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Review

Strategies for Optimizing the Morphology of CsSnI3 Perovskite Solar Cells

by
Minhao Zhang
1,†,
Kunli Chen
1,†,
Yunxiao Wei
1,
Wenzheng Hu
1,
Ziyu Cai
1,
Junchi Zhu
1,
Qiufeng Ye
1,2,*,
Feng Ye
3,*,
Zebo Fang
1,2,*,
Lifeng Yang
1,2,4,* and
Qifeng Liang
1,2
1
Department of Physics and Zhejiang Engineering Research Center of MEMS, Shaoxing University, Shaoxing 312000, China
2
ShaoXin Chip Lab, Shaoxing 312000, China
3
Shangyu College, Shaoxing University, Shaoxing 312300, China
4
Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Crystals 2023, 13(3), 410; https://doi.org/10.3390/cryst13030410
Submission received: 30 January 2023 / Revised: 19 February 2023 / Accepted: 21 February 2023 / Published: 27 February 2023
(This article belongs to the Special Issue Multi-Dimensional and Multi-Scale Applications of Perovskite Materials)
Browse Figures
Versions Notes

Abstract

:
Over the past decade, organic–inorganic hybrid perovskite solar cells (PVSCs) have shown unprecedented growth in power conversion efficiency (PCE) from 3.8% to 25.7%. However, intrinsic thermal instability and lead toxicity are obstacles limiting its large–scale commercialization. Thus, all-inorganic CsSnI3 perovskite has drawn remarkable interest owing to its nontoxicity, excellent thermal stability, low-cost fabrication, and spectacular photoelectric characteristics, including ideal bandgap range, long carrier lifetime, and large absorption coefficient. Many studies have shown that the device performances are closely related to the morphology and crystallinity of perovskite films. In this review, the physical properties of CsSnI3 perovskite are summarized. Furthermore, this review primarily narrates the recent progress in optimizing the morphology by various strategies such as additive engineering, composition regulation, and deposition techniques, emphasizing their effects on grain sizes, film uniformity, grain boundary, and defect passivation.

1. Introduction

Perovskite solar cells (PVSCs) have surged in popularity over the past decade, rapidly increasing their power conversion efficiency (PCE) from 3.8% to 25.7% [1,2,3,4,5,6,7,8]. However, the most influential organic–inorganic hybrid PVSCs are still in the immature stage of commercialization mainly due to the thermal instability of the organic composition and the toxicity of lead, which need to be treated well urgently [9,10,11,12]. The replacement of organic elements with stable inorganic components and replacing lead with nontoxic elements without sacrificing excellent photoelectric properties can be better adapted to applications [13,14,15]. Therefore, considerable environmentally benign lead-free perovskite materials have been developed. Among them, the full-inorganic CsSnI3 perovskite can simultaneously achieve the advantages of nontoxicity as well as excellent thermal stability with a high decomposition temperature (~450 °C) [16,17].
In recent years, CsSnI3 perovskite has been proven to be one of the most competitive lead–free perovskite materials for PVSCs [18,19,20]. In addition to the characteristics of thermal stability and low toxicity, CsSnI3 has a band gap (Eg) of 1.3 eV, different from the Eg of lead-based perovskite (from 1.5 to 2.3 eV) and closer to the ideal photovoltaic Eg of 1.34 eV, which is more conducive to achieve the excellent device performance to reach the theoretical efficiency limit; the potential of CsSnI3 exceeds that of hybrid perovskite [21,22]. Meanwhile, the low–cost fabrication of CsSnI3 possesses spectacular photoelectric characteristics, including high absorption coefficient, low exciton binding energy, and long carrier diffusion length [23,24,25]. Therefore, efficient performance can be realized within the CsSnI3 PVSCs.
Currently, CsSnI3 PVSCs have achieved significant progress. In 2012, CsSnI3 perovskite solar cells with a PCE of 0.9% were firstly fabricated by Chen et al. with an ITO/CsSnI3/Au/Ti device structure [26]. In 2016, Marshall et al. codeposited perovskite precursors with SnCl2, which can remove the electron barrier and suppress the pinhole density in the perovskite layer, achieving a stronger stability than MAPbI3 and a PCE of 3.56% [27]. Since then, CsSnI3 perovskite did not make outstanding achievements. In 2021, Yin et al. used the surface passivation strategy to achieve a photovoltaic device with 8.2% PCE [28]. Additionally, most recently, the efficiency of CsSnI3, further improved by Ye et al., exceeded 10%, showing a bright promise [29]. In pursuing the development of CsSnI3 PVSCs, besides solving the issues of oxidation of Sn2+ to Sn4+, interfacial engineering, composition optimization, and morphology improvement have been studied as the main focus.
More importantly, the improvement of morphology and crystallinity of the perovskite layers are the pivotal factors to reduce defect density, leakage current, and contact resistance, aiming to prepare efficient and stable solar cells [30,31,32,33,34]. The purpose of morphology control is to obtain films with better uniformity, compactness, pinhole freeness, large crystal size, and excellent crystallinity. In this review, the physical properties of CsSnI3 perovskite are systematically analyzed, and moreover, the present review focuses on the recent progress in optimizing the morphology by various strategies such as additive engineering, compositional regulation, and deposition methods focusing on grain sizes, film uniformity, grain boundary, and defect passivation, as illustrated in Figure 1.

2. Crystal Structure and Phase Transition of the CsSnI3 Perovskite

2.1. Crystal Structure

Goldschmidt toleration factors can predict the structural stability of an ABX3 perovskite depending on the degree of distortion of the BX6 octahedron [35,36]. According to well–known theoretical calculations, perovskites are stable at t values of 0.8–1.1, whereas cubic perovskites form at t values of 0.9–1.0 [37]. Compared to CsPbI3 perovskite, the CsSnI3 has a t value closer to 1 because Sn2+ (0.112 nm) has a smaller ionic radius than Pb2+ (0.119 nm) [38]. Thus, all-inorganic CsSnI3 perovskite is more robust and environmentally stable than lead–based perovskites (e.g., CsPbI3). However, the structural stability of the CsSnI3 perovskite remains a major challenge owing to the small ionic radius of Cs+, resulting in rotation and tilt of the [SnI6]4− octahedral [39,40].

2.2. Phase Transition

Similar to other phase–change materials, such as CsPbI3, CsSnI3 exhibits four polymorphs [41,42]. There are two types of polymorph: one is yellow in color (Y) with a one-dimensional double-strand structure, and the other is black in color (B) with a three-dimensional structure [43]. As shown in Figure 2a, at a high temperature of above 425K, CsSnI3 perovskites display a phase change from the yellow Y phase to a black cubic B-α phase; with the subsequent cooling, it undergoes a degeneration in symmetry and will transform to a black tetragonal B–β phase at 380 K and a black orthorhombic B–γ phase at around 300K. When perovskite is exposed to air, the black phase (B–α, B–β, B–γ) will transform to the yellow phase (Y–phase), owing to the degree of oxidation of Sn. The local coordination environment of Cs atoms in different CsSnI3 phases was also studied by Chungs et al. [43], as shown in Figure 2b–e, which can reveal the key driver behind these phase transitions. They found that the Cs atoms are randomly distributed in the perovskite lattices and the Sn–I–Sn bond is flexible enough to distort the lattice as well as to shorten the distance between the Cs and I atoms. For the cooling process of CsSnI3 perovskite, the Cs atom is in the force-balanced position in the cuboctahedral geometry at 500K. At 380K, the Cs atom will have an elliptical vibration, making it have the distorted cuboctahedral geometry; thus, the distance between Cs and I will be closer. At room temperature, the Cs and I atoms will split, and the Cs atoms will deviate to one side to strongly interact with I atoms. After perovskite exposed to air and transformed to the Y phase, the Cs atoms were no longer in the lattice cage but eventually settled into a nine–coordinated tricapped trigonal prism geometry. Therefore, the local coordination environment of Cs atoms provides valuable information for the phase transition of CsSnI3 perovskite.

3. Morphology Control

The surface morphology of the CsSnI3 perovskite has become a significant issue. High–quality film morphology plays an important role in obtaining efficient and stable CsSnI3 PVSCs simultaneously. For CsSnI3 PVSCs, a high–quality film morphology is effective in reducing defects and carrier traps as well as in preventing the oxidation of Sn2+ by oxygen and water through the pinholes. Device parameters that have a great influence on the performance of CsSnI3 PVSCs, such as carrier separation ability, diffusion coefficient, and diffusion length, are also mainly determined by the crystallinity quality of perovskite films [44,45,46,47]. Through the component regulation, the introduction of various additives, and the improvement of fabrication procedures, systematic progress has been made on the perovskite morphology. Optimization of the morphology is determined by in-depth study of the characterization of the perovskite film and the photoelectric characteristics of the device, with the aim of ultimately improving the performance of PVSCs [48,49,50]. Generally speaking, the crystallinity of the final film depends on plentiful factors such as additive type, chemical composition, deposition techniques, and hydrophobicity. Through the morphology control strategy, the crystallinity in perovskite films can be effectively optimized. Specifically, uniform grains and high surface coverage can be achieved, which is conducive to carrier transport.

3.1. Additive Engineering

In 2012, Chen et al. first applied inorganic CsSnI3 perovskite without any additives as the light-sensitive layer to fabricate a Schottky device [26]. The relatively poor device performance, with PCE less than 1% and a large ideality factor n of 2.8, which is related to the charge recombination, proved the inferior film quality and surface state [51,52,53]. Doping various additives in the CsSnI3 perovskite film, such as inorganic halide salts such as SnX2 (X=F, Cl, Br, I) and organic reducing additives such as some polymers and organic salts, is one of the effective ways to improve its film quality and surface morphology. Different additives have their own unique effects on morphology; generally speaking, additives can improve film crystallinity, reduce defects, improve carrier transport capacity, and ultimately affect the device performance [54,55].

3.1.1. SnX2 (X=F, Cl, Br, I)

The introduction of SnX2 additives can mainly precipitate more crystal growth nuclei, form a thin nanolayer at the grain boundaries, and modify the surface state, contributing to make the film more uniform and of higher coverage. This modification can result in the optimization of the film morphology and therefore can be considered as an effective strategy to improve the quality of the CsSnI3 perovskite.
SnI2: Marshall et al. reported a promising method to simultaneously improve the efficiency and stability of CsSnI3 devices by using excessive SnI2 during the synthesis of CsSnI3. The optimized CsSnI3 perovskite films showed a larger mean crystallite size, fewer pin holes, and more reduced background carrier density. As a result, at the optimal content of excessive SnI2, the PVSCs achieved a PCE of 2.76% and a high Voc of 0.55 V [56]. Inspired by the work of Marshall et al., the effect of excessive SnI2 on the performance of the CsSnI3 perovskite device was also explored by Song et al. [57]. They argued that SnI2 does not disturb the formation of CsSnI3 film, but disperses uniformly within the films or forms a very thin nanolayer at the grain boundaries of CsSnI3 to promote a film with better coverage and fewer pinholes, as shown in Figure 3a. At the optimal CsI/SnI2 ratio of 0.4, the modified CsSnI3 PVSC under hydrazine atmosphere realized a PCE of 4.81%.
SnBr2: In 2018, Heo et al. compared the impact of SnX2 additives (X=F, Cl, Br) in CsSnI3 PVSCs. As shown in Figure 3b, when the SnBr2 additive is added, the surface adsorption energy of CsSnI3 perovskite is minimal, and the surface formation energy of perovskite is also reduced, greatly suppressing the defects in the film. Furthermore, as shown in Figure 3c–f, the addition of SnBr2 can inhibit phase separation and thus further improve the film morphology. As a result, devices with the SnBr2 additive are more stable and efficient, showing 100 h of storage stability as well as a PCE of 4.3 % [58].
SnCl2: Marshall et al. also studied the impact of the SnX2 (X = F, Cl, Br, I) additives on the morphology of the CsSnI3 films and the performance of the CsSnI3 PVSCs. Figure 3g shows the scanning electron microscope (SEM) of the films with the different SnX2 additives (X=F, Cl, Br, I); all additives showed significant improvements in the quality of the perovskite films. Among the SnX2 (F, Cl, Br, I) additives, the authors argued that SnCl2 could form a thin film on the surface of the perovskite, and therefore modified the surface state. It acted as a desiccant and sacrificial layer to absorb water, as schematically illustrated in Figure 3g. Finally, by adding SnCl2, the perovskite device delivered a PCE of 3.56% and excellent stability, as shown in Figure 3h–i [27].
SnF2: SnF2 is commonly applied to improve the morphology and coverage of Sn–based perovskite as well as play the role of antioxidant [59]. Liao et al. found that the flower-shaped structure of the initial film without additives disappears with the increase in SnF2 in FASnI3 perovskite. Meanwhile, SnF2 also inhibits the oxidation of Sn2+, maintaining a reduction environment. Inspired by this idea, Kumar et al. added SnF2 into CsSnI3 and improved the quality of perovskite. The background carrier density and tin vacancy were reduced. Finally, the modified solar cells with the structure of FTO/compact TiO2/mesoporous TiO2/CsSnI3/HTM/Au were achieved with a top PCE of 2.02% [60]. What’s more, it has been found that adding the SnF2 additive to CsSnI3 is helpful to improve light stability and optimize band alignment at the perovskite interfaces.

3.1.2. Reductive Organic Additives

As mentioned above, the introduction of the SnX2 additive can improve the morphology quality of perovskite films, which is extremely important for the performance of CsSnI3 PVSCs. However, in terms of this single strategy, the improvement in PVSCs’ performance is not significant enough (PCE < 5%), mainly due to the fact that Sn2+ tends to oxidize to Sn4+ even in the presence of SnX2 during and after CsSnI3 film formation [61]. Meanwhile, SnX2 will also cause phase separation of halide perovskite [62]. Fortunately, it was found that reductive organic additives can further provide a strong reduction environment for Sn2+, effectively inhibiting the oxidation of Sn2+ and the generation of Sn vacancy. More significantly, organic additives can improve the morphology of perovskite films and promote the performance of devices.
Wang et al. used a coadditive 2–aminopyrazine (APZ) as a secondary addition to the CsSnI3–20% SnF2 perovskite solution to form the APZ–SnF2 complex, with the aim of inhibiting the oxidation of Sn2+, achieving uniform films, and finally improving the CsSnI3-based device performance. Figure 4a illustrates the process for the preparation and morphology of CsSnI3 films with different additives (SnF2 alone and the SnF2–APZ complex). This strategy can efficiently enable the homogeneous spread of SnF2 in perovskite films and promote the ordered crystallization and growth of the perovskite layer. The introduction of APZ also greatly inhibits the oxidation of Sn2+ in CsSnI3 perovskite owing to the electron-donating effect contributed by the amino group in APZ (Figure 4b). More importantly, as shown in Figure 4c, compared to the initial perovskite morphology with cobblestone-like aggregations and many pinholes on the surface, the resulting film with the APZ–SnF2 complex showed a denser and smoother surface. Consequently, optimized CsSnI3 PVSCs that used a printable c–TiO2/m–TiO2/Al2O3/NiOx/carbon structure achieved a PCE of 5.12% [63]. The reductive additive cobaltocene (denoted CoCp2) was also utilized by Wang et al. to modify the CsSnI3 perovskite, as illustrated in Figure 4d, which achieved enhanced performance, suppressed Sn2+ oxidation, and improved film morphology, as shown in Figure 4e [64].
Due to the advantages of reductive organic additives, organic compounds of reductive properties and containing multiple electron–donating groups are applied to CsSnI3 perovskite to inhibit the Sn2+ oxidation to decrease defect density and improve film morphology. Ye et al. introduced N,N’–methylenebis(acrylamide) (MBAA) with the –NH and –CO groups into the B–γ CsSnI3 perovskite layer. The lone electron pairs on the –NH and –CO groups can form a coordination bond with Sn2+, thus causing a reduced defect (Sn4+) density and a smoother and denser morphology, as shown in Figure 4f. Correspondingly, a high PCE of 7.5% was achieved with excellent device stability [65]. Phthalimide (PTM) that has the same chemical groups of –NH and –CO was also utilized by the Ye group into the B–γ CsSnI3 perovskite layer, as illustrated in Figure 4g. PTM could uniformly cover the CsSnI3 perovskite particles. Meanwhile, the strong coordination effect between PTM and perovskite makes it act as an adhesive to bind adjacent perovskite grains and form a compact and smooth crystal perovskite film. Optimized PTM–CsSnI3 PVSCs with a device structure of ITO/PEDOT:PSS/PTM–CsSnI3/indene-C60 bis–adduct (ICBA)/BCP/Ag showed a record PCE of 10.1% with excellent thermal and light stability [29].
In addition to the organic compounds mentioned above, other reductive organic additives such as piperazine (Figure 4h), ascorbic acid, thiosemicarbazide (TSC), and hydrazine [28,66,67,68] with Lewis base groups have also been widely used to improve the quality of perovskite films. Therefore, these encouraging results provide a promising strategy for the preparation of environmentally friendly high–performance CsSnI3 PVSCs.

3.2. Composition Regulation

Unlike additive engineering, component regulation is the optimization of the component elements in each position of the perovskite. It is well–known that lead-based ABX3 perovskite can be partially substituted by other ions to modify the photoelectric performance by enhancing the quality of the perovskite film and the stability of solar cells [69,70,71,72,73,74]. Therefore, these strategies are also used in CsSnI3 perovskite to adjust tolerance factors, optimize film morphology, and reduce defect density.
A site: Marshall et al. systematically studied the effect of Rb+ partial replacement of Cs+ on the CsSnI3 perovskite. The Cs+ ion is located in the center of the ABX3 perovskite cage. If it is replaced by Rb+ of a smaller ionic radius, the crystal will tighten and the lattice will be distorted, which indirectly affects the band gap of the perovskite and the stability of perovskite. With a finite range of Rb+ density, it is found that perovskite is stable enough to be used as the absorption layer when the density of Rb+ is less than 0.2, and when Rb+ exceeds 50%, Cs1-xRbxSnI3 will not form a three–dimensional perovskite structure. As a result, the optimized PVSC based on the Cs1-xRbxSnI3 film showed an increase in Voc, demonstrating that the doping of Rb into B–γ CsSnI3 perovskite is an effective strategy to increase device performance without obviously sacrificing light absorption [75].
B site: Cations at the B site occupy the lattice corner of the ABX3 perovskite and have a significant effect on the electronic structure of the conduction and valence bands. Therefore, partial ion substitution at the B site can also optimize the properties and film quality of perovskite. Germanium (Ge) is in the same group of Sn and has a similar elemental property. Therefore, Ge could be used to replace Sn to improve the performance and stability of CsSnI3 perovskite [76]. Most recently, Chen et al. introduced Ge into CsSnI3 and used the solid solution perovskite as the light absorber. As shown in Figure 5a–b, they found that due to the extremely high oxidation activity of Ge, an ultrathin and uniform primary oxide surface passivating layer formed on the surface of the perovskite, which could inhibit the recombination of interface carriers and enhance the thermodynamic stability of the perovskite. Meanwhile, the perovskite layers prepared by the melt–crystallization method have ultrasmooth surfaces, as illustrated in Figure 5c. Finally, fabricated perovskite solar cells showed a PCE of 7.11%. More importantly, PVSCs also showed excellent stability, maintaining above 90% PCE after 500 h of continuous operation in N2 atmosphere under one-sun illumination [77].
In addition to Ge, doping with other metal elements, such as Sb and Bi doping, are also an effective means to improve the crystallinity and performance of lead-based perovskite solar cells [78,79,80]. Inspired by this, Lee et al. studied the effects of Sb and Bi doping on CsSnI3 perovskite. They found that MI3 (M=Sb, Bi) doping can improve crystallinity and reduce the formation energy of B–γ phase CsSnI3, thus ensuring that the stability of the perovskite can be greatly improved. As a result, the MI3–doped CsSnI3 PVSCs became thermodynamically more stable. B–γ CsSnI3 perovskites doped with 3 mol% SbI3 and BiI3 maintained 96% and 77% of their crystal structure, respectively, after exposure to air with a relative humidity of 45–55% for 12 h [81].
X site: X are mainly halide anions located in the vertices of the [BX6]4+ octahedral, which have an impact not only on the crystal structure of perovskite materials but also on their film quality and photoelectric properties [82,83,84,85]. Therefore, halide anions doping is an effective strategy for regulating the quality of perovskite films and device performance. Sabba et al. modulated the open–circuit voltage and photoelectric properties of CsSnI3 by chemically doping Br to form the CsSnI3-xBrx (0 ≤ x ≤ 3) perovskite. As can be seen in Figure 5d–g, when perovskite is in the form of CsSnI2Br, the original film of CsSnI3 perovskite with multiple pinholes becomes dense and smooth. A further increase in Br doping amount will lead to some island-shaped grains forming on the surface of the perovskite until it becomes rough for CsSnBr3. Therefore, there exists an optimal Br doping, at which the morphology of thin films is significantly improved. Figure 5h shows that the bandgap of CsSnI3-xBrx increased gradually with increasing Br content, which will reduce light absorption but will be beneficial for enhancing Voc. As a result, a Voc enhancement was achieved in CsSnI3-xBrx PVSCs, as the I ions were gradually substituted by Br [86]. Zhao et al. modified the crystal structure of B–γ CsSnI3 perovskite by chemically doping Br to form the CsSnI3-xFx (0 ≤ x ≤ 3) perovskite. Figure 5i shows the macroscopic images of the color evolution in the CsSnI3-xFx films, and it can be observed that the appearance of the perovskite film becomes visually lighter as the I ions are gradually substituted by F. It is found that the evolution of color slows down as the F ratio in the films increases, which indicates that the stability is enhanced [87].

3.3. Deposition Techniques

Homogeneous and dense perovskite thin films are essential for the preparation of highly efficient and stable CsSnI3 PVSCs, and the quality of perovskite thin films is closely related to the deposition techniques. Therefore, the development of advanced and highly reproducible deposition techniques is the key to the preparation of high–performance devices. Currently, the commonly used deposition methods for CsSnI3 PVSCs include the conventional solution technique, evaporation–assisted solution (EAS) technique [88], vacuum flash–assisted solution processing (VASP) technique [89], and pulsed–laser deposition (PLD) technique [90].
Solution techniques fall into two categories: the one–step solution technique and the two–step solution technique [91,92]. Because the latter has a poorer effect on CsSnI3 perovskite, it is rarely used. The one–step solution technique has become the most frequently used deposition technique for the preparation of CsSnI3 perovskite films because of its simple preparation technique, high film–forming reproducibility, and environmental friendliness. The quality of CsSnI3 perovskite films can be improved during spin coating with the help of antisolvent [93]. However, the fabrication of high–quality CsSnI3 perovskite films using the conventional solution technique remains a challenge. Thus, it is significant for the development of new deposition techniques.
In 2016, a new technique of thermally driven solid-state coarsening was applied by Wang et al. to achieve high-quality B–γ CsSnI3 perovskite films [94]. Figure 6a shows the preparation of B–γ–CsSnI3 thin films and the morphology evolution of deposited B-γ-CsSnI3 thin films driven by coarse graining with an increase in the temperature of heat treatment. They dissolved solid CsSnI3 perovskite in a mixture of methoxyacetonitrile, dimethyl formamide, and acetonitrile, then spin-coated on the substrate. After that, different annealing temperatures are set to the as-deposited films to facilitate crystallization. As shown in Figure 6b, with the increase in annealing temperature, the grain size of B–γ CsSnI3 perovskite gradually grew, and the grain boundary decreased. As a result, at a moderate temperature of 150 °C, the CsSnI3 perovskite film with large grain size and extremely smooth morphology were realized, and the optimized device with a structure of ITO/NiOx/CsSnI3/PCBM/Al achieved a PCE of 3.31%. Similar annealing methods for controlling solvent growth were also been used by Ban et al. in 2022. The group proposed an effective two-step thermal annealing method to modulate the dynamic balance between the perovskite crystals’ growth and nucleation for the black B–γ phase CsSnI3 perovskite, as shown in Figure 6c. The well-crystallized and smooth CsSnI3 thin films can be obtained after a two-step temperature annealing process with temperatures of 40 °C and 70 °C, respectively, as shown in Figure 6d. With the further introduction of 1–(4–carboxyphenyl)–2–thiourea to coordinate with surface undercoordinated Sn2+ cations of CsSnI3 thin films, the defect density decreased significantly, and the modified device with a printable c–TiO2/m–TiO2/Al2O3/NiOx/carbon mesoporous framework achieved a PCE of 8.03%. The device showed excellent stability, as it maintained 90% of the initial efficiency after 3000 h of storage in a N2–filled glovebox [95].
The evaporation–assisted solution (EAS) technique provides a new way to prepare high–quality perovskite films. In 2018, Zhu et al. employed the EAS technique, combining thermal evaporation with the solution technique to produce rather uniform, dense, and pinhole–free CsSnI3 films. As shown in Figure 6e, the SnI2 solution with SnF2 was first spin coated on the mesoporous TiO2. After annealing, the resultant film was transferred to a vacuum chamber for the next step of the CsI deposition. After precisely controlling the evaporation rate and the final thickness of the CsI was deposed onto the SnI2–SnF2 layer, the prefabricated film was eventually annealed at 150 °C and formed a black B–γ CsSnI3 perovskite. By comparing the morphology of CsSnI3 perovskite films with different CsI thicknesses, as illustrated in Figure 6f, it is obvious that the quality of the perovskite film has improved remarkably when the thicknesses of CsI are optimized. As a result, the CsSnI3 PVSCs fabricated by the EAS technique delivered a best PCE of 2.23% [88].
The evaporation technique is a better method for the preparation of a large-area perovskite without the bondage of the solution. An effective passivator–assisting sequential vapor deposition (PASVD) strategy was demonstrated by Yin et al. [28], combining the synergistic advantage of vapor deposition and surface passivation, as shown in Figure 6g. The fabricated CsSnI3 perovskite with low densities in the deep trap state was achieved, which was attributed to the smoother compact morphology (Figure 6h) and the low molecular weight organic passivators: thiosemicarbazide (TSC). With this strategy, the CsSnI3 PVSCs with an inverted ‘p–i–n’ structure of ITO/PEDOT:PSS/CsSnI3/C60/BCP/Cu achieved a high PCE of 8.2%, as shown in Figure 6j.

4. Conclusions and Summary

The surface morphology has become the key issue that affects the performance of CsSnI3 PVSCs and is now the subject of the greatest attention. Through the morphology control strategy, the crystallinity of the perovskite film can be effectively improved. Specifically, uniform grains and high surface coverage can be achieved, which is conducive to carrier transport and finally has a positive impact on the efficiency and stability of the solar cells. Herein, the physical properties of CsSnI3 PVSCs were summarized. In addition, we focused on the recent progress in optimizing morphology using various strategies such as additive engineering, compositional regulation, and deposition techniques as shown in Table 1, especially their effects on grain sizes, film uniformity, grain boundary, and defect passivation.
Although there has been much research on improving the morphology of CsSnI3 perovskite, the mechanism of additive engineering and component engineering is still unclear. We emphasize that the oxidation resistance and morphology control of CsSnI3 are two key points for the improvement of device performance. For the practical application of CsSnI3 in solar cells, there is still a lot of work to do for the stability of the B–γ CsSnI3 phase. We hope to find more methods to stabilize and optimize black B–γ CsSnI3 films in future studies and gain a deeper understanding of their mechanism.

Author Contributions

Writing—original draft preparation, Q.Y. and Q.L.; Writing—review and editing, M.Z., Q.Y., Z.F., F.Y., L.Y. and Q.L.; Information collection, Y.W., W.H., Z.C., K.C. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Natural Science Foundation of Zhejiang Province, China (Grant Nos. LQ22F040001), China Postdoctoral Science Foundation (Grant No. 2022M723281), the Zhejiang Province Public Welfare Technology Application Research Project (Grant Nos. LGG20E030003), and the National Natural Science Foundation of China (Grant Nos. 12204313, 51872186).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00410 g001 550
Figure 1. Classification of morphology engineering technologies for CsSnI3 PVSCs.
Figure 1. Classification of morphology engineering technologies for CsSnI3 PVSCs.
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Crystals 13 00410 g002 550
Figure 2. (a) Phase transition diagrams of four CsSnI3 polycrystalline types; coordination environment of Cs atoms at different temperatures in the black phase (b) 500K (B–α), (c) 380K (B–β), and (d) 300K (B–γ); (e) coordination environment of Cs atoms in the yellow phase (Y) [43].
Figure 2. (a) Phase transition diagrams of four CsSnI3 polycrystalline types; coordination environment of Cs atoms at different temperatures in the black phase (b) 500K (B–α), (c) 380K (B–β), and (d) 300K (B–γ); (e) coordination environment of Cs atoms in the yellow phase (Y) [43].
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Figure 3. (a) SEM pictures of CsSnI3 perovskite films with different molar ratios of CsI/SnI2 [57]. (b) Total energy discrepancy between different phases of CsSnI3. (c) Performances of CsSnI3 PVSCs with different additives. The SEM surface pictures of CsSnI3 perovskite films with different additives (d) SnF2, (e) SnCl2, and (f) SnBr2 additives [58]. (g) SEM images of CsSnI3 films prepared under different conditions containing no tin halide additive, 10 mol% SnI2, 10 mol% SnBr2, 10 mol% SnF2, and 10 mol% SnCl2. (h) J–V performance and (i) stability of CsSnI3 PVSCs prepared under different conditions containing without additive, 10 mol% SnI2, 10 mol% SnBr2, 10 mol% SnF2, and 10 mol% SnCl2 [27].
Figure 3. (a) SEM pictures of CsSnI3 perovskite films with different molar ratios of CsI/SnI2 [57]. (b) Total energy discrepancy between different phases of CsSnI3. (c) Performances of CsSnI3 PVSCs with different additives. The SEM surface pictures of CsSnI3 perovskite films with different additives (d) SnF2, (e) SnCl2, and (f) SnBr2 additives [58]. (g) SEM images of CsSnI3 films prepared under different conditions containing no tin halide additive, 10 mol% SnI2, 10 mol% SnBr2, 10 mol% SnF2, and 10 mol% SnCl2. (h) J–V performance and (i) stability of CsSnI3 PVSCs prepared under different conditions containing without additive, 10 mol% SnI2, 10 mol% SnBr2, 10 mol% SnF2, and 10 mol% SnCl2 [27].
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Figure 4. (a) Illustration for the preparation of CsSnI3 films with SnF2 and SnF2–APZ complex. (b) Electrostatic potential surfaces for SnF2 and APZ. (c) Top–view SEM pictures of CsSnI3 films with 0% APZ and 10 mol% APZ additives [63]. (d) Schematic illustration of CoCp2’s redox property and J–V curves of CsSnI3 PVSCs with and without CoCp2. (e) Top–view SEM images of CsSnI3 films without and with different content of CoCp2: 0%, 0.5%, 1%, and 2% [64]. (f) The images show the color changes of the CsSnI3 films with and without MBAA [29]. (g) Surface SEM images and performance of the CsSnI3 and CsSnI3–PTM samples, respectively [66]. (h) SEM images and I–V characteristics of 0.4–CsSnI3 perovskite films prepared with different concentrations of piperazine; scale bars are 2 µm [65].
Figure 4. (a) Illustration for the preparation of CsSnI3 films with SnF2 and SnF2–APZ complex. (b) Electrostatic potential surfaces for SnF2 and APZ. (c) Top–view SEM pictures of CsSnI3 films with 0% APZ and 10 mol% APZ additives [63]. (d) Schematic illustration of CoCp2’s redox property and J–V curves of CsSnI3 PVSCs with and without CoCp2. (e) Top–view SEM images of CsSnI3 films without and with different content of CoCp2: 0%, 0.5%, 1%, and 2% [64]. (f) The images show the color changes of the CsSnI3 films with and without MBAA [29]. (g) Surface SEM images and performance of the CsSnI3 and CsSnI3–PTM samples, respectively [66]. (h) SEM images and I–V characteristics of 0.4–CsSnI3 perovskite films prepared with different concentrations of piperazine; scale bars are 2 µm [65].
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Figure 5. (a) Schematic illustration of the device structure of the CsSn0.5Ge0.5I3 PVSC. (b) Corresponding device energy-level diagram. (c) Schematic illustration showing the deposition of ultrasmooth CsSn0.5Ge0.5I3 perovskite film [77]. (d–g) Top–view SEM images of CsSnI3, CsSnI2Br, CsSnIBr2, and CsSnBr3 displaying the morphological evolution. (h) The band gap variation with Br concentration [86]. (i) Macroscopic images of the color evolution in CsSnI3-xFx films [87].
Figure 5. (a) Schematic illustration of the device structure of the CsSn0.5Ge0.5I3 PVSC. (b) Corresponding device energy-level diagram. (c) Schematic illustration showing the deposition of ultrasmooth CsSn0.5Ge0.5I3 perovskite film [77]. (d–g) Top–view SEM images of CsSnI3, CsSnI2Br, CsSnIBr2, and CsSnBr3 displaying the morphological evolution. (h) The band gap variation with Br concentration [86]. (i) Macroscopic images of the color evolution in CsSnI3-xFx films [87].
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Figure 6. (a) Illustration of one-step solution fabrication and the grain-coarsening driven morphology evolution of as–deposited B–γ–CsSnI3 thin film with increasing heat–treatment temperature. (b) SEM pictures of the CsSnI3 films annealed at different temperatures [94]. (c) Illustration of the two-step temperature annealing process of B–γ–CsSnI3 thin films. (d) Surface SEM images of CsSnI3 films employing different annealing procedures [95]. (e) Schematic representation of the evaporation–assisted solution technique. (f) Top–view SEM pictures of annealed CsSnI3 films with different CsI thicknesses [88]. (g) Schematic representation of the PASVD technique for fabricating CsSnI3 perovskite films. (h,i) SEM pictures of CsSnI3 films without and with TSC addition. (j) The best performance for with and without TSC passivated of CsSnI3 PVSCs [28].
Figure 6. (a) Illustration of one-step solution fabrication and the grain-coarsening driven morphology evolution of as–deposited B–γ–CsSnI3 thin film with increasing heat–treatment temperature. (b) SEM pictures of the CsSnI3 films annealed at different temperatures [94]. (c) Illustration of the two-step temperature annealing process of B–γ–CsSnI3 thin films. (d) Surface SEM images of CsSnI3 films employing different annealing procedures [95]. (e) Schematic representation of the evaporation–assisted solution technique. (f) Top–view SEM pictures of annealed CsSnI3 films with different CsI thicknesses [88]. (g) Schematic representation of the PASVD technique for fabricating CsSnI3 perovskite films. (h,i) SEM pictures of CsSnI3 films without and with TSC addition. (j) The best performance for with and without TSC passivated of CsSnI3 PVSCs [28].
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Table
Table 1. Summary of morphological engineering strategies for CsSnI3 PVSCs.
Table 1. Summary of morphological engineering strategies for CsSnI3 PVSCs.
Morphological Engineering StrategiesDevice PCERef.
Additive Engineering
10 mol% excess SnI22.76%[56]
CsI/SnI2 ratio of 0.44.81%[57]
Perovskite film doped with SnBr24.3%[58]
Perovskite film doped with SnCl23.56%[27]
Perovskite film doped with SnF22.02%[60]
Coadditive 2–aminopyrazine (APZ)5.12%[63]
Cobaltocene (denoted CoCp2)3.0%[64]
N,N′–methylenebis(acrylamide) (MBAA)7.5%[65]
Piperazine3.83%[66]
Thiosemicarbazide (TSC)8.2%[28]
Hydrazine4.81%[68]
Phthalimide (PTM)10.1%[29]
Composition Regulation
CsSn0.5Ge0.5I37.11%[77]
Deposition Techniques
Thermally driven solid–state coarsening technique3.31%[94]
Two–step thermal annealing process8.03%[95]
Evaporation–assisted solution (EAS) technique2.23%[88]
Passivator–assisting sequential vapor deposition (PASVD) technique8.2%[28]
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MDPI and ACS Style

Zhang, M.; Chen, K.; Wei, Y.; Hu, W.; Cai, Z.; Zhu, J.; Ye, Q.; Ye, F.; Fang, Z.; Yang, L.; et al. Strategies for Optimizing the Morphology of CsSnI3 Perovskite Solar Cells. Crystals 2023, 13, 410. https://doi.org/10.3390/cryst13030410

AMA Style

Zhang M, Chen K, Wei Y, Hu W, Cai Z, Zhu J, Ye Q, Ye F, Fang Z, Yang L, et al. Strategies for Optimizing the Morphology of CsSnI3 Perovskite Solar Cells. Crystals. 2023; 13(3):410. https://doi.org/10.3390/cryst13030410

Chicago/Turabian Style

Zhang, Minhao, Kunli Chen, Yunxiao Wei, Wenzheng Hu, Ziyu Cai, Junchi Zhu, Qiufeng Ye, Feng Ye, Zebo Fang, Lifeng Yang, and et al. 2023. "Strategies for Optimizing the Morphology of CsSnI3 Perovskite Solar Cells" Crystals 13, no. 3: 410. https://doi.org/10.3390/cryst13030410

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