1Northwestern Polytechnical University, School of Materials Science and Engineering, Center for Nano Energy Materials, State Key Laboratory of Solidification Processing, Xi’an, China
2Northwestern Polytechnical University, Chongqing Innovation Center, Chongqing, China
3Northwestern Polytechnical University, School of Microelectronics, Xi’an, China
Feng Yang, Yu Tong, Kun Wang, Yali Chen, Ziyong Kang, Hongqiang Wang, "Recent advances in tin perovskites and their applications," Adv. Photon. 7, 034003 (2025)
Copy Citation Text
【AIGC One Sentence Reading】:Tin perovskites (TiPes) offer reduced toxicity & unique properties, showing promise in solar cells & other applications, with challenges addressed for future development.
【AIGC Short Abstract】:Tin perovskites (TiPes) offer a less toxic alternative to lead perovskites, boasting favorable optical and electronic properties. Recent advances in TiPes cover crystal structure, optoelectronic properties, fabrication, and diverse applications. Despite challenges, potential solutions are proposed, paving the way for their future development in high-performance solar cells and beyond.
Note: This section is automatically generated by AI . The website and platform operators shall not be liable for any commercial or legal consequences arising from your use of AI generated content on this website. Please be aware of this.
Abstract
Lead halide perovskites have started a new era for solar cells. However, the toxicity of lead poses a challenge for their practical applications. Replacing lead with tin provides a feasible way to reduce the toxicity of lead perovskites and can further promote the applications of perovskites. Due to their reduced toxicity with advantageous optical and electronic properties compared to their lead counterparts, tin (II) perovskites (TiPes) have attracted significant interest in recent years, not only for pursuing high-performance solar cells but also in other application fields. We aim to provide a comprehensive overview of recent advances in TiPes, covering fundamental crystal structure, optoelectronic properties, fabrication methods, and applications. A detailed comparison with lead perovskites is included, emphasizing TiPes’ unique strengths while presenting their application challenges. Finally, potential solutions to the challenges are proposed, along with a vision for their future development and potential.
Metal halide perovskites have emerged as a class of semiconductor materials suitable for various electronic or optoelectronic applications.1–4 In the field of photovoltaics, perovskite solar cells (PSCs) have witnessed a rapid increase in power conversion efficiency (PCE) from 3.8% to over 26% in the past decade.5–7 Owing to the easy processibility and exceptional carrier properties of perovskites, their applications have also been extended to light-emitting diodes (LEDs), photodetectors (PDs), lasers, and even transistors.8–11 However, concerns remain within the scientific community regarding the practical application prospects of perovskites due to the high toxicity of water-soluble lead salts in their degradation products.12,13 Although physical or chemical encapsulation provides the feasibility to decrease lead leakage from perovskites to the environment, challenges still remain in the ultimate elimination of lead release. Substituting Pb with low-toxicity elements, such as tin (Sn), germanium (Ge), copper (Cu), or manganese (Mn), could be an alternative way to solve this problem. Among the low-toxicity elements, Sn, with its similar electronic configuration and ionic radius to Pb, is considered one of the most promising substitutes for lead counterparts.14–17
Tin (II) perovskites (TiPes) possess unique properties that make them attractive candidates for a wide range of applications. For instance, bulk TiPes feature a narrower bandgap compared with their lead counterparts, thus facilitating a broader spectrum coverage of the whole ultraviolet-visible-near infrared range.18,19 Furthermore, TiPes offer potential properties, such as a long hot carrier lifetime, which provides the possibility of breaking the Shockley–Queisser limit for solar cells.20–22 The rise of TiPes stems from the need to replace toxic lead in perovskite solar cells. After development in the past decade, tin PSCs have achieved PCE exceeding 15%, representing the most advanced heavy metal-free solar cells to date.23 Alongside these breakthroughs, research on TiPes has expanded into other applications traditionally dominated by lead perovskites, such as LEDs, PDs, lasers, and transistors.24–27 The rapid advancement of TiPes is largely attributed to the fundamental research on lead perovskites, with many key achievements built upon the latter’s successes. However, most existing review articles predominantly highlight the potential application advantages of TiPes in specific areas, often neglecting a thorough, one-to-one comparative analysis of the structure, property, and application differences between tin and lead perovskites.28–31
This review provides a comprehensive summary of TiPes, highlighting their properties and recent advancements across various applications. It covers the crystal structure and optoelectronic properties of TiPes, methods for preparing thin films, single crystals, and nanocrystals, as well as their current applications in solar cells, LEDs, PDs, lasers, and transistors. In addition, a detailed comparison with lead perovskites is presented, clarifying the strengths and weaknesses of TiPes and offering clear guidance for material selection in specific application scenarios. Finally, this review discusses the key challenges faced by TiPes, proposes potential solutions, and envisions their future development trajectory.
Sign up for Advanced Photonics TOC. Get the latest issue of Advanced Photonics delivered right to you!Sign up now
2 Structure and Properties
Perovskites are a class of compounds with chemical formula, where represents monovalent cations, primarily includes Pb or Sn ions, and stands for halogen anions.32 The perovskite structure, as depicted in Fig. 1(a), consists of a three-dimensional (3D) inorganic network created by co-angled octahedra, with -site cations occupying the octahedral voids. Lead perovskites derive their excellent physical properties from the lone pair of electrons of . Being the same group element, Sn shows similar electron arrangements and properties to Pb, and due to its low biological toxicity, it is considered a promising substitute for Pb.35 Although TiPes exhibit structural and property similarities to their lead counterparts, they also possess distinctive performance characteristics, attracting considerable interest from researchers.36 To provide a clearer comparison, we have compiled a summary table highlighting the differences between tin and lead perovskites in terms of optoelectronic properties and room-temperature (RT) crystal structures (Table 1). TiPes have garnered significant attention due to their higher light absorption coefficients and broader bandgap tunability, in addition to their low toxicity, which is a key driver for research interest.17 The table provides an overview of these fundamental properties, which will be discussed further in detail.
Figure 1.Crystal structure of perovskites. (a) Crystal structure of perovskites. (b) Tolerance factors () of a series of halide perovskites. (c) Schematic diagram of the temperature-induced phase transition of . Reproduced with permission from Ref. 33. (d) XRD patterns of during two sequential compression−decompression cycles. Reproduced with permission from Ref. 34.
Crystal structure stability is the key to the performance of TiPes devices, and the well-known Goldschmidt tolerance factor () is usually used to roughly estimate the stability of the perovskite phase structures, as illustrated in Eq. (1).48where , , and represent the ionic radii of the , , and ions, respectively. Empirical data reveal that maintaining values between 0.8 and 1 preserves the 3D perovskite structure, with the ideal cubic structure achieved between 0.9 and 1. Deviation from this range can result in phase transitions or lower-dimensional structural transformations. Among the currently spotlighted TiPes, the -site of the formula is often occupied by spacer organic cations such as methylamine ions [, Å] or formamidine ions [, Å]. Inorganic cations are typically introduced into octahedral voids to replace organic cations and enhance thermal stability.49–51 The halogen ions that occupy the -site anion are usually , , and . Furthermore, given the extensive use of for tin compensation, it is important to examine the impact of on the TiPes crystal structure. For instance, Pascual et al. showed that fluoride plays a crucial role in the reduction of . selectively forms complexes with , resulting in , and the likelihood of introducing complexes into perovskite lattice is lower compared to .52 Although substituting Pb with Sn appears to increase and should theoretically promote the formation of a stable cubic structure [Fig. 1(b)], poor stability is generally observed in the cubic TiPes. It is essential to note that the -value alone is a necessary but not sufficient condition for determining the stability of the perovskite structure. Other non-geometric factors, such as bond valence and chemical stability, are also crucial.53,54
As is known, perovskites could undergo structural and symmetry changes with temperature, and high temperatures typically favor an improved cubic symmetry. For TiPes, their phase transition from cubic to tetragonal, orthorhombic, or further to rhombohedral or monoclinic, usually occurs when the temperature is decreased. A few soft-lattice TiPes experience more intricate transitions. For example, undergoes a continuous phase transition from cubic (), through rhombohedral () and monoclinic (), to triclinic () in a narrow temperature range near 300 K.55 Temperature-induced phase transition behavior of in a single crystal with a size of about 0.1 mm was investigated by Kahmann et al.33 As shown in Fig. 1(c), has a cubic () crystal structure at room temperature, where FA molecules are free to rotate and in a completely disordered state. Upon cooling, FA molecules are gradually oriented in the ab plane and the crystal undergoes a phase transition to a tetragonal () system at 255 K. With further temperature reduction, FA molecules become fully ordered, resulting in a transition to space group at 155 K. However, this is partially inconsistent with the earlier report on single crystals by Stoumpos et al., in which a transition of from an orthorhombic (Amm2) crystal system at room temperature to a low-symmetry Imm2 space group at 180 K was demonstrated.56 Besides single crystal, a phase transition from a tetragonal (P4/mbm) to an orthorhombic (Pnma) crystal system at 100 K has been reported.57 This difference in crystal structure may be inextricably linked to defects and the size of the crystal.33 Besides temperature, pressure also serves as an external factor that governs the crystal structure and symmetry. Figure 1(d) illustrates the XRD patterns of during successive compression and decompression. For , the tilt of the octahedra gradually increases with pressure, promoting the crystal structure shift from tetragonal to orthorhombic. As pressure exceeds 3 GPa, shows a disordered crystal structure and transforms into the amorphous phase. Remarkably, the crystalline properties of are preserved even above 30 GPa during secondary compression, showcasing a significant enhancement in structural stability.34 This stability improvement is ideal for application in optoelectronic devices, but the introduction of exogenous stresses is difficult. Fortunately, this can be alternatively realized by external doping, which introduces stresses, thus improving the stability.58,59
In recent years, the attractive physical properties unlocked by nanoscale effects have fueled a rapid surge in research aimed at reducing the size and dimension of perovskites [Fig. 2(a)].60 Perovskites with various sizes and dimensions, including microcrystals, nanosheets, nanowires, nanocrystals, and quantum dots, have been effectively synthesized.61–64 Usually, “smaller” TiPes tend to maintain the structural framework and retain the crystal structure of the parent bulk material.65 For example, in recent work, it was verified that nanocrystals with a lateral size of 10 nm possess the same stable -orthorhombic phase as the parent bulk.66 Based on the framework, the introduction of long-chain ammonium cations [typical examples shown in Fig. 2(b)] leads to a reduction in the dimensionality of TiPes, e.g., down to two or quasi-two dimensions (2D or quasi-2D). Their typical structural formula is (RP phase) or (DJ phase), with representing the introduced long-chain cation, and indicating the number of octahedral layers.67 Organic cations with different chain lengths lead to partial distortion of the inorganic octahedra, reducing the symmetry of the TiPes crystal structure and also affecting the TiPes crystal orientation, size, and growth distribution.68 Low-dimensional TiPes have garnered significant attention for their exceptional stability, primarily attributed to the excellent moisture resistance offered by the organic intercalation layer. Moreover, this intercalation layer serves as a barrier, to effectively suppress ionic migration and further enhance the stability. In addition, the quantum well structure and energy funneling effect endow them with potentially good energy transfer capability and high radiative recombination efficiency, which could be beneficial for photovoltaic and optoelectronic applications.69 Nevertheless, the out-of-plane charge transfer in these materials is constrained, necessitating precise adjustments in their composition and crystalline orientation for practical applications.
Figure 2.TiPes with different dimensions. (a) Schematic diagram illustrating the reduction in size and dimensionality of TiPes. (b) Typical examples of ammonium cations used to reduce the dimensionality of TiPes.
For perovskites, under external excitation (light, voltage, radiation, etc.), electrons in the valence band absorb energy () and are excited to the conduction band, yielding free charge-carriers or excitons.70 Some of the carriers are excited to energy levels higher than the valence band maximum (VBM) or conduction band minimum (CBM), forming hot carriers. These hot carriers rapidly lose energy and relax back to the VBM or CBM through coupling with lattice vibrations (phonons). Four stages are usually involved in this process, i.e., hot carrier generation, thermalization (reaching thermal equilibrium through interactions and energy exchange), dephasing (eliminating coherence between ground and excited states), and cooling (interaction with the lattice, releasing heat) [Fig. 3(a)].72,73 If the hot carriers can be extracted for application before they relax to the band edge, it would overcome the limit and bring a revolutionary breakthrough for photovoltaic technology.20,73,74
Figure 3.Charge carrier and exciton properties. Schematic diagram of (a) carrier generation (including hot carrier generation) and (b) carrier recombination. (c)–(e) Energy-dependent and time-dependent photoluminescence spectra of perovskite thin films at low and high excitation densities for , , and , showing that the thin films exhibit a much stronger band-filling effect and prolonged hot carrier emission. Reproduced with permission from Ref. 21. (f) Dark conductivity spectra (real part). (g) OPTPS measurements of the charge-carrier recombination dynamics of thin films treated with . Reproduced with permission from Ref. 71.
Fang et al. discovered the photoluminescence of long-lived hot carriers (a few ns) in and observed a large blue shift with increasing excitation energy in the time-integrated photoluminescence energy spectrum.21 They attributed this phenomenon to the slow hot carrier relaxation and strong dynamic band-filling effects. Notably, the hot carrier lifetime in is longer than that of lead counterparts, highlighting its potential for hot carrier applications in TiPes [Figs. 3(c)–3(e)].21,75 In addition, Dai et al. reported that for small-sized nanocrystals with quantum confinement, the high-energy states are unaffected by the fast relaxation and exhibit a separated energy level structure. At low injected carrier densities ( carrier pair per nanoparticle), the relaxation of hot carriers in nanocrystals is slowed down by two orders of magnitude, and the hot carriers are preserved up to .22 Moreover, studies have shown that doping tin into lead perovskite nanocrystals significantly retards the hot carrier relaxation process, which can be primarily attributed to several aspects as follows. First, the introduction of tin creates a new high-energy band structure that alters energy relaxation pathways and effectively suppresses rapid cooling. Second, tin doping weakens the coupling between carriers and optical phonons, reducing the efficiency of energy transfer to the lattice. Third, it decreases phonon-phonon scattering (specifically, the decay of optical phonons into acoustic phonons, known as Klemens decay), further slowing the lattice’s energy absorption. Fourth, tin doping lowers the lattice thermal conductivity, particularly the transport efficiency of acoustic phonons, thereby reducing the dissipation of thermal energy. These synergistic effects significantly slow down the hot carrier relaxation in tin-doped lead nanocrystals, further highlighting the unique potential of tin perovskites for hot carrier-related applications.76
Bulk perovskites are known for their relatively small exciton binding energy. Similarly, TiPes exhibit comparable or even lower exciton binding energies. For example, it has been reported that the exciton binding energies for the , , and are , , and , respectively. This means that excitons in TiPes are more likely to dissociate into free carriers, which is beneficial for photovoltaics and photodetection applications.71,77 Moreover, TiPes are theoretically predicted to exhibit superior electrical properties compared to their lead counterparts. For instance, demonstrates intrinsic electron and hole mobilities exceeding 2000 and , respectively, which are significantly higher than those of .78,79 However, the spontaneous -type doping commonly observed in TiPes negatively impacts carrier performance, resulting in the actual mobility of TiPes being lower than that of their lead counterparts (Table 1). Theoretical and experimental studies show that in , when the -doping density is , the carrier diffusion length can exceed ; when the -doping density increases to , the carrier diffusion length rapidly decreases to .80
The exciton binding energy of 3D TiPes is small, and the excitons can dissociate rapidly into free carriers at room temperature, which dominates the carrier recombination process.81 In an ideal case with other factors excluded, all carriers are expected to undergo radiative recombination by emitting photons. However, in the practical situation, there are many non-radiative recombination channels in perovskites, and most carriers usually take the priority to recombine via non-radiative processes, such as defect-assisted recombination, interface-induced recombination, Auger recombination, and band-tailed recombination [Fig. 3(b)].72 Because the radiative recombination is responsible for the luminescence performance, the photoluminescence quantum yields (PLQYs) of 3D perovskites are typically very low.37 From the lesson of lead perovskites, researchers have used dimensionality reduction or crystal size reduction to excite quantum and/or dielectric confinement effects in TiPes, which in turn increase the carrier density and exciton binding energy to induce high PLQYs.82–84 Compared with other recombination processes, Auger recombination is usually weaker and has less impact on the device performance. The other three non-radiative pathways can be grouped into one category, namely single molecule relaxation. The rate constant of single-molecule complexation in TiPes is reported to be three orders of magnitude higher than that of lead counterparts, which stems from the instability and severe -type self-doping of TiPes.36 This can be one of the reasons for the inferior performance of TiPes devices compared with the lead perovskite devices. Small amounts of introduction typically contribute significantly to complementing and passivating defects.52,85,86 For instance, a decrease in dark current and a slowdown in photoconductivity decay are observed with the addition of small amounts of to films, indicating a reduced doped hole concentration and an extended carrier lifetime [Figs. 3(f) and 3(g)].71 However, a particular note of caution is that the over-doping of should be avoided because it may introduce additional non-radiative recombination channels.87 In recent years, many attempts have been made to improve the stability and reduce the doping level of TiPes, such as using reductive agents, introducing low-dimensional components, and adding passivation, which are detailed in the subsequent sections.88–90
2.3 Band Structure and Optical Properties
In general, TiPes exhibit a similar energy band structure to lead perovskites. For example, in , the VBM is contributed by the antibonding orbitals of the Sn orbital and the I orbital, dominated by I ; and the CBM is composed of the antibonding orbitals of the Sn orbital and the I orbital, dominated by Sn [Fig. 4(a)].91,96 Various theoretical models have been applied to calculate the band gap of perovskites, and introducing spin-orbit coupling (SOC) effects into an efficient GW (G stands for Green’s function, and W denotes the dynamically screened Coulomb interaction) scheme achieves greater success in deriving the band gap of TiPes.97–101 Angelis et al. used this approach to calculate the electronic density of states (DOS) of [Fig. 4(b)] and compared it with the lead perovskites. The substitution of Pb with Sn induces an upward shift of the atomic energy level, implying an upward shift of both VBM and CBM. However, the smaller splitting of the and states in the Sn atom makes the VBM more upshifted than the CBM, leading to a narrower band gap of than .65,92,93 The energy band structure of calculated by SOC-GW is shown in Fig. 4(c), indicating that it has a band gap of in the Brillouin zone, which is close to the experimental value (). Similar to lead perovskites, the band gap of TiPes can be tuned directly by varying the -site and -site ions.102 Tao et al. established the band gap of the extensively studied tin and lead perovskites, as shown in Fig. 4(d).93 The effect of the halogen elements on the band gap is predominant, with the band gap increasing substantially when the -site changes from I to Br and to Cl. The primary reason for this phenomenon is the downward shift of the level in the halide, which results in a significant downward shift of the VBM. The octahedral tilt and distortion caused by the -site ion is a secondary factor affecting the band gap, and this factor has a greater impact on TiPes than lead perovskites due to the smaller -site ion radius. In TiPes with lattice structures showing large deviations from the ideal cubic one (, , and ), the -site can impose an even larger influence on the band gap than the -site substitution.103 With these features, TiPes usually exhibit a narrower bandgap and broader bandgap tuning range in comparison with their lead counterparts, which makes it even more advantageous in meeting the ideal bandgap requirements for single-junction/multi-junction photovoltaic devices and tuning the emission colors of LEDs.
Figure 4.Band structure and optical properties. (a) Energy band of and . Reproduced with permission from Ref. 91. Based on SOC-GW calculation: (b) electronic DOS of and (c) energy band structure of and . Reproduced with permission from Ref. 92. (d) Schematic energy level diagram of tin and lead perovskites. Reproduced with permission from Ref. 93. (e) Optical absorption and steady-state PL spectra of . Reproduced with permission from Ref. 94. (f) Optical absorption spectra and (g) PL emission spectra of TiPes [ represents the percentage of substituted by ethylenediamine ions (en)]. Reproduced with permission from Ref. 95.
As is well known in perovskites, halogen engineering is considered an effective option for tuning the band gap and optical properties, which is typically achieved by changing the precursor halide ratio or by anion exchange reactions. Figure 4(e) shows the PL emission of (, , Br, , I), covering the whole visible to the near-infrared (NIR) range.94 Compared with lead counterparts, TiPes have greater potential for applications in the NIR region (e.g., lasers and PDs).104,105 However, the PL intensity is extremely low and the stability obtained is unsatisfactory. Spanopoulos et al. substituted with ethylenediamine cations to trigger Sn and I vacancies without changing the structure to obtain 3D hollow TiPes.95 At room temperature, all hollow perovskites show good absorption of visible light with intensive PL emission, and the emission wavelength can be tuned from 991 to 718 nm [Figs. 4(f) and 4(g)]. The introduction of Sn and I vacancies leads to the formation of spatially confined domains in the perovskite structure, which in turn leads to lattice distortion and the generation of stable excitons, thus enhancing the PL.106 In addition, the introduction of the element significantly improves the stability, and the resultant 3D hollow TiPes can stay stable in the air for at least 9 days. Apart from the doping modification of the native, reducing the dimensionality is also an effective approach to enhance the stability and optical properties.17 For example, 2D-layered perovskite (OCTAm for octylammonium cation) shows bright orange emission at 600 nm with a near-unit value of absolute. Notably, the PLQYs of this perovskite can remain unchanged after 240 days of storage at room temperature under ambient air and humidity conditions.107 This 2D structure provides spatial confinement for electrons and holes, leading to strong coulombic interactions, thus increasing the probability of radiative recombination. Moreover, the organic cations provide a barrier for hindering the oxygen and moisture invasion, thereby improving the stability.108 Besides, the lower-dimensional structure of TiPes also exhibits luminescence enhancement due to self-trapped exciton emission.81,82 It is also worthy to highlight the advantages of single-halogen TiPes in the field of pure red luminescence, with PL emission peak located in the range of 630 to 640 nm.83,109 Although higher external quantum efficiency (EQE) red LEDs have been reported in lead perovskites, they generally suffer from color impurity and phase separation.110,111 However, TiPes LEDs can avoid these issues and yield color-pure and stable red emission.
2.4 Environmental and Operational Stability
The stability of perovskites is influenced by both intrinsic and extrinsic factors. Intrinsic factors primarily involve ion migration within the perovskite, whereas extrinsic factors include the combined effects of environmental conditions such as water, oxygen, and temperature. Halogen ion migration is a critical factor impacting the stability of lead perovskites, whereas this issue is significantly suppressed in TiPes, which can be attributed to the unique defect environment of TiPes, i.e., tin vacancies.35 Theoretically, TiPes are expected to exhibit superior stability; however, extrinsic factors have a significantly detrimental effect on TiPes, especially in the presence of oxygen.112 For Pb, the insufficient shielding of the nuclear charge by the electrons induces a lanthanide contraction, resulting in a smaller atomic radius and stronger binding of the electrons [Fig. 5(a)]. By contrast, Sn, with a higher atomic energy level and lower electronegativity, has a smaller energy gap between the and orbitals and lacks the lanthanide contraction effect as Pb has.113 As a result, Sn’s outer electrons escape more easily, leading to an increased tendency for oxidative decomposition.31 As the humidity or temperature increases, the decomposition trend of perovskite is exacerbated, a phenomenon that is also observed in lead perovskites.113Figure 5(b) illustrates the degradation pathway of a typical TiPes device, which occurs in three main stages: precursor, perovskite, and device. Consequently, stabilization strategies are applied at each of these three stages.
Figure 5.Environmental and operational stability. (a) Electronic structure of Sn and Pb atoms. (b) Schematic of typical TiPes device degradation. Reproduced with permission from Ref. 113. (c) Schematic diagram of vanillin reduction of TiPes. Reproduced with permission from Ref. 114. (d) Schematic diagram of 2D DJ-phase TiPes. Reproduced with permission from Ref. 115. (e) Schematic of the encapsulation of TiPes with poly(methyl methacrylate) and . Reproduced with permission from Ref. 116.
For precursors, several stabilization strategies have been developed. These include tin compensation strategies, such as adding to offset the loss of due to oxidation; tin reduction strategies, such as introducing reductants to reduce [Fig. 5(c)]; ligand incorporation, for example, using organic ammonium salts to coordinate with ; and solvent substitution strategies, where the commonly used DMSO—known to oxidize —is replaced with alternatives such as -diethylformamide (DEF) and -dimethylpropylene urea (DMPU).52,96,114,117 By contrast, lead precursors typically do not require oxidation-focused stabilization strategies. Instead, their stabilization primarily relies on ligand engineering, with Lewis bases coordinating with to enhance precursor stability.118 Although this approach resembles the ligand incorporation strategy used for tin precursors, important differences exist. , as a soft Lewis acid, preferentially coordinates with soft bases, such as sulfur- or nitrogen-containing ligands. On the contrary, , which lies between soft and hard Lewis acids, can coordinate with both soft bases and moderately hard bases, such as oxygen-containing ligands, to form stable coordination complexes.119 These differences in chemical behavior highlight the distinct requirements for stabilizing tin and lead precursors.
Component engineering is an important tool for stabilizing perovskite. In TiPes, introducing large ammonium ions into the -site is a commonly used and effective strategy to enhance material stability.120 As mentioned earlier, the introduction of ammonium cations with different valences results in the formation of distinct phases: the +1 valence ammonium cations lead to the RP phase, whereas the +2 valence cations form the DJ phase. Compared with RP-phase perovskites, DJ-phase generally exhibits improved stability. Park et al. incorporated diammonium organic spacers, 3-(aminomethyl)piperidinium iodide () and 4-(aminomethyl)piperidinium iodide (), into TiPes, resulting in the formation of 2D DJ-phase perovskites, and [Fig. 5(d)].115 Specifically, and retain their crystal structures after 24 h of air exposure. By contrast, the 2D RP-phase undergoes oxidation, as evidenced by the formation of a phase. The DJ-phase perovskite exhibits excellent stability; however, the anchoring of diammonium cations to the perovskite slabs increases the rigidity of the structure, which may lead to significant structural distortion, thereby affecting the performance of the perovskite devices.81 In addition to ammonium cations, the incorporation of also contributes to enhancing the thermal stability of perovskites. Particularly in the field of lead PSCs, efficient and stable perovskite formulations often rely on a combination of and .121,122 However, this strategy is more challenging to apply in TiPes because is not resistant to oxidation and can be rapidly converted to the 1D orthorhombic phase in a humid environment.123 Nevertheless, studying remains valuable, as it has the potential to contribute to improved thermal stability.
Proper device structure design is also an effective means to improve stability. Unlike lead perovskites, variable-valence metal oxides are not suitable as transport layer materials in tin perovskite devices, as these may induce the oxidation of .81 In general, organic transport materials exhibit better compatibility with TiPes. Furthermore, the interface between TiPes and transport layers requires careful modulation. For instance, introducing FACl can remove surface doping, whereas evaporating a thin Sn layer can reduce interfacial .124,125 Interface engineering significantly benefits the suppression of tin oxidation and isolation from water and oxygen; however, research in this area for TiPes remains insufficient. In addition, drawing on the experience of encapsulating lead perovskites can also boost tin device stability. For example, Gahlot et al. achieved long-term stability in air for 15 days by encapsulating TiPes with a combination of the long-chain organic material poly(methyl methacrylate) and the inorganic material [Fig. 5(e)].116
Beyond oxygen, moisture ingress poses another critical threat to perovskite stability. Water infiltration induces crystal lattice distortion, surface state degradation, defect proliferation, and material decomposition, severely compromising device efficiency and stability.126 Conventional encapsulation strategies (e.g., polymer or inorganic oxide coatings) could block moisture but may also hinder charge carrier transport due to insulating layers.127,128 Developing a suitable ligand engineering strategy can help solve this problem, in which Rao’s team suggests two principles for ligand design: (i) designing suitable carbon chain length to balance the need for hydrophobicity and charge transport and (ii) designing suitable terminal groups for robust binding to the perovskite surface.129–131 Taking (11-mercaptoundecyl)--trimethylammonium bromide (MUTAB) as an example, this ligand modifies oleylamine/oleic acid-capped nanocrystals via a ligand exchange strategy.132 One end of the molecule passivates surface defects via strong Pb–S bonds, whereas the other end repels water molecules through steric hindrance, achieving dual-functional protection. nanocrystals based on this strategy can maintain stability in water for over 60 days while retaining excellent charge generation and transport efficiency. This strategy provides a paradigm for water stability research in lead-based perovskites. Although TiPes are still in the early stages of water stability research due to the easily oxidizable nature of , the ligand engineering strategy can draw on the experience from lead-based systems, pointing the way toward the development of stable lead-free perovskite devices.
2.5 Toxicity and Sustainability
As research on metal halide perovskite advances, concerns have been raised regarding their potential toxicity. Babayigit et al. systematically investigated the environmental impacts of TiPes and their lead counterparts, identifying and as their primary degradation products, respectively.133 To assess the toxicity of these degradation products, the researchers introduced equal amounts of and into the aquatic environment of zebrafish Danio rerio embryos, regularly monitoring the embryos for lethality and defective phenotypes (collectively referred to as malformations). The results showed that both the lethal and teratogenic concentrations of could show higher toxicity risk. Importantly, the study proposed a unique toxicity mechanism distinct from conventional heavy metal lead poisoning. The toxicity of primarily arises from its degradation process: in the presence of and , breaks down into , accompanied by the formation of the intermediate product HI. The release of HI rapidly acidifies the aquatic environment by lowering the pH of the growth medium, thereby inducing harmful effects. The proposed reaction for in aqueous medium is
This mechanism provides valuable insights into the environmental risks posed by TiPes and highlights the need for further investigation into their stability and safety.
However, this experimental result is based on the assumption that is the primary degradation product of TiPes. In reality, unlike , is highly susceptible to oxidation due to the absence of the lanthanide contraction effect. In an oxygen-rich environment, TiPes preferentially undergo the following reactions.134,135 can further react with AI, allowing it to be consumed:
Owing to this competitive response mechanism between Eqs. (3) and (5), the actual generation of HI is greatly reduced. Concerning HI is also produced during the degradation of lead perovskites, the induced impact difference of HI between TiPes and lead perovskites is decreased as well. Notably, the significant impact is the heavy metal issue with extreme toxicity in lead perovskite, regardless of its valence state. Moreover, Pb can persist in soil and water, accumulating over time and causing irreversible harm to biological systems.15,136 Therefore, TiPes is considered to be one of the good substitutes for lead perovskites given their heavy metal-free nature. It is worth mentioning that despite TiPes exhibiting lower toxicity compared with their lead counterparts, their environmental impact still needs to be further studied.
Beyond toxicity, cost considerations are also crucial for practical applications. In the case of lead PSCs, for example, their high light absorption coefficient allows them to achieve high light absorption at a relatively thin film thickness, thus reducing the amount of Pb required per square meter of module to less than 1.4 g. If 1 terawatt of photovoltaic modules is deployed, it is estimated that only of Pb will be needed. Given the abundant Pb reserves and mature recycling technologies, material costs can be significantly minimized.137 In comparison, TiPes have a higher light absorption coefficient and require a lower theoretical amount than lead.38 Although global tin reserves are lower than those of lead, they are still sufficient to meet industrial demand. Compared with its reserves, the greater concern lies in the high manufacturing cost of TiPes, primarily due to Sn’s susceptibility to oxidation. With the advancements in preparation processes and antioxidant technologies, the cost disadvantage of TiPes is expected to gradually diminish, rendering TiPes more competitive in future sustainable development.
3 Fabrication Methods
Generally, fabricating high-quality perovskite is a prerequisite for achieving high-performance devices, which means that the optimization of the TiPes preparation method is very important. This section provides an overview of the various types of approaches for preparing TiPes with different morphologies, including polycrystalline films, single crystals, and nanocrystals.
3.1 Polycrystalline Thin Films
TiPes films are typically prepared by a one-step deposition method or a sequential deposition method from their solution with precursor salts (e.g., , FAX, MAX, CsX) dissolving in polar aprotic solvents. The deposition of the solution on the substrate is generally assisted by a spin-coating process. The one-step deposition method for TiPes films usually suffers from rapid crystallization due to the higher Lewis acidity of than , resulting in poor film quality with large amounts of pinholes and defects. Therefore, more factors need to be considered for TiPes film preparation. Hao et al. found that using dimethyl sulfoxide (DMSO) in the one-step deposition of films could create a intermediate phase, promoting uniform nucleation and slowing down the film growth [Fig. 6(a)].138 However, DMSO’s low evaporation rate and high viscosity make it challenging to obtain high-quality perovskite films via spin coating. It is often used in combination with more volatile -dimethylformamide (DMF) to improve the film quality.140,141 Besides, an anti-solvent, such as chlorobenzene (CB), toluene (TL), and diethyl ether (DE), is generally used to rapidly initiate nucleation and induce the formation of uniform, smooth, and dense perovskite films,142–144 among which, CB is one of the most widely used anti-solvents in improving the film quality.140 Considering the high toxicity of CB, the use of green anti-solvent and the development of anti-solvent-free processes are widely appreciated. He et al. developed an innovative cooling procedure combining thermal convection and spontaneous nucleation to prepare inverted PSCs with a PCE of 8.9% without anti-solvent.61 Zhang et al. substituted CB with green diethyl carbonate (DEC) in a one-step preparation process to obtain thin films. By controlling crystal orientation and suppressing defects, the film quality was enhanced, resulting in solar cells with a champion PCE of 14.2%.145 Unlike the one-step method, sequential deposition typically involves two steps deposition of and AX without the assistance of anti-solvents.146 However, the rapid reaction between the two precursors (e.g., and FAI), coupled with the high solubility of in the conventional solvent isopropanol, leads to poor film quality. Zhou et al. addressed this issue by using reducing formic acid as a solvent for the organic ammonium salts. This approach not only mitigated the erosion of but also slowed down the crystallization rate through hydrogen bonding between formic acid and FAI, enabling the successful preparation of high-quality TiPes films. In addition, by leveraging the reducing properties of dithiothreitol, the PCE of the perovskite device was increased to 12.68%.147 Such progress in solvent and additive engineering provides promising opportunities for the broader adoption of the sequential deposition method.146,148–150
Figure 6.Fabrication of polycrystalline TiPes films. (a) Schematic diagram of one-step deposition method for film preparation. Reproduced with permission from Ref. 138. (b) Schematic diagram of film preparation based on ion exchange/insertion reaction. Reproduced with permission from Ref. 139.
Vapor phase deposition complements the conventional spin coating process. Compared with the solution processing methods, the perovskite films obtained by vapor deposition usually possess smooth surfaces, high coverage, and good reproducibility independent of the operator.151,152 For example, Yokoyama et al. deposited MAI on substrates using a low-temperature vapor-assisted solution process (LT-VASP) to obtain films with extremely high coverage and good uniformity; such films overcome the short-circuiting behavior seen in conventional solution methods.153 Alternatively, depositing on the FAI substrate generates films, and the introduction of PEAI into this system further enhances overall film quality.152 Wang et al. deposited MAI on a substrate pre-spin-coated with /poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and obtained thin films with the aid of an ion exchange/insertion method [Fig. 6(b)].139 The films were homogeneous and dense without pinholes, and a high PCE of 7.78% was obtained when applied to planar PSCs. The vapor phase deposition possesses a large degree of freedom in operation, allowing free choice between procedures such as simultaneous evaporation of precursors, spin coating followed by evaporation, and direct evaporation of perovskite. Therefore, currently, it has been widely used in the field of high-quality TiPes film preparation.154–156
Recently, scalable production processes such as blade coating, spraying coating, inkjet printing, screen printing, and electrodeposition have been found to be highly productive in controlling large-area film formation of perovskites.157,158 For example, the maximum PCE of lead PSCs obtained by spraying or scratch coating can reach 19.4% and 23.19%, respectively, demonstrating the potential for scalable production.159,160 The research progress of TiPes in related fields has been relatively slow due to the challenges in controlling the crystallization rate. Zuraw et al. pioneered the application of the blade coating technique to the preparation of TiPes thin films. By finely tuning the composition of the system, they successfully fabricated high-quality thin films and achieved a PCE of 5.7% on flexible modules with an area of up to .161 Similarly, the blade coating technique has also been applied to the scalable fabrication of tin-lead hybrid PSCs, further demonstrating the potential of TiPes for large-area production.162,163 However, there remains a significant performance gap in PCE between the blade-coating process and the spin-coating method. To further enhance the performance of large-area TiPes devices, greater emphasis should be placed on improving the film quality and homogeneity during the fabrication process.
3.2 Single Crystals
The synthesis processes of TiPes single crystals mainly include slow cooling crystallization (SCC) or inverse temperature crystallization (ITC).39,164 In SCC, the temperature is gradually decreased to trigger the supersaturation of the precursor solution, ultimately yielding perovskite single crystals. For example, as shown in Fig. 7(a), Han et al. synthesized single crystals in ethylene glycol solvent using a slow solution temperature lowering method. Compared with the single crystals in HI, the quality of the obtained single crystals was higher due to the ligand effect of ethylene glycol.165 To grow perovskite single crystals with larger sizes, a combination of top seeding methods and SCC can be used.167 Based on this method, Dang et al. successfully grew and single crystals with crystal sizes up to and , respectively, in mixed solution.50 In ITC, millimeter-sized perovskite single crystals can be obtained by raising the temperature to supersaturate the precursor solution and allowing it to crystallize over an extended period. For instance, Bakr et al. used ITC to produce single-crystal films with a surface area of up to , and the single-crystal thickness can be controlled from 5 to by surface tension; such large-size single-crystal films are desirable for PD applications.168,169 Yuan et al. demonstrated the synthesis of single crystals with 3D dimensions in the micron range in a GBL solvent, leveraging spatial constraints. These single crystals exhibit sharp absorption edges and strong optical absorption, featuring an ideal optical band gap of 1.24 eV, which provides the opportunity to fabricate efficient single-junction tin PSCs.170 In addition, single-crystal nanowires were fabricated through a slow evaporation of the solvent, resulting in perfect lattice structure, low carrier trap density, long carrier lifetime, and excellent carrier mobility. These characteristics position them as attractive candidates for applications in flexible optoelectronic devices.63
Figure 7.Fabrication of single crystals, nanocrystals, and quantum dots. (a) Schematic diagram of single crystals prepared by slow solution temperature lowering method. Reproduced with permission from Ref. 165. (b) Schematic for the synthesis of nanocrystal by the hot injection. (c) TEM images of nanocrystal with different Cs:Sn ratios. Reproduced with permission from Ref. 166.
TiPes nanocrystals or quantum dots can be synthesized by hot injection and ligand-assisted reprecipitation (LARP), which are similar to lead perovskites.157 The hot injection method is a widely used approach for the preparation of TiPes nanocrystals/quantum dots.22,65,171 Long-chain organic ligands, such as aliphatic carboxylic acids and amines, are usually used, and the size and morphology can be regulated via controlling the ligand chain length and reaction temperature.157As shown in Fig. 7(b), cubic-phase nanocrystals were synthesized via a hot-injection method, where a tri--octylphosphine (TOP) solution containing tin(II) 2-ethylhexanoate [] and trimethylsilyl iodide (TMSI) was injected into a -rich precursor solution. The nanocrystal size could be precisely tuned by adjusting the molar ratio of Sn to Cs precursors [Fig. 7(c)].166 This method not only produces regular and uniform cubic nanocrystals but also enables the formation of well-ordered nanoplate structures through further surface ligand modulation.172 In parallel, Li et al. conducted an in-depth investigation into the oxidation mechanism of and proposed a strategy combining TOP with metallic tin powder. This approach effectively mitigates tin oxidation driven by oxygen and solvents, enabling the preparation of monodisperse films. As a result, the carrier lifetime is significantly extended by two orders of magnitude, reaching 278 ns.173 LARP is another effective strategy to synthesize perovskite nanocrystals, and the main principle is to use a polar solvent containing the respective ions and a nonpolar solvent for rapid mixing, and to exploit the large solubility difference of the ions in the two types of solvents to provide the crystallization driving force.157,174 LARP has been utilized for the preparation of lead counterparts of various morphologies and sizes, but there are only a few reports on the TiPes.62,64 Weidman et al. prepared and (BA: butylamine) nanoplates by dropping TL into the solutions with BAI, CsI, and in DMF.62 The thickness of such nanoplatelets can be controlled by changing the ratio of precursors. nanocrystals were successfully synthesized using LARP and the stability of the nanocrystals is improved by introducing .64 However, it is worth noting that, unlike the lead perovskites, the synthesis of TiPes nanocrystals and quantum dots has a higher requirement of inert-atmosphere protection to prevent oxidation.
Similar to lead perovskite nanocrystals, TiPes nanocrystals can also be synthesized indirectly by leveraging ion exchange properties. Gahlot et al. demonstrated the successful preparation of 3D nanocrystals by utilizing 2D and cationic oleates under diffusive conditions at the liquid-solid interface at room temperature. Similarly, anion exchange of and can also be achieved using benzoyl halides as halogen sources.175 However, compared with lead perovskites, TiPes typically exhibit lower ion exchange efficiency. In addition, the instability of makes it prone to oxidation during the exchange process, which further accelerates crystal degradation. Consequently, the fabrication of high-quality TiPes nanocrystals still primarily relies on the direct synthesis methods described earlier.
4 Applications
Similar to lead perovskites, the extensive investigation of TiPes starts with a focus on the application in solar cells, where the TiPes were used to replace toxic lead perovskites as light absorbers and significant progress has been made in this area. With further exploration of the optoelectronic properties of TiPes, their applications have been gradually extended to LEDs, PDs, lasers, and other fields. In this section, we overview the progress of the application of TiPes in various fields, with special emphasis on the recent advancements of high-performance devices based on TiPes.
4.1 Solar Cells
Table 2 summarizes the device structures and photovoltaic performance parameters of representative tin and lead perovskites, providing a clear insight into the latest research advancements in tin PSCs. It is known that two device configurations are typically used for constructing PSCs, i.e., regular () and inverted () structures [Fig. 8(a)].192 In contrast to the balanced development of regular and inverted lead PSCs, high-efficiency tin PSCs are typically designed with structures. This preference primarily arises from the challenges associated with the common structure, where the electron transport layer (ETL) tends to oxidize the TiPes film and excessive -type doping leads to imbalanced carrier transport.183 Li et al. attempted to replace the conventional metal-oxide electron transport layer (ETL) with the mixed chalcogenide compound , achieving an improvement in the PCE of the structure to 11.78% by suppressing the generation of Sn4+.177 However, this efficiency remains lower than the high PCE of over 15% achieved in the structure. Currently, research on tin PSCs continues to focus primarily on the structure.40 The PCE of device configuration tin PSCs remains significantly lower than that of lead counterparts, particularly in terms of fill factor (FF) and open-circuit voltage (). This performance gap arises from challenges such as rapid crystallization, oxidation, energy level mismatches in the transport layers, and inefficient carrier transport at the interfaces.195
Figure 8.Application of TiPes in solar cells. (a) Schematic diagram of formal and inverted solar cell structure. Reproduced with permission from Ref. 192. (b) Schematic crystal structures of trans-2, trans-3, trans-4, and e. Reproduced with permission from Ref. 193. (c) Schematic illustration of the interface between , anchored PEDOT:PSS and TiPes. (d) Current density voltage curves. (e) EQE spectra of tin PSCs with/without . Reproduced with permission from Ref. 194. (f) Schematic diagram of FPEABr molecular dipole. (g) Ultraviolet photoelectron spectra (UPS) of films with and without FPEABr. (h) Schematic diagram of interface dipole and (i) energy level alignment. Reproduced with permission from Ref. 23.
To address these issues, careful selection of transport layers is critical. For the hole transport layer (HTL), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is commonly used due to its polar surface components, which strongly interact with perovskite precursors and promote crystallization. However, the hygroscopic nature of PEDOT:PSS can lead to the rapid degradation of TiPes. By contrast, inorganic HTLs offer better stability, but the presence of trivalent Ni can oxidize the TiPes, leading to poor PCE. Li et al. addressed this issue by introducing a self-assembled monolayer (SAM) of (4-(7H-dibenzo[]carbazol-7-yl)ethyl)phosphonic acid (2PADBC) at the perovskite- interface.178 The phosphonic acid group in 2PADBC anchors to under-coordinated Ni cations, inhibiting their reaction with the perovskites and boosting device efficiency to 14.19%. SAM can be directly used as an HTL, achieving efficiencies exceeding 26% in lead PSCs.122 However, in tin PSCs, its highest reported efficiency remains below 10%, primarily due to uneven coverage of SAM on the conductive glass surface.179 For the ETL, PCBM and are commonly used. However, due to their better energy level alignment with TiPes, new fullerene derivatives are increasingly attracting attention. Jiang et al. used indene- bisadduct (ICBA) with higher CBM as ETL to obtain up to 0.94 V and boost the PCE of tin PSCs to 12.4%.196 Sun et al. applied four regioisomers (trans-2, trans-3, trans-4, and e) of diethylmalonate- bisadduct (DCBA) as ETL, among which, trans-3 exhibits the best energy level alignment and enhanced interfacial interactions with the TiPes layer, achieving a certified PCE of 14.3% [Fig. 8(b)].193
Beyond transport layers, bulk phase engineering has been extensively explored, with Lewis bases emerging as particularly important additives in TiPes. Their significance lies in the ability to modulate the crystallization process. For example, Chen et al. demonstrated the effectiveness of the small-molecule Lewis base urea in enhancing the PCE of tin PSCs.197 DFT theoretical calculations further reveal that Lewis bases effectively passivate deep energy level traps in TiPes, which in turn suppresses non-radiative recombination and enhances carrier transport efficiency.198 Building on this, Zhou et al. doped TiPes with 2,8-dibromo-dibenzothiophene-S,S-dioxide (BrDS) molecules. By effectively modulating crystallization and passivating defects, BrDS significantly improved the from 73.91% to 79.45% and boosted the PCE from 11.9% to 14.98%.176 Chen et al. further advanced the role of Lewis base additives by synthesizing two pyridyl-substituted fullerene derivatives: cis-(CPPF) and trans-(TPPF).181 The spatial configurations significantly affect their electron density distribution, thereby influencing the interaction between the Lewis basic groups and TiPes. CPPF’s two nitrogen atoms can coordinate with one tin colloidal cluster, whereas TPPF can coordinate with two, leading to larger clusters in the case of TPPF. This reduces the number of nucleation sites, resulting in higher-quality TiPe films. The tin PSCs using TPPF achieve a PCE of 15.38% (certified 15.14%), representing a substantial improvement.
Interfacial engineering is also essential for improving the performance of tin PSCs.199 In addition to the role of the buried interface in regulating crystallization and the upper interface in protecting the perovskite layer, its critical contribution to improving carrier extraction efficiency is particularly noteworthy.200,201 It is known that tin perovskite films usually suffer from lower carrier mobility and shorter carrier lifetime due to unfavorable film quality. Besides, the higher density of interfacial defects and poor energy level alignment bring more challenges.196 To address these challenges, Teng et al. developed silane coupling agents with varying molecular dipole moments by designing different terminal groups. Specifically, the electron-withdrawing −F in creates a dipole moment directed toward the HTL, whereas the electron-donating group in -TMOS generates an opposing dipole moment [Fig. 8(c)]. Notably, the incorporation of as a bottom interface modifier significantly improves device performance. The modified HTL induces downward energy band bending, effectively optimizing the energy level alignment between the HTL and the perovskite layer. As a result, the device achieves a of 0.91 V and a PCE of 14.67% [Figs. 8(d) and 8(e)].194 Similarly, interfacial engineering at the upper interface has demonstrated notable performance enhancements. For example, fluorophenethylamine hydrobromide (FPEABr) forms molecular dipoles on the TiPes surface while passivating surface defects [Figs. 8(f) and 8(h)]. This modification resulted in improved energy level alignment at the upper interface, leading to an increase in to 0.974 V and achieving a PCE of 15.7% [Figs. 8(g) and 8(i)].23
Stability is another key factor in advancing the practical applications of PSCs. Table 3 presents a summary of stability studies on tin PSCs and offers a comparative analysis with their lead counterparts. The stabilization strategies for TiPes primarily focus on preventing the rapid oxidation of . For instance, Yang et al. developed a two-stage reduction strategy using , which not only reduces to in the precursor but also converts to during the film formation process. This approach significantly enhances device stability, enabling unencapsulated devices to retain 90% of their initial efficiency after 628 h of maximum power point (MPP) tracking in a environment.204 Beyond oxygen, water-induced degradation poses another critical challenge for TiPes.
Perovskite
Configuration
Strategy
Test conditions
PCE (%)
Stability
Ref.
ITO/PEDOT:PSS/PVK/C60/BCP/Ag
Antioxidant, DipI + NaBH4
MPP tracking, , unencapsulated
10.61
1300 h, 96% PCE retained
202
ITO/NiOx/PVK/PCBM/ZrAcac/Ag
Antioxidant, DMKO
Shelf stability, , 85°C, unencapsulated
11.2
720 h, 70% PCE retained
203
FTO/PEDOT:PSS/PVSK/ICBA/BCP/Ag
Antioxidant, K2S2O3
MPP tracking, , unencapsulated
14.78
628 h, 90% PCE retained
204
ITO/PEDOT:PSS/PVK/PCBM/BCP/Ag
Antioxidant, TPPF
MPP tracking, , unencapsulated
15.38
500 h, 93% PCE retained
181
ITO/PEDOT:PSS/PVK/C60/BCP/Ag
Component regulation, TEABr
MPP tracking, , unencapsulated
9.4
2000 h, 95% PCE retained
205
ITO/PEDOT:PSS/PVK/PCBM/BCP/Ag
Component regulation, PEABr
Shelf stability, air, 25% to 30% RH, unencapsulated
7.96
300 h, 80% PCE retained
206
ITO/PEDOT:PSS/PVK/PCBM/BCP/Ag
Crystallization regulation, urea
Shelf stability, , unencapsulated
14.22
200 h, 90% PCE retained
197
ITO/PEDOT:PSS/PVK/CF3PEAI/PCBM/BCP/Ag
Interface passivation, CF3PEAI
Shelf stability, air, 15% RH, unencapsulated
10.35
150 h, 70% PCE retained
207
ITO/PEDOT:PSS/PVK/C60/BCP/Cu/Bi
Electrode covering, Bi
MPP tracking, air, 43°C, 30% to 70% RH, unencapsulated
6.6
, 70% PCE retained
208
ITO/PPr-SBT-14/PVK/C60/BCP/Ag
HTL regulation
Shelf stability, , unencapsulated
7.6
6000 h, PCE retained
209
FTOc-TiO2/PVK/MeO-PEAI/Spiro-OMeTAD/Au
Ion migration inhibition, 0.1% water in anti-solvent
Quasi-2D structures play a significant role in this field, as the capping layers they form can effectively block the intrusion of water and oxygen.120 To date, various cations have been employed to construct quasi-2D structures for TiPes, including , , , , and .23,205,206,215,216 For example, a device prepared by partially substituting the -site with maintains 80% of its initial efficiency after 300 h in an air environment (25% to 30% RH), whereas the PCE of a conventional device drops rapidly to below 20% within just 5 h.206 Compared with TiPes, lead perovskites are less sensitive to oxygen, and their stabilization strategies primarily focus on preventing moisture intrusion, inhibiting the deprotonation of -site ions, and addressing the widespread issue of ion migration. The first two strategies are highly relevant to TiPes; however, studies have shown that TiPes are less affected by ion migration.35 Stability testing for lead perovskites has already become increasingly standardized, but significant progress is still needed to establish comparable testing protocols for TiPes.
In the field of conventional bandgap PSCs, lead perovskites remain dominant in terms of both efficiency and stability.211 TiPes, on the contrary, shows significant potential due to its low toxicity, making it suitable for indoor photovoltaic applications.217 Moreover, its tunable bandgap to a narrow value offers opportunities to extend the spectral response range of tandem solar cells.218 However, to fully harness the advantages of TiPes, substantial progress in efficiency is still needed.
4.2 Light-Emitting Diodes
The application of TiPes in LEDs began in 2016, initially targeting the NIR spectrum. Since then, it has expanded to red and blue light emission. Table 4 provides a comparison of the key performance parameters between state-of-the-art tin and lead perovskite LEDs. In terms of efficiency and luminance (or radiance), TiPes LEDs do not demonstrate significant advantages. However, TiPes LEDs show advantages in their tunable emission in the NIR-II region.83,229,230 Zhang et al. achieved high-quality TiPes films through a vapor-assisted spin-coating method. Solvent vapors were shown to lead to in situ recrystallization of TiPes during the film formation process, which significantly improves the crystal quality and reduces defects. By further adding the reducing agent phenylhydrazine hydrochloride (PH), the NIR LED devices based on ITO/PEDOT: PSS/perovskite/TmPyPB/LiF/Al structure can achieve a high EQE of 5.3% [Figs. 9(a)–9(c)].231 This performance is mainly affected by the rapid aggregation of TiPes emitters during the preparation process, which would cause imperfect alignment of nanoparticles and formation of defects, thus leading to severe non-radiative recombination.
Configuration
Luminance ()
Radiance ()
FWHM (nm)
EL (nm)
EQE (%)
Stability
Ref.
ITO/PEDOT:PSS/Poly-TPD/PEA2SnI4/TPBi/LiF/Al
451
—
23
630
3.51
219
ITO/NiOx/TFB/(BrPMA)2SnBr4/TPBi/LiF/Al
800
—
70
467
1.3
220
ITO/PEDOT:PSS/CsSnI3/B3PYMPM/LiF/Al
—
—
—
931
5.6
221
ITO/m-PEDOT:PSS/CsSnI3/SPPO13/B3PYMPM/LiF/Al
—
—
—
945
7.6
222
ITO/PEDOT:PSS/CsSnI3/TPBi/LiF/Al
—
226
71
948
2.63
223
ITO/PEDOT:PSS/PEAI-FA0.9Cs0.1SnI3/TPBi/LiF/Al
—
—
8.3
—
18
ITO/PEAI-FA0.9Cs0.1SnI3/TPBi/LiF/Al
—
89
—
11.6
—
224
ITO/PEDOT:PSS/TEA2SnI4/TPBi/LiF/Al
—
—
24.9
630
20.29
24
ITO/TFB/PVK/PEABr-CsPbBr3/BPBiPA/LiF/Al
11,370
—
24
516
32.1
225
/TPBi/PO-T2T/LiF/Al
—
—
—
480
26.4
—
226
270
—
—
648
29.04
227
ITO/PEIE-ZnO /FAPbI3-5AVA-PyNI/TFB/MoOx/Au
—
390
—
805
32
228
Table 4. Summary of key parameters of state-of-the-art tin and lead perovskite LEDs.
Figure 9.Application of TiPes in light-emitting diodes. (a) Schematic of the LED device structure. (b) Cross-sectional SEM image of LED device (scale bar: 100 nm). (c) Current density and radiance-dependent EQE of TiPes LED. Reproduced with permission from Ref. 231. (d) Crystal structure image of perovskite and electrostatic potential image of GSH. (e) Photos of air aging (40% RH, 25°C) of perovskite with and without GSH addition. Reproduced with permission from Ref. 232. (f) EL spectra of LED with moisture treatment. (g) Statistical results and (h) operation lifetime test results of EQE with and without moisture treatment for LEDs. Reproduced with permission from Ref. 222.
The formation of quasi-2D materials has been shown to effectively passivate defects and reduce non-radiative recombination. In addition, they can self-assemble into multi-quantum well structures and provide energy transfer channels, enhancing the light emission efficiency of LEDs. Min et al. compared the in situ PL images of 3D before and after the introduction of PEAI and found that the incorporation of PEAI significantly increased the lifetime of the 3D TiPes emitter, confirming the suppression of non-radiative recombination by the quasi-2D structure.18 In addition, the introduction of vitamin B1 (VmB1) can further benefit the PL and the LEDs fabricated using PEAI-VmB1-based exhibited an emission peak at and achieved an impressive EQE of up to 8.3%. Quasi-2D structures are widely and favorably used in TiPes LEDs. It is crucial to note that the crystallization of perovskites with low-dimensional phases requires precise tuning, as varying -values in these phases can lead to disordered heterogeneous interfaces and hinder efficient energy transfer. Wang and colleagues discovered that the strong interaction between and PEAI inhibits the formation of 2D perovskites. However, upon introducing tryptophan, a stronger interaction occurs between tryptophan and , leading to the partial release of ions. This facilitates the formation of 2D/3D mixed-dimensional bilayer perovskites during sequential annealing. As a result, the corresponding TiPes LEDs achieve a high EQE of 11.6%.224
Compared to lead perovskite LEDs, TiPes LEDs also demonstrate superior red color purity. For instance, an early study on the system reported a red TiPes LED with chromaticity coordinates of (0.706, 0.294) on the Commission Internationale de L’Eclairage (CIE) chromaticity coordinate chart, which are remarkably close to the ideal red emitter coordinates of (0.708, 0.292). However, the device suffered from a low EQE of only 0.3% and a luminance of , with this underperformance primarily attributed to the inherent instability of .229 Bai et al. utilized L-glutathione reduced (GSH) as a surface ligand to inhibit the oxidation of and passivated the defects to construct high-quality perovskite films [Fig. 9(d)]. The additive not only led to a significant increase in device EQE to 9.32% but also enhanced the stability. As shown in Fig. 9(e), the target film still emits bright red light under UV light after air aging for 90 min.232 Recent findings by Han et al. demonstrated that poly-functional cyanuric acid (CA) can establish strong chemical interactions on the surface of , inducing an electron localization effect that generates stable dimer/trimer superstructures.24 This superstructure may form strong chemical coordination with , stabilizing as well as improving the crystal ordering of . Notably, the champion EQE of the target device reached 20.29%, rendering it on par with state-of-the-art lead counterparts.233,234
Due to the tunability of the bandgap, TiPes LEDs can theoretically be applied to green and blue light emission, similar to lead perovskite LEDs. Han et al. achieved blue light emission in quasi-2D TiPes by modulating cations, which enhanced lattice rigidity and reduced the electron-phonon coupling effect.220 A blue LED based on demonstrated an EQE of 1.3% and a maximum brightness of . However, this remains the only report in the field to date, and its EQE of 1.3% falls far short of the 26.4% EQE achieved by state-of-the-art lead perovskite LEDs.226 This performance gap is primarily attributed to the inferior crystal quality and higher defect density of Br-containing tin-based systems, which significantly reduce luminous efficiency. To address these challenges, further research is needed to optimize the crystalline quality of TiPes and minimize defect concentrations, thereby overcoming existing technological barriers and improving device performance.
In terms of stability research, the gap between TiPes LEDs and lead perovskite LEDs is gradually diminishing, particularly in studies focused on the NIR region. Yuan et al. reported highly stable, bright inorganic TiPes LEDs by incorporating -phenylthiourea (NPTU) and .223 They proposed that, prior to Auger recombination becoming dominant at higher doping concentrations, the presence of free background holes from -type doping could enhance the PLQY and promote radiative recombination. By skillfully leveraging the crystallization control of NPTU and , they managed to regulate -doping density and reduce trap density, thereby boosting both radiative efficiency and stability. The modified achieved a radiance of at and a record of 39.5 h at for NIR LEDs with emission .235,236 Guan et al. developed an innovative moisture-triggered hydrolysis strategy to eliminate -induced defects in thin films, effectively reducing background carrier density while providing persistent protection through the formation of .222 The moisture-treated exhibited stable electroluminescence (EL) at 945 nm [Fig. 9(f)], achieving a peak EQE of 7.6% with excellent reproducibility [Fig. 9(g)]. Notably, this treatment significantly extended the operational lifetime of by 3.7-fold, reaching 82.6 h under a constant current density of [Fig. 9(h)].
In the LED field, lead perovskites have achieved significant breakthroughs in EQE, reaching 26.4%, 32.1%, and 29.04% for blue, green, and red light emissions, respectively. In the NIR range (805 nm), lead perovskites have also recently achieved a high EQE of 32%.225–228 By contrast, current research on TiPes primarily focuses on red and NIR emissions, with EQE values still lagging behind those of lead perovskites. However, TiPes hold promise for surpassing lead perovskites in the NIR region, particularly in the range, an area where pure lead perovskites are less effective and difficult to apply.223 Furthermore, the inherent low ion migration probability of TiPes promises higher stability of LEDs compared with lead perovskites, which is a critical advantage for practical applications.
4.3 Photodetectors
In halide perovskites, applying a constant voltage results in the accumulation of halide ions at the interface, which can help reduce the barrier for collecting photogenerated carriers [Fig. 10(a)]. This will not only contribute to enhancing the photocurrent but, at the same time, will also lead to increased dark current due to ion shielding effects. Besides, the perovskite photosensitive layer is a polycrystalline film with carriers moving along different pathways. Enhancing vertical carrier transport by reducing grain size promotes increased charge recombination at defect sites, which suppresses dark current. However, this comes at the expense of photocurrent. Achieving optimal photodetection performance necessitates striking a balance between increasing photocurrent and decreasing dark current.27 Compared with lead perovskites, TiPes have a stronger binding affinity with halogen ions and lower ion mobility. In addition, TiPes have high carrier mobility, which is beneficial for the transport of photo-induced charge carriers. These inherent advantages of TiPes make them suitable candidates for PD applications.14,239
Figure 10.Application of TiPes in photodetectors. (a) Schematic diagram of the effect of halogen ion migration on the barrier. Reproduced with permission from Ref. 27. (b) Schematic diagram of fabrication of PDs. Reproduced with permission from Ref. 237. (c) Responsivity of the flexible PDs with and without CNI treatment. (d) Schematic diagram of photoplethysmography test. (e) Heart rate results of rigid (blue) and flexible (red) PDs with CNI treatment at 0 V and light intensity. Reproduced with permission from Ref. 238.
Han et al. reported the application of all-inorganic TiPes in NIR PDs.240 The nanowire arrays were deposited by solid-source CVD on mica substrates with a narrow band gap of 1.34 eV. Assembly of the nanowire arrays into an NIR PD achieved a responsivity () of and a detection rate (*) of with response times of (rise/decay). The formation of the oxidized phase is a crucial factor constraining PDs’ performance, and most subsequent studies focus on preventing the generation of the oxidized phase. For example, Cao et al. introduced ascorbic acid as a reductant in to suppress the oxidation, with the device preparation process shown in Fig. 10(b).237 The resultant PDs exhibited enhanced responsivity and stability over a wide spectral range of 350 to 1000 nm; the optimal was , and the response time was .
Structures in 2D and quasi-2D have exhibited enhanced moisture resistance, leading to improved stability and detection performance. Yang et al. investigated the photoelectric response of 2D and quasi-2D under 405-nm optical radiation.241 These structures achieved high on/off ratios of 458 and 1108, respectively, at an irradiation intensity of . Notably, the long-term performance of 2D and quasi-2D PDs remained stable even under extended irradiation. At lower light intensities, these 2D/quasi-2D PDs exhibited remarkable and * values of and , respectively. The ultra-high stability, detection capability, and on/off ratio of low-dimensional structures are difficult to achieve with conventional TiPes PDs. However, disordered quantum well structures may affect carrier mobility, thereby impacting the detector’s fast response. Therefore, the application of low-dimensional materials should be based on more precise control of layer numbers and crystallization.
Despite notable progress, the sensitivity and response speed of TiPes in the conventional wavelength range (300 to 800 nm) still lag significantly behind those of lead perovskites, primarily due to their inherent instability. However, the band gap limitation of lead perovskites makes NIR detection challenging, creating opportunities for TiPes in imaging and medical applications.242 For example, self-powered PDs based on quasi-single-crystal thin films exhibit high detection rates in the NIR range (780 to 890 nm) exceeding .243 In addition, TiPes possess low-temperature processing capabilities, offering significant advantages in the field of flexible PDs. Liu et al. demonstrated the potential of TiPes in flexible PDs. They found that flexible PDs treated with 2-cyanoethan-1-aminium iodide (CNI) exhibited excellent responsivity in the NIR region, achieving at 785 nm [Fig. 10(c)]. In addition, the PDs showed high detectivity ( at 785 nm) and fast response times (). These photodetectors are well-suited for wearable electronic devices, enabling real-time monitoring of human heart rate under low-light and zero-bias conditions [Figs. 10(d) and 10(e)].238 Therefore, with advancements in material design, defect passivation, and device optimization, TiPes are poised to play a crucial role in NIR and flexible detection.
4.4 Lasers
TiPes are also utilized for fabricating tunable lasers. Xing et al. concluded that lasing originates from free electron-hole bimolecular recombination, and ( treatment) has an extremely high recombination rate for lasing applications. Ultra-stable NIR coherent emission (700 to 1000 nm) can be achieved at room temperature through halogen modulation. This wavelength range, however, is challenging to achieve with lead perovskites.104 Chirvony et al. found the NIR random laser behavior in polycrystalline films. A high-quality factor () with a low amplified spontaneous emission threshold () was obtained at 20 K.244 Interestingly, the random lasing (RL) mode generated in polycrystalline films exhibits exceptional spectral stability; as the excitation flux increases, the spectral position of the narrow PL lines remains unchanged [Figs. 11(a) and 11(b)]. This stands in stark contrast to the disordered variations seen in the RL spectra of lead perovskites. This suggests that TiPes are promising for practical applications with narrow laser lines and high spectral stability.
Figure 11.Application in lasing. (a) RL spectra and (b) normalized RL spectra of thin film at different excitation fluences (20 K, excitation by 532 nm pulses). Reproduced with permission from Ref. 244. (c) Temperature-dependent PL spectra of at an excitation fluence of and (d) FWHM of the ASE peak. Reproduced with permission from Ref. 245. (e) Schematic presentation of substitution by conjugated monomers. (f) Lasing spectra for at , at , and at [Inset is a lasing image of ]. (g) Temperature-dependent PL spectra of at different excitation fluences. Reproduced with permission from Ref. 25.
2D perovskites exhibit higher radiative recombination efficiencies, making them highly promising candidates for laser applications. A series of 2D lead perovskites (), such as , has demonstrated intrinsic laser emission in the visible region without requiring an external cavity.246 However, due to the limitations of high Auger recombination rates and strong exciton-phonon coupling in 2D TiPes, their laser design requires a more flexible and meticulous approach. Petrozza and his colleagues successfully synthesized through precise crystal engineering, achieving amplified spontaneous emission (ASE) at an exceptionally low threshold with an optical gain exceeding at 77 K.245 They observed significant thermal sensitivity in and found that higher temperatures resulted in reduced ASE intensity and broadened FWHM [Figs. 11(c) and 11(d)]. Even at room temperature, albeit with a weaker signal, ASE was still detectable, underscoring the potential utility of room temperature lasers based on 2D TiPes using . Li et al. employed a mixed-solvent strategy to synthesize a series of RP-phase TiPes. By skillfully tuning the quantum well thickness and organic ligand design, they achieved modulation of broad-spectrum optoelectronic properties.25 It has been shown that substituting with conjugated ligands (, , ) enables optically pumped lasing in 2D TiPes [Fig. 11(e)]. By contrast, RP-phase TiPes can achieve optically pumped lasing at , whereas lead perovskites require . This further demonstrates the superiority of TiPes [Fig. 11(f)]. Furthermore, the temperature-dependent lasing properties of monolayer TiPe, i.e., , have been studied, which clearly demonstrate that the temperature has a huge impact on the modes and threshold of lasing [Fig. 11(g)].
TiPes offer significant advantages for achieving room-temperature NIR coherent emission, with notable breakthroughs enabled by low-dimensional and nanomaterials.25,246 Despite significant progress, TiPes continue to face challenges in achieving high phase purity and precise control over shape and size, especially when compared to lead perovskites. Furthermore, the relationship between the crystal structure, carrier dynamics, and lasing performance of TiPes requires further in-depth investigation.
4.5 Transistors
TiPes are promising candidates for realizing high-performance transistors. Their intrinsically high hole mobility offers a significant opportunity to drive the development of high-performance -channel (hole) field-effect transistors (FETs) and complementary metal-oxide-semiconductors (CMOSs).247 Shao et al. first demonstrated the feasibility of employing as a semiconductor channel in FETs. FETs utilizing exhibit exceptional hole mobility (), impressive on and off current ratio (, ), and relatively low threshold voltages (, 2.8 V).248 Zhu et al. successfully enhanced the quality of thin films and improved the device performance of thin-film transistors (TFTs) through halogen ion modulation.26 The introduction of a small quantity of Br ions could enhance coordination with , effectively governing the nucleation and crystallization kinetics of TiPes. This led to improved crystallinity and orientation while reducing defects such as pin-holes. The introduction of Cl ions promoted the formation of TiPes, resulting in a smoother film surface. The effective synergistic combination of these two halogens led to a significantly enhanced performance of TFTs, achieving a high hole mobility of , an ratio as high as , and the elimination of a bias voltage to deactivate the transistor. It is important to clarify that the hysteresis phenomenon, primarily caused by ion migration, is a major concern in lead perovskites but is dramatically suppressed in TiPes. The hysteresis of TiPes TFTs, on the other hand, is mainly related to minority carrier traps, i.e., vacancies (). However, in composite halide TiPes, the presence of Br/Cl has a passivating effect on , resulting in negligible hysteresis in TFTs [Figs. 12(a) and 12(b)].
Figure 12.Application of TiPes in transistors. (a) TFTs structure. (b) Transfer characteristics of TFTs with different halogen-substituted TiPes. Reproduced with permission from Ref. 26. (c) Inverter structure and optical image. (d) Butterfly voltage transfer characteristic (); noise margin (NM) in blue color. (e) Gain . Reproduced with permission from Ref. 249.
The stability of perovskite can also be improved by incorporating a 2D structure for better application in transistors, for example, the introduction of the diammonium cation and dramatically improves the stability of TiPes in air.115 In addition, the 2D layer is capable of passivating defects, and rational crystallization modulation can enhance the charge carrier mobility of TiPes. Zhu et al. developed -type TFTs using TiPes with a hybrid cation engineering approach involving Cs, FA, and PEA. The gradual introduction of PEA and Cs ions had a beneficial effect on the crystallization process of the FA film, resulting in reduced surface roughness and improved film quality.249 The ternary film exhibited a low density of charged defects, achieving a remarkable field-effect mobility up to , and demonstrated an ratio of . These achievements represent leading performance in perovskite TFTs. By employing cation engineering, the presence of iodide interstitials/vacancies in TiPes was reduced, leading to diminished dual-sweep hysteresis. In addition, the combination of TFTs with -type enabled the creation of complementary semiconductor inverters [as depicted in Fig. 12(c)]. The inverter displays typical rail-to-rail voltage transition characteristics, featuring a sharp transition when the input voltage reaches , signifying a logic state shift from “1” to “0” [Fig. 12(d)]. With a of 12.5 V, the inverter exhibits an impressive gain of 370, surpassing the performance of the lead-based integrated inverter [Fig. 12(e)]. In the realm of fully inorganic TiPes, exceptional field-effect hole mobility exceeding and an ratio surpassing have been realized.250
Due to -type doping in TiPes, it is well-suited for applications in -channel field-effect transistors. In addition, when combined with -channel metal-oxide transistors, perovskite can be used to create CMOS electronics, offering enhanced noise resistance and low power consumption.247 However, their performance remains difficult to match that of -type FETs due to the challenges of high -type doping, whereas -type transistors can be readily achieved using lead perovskite.251
4.6 Other Applications
Apart from the abovementioned applications, TiPes, known for their exceptional semiconductor properties, have also seen a gradual expansion of application in other areas. In the study conducted by Qian et al., an device structure was assembled to evaluate its memory capacitance characteristics, as depicted in Fig. 13(a).252 When subjected to periodic bias voltage sweeping, the device exhibits significant hysteresis loops in the capacitance–voltage () curve, whereas the charge–voltage () curve displays two distinctive pinched loops [illustrated in Fig. 13(b)]. These observations validate the device’s suitability as an ideal memory capacitor. Besides, the oxygen sensitivity of TiPes offers potential for oxygen sensing applications. Cai et al. leveraged this trait and created an innovative oxygen sensor by pairing 2D with a tilted fiber Bragg grating. This sensor exhibits high sensitivity and rapid response [Figs. 13(c) and 13(d)], showcasing TiPes’ potential for other sensor applications.253 Moreover, TiPes, benefiting from their excellent Seebeck coefficient and ultra-low thermal conductivity, have achieved an optimal thermoelectric power factor of in thin films, highlighting their promising potential in thermoelectric applications.254
Figure 13.Applications of TiPes in memory capacitor and sensor. (a) Schematic diagram of device structure. (b) Typical and loops detected at 1 MHz. Reproduced with permission from Ref. 252. (c) Sensitivity of modified to various gases (gas concentration of 0.5%). (d) Response of the TiPes sensor to oxygen in a wet or dry atmosphere. Reproduced with permission from Ref. 253.
With their outstanding optical and electrical properties, TiPes have become very promising lead-free semiconductors and significant progress has been achieved in their applications such as solar cells, LEDs, PDs, and lasers.23,243,255 However, the instability of TiPes remains the greatest obstacle to their replacement of lead perovskites. This instability is primarily caused by external factors such as oxygen, moisture, and temperature, which accelerate degradation. Various strategies have been proposed to enhance the stability of TiPes, spanning from precursor protection to perovskite optimization and device fabrication. Drawing on the insights into the stability of lead perovskites, we believe the most promising development strategies to improve the operational stability of TiPes can be identified. (i) Component regulation: efficient and stable lead perovskites are commonly achieved through the combination of and ions.121,122 However, this approach is limited in the case of TiPes due to the sensitivity of to oxygen and moisture.123 We believe that adjusting the ratio of these two ions with 2D-phase organic ammonium ions is desirable, as it can effectively isolate moisture and oxygen while simultaneously improving the thermal stability of the devices. (ii) Interfacial engineering: the interface of TiPes is typically the region where defects concentrate and degradation initiates. Therefore, the formation of a surface passivation layer is crucial to enhancing the stability of TiPes. In this regard, lead perovskites provide valuable insights, particularly in terms of passivating uncoordinated divalent metal ions and forming protective layers to block water and oxygen. Both systems share these common characteristics. However, the implementation of this approach must take into account the unique properties of TiPes, such as the coordination activity of and the corrosive effect of the commonly used solvent isopropanol (IPA). Feasible strategies include utilizing the soft Lewis acid characteristics of TiPes to select S- or N-containing groups for coordination. In addition, it may be beneficial to introduce groups that can coordinate with the active sites (−OH) of IPA or use mixed solvents to reduce the polarity-induced corrosion of IPA on the TiPes.
Although some issues remain unresolved, considering the unique features of TiPes, it still shows great potential for making breakthroughs in the following areas in the future.
1.Extraction and utilization of hot carriers
In TiPes, longer hot carrier lifetimes than those of lead counterparts have been reported. These hot carriers are present under both pulsed excitation and continuous wave excitation, confirming the possibility that hot carriers can be extracted for solar cell applications.21,76 Currently, hot carrier-based extraction applications are facing a dual challenge: the standard solar irradiation energy makes it difficult to meet the output demand of hot carriers, and in addition, the relaxation time of hot carriers needs to be further slowed down to enhance the probability of their extraction. To address these challenges, several potential solutions have been proposed: (i) achieving an appropriate perovskite grain size for slow relaxation of hot carriers at low injected carrier densities, (ii) designing device structures that maximize the use of sunlight, and (iii) selecting effective passivation agents to eliminate deep energy level defects that can directly trap hot carriers.22,256
2.Applications in narrow bandgap field
The application of TiPes in narrow band gap areas provides an important opportunity to break the monopoly of lead perovskites. These areas include (i) solar cells: narrow bandgap TiPes can broaden the spectral absorption range of conventional solar cells and thus have the potential to be used in all-perovskite tandem solar cells. (ii) NIR emission and detection: due to the limitations of the bandgap, lead perovskites are unable to achieve effective light emission or detection beyond 850 nm.28 The addition of narrow-bandgap TiPes fills this gap. In the field of light emission, -based LED devices can extend the emission wavelength to over 940 nm. In addition, compared with lead perovskites, TiPes exhibit lower Auger recombination, meaning they can maintain ultra-low efficiency roll-off at high current densities.223 In the field of light detection, TiPes can extend the detection range to 1000 nm, achieving a detectivity of over and microsecond-level fast response. Although TiPes show potential for application in these fields, their relatively low PCE, as well as light emission and detection efficiencies, still limits their practical use. One feasible solution is to use tin-lead mixed perovskites.242,257,258 From the perspective of TiPes, adding a small amount of lead can improve device efficiency while maintaining a low bandgap. From the perspective of lead perovskites, this approach can reduce the use of toxic lead and effectively narrow the bandgap of lead perovskites.
3.Applications in low toxicity field
Due to the low toxicity of TiPes, they present promising opportunities and breakthroughs in various fields, particularly in applications that are integrated into daily life, where concerns about toxicity are more significant. Specifically, (i) indoor photovoltaics: common indoor light sources emit light in the 400 to 700 nm range, and the optimal bandgap for indoor photovoltaics is between 1.7 and 1.9 eV. TiPes can achieve this bandgap range through halide composition adjustments. Moreover, the halide ion migration in TiPes under light is minimal, which suggests that their application in indoor photovoltaics could offer improved stability. Devices based on perovskites have already achieved a PCE of 20.12% under indoor lighting (3000 K, 1000 lux).217 (ii) Wearable flexible electronics: TiPes offer a distinct advantage over lead perovskites in low-temperature film formation, making them more suitable for fabrication on flexible substrates. Studies have been reported on flexible TiPes optoelectronic devices, such as flexible solar cells and photodetection.161,238 These are expected to boost the application of TiPes in wearable self-powered health detection devices. It is important to note that the above application scenarios are all based on low-light conditions, making it essential to achieve high light utilization efficiency in such environments. Combining light-capturing technologies, such as metamaterial nanostructures and gratings, to further enhance light absorption efficiency could be crucial to fully harness the excellent optical properties of TiPes.
Feng Yang received his MS degree from Nanjing University of Aeronautics and Astronautics (NUAA) and is now advancing his research as a PhD candidate under the supervision of Prof. Yu Tong at Northwestern Polytechnical University. His research focuses on tin perovskite solar cells.
Yu Tong is currently a professor at School of Materials Science and Engineering, Northwestern Polytechnical University. He received his PhD from Ludwig-Maximilians-University Munich in 2018 under the supervision of Prof. Jochen Feldmann. His recent research interest is novel semiconductor optoelectronic functional materials and devices.
Kun Wang is an associate professor at School of Microelectronics, Northwestern Polytechnical University. She received her PhD from Technical University of Munich in 2019 under the supervision of Prof. Peter Müller-Buschbaum. Her research primarily focuses on optoelectronic conversion materials and devices.
Hongqiang Wang is a professor at the School of Materials Science and Engineering, Northwestern Polytechnical University. Before then, he sequentially worked in National Institute of Advanced Industrial Science and Technology, Japan (postdoc), Max Planck Institute of Colloids and Interfaces (Alexander von Humboldt Fellow), and University of Liverpool (Marie Curie Fellow). His research focuses on advanced energy and catalytic materials, encompassing both fundamental and applied studies.
Biographies of the other authors are not available.
Feng Yang, Yu Tong, Kun Wang, Yali Chen, Ziyong Kang, Hongqiang Wang, "Recent advances in tin perovskites and their applications," Adv. Photon. 7, 034003 (2025)