreased from 49/51 to 20/80 after TA treatment. The untreated Sb₂S₃ film showed hole traps (E_v + 0.52 eV) with a defect density of about 2 to 5 × 10¹⁴ cm⁻³. The average PCE of the TA-treated devices (∼5.5%) was about 0.8% higher than that of the untreated devices (∼4.7%). To improve the performance of Sb₂S₃-sensitized solar cells obtained by TA treatment, they studied the Sb₂S₃ CBD reaction time. The most efficient devices were obtained after 2 h 20 to 25 min. The best device exhibited a PCE of 7.5%, which is the highest reported for Sb₂S₃-based solar cells to date (Fig. 6c).

Fig. 6 Effects of TA sulfurization on the surface states of Sb₂S₃. High resolution XPS spectra of (a) Sb_3d3/2 and (b) S_2p of samples with TA and without TA. (c) J–V characteristics of the best cell. Reprinted from Choi et al.,¹³ with permission.

As mentioned above, oxide impurities are sometimes considered to act as a passivation layer and have beneficial effects on Sb₂S₃ solar cells.^31,77,78 Gödel et al.⁷⁷ investigated the effects of post heat treatment of CBD-deposited Sb₂S₃ samples on solid Sb₂S₃-sensitized solar cells in air for different times from 0 to 20 min. UV-vis measurements did not show obvious changes; also, the peaks of antimony oxide did not appear in the XRD spectra. The author pointed out that absorption spectroscopy measures the bulk properties of a material sensitively instead of the thin surface layers. Also, antimony oxide did not exist as crystals. XPS measurements of the films showed a Sb_3d3/2 peak at 539.5 eV corresponding to antimony oxide (Sb_xO_y); the intensity of the peak increased with increasing annealing time. From the J–V curves of the samples, short annealing times were beneficial to the PCE; the best PCE of 2.4% was obtained after annealing for 1 min at 200 °C in air (Fig. 7). Itzhaik et al.⁷⁸ estimated the Sb₂O₃ overlayer thickness in a TiO₂/In-OH-S/Sb₂S₃ film to be 3 to 5 nm or 1 nm according to XPS and energy-dispersive X-ray spectroscopy (EDS), respectively, for the standard air-cooled Sb₂S₃ film after CBD and N₂ annealing. A grain radius of Sb₂S₃ of 10 nm was used for the EDS thickness evaluation, while the authors noted that in reality, Sb₂S₃ grains appeared in clusters with radii ca. 40 nm; some even had radii of 100 nm. They further pointed that the Sb₂O₃ thin layer effectively presented a ca. 1 eV barrier to hole transport according to the raw XPS valence band spectra of the Sb₂S₃ film (Fig. 8).

Fig. 7 (a) J–V characteristics of an unannealed and an annealed (1 min) solar cell under 1 Sun illumination (full symbols) and in the dark (open symbols). The inset shows the averaged efficiencies for different annealing times in air. (b) Surface oxidation by controlled heat treatment in air leads to improved Sb₂S₃-sensitized solar cells by decreasing the charge carrier recombination. Reprinted from Gödel et al.,⁷⁷ with permission.

Fig. 8 Sb_3d3/2 XPS spectra of the as-deposited (top) and annealed (bottom) Sb₂S₃ film on TiO₂/In-OH-S substrate. The Sb signal is composed of both Sb₂S₃ and Sb₂O₃ components. Reprinted from Itzhaik et al.,⁷⁸ with permission.

The reactions of conventional CBD must be slowed by cooling below 10 °C to avoid immediate precipitation.⁷⁹ This is disadvantageous for large-scale applications at low temperatures. Gödel et al.⁸⁰ presented a room temperature deposition route of Sb₂S₃ using the same precursor materials (SbCl₃ and Na₂S₂O₃) as the conventional CBD method. They changed the order of reactant addition at room temperature to slow the formation of Sb₂S₃ on 1 μm thick TiO₂ mesoporous films composed of 50 nm particles. The average Sb₂S₃ crystallite sizes were calculated to be 40 nm and 35 nm for room temperature and conventional low temperature deposition, respectively, from the XRD patterns (Fig. 9a). These values are relatively large compared with the average particle and pore sizes of the mesoporous TiO₂ film (∼50 nm). The RT sample had a marginally higher oxide content compared to the low-T material by XPS measurements (Fig. 9b). There was a clear decrease in the density of deep-trap states for the RT-deposited Sb₂S₃ based on photo-thermal deflection spectroscopy (PDS) (Fig. 9c). The best device of the FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/PCPDTBT:PCBM/PEDOT:PSS/Au structure achieved a PCE of 5.1% using the room temperature deposition method.

Fig. 9 (a) XRD pattern of crystalline Sb₂S₃, synthesised via the low temperature deposition method (upper trace) and the RT technique (center trace). The lower trace shows the difference of the two patterns. The stibnite reference pattern was taken from ref. 15. (b) XPS Sb_3d3/2 peaks of the films obtained from both deposition methods. The solid lines show the least squares fit of a superposition of two Gaussian curves. (c) PDS measurements showing the sub-band-gap energy levels for both deposition methods. The inset shows the corresponding Urbach energies. Reprinted from Gödel et al.,⁸⁰ with permission.

2.2 Spin-coating

The conventional CBD method requires a relatively long deposition time, and control of the size or the thickness of the Sb₂S₃ layer is difficult. The formation of impurities, including SbOCl and antimony oxides, cannot be avoided thoroughly, resulting in deep traps and causing backward recombination of the generated charge carriers. The spin-coating method has the advantages of fast processes and has been extensively investigated in organic and inorganic thin film preparation.^81–83 Especially, this method has earned great success in the fabrication of lead halide perovskite optoelectronic devices^44,84–86 and shows enormous room for further improvement.

Choi et al.⁵² first spin coated a SbCl₃-thiourea (Sb-TU) complex solution on a mesoporous TiO₂ film to fabricate efficient Sb₂S₃-sensitized solar cells. The Sb₂S₃ loading amounts were simply controlled by tuning the mole concentrations of SbCl₃ and thiourea in solution, enabling them to obtain an exact ratio of S/Sb in the final product (Fig. 10a). The EDX line scan profile showed the uniform distribution of Sb and S elements along the mesoporous TiO₂ layer, as observed in Fig. 10b and c, confirming the uniform deposition of Sb₂S₃ inside the pores of the mp-TiO₂ layer. The most efficient device was obtained under specific conditions of the ratio of SbCl₃ : TU = 1 : 1.8. The best device exhibited a high device efficiency of 6.4%.

Fig. 10 (a) Schematic of the fabrication process of our devices via Sb-TU complex solution processing. (b) Cross-sectional FESEM image of a typical device and (c) EDX line scan profile along the L1 layer. For the EDX line scan profile analysis, the sample was deposited on glass (not FTO) to avoid misinterpretation between two elements, i.e., Sb of Sb₂S₃ and Sn of FTO. Reprinted from Choi et al.,⁵² with permission.

A vacuum annealing process just after the spin-coating process was employed to fabricate highly efficient solar cells.⁴¹ SbCl₃ and thiourea source materials were dissolved in 2-methoxyethanol. The solution precursors were spin coated onto TiO₂ nanorod arrays in order to fabricate a Sb₂S₃-sensitized device architecture. The optimized pre-annealing temperature was 125 °C for the vacuum-assisted solution process (VASP) approach. Finally, the film was annealed at 270 °C under a N₂ atmosphere to improve the crystallinity (Fig. 11). The highest PCE of the FTO/bl-TiO₂/TiO₂ nanorods array/Sb₂S₃/Spiro-OMeTAD/Au device reached 6.78%.

Fig. 11 Schematic of the fabrication process of Sb₂S₃-based devices via vacuum-assisted solution processing. Reprinted from Tang et al.,⁴¹ with permission.

The modified spin-coating method using 2-methoxyethanol as the precursor solvent was also applied in planar solar cells with the structure of FTO/c-TiO₂/Zn–Sb₂S₃/Spiro-OMeTAD/Au.²⁸ In addition, ZnCl₂ was introduced into the precursor solution to dope Sb₂S₃ films. The Zn doping considerably increased the number of sulfur vacancies, which in turn increased the electron concentration in the final Sb₂S₃ film. In addition, the doping of Zn ions regulated the film growth and enhanced the crystallinity. However, a much higher doping concentration caused an increase of pinholes in the film (Fig. 12). The final PCE of the Sb₂S₃ planar solar cells reached 6.35%.

Fig. 12 SEM images of the Sb₂S₃ film by spin-coating: as-synthesized pristine (a) and with Zn doping of 1.5% (b), 4.8% (c), and 8.6% (d). Reprinted from Tang et al.,²⁸ with permission.

In addition to the precursor materials of SbCl₃ and thiourea, Wang et al.²⁷ developed a novel precursor for spin-coating Sb₂S₃ films. They used Sb₂O₃ as a Sb-source that was dissolved in carbon disulphide (CS₂) and n-butylamine mixed solution, forming an antimony-complex precursor solution. By spin-coating the precursor solution on a substrate and annealing at 300 °C for only 2 min, Sb₂S₃ film with a lateral grain size of up to 12 μm could be obtained; the average size was about 6 μm and the thickness of the Sb₂S₃ film was ∼137 nm (Fig. 13a and b). A large grain size was favorable for charge transport across the film, and the resulting planar FTO/bl-TiO₂/Sb₂S₃/Spiro-OMeTAD/Au solar cell delivered a PCE of 4.3%. Subsequently, the same group used the spin-coating approach to deposit Sb₂S₃ films; V₂O₅ ⁵⁵ and NiO_x ⁸⁷ acted as HTMs. The PCE of devices based on Sb₂S₃/V₂O₅ and Sb₂S₃/NiO_x reached 4.8% and 3.51%, respectively. Recently, they modified the method by applying ethanol and dimethyl sulfoxide as dissolution solvents to increase the boiling point of the mixed solvent and introduced alkali hydroxides (LiOH, NaOH, KOH, RbOH, CsOH)³⁶ into the precursor solution to dope Sb₂S₃ films. It was found that K, Rb and Cs were beneficial to large Sb₂S₃ grain growth and better morphology (Fig. 13c) along with an upshift of the Fermi energy level, which facilitated the charge transport from Sb₂S₃ to TiO₂. The Cs-doped planar FTO/c-TiO₂/Cs–Sb₂S₃/Spiro-OMeTAD/Au solar cell delivered the best PCE of 6.56%, which is the record efficiency for planar Sb₂S₃ solar cells.

Fig. 13 (a) Schematic of the synthesis of the Sb₂S₃ film and device assembly. (b) SEM images of the Sb-complex films annealed at different temperatures and cross sectional SEM images of the devices containing FCA and CBD-fabricated Sb₂S₃ films. (c) SEM image of the Cs-doped Sb₂S₃ film. Reprinted from Wang et al.,^27,36 with permission.

The same group⁸⁸ reported an aqueous precursor solution composed of Sb₂O₃, 3-mercaptopropionic acid and aqueous ammonia. 3-Mercaptopropionic acid acted as both the solvent and sulfur source, while aqueous ammonia facilitated the formation of a homogenous thiol-containing Sb-complex solution. Thermal decomposition at 270 °C after spin-coating produced compact, flat, uniform and well-crystallized Sb₂S₃ films (Fig. 14). A planar Sb₂S₃ solar cell with the structure of FTO/bl-TiO₂/Sb₂S₃/Spiro-OMeTAD/Au delivered a PCE of 5.57%.

Fig. 14 SEM image of a Sb₂S₃ film prepared with an annealing temperature of 270 °C; cross section of the fabricated Sb₂S₃ solar cell. Reprinted from Li et al.,⁸⁸ with permission.

A convenient two-step sequential spin-coating approach was suggested by Zhang et al.⁸⁹ Antimony acetate (Sb(Ac)₃) solution using DMSO as a solvent was first spin-coated; then, thiourea solution, also using DMSO as the solvent, was spin-coated. Annealing at 320 °C caused Sb(Ac)₃ and thiourea to react to form a Sb₂S₃ film very quickly, in less than 2 min (Fig. 15). By optimizing the thiourea solution concentration, a planar FTO/bl-TiO₂/Sb₂S₃/Spiro-OMeTAD/Au device delivered a PCE of 5.69%.

Fig. 15 Schematic of the film deposition and device fabrication. (a) Spin-coating Sb(Ac)₃ in DMSO solution for the fabrication of the Sb(Ac)₃ film (film A). (b) Deposition of thiourea (TU) on the surface of film A, followed by dissociating the organic molecules using a vacuum pump (film B). (c) Annealing of film B at an elevated temperature for the formation of the crystallized Sb₂S₃ film and device assembly. (d) High magnification surface SEM images of the Sb(Ac)₃ film reacted with spin-coated TU with concentrations of 6 M TU, followed by annealing at 320 °C for 2 min. Reprinted from Zhang et al.,⁸⁹ with permission.

Zheng et al.⁹⁰ developed a sequential solid–gas deposition method to prepare solid Sb₂S₃ sensitized solar cells, first spin-coating SbCl₃ aqueous solution followed by reaction with H₂S gas and further annealing (Fig. 16a). The loading amount of Sb₂S₃ within the porous TiO₂ film could be easily controlled through adjusting the concentration of SbCl₃ solution. From the SEM results, Sb₂S₃ was homogeneously coated within the mesoporous TiO₂ film, which was attributed to the in situ solid–gas reaction between SbCl₃ pre-coated on the TiO₂ film and H₂S, in contrast to the heterogeneous nucleation and growth process in the CBD process. Moreover, the method afforded pure Sb₂S₃ and no Sb₂O₃ on the surface based on XPS (Fig. 16b). A PCE of 6.27% was achieved for the optimized Sb₂S₃ sensitized solar cells.

Fig. 16 (a) Schematic of the deposition of a Sb₂S₃ layer on the TiO₂/FTO substrate by the sequential deposition process. (b) XPS Sb_3d spectra of Sb₂S₃ obtained from sequential deposition (with a SbCl₃ concentration of 16 mg ml⁻¹) and CBD. Reprinted from Zheng et al.,⁹⁰ with permission.

Spin-coating of pre-synthesized nanomaterials has been extensively investigated in the field of quantum dot optoelectronics.^91,92 Wang et al.⁹³ reported the synthesis of colloidal Sb₂S₃ nanoparticles by a hot-injection method; they applied the pre-synthesized Sb₂S₃ nanoparticles in planar solar cells by spin-coating on TiO₂-covered FTO substrates. Solar cells based on these nanoparticles achieve a PCE of 1.5%, which is the record efficiency for planar hybrid solar cells based on Sb₂S₃ nanoparticles. It is clear from the SEM images that there is much room to achieve uniform and dense Sb₂S₃ nanoparticle films (Fig. 17).

Fig. 17 SEM images of Sb₂S₃ NPs on TiO₂-covered FTO substrates (a) before and (b) after annealing. Reprinted from Wang et al.,⁹³ with permission.

2.3 Vacuum deposition

During the early stages, when Sb₂S₃ found applications in television cameras, microwave devices, etc., Sb₂S₃ films were prepared by the vacuum evaporation technique.^94–96 Compared with the Sb₂S₃ films obtained from solution processes such as CBD and spin-coating, the Sb₂S₃ films deposited by vacuum routes were formed with smooth and compact surface morphologies, free of impurities such as antimony oxides. However, TE,^{34,58–60,66,97} sputtering (SP)^64,65 and ALD^35,60,61,98 based on vacuum technology have been used to prepare Sb₂S₃ solar cells only recently.

ALD essentially combines the advantages of fine thickness control and high purity. Wedemeyer et al.⁶⁰ first employed the method to prepare solid Sb₂S₃-sensitized solar cells. With accurate growth rate control, the thickness of the Sb₂S₃ layer was tuned from 3 to 13 nm. The disappearance of the initially visible sharp edges of the TiO₂ crystals demonstrated the conformal coating (Fig. 18). A PCE of 2.6% was achieved for the 10 nm Sb₂S₃-sensitized solar cells. Kim et al.³⁵ later employed ALD to fabricate planar FTO/bl-TiO₂/Sb₂S₃/P3HT/PEDOT:PSS/Au solar cells; the optimal thickness of the Sb₂S₃ layer was 90 nm. There was significant improvement from the aspects of Sb₂S₃ thickness control, deposition reproduciblity and purity against oxides based on the tranmission electron microscopy (TEM), spectroscopic ellipsometry and XPS measurements (Fig. 19a and b). The optimized device delivered a PCE of 5.77%.

Fig. 18 Microstructures of mesoporous solar cells based on Sb₂S₃ obtained by ALD. Changes observed by SEM (top view) upon ALD and annealing. Reprinted from Wdemeyer et al.,⁶⁰ with permission.

Fig. 19 XPS Sb_3d spectra of Sb₂S₃ obtained from the (a) conventional CBD process and (b) ALD process. The solid line is the experimental data and the dotted line is the fitted data. 1 (Sb₂S₃) = Sb_3d5/2, 1′ (Sb₂S₃) = Sb_3d3/2, 2 (Sb₂O₃) = Sb_3d5/2, 2′ (Sb₂O₃) = Sb_3d3/2, 3 = O_1s. Reprinted from Kim et al.,³⁵ with permission.

Thermal evaporation is a conventional cost-effective vacuum deposition method that has been successfully used to fabricate various planar Sb₂S₃ solar cells.^{47,58,59,66,97,99–101} Recently, Yin et al. reported the composition engineering of Sb₂S₃ film by co-evaporation of sulfur or antimony with Sb₂S₃ (Fig. 20). By controlling the ratios between Sb₂S₃ and S or Sb during thermal evaporation, the sulfur-rich Sb₂S₃ film-based device showed high carrier concentration and decreased recombination probability compared with the pristine and antimony-rich Sb₂S₃-based devices. With the thermally evaporated Sb₂S₃ thin films, the best PCE was 5.8% for the glass/FTO/bl-TiO₂/TiO₂ array/Sb₂S₃/Spiro-OMeTAD/Au planar devices.¹⁰² In addition, post-treatment in Se atmosphere was combined with rapid thermal evaporation to improve junction quality, passivate absorber defects, decrease the back contact barrier and therefore enhance the solar cell PCE.^47,101

Fig. 20 Schematic of the fabrication process of the Sb₂S₃-based device via thermal evaporation. Reprinted from Yin et al.,¹⁰² with permission.

Deng et al.⁴⁸ proposed a facile strategy to achieve quasiepitaxy growth of Sb₂S₃ film on TiO₂ surfaces for the first time. Engineering the TiO₂ exposure facets through the annealing temperature induced quasiepitaxial growth of vertical orientation Sb₂S₃ films by rapid thermal evaporation. The authors systematically investigated the epitaxial effect by morphology, lattice structure and physical characterization. The interface defects and Sb₂S₃ absorber transport loss were overcome, and the optimal device delivered a PCE of 5.4%, which is the best reported for full-inorganic Sb₂S₃ solar cells.

3. Electron transporting materials (ETM)

Sb₂S₃ solar cells usually employ a sandwiched structure of ETM/Sb₂S₃/HTM. These devices need the assistance of ETM and HTM to extract the carriers. By choosing different electron conductors, such as SnO₂,^29,62,103 CdS,^{33,47,58,73,99,104} ZnO,^56,57,61,63 and TiO₂,^{13,31,37,38,105–108} much progress has been achieved in Sb₂S₃ solar cells.

SnO₂ is a wide band-gap metal oxide semiconductor (∼3.8 eV);¹⁰⁹ the carrier mobilities of single crystal SnO₂ and nanostructured SnO₂ are ∼250 cm² V⁻¹ s⁻¹ (ref. 110) and ∼125 cm² V⁻¹ s⁻¹,¹¹¹ respectively. Early in 1992, Savadogo and Mandal fabricated Sb₂S₃ solar cells with a glass/SnO₂/Sb₂S₃/graphite structure. The efficiency reached 3.9% under 40 mW cm⁻² illumination.²⁹ Subsequently, SnO₂ has rarely been used as an ETM. Lei et al.¹⁰³ employed ∼40 nm thick SnO₂ film as an ETM to fabricate FTO/SnO₂(∼40 nm)/Sb₂S₃/P3HT/Au devices in 2016. The best-performing solar cells reached a PCE of 2.8%.

ZnO has a high electron mobility (μ_e = ∼200 cm² V⁻¹ s⁻¹) and a wide band gap (E_g = ∼3.2 eV).¹¹² However, the PCE of the Sb₂S₃ solar cells with ZnO as the ETM was relatively low. Liu et al.⁵⁷ prepared polycrystalline thin films of ZnO with a thickness of ∼60 nm by spin casting followed by thermal annealing in air at 350 °C for 20 min. The best PCE of the ITO/ZnO(n)(∼60 nm)/Sb₂S₃(i)(50 to 350 nm)/P3HT(p)/Ag structure device was 2.4%. Jo et al.⁶¹ prepared a∼ 30 nm thick ZnO layer using the ALD method. They indicated that the ALD-processed ZnO had good crystal quality. The device structure was FTO/ZnO/Sb₂S₃/P3HT/Au, and an efficiency of 2.81% was delivered. Although the electron mobility of the ZnO layer is high, the hole mobility of P3HT is low (10⁻⁵ to 10⁻³ cm² V⁻¹ s⁻¹),¹¹³ which hinders the efficiency enhancement.

Most commonly, a mesoporous TiO₂ layer (mp-TiO₂)^{13,31,37–39,76–78,80,105,114–117} or compact TiO₂ (c-TiO₂) layer^{28,53–55,59,87,97,100,101} is used as the ETM for Sb₂S₃ solar cells. In the first attempt to fabricate Sb₂S₃-sensitized solar cells by Itzhaik et al.,³¹ they designed the structure of FTO/c-TiO₂/mp-TiO₂/In-OH-S/Sb₂S₃/KSCN/CuSCN/Au using a nanoporousTiO₂ film (1 to 2 μm thick, 25 nm particles) as the ETM. The deposition often covered several TiO₂ particles, as shown in Fig. 21. Subsequently, Moon et al.¹¹⁴ also deposited Sb₂S₃ onto the nanoporous TiO₂ substrate. The thickness and particle size of the nanoporous TiO₂ film were ∼2 μm and 30 nm, respectively. The Sb₂S₃ did not cover each TiO₂ particle conformally but rather coated clusters of TiO₂ (Fig. 22). A TiO₂ film composed of 20 nm-sized nanoparticles had a smaller pore size than films comprising 30 nm particles; the device made of 20 nm TiO₂ nanoparticles yielded an efficiency of less than 1%. The particle size of the nanoporous TiO₂ film was increased to 60 nm by Chang et al.;³⁷ the cells of the FTO/dense-TiO₂/mp-TiO₂/Sb₂S₃/P3HT/PEDOT:PSS/Au structure reached 5.13% PCE (Fig. 23).

Fig. 21 SEM images of an Sb₂S₃ film on P25/In-OH-S substrate. Reprinted from Itzhaik et al.,³¹ with permission.

Fig. 22 SEM pictures of (a) bare 30 nm TiO₂ particles and (b) the Sb₂S₃ film deposited and annealed on a 30 nm TiO₂/In-OH-S substrate. Reprinted from Moon et al.,¹¹⁴ with permission.

Fig. 23 TEM photographs of TiO₂ nanoparticles. Reprinted from Chang et al.,³⁷ with permission.

Gui et al.⁷⁶ fabricated devices of FTO/c-TiO₂/mp-TiO₂/In-OH-S/Sb₂S₃/P3HT/PEDOT:PSS/Au with larger TiO₂ nanoparticles (∼50 nm and ∼100 nm) for comparison. The porosities and surface areas of the TiO₂ films were characterized with BET. The surface area and pore size of the film consisting of 50 nm TiO₂ nanoparticles were 128.31 m² g⁻¹ and 52 nm, respectively. The film consisting of ∼100 nm nanoparticles showed a surface area of 13.36 m² g⁻¹ and a pore size of 120 nm. For larger-size nanoparticle films, a larger porosity probably facilitates access of the precursor into the TiO₂ pores.

The thickness of nanoporous–TiO₂ films is generally controlled to be about 1 to 3 μm. Boix et al.¹¹⁵ decreased the TiO₂ film thickness and studied the effects on the performance of FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/P3HT/Au(NS) and FTO/c-TiO₂/Sb₂S₃/P3HT/Au (flat) solar cells by impedance spectroscopy. The NS solar cells presented higher performance than the flat cells due to their higher J_sc and fill factor (FF). Conversely, V_oc was higher for the flat device. The impedance spectroscopy analysis showed that decreasing the effective surface area of the flat cells decreased the recombination rate and increased the open circuit potential, V_oc; meanwhile, charge compensation problems as a consequence of inefficient charge screening in the flat cells increased the hole transport resistance, severely decreasing the cell J_sc and fill factor. Muto et al.¹¹⁶ obtained a similar relationship between the cell performance and the TiO₂ film thickness. Tsujimoto et al.¹⁰⁵ also found variation of the PCE of FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/KSCN/CuSCN/Au solar cells related to the TiO₂ particle size. The optimized thickness of the nanoporous–TiO₂ films was fixed as 2.5 μm. When the particle size was increased to 30 nm, the conversion efficiency increased to 3.65% and then decreased with further increase of the particle size. The larger TiO₂ particles led to a smaller surface area, resulting in lower photon absorption and a lower photocurrent density.

The surface treatment of ETM nanoparticles is expected to improve the charge extraction at the TiO₂/Sb₂S₃ interface. Tsujimoto et al.¹⁰⁵ treated the mesoporous–TiO₂ surface by Mg²⁺, Ba²⁺, and Al³⁺. The short-circuit current density was improved with Mg²⁺. The FF was improved with Ba²⁺. The open-circuit voltage was improved with both Al³⁺ and Ba²⁺. However, the starting material for Al³⁺ treatment, AlCl₃, was quite unstable, resulting in inconsistent results. Excluding Al³⁺, therefore, Mg²⁺ and Ba²⁺ were chosen for double treatment. The best efficiency obtained using Ba²⁺/Mg²⁺ double treatment was 4.1%. Fukumoto et al. treated Sb₂S₃-sensitized TiO₂ films with DPA to decrease recombination. The DPA-treated devices exhibited increased V_oc and FF.¹¹⁷ Xu et al.¹⁰⁷ utilized ultrathin SiO₂ films with thicknesses of 0.6 to 1.6 nm (Fig. 24) as a blocking layer to suppress the interfacial recombination at the TiO₂/Sb₂S₃ interface, which was the major recombination path in the solar cells as confirmed by impedance spectra and open-circuit voltage-decay analysis. The efficient injection of photoinduced electrons from Sb₂S₃ to TiO₂ was not impeded due to the ultrasmall thickness of the SiO₂ layer. The SiO₂ passivated liquid Sb₂S₃-sensitized solar cells exhibited increased PCE from 3.00% to 3.96%.

Fig. 24 HRTEM images of (a) bare TiO₂, (b) TiO₂/SiO₂, (c) TiO₂/crystalline Sb₂S₃ and (d) TiO₂/SiO₂/crystalline Sb₂S₃. Reprinted from Xu et al.,¹⁰⁷ with permission.

In addition, the surface morphology structure of TiO₂ films has an important effect on the performance of the planar Sb₂S₃ solar cells. Sung et al.⁵⁴ reported that a smooth surface morphology of TiO₂ layers (BL1) was unfavorable to the formation of uniform and dense high-quality Sb₂S₃ absorber layers during the spin-coating process (Fig. 25). In contrast, due to the lower surface energy, more compact Sb₂S₃ films formed on the rough TiO₂ layers (BL2). A high-quality Sb₂S₃ absorber layer could improve the photovoltaic performance of planar Sb₂S₃ solar cells, with the cell corresponding to a thiourea/Sb ratio of 2.0 showing the best performance of 3.8% PCE.

Fig. 25 SEM images of FTO, BL1, and BL2 blocking layers. Reprinted from Sung et al.,⁵⁴ with permission.

To reduce contact between Sb₂S₃ and the oxide ETMs (TiO₂, SnO₂ or ZnO), Tang's group fabricated planar Sb₂S₃ solar cells using CdS as an ETM without a HTM; the cell structure was ITO/CdS/Sb₂S₃/Au. The thickness of the CdS was about 60 nm. The CdS/Sb₂S₃ solar cells achieved a PCE of 4.17%, which is the highest efficiency Sb₂S₃ solar cell using CdS as the ETM at present.

4. Hole transporting materials (HTM)

HTMs are used to extract holes from Sb₂S₃. Different HTMs, including poly(3-hexylthiophene) (P3HT),^{35,37,90,107,108,115} 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD),^{27,41,66,114,118} poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzo-thiadiazole) (PCPDTBT),^39,80 poly(triarylamine) (PTAA), CuSCN,^31,78,105 V₂O₅,⁵⁵ NiO_x ⁸⁷ and graphene,¹¹⁹ have been employed in Sb₂S₃ solar cells. Enhancing the hole-transporting properties of Sb₂S₃ solar cells is important for further increasing their efficiency.

As an efficient hole-transporting material, spiro-OMeTAD has been widely applied in solid-state dye-sensitized solar cells and perovskite solar cells.⁴² Spiro-MeOTAD was first employed as the HTM in solid Sb₂S₃ sensitized solar cells by Moon et al.;¹¹⁴ it was coated on the TiO₂/Sb₂S₃ film by spin coating. Due to the fact that the conductivity of spiro-OMeTAD is significantly improved with suitable p-type dopants,¹²⁰ Chung et al.¹¹⁸ employed DPA as an effective p-doping additive in the presence of lithium bis(trifluoromethyl-sulfonyl)-imide (LiTFSI) to oxidize spiro-OMeTAD. The LiTFSI + DPA device exhibited the best performance, with significantly improved J_sc and FF with respect to those without DPA. The best device with a structure of FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/spiro-OMeTAD/Ag showed a PCE of 6.0%.

P3HT is a hole-conducting material that can strongly absorb light in the visible range. In order to compensate the external quantum efficiency (EQE) lost by P3HT coabsorption, Chang et al.³⁹ provided PCBM as a newly constructed electron channel to bridge P3HT and mp-TiO₂, transporting the generated charge carriers in P3HT to mp-TiO₂ as a result of more efficient charge transfer in the PCBM/P3HT interface. From the EQE spectra of the FTO/mp-TiO₂/Sb₂S₃/P3HT (T/S/P) and FTO/mp-TiO₂/Sb₂S₃/P3HT/PCBM (T/S/P–P) devices, the PCBM electron channel substantially recovered the EQE loss caused by absorption of P3HT. The use of P3HT assisted by PCBM electron channel enabled a PCE of 6.3% (Fig. 26).

Fig. 26 Schematic of the device structure (a). EQE spectra: the region marked by the blue lines is the EQE difference between the T/S/P–P and T/S/P samples (b). Reprinted from Chang et al.,³⁹ with permission.

The porously structured P3HT layer is expected to decrease the generated charge carrier recombination in P3HT and to decrease the hole transfer series resistance at the interface of P3HT/Au. Therefore, Lim et al.¹²¹ added AIBN as a nanopore-generating agent to the P3HT solution to create a porous P3HT layer in the FTO/bl-TiO₂/mp-TiO₂/Sb₂S₃/AIBN:P3HT/Au cell. A porously nanostructured P3HT film was formed by the thermal decomposition of AIBN. Compared to the reference cell, the device efficiency was improved by about 16%, exceeding 4.5%.

To enhance the P3HT hole transport mobility, Gamboa et al.⁶⁷ mixed P3HT with NiO_x nanoparticles (NiO_x-NP) as an HTM. The solid Sb₂S₃-sensitized solar cells assembled from FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/P3HT or P3HT:NiO_x-NP/Ag delivered PCE from 2.22% to 3.34% using 2% NiO_x-NP as the optimal concentration.

At a wavelength of 650 nm, the EQE of the Sb₂S₃ solar cells with P3HT usually exhibits a depression. PCPDTBT was used to overcome this shortcoming instead of P3HT. The mesoporous Sb₂S₃-sensitized FTO/c-TiO₂/mp-TiO₂/Sb₂S₃/PCPDTBT/Au solar cells did not exhibit a depression at a wavelength of 650 nm because the band gap of PCPDTBT was positioned at approximately 1.4 eV (Fig. 27). Accordingly, the device performance was superior to that of P3HT because of the higher current density that resulted from the decreased absorption loss. The best PCE reached 6.18%, with a J_sc of 15.3 mA cm⁻², V_oc of 616 mV, and FF of 65.7%.³⁸

Fig. 27 Energy level diagram of the corresponding materials used in devices. Reprinted from Im et al.,³⁸ with permission.

Considering the stability of solar cells, all-inorganic solar cell devices may be more desirable. CuSCN is the most commonly used inorganic HTM in Sb₂S₃ solar cells. CuSCN has p-type conductivity of 10⁻² to 10⁻³ Ω⁻¹ cm⁻¹.¹²² The thiocyanate treatment increases the p-type conductivity of CuSCN. Itzhaik et al.³¹ reported Sb₂S₃/(KSCN)CuSCN solar cells with a PCE of 3.37%. These Sb₂S₃ solar cells were treated with KSCN solution before CuSCN deposition.^{79,108,123–125} After SCN⁻ treatment, the phase changes of CuSCN were restricted, facilitating the diffusion of K⁺ ions into the CuSCN layer as a p-dopant to align the energy levels between Sb₂S₃ and the HTM layer. Moreover, K-doping passivated the surface trap states of CuSCN and resulted in decreased series resistance of the HTM and enhanced device performance. The treatment worked just as well in improving photovoltaic performance when carried out after CuSCN deposition (post-treatment).⁷⁸ In addition, Tsujimoto et al.¹⁰⁵ reported the effects of CuSCN thickness on the PCE of TiO₂/Sb₂S₃/(KSCN)CuSCN cells. In the range of 0.68 to 1 μm, the PCE increased with increasing CuSCN thickness; the best efficiency of 3.71% was obtained with 1 μm thickness.

Among various inorganic semiconductors, V₂O₅ has received particular attention because of its outstanding physical properties, including suitable optical and electrical properties and ambient stability. It has been widely used as a hole transporter in organic photovoltaic devices. V₂O₅ served as a hole transporting material to substitute organic transporting materials for Sb₂S₃ solar cells. The PCE of the optimized device based on the Sb₂S₃/V₂O₅ heterojunction reached 4.8% (Fig. 28).⁵⁵

Fig. 28 (a) SEM image of V₂O₅ film deposited on Sb₂S₃ and (b) cross-sectional SEM image of a complete device. (c) Illustration of the device with V₂O₅ as the hole transporting material. Reprinted from Zhang et al.,⁵⁵ with permission.

Inorganic p-type NiO_x is believed to act as an efficient HTM for planar heterojunction Sb₂S₃ solar cells. The deposition of the NiO_x HTM was thus conducted by spin-coating the nanoparticle ink in air, followed by low temperature annealing⁸⁷ (Fig. 29). Upon O₂ plasma treatment of the NiO_x layer, the valence band maximum of NiO_x shifted from −4.9 eV to −5.25 eV, giving rise to excellent energy-level alignment with Sb₂S₃ and the Au electrode. The device performance was thus significantly enhanced to 3.51% (Fig. 30).

Fig. 29 Schematic flow of fabricating the planar heterojunction Sb₂S₃ solar cell using NiO_x hole transporting materials. Reprinted from Jin et al.,⁸⁷ with permission.

Fig. 30 (a) Surface SEM image of the as-prepared Sb₂S₃ film deposited on FTO/TiO₂. (b) Cross-sectional SEM image of the most efficient planar heterojunction Sb₂S₃ solar cell using the O₂ plasma-treated NiO_x HTM. Reprinted from Jin et al.,⁸⁷ with permission.

5. Conclusions and perspectives

Nearly thirty years have passed since the first report on Sb₂S₃ solar cells. Various device structures, mainly mesoporous sensitized solar cells and planar heterojunction solar cells, were extensively investigated. CBD, spin-coating, thermal evaporation, ALD, etc. have been used to achieve compact, uniform and impurity-free Sb₂S₃ absorbers. Simultaneously, novel ETM and HTM materials have been proposed and synthesized to assist the solar energy conversion and carrier transport processes. However, the PCE of Sb₂S₃ solar cells still lags greatly behind those of other thin film solar cells, such as cadmium telluride (CdTe) solar cells, copper indium gallium diselenide (CIGS) solar cells and perovskite solar cells. We suggest further improvement of Sb₂S₃ solar cells in the following aspects: (1) more knowledge of the defect properties both from theoretical and experimental aspects is needed. Theoretical simulation work on the electronic and band structures of Sb₂S₃ is rather limited; meanwhile, experimental characterization such as deep level transient spectrocopy, thermal admittance spectroscopy, and space charge limited current have been rarely reported. Good understanding and healing engineering of defects will play a key role in the development of Sb₂S₃ solar cells. (2) Novel synthesis of Sb₂S₃ nanostructured materials. To date, it is still difficult to synthesise high quality Sb₂S₃ nanostructures due to the ribbon-like (Sb₄S₆)_n chains along the [001] direction. Despite this, we suggest that successful nano-synthesis of Sb₂S₃ will be benefical for solar cells, especially sensitized solar cells. The nuleation and growth process at suitable temperatures in organic solvent helps avoid the appearance of oxides, such as Sb₂O₃, which are usually formed in processes such as CBD. (3) Elucidating the device physics. Based on the knowledge of the material optoelectronic properties of Sb₂S₃, ETM and HTM, the device physics should be investigated either through numerical modelling or through capacitance–voltage studies, impedance spectroscopy, etc. Whether the p–i–n or p–n configuration is more favorable for performance and the role of the HTM must be clarified.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (SQ2017YFGH001237), the Distinguished Youth Foundation of Anhui Province (1708085J09), and the Fundamental Research Funds for the Central Universities (JZ2018HGPB0276). The Key Lab of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences is gratefully acknowledged by the authors.

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