Visible-blind ultra-violet (UV) photodetectors have great application prospects in military and civilian fields. Hence, it is important to develop high-efficiency UV photodetectors. Perovskite materials have been widely applied in many fields, such as solar cells, light-emitting diodes, and photodetectors, owing to their excellent optical properties. The CsPbCl₃ perovskite has a great potential in visible-blind UV photodetectors owing to its stable chemical properties and a suitable band gap. However, due to the extremely poor precursor solubility of CsPbCl₃, it is difficult to find a suitable solvent to prepare CsPbCl₃ films. In this study, we developed a two-step diffusion method to prepare CsPbCl₃ films. PbCl₂ and CsCl were dissolved in different solvents to overcome the solubility problem of CsPbCl₃. First, the PbCl₂ film was spin coated on a soda-lime glass. The CsCl solution was then continually dropwise spin-coated on the PbCl₂ film to form the CsPbCl₃ film. By controlling the morphology of PbCl₂ through different annealing temperatures, the influence of different PbCl₂ precursor films on microstructure of the CsPbCl₃ film was investigated. The X-ray diffraction and scanning electron microscopy profiles of the PbCl₂ precursor film and the CsPbCl₃ film suggested that when the annealing temperature was too low, excess dimethyl sulfoxide (DMSO) remained in the PbCl₂ precursor film, which hindered the transformation of CsPbCl₃. However, when the annealing temperature was too high, voids appeared in the CsPbCl₃ film due to excess DMSO volatilization, which caused pinholes in the CsPbCl₃ film. Finally, a pinhole-free, smooth CsPbCl₃ film with full grains was obtained when PbCl₂ was annealed at 80 ℃. To the best of our knowledge, this is the first time a high-quality CsPbCl₃ film with 1 μm grains was prepared using a two-step diffusion method. Photoelectrical characterizations, such as ultraviolet-visible absorption, steady-state photoluminescence (PL), and time-resolved PL characterizations, were studied to evaluate the quality of the film. The as-prepared CsPbCl₃ film had strong UV absorption and PL intensity, and its longer PL lifetime indicated that the film had fewer defect states. Furthermore, a UV photodetector with a lateral structure based on the CsPbCl₃ film was fabricated. The related electrical characterizations, such as current voltage curves, showed a good responsivity of 0.75 A·W^-1 and an outstanding detectivity of 7.8 × 10¹² Jones, which are better than those of contemporary commercial photodetectors. The findings of this study show that a two-step diffusion method can be used to prepare CsPbCl₃ films with better features and that it has a great potential in the development of UV photodetectors in the future.

Tin oxide (SnO₂) thin films are widely used as electron transport layers in planar perovskite solar cells (PSCs) and commonly prepared using solution-processed spin-coating. However, obtaining full coverage and pinhole-free surfaces for the spin-coated (SC) SnO₂ is challenging because the nanocrystals in the precursor solution can undergo aggregation, wherein the precursor solution may contain dust particles, and the desired film thickness is rater small. Since dense electron transport layer films without pinholes are crucial in suppressing the non-radiative recombination of charge carriers in PSCs, developing deposition methods to prepare high-quality SnO₂ films is important to improve the performance of planar PSCs. In this study, we investigated the application of electrophoresis (EP) in depositing compact and pinhole-free SnO₂ thin films. We conducted electrophoresis to deposit a dense nanocrystalline film on the surface of Indium tin oxide (ITO) and employed a spin-coating step to adjust the thickness of the film and remove the residual SnO₂ nanocrystalline precursor solution. This method was denoted as EP-SC. In the electrophoresis, the negatively charged SnO₂ nanocrystals, caused by a strong electric field, migrated towards the surface of the ITO anode and formed more compactly packed thin films than that of the spin-coated SnO₂. The atomic force microscopy (AFM) measurements demonstrated that the surface of EP-SC SnO₂ was more uniform than that of SC SnO₂ and there were no streaks and aggregated particles on the surface. This may have been due to the fact that the surface charge properties of the aggregated and dust particles in the precursor solution was different from that of the desired SnO₂ nanocrystals. Hence, electrophoresis can selectively deposit SnO₂ nanocrystals. Specifically, high-quality perovskites and electron transport layer interfaces can be achieved using this method. Both the electrochemical impedance spectroscopy (EIS) and dark J–V measurements showed that the PSCs using SnO₂ prepared by electrophoresis followed by spin-coating demonstrated remarkably suppressed non-radiative recombination. As a result, the photoelectric power conversion efficiency increased from 18.17% (based on SC) to 19.52% (based on EP-SC) due to the enhanced short-circuit current and open-circuit voltage. The hysteresis of the device was eliminated. More importantly, the long-term stability measurements demonstrated that our device can maintain up to 71% of the initial efficiency after 960 h of continuous operation at the maximum power point (MPP) under one sun illumination. Whereas the device based on spin-coated SnO₂ can maintain only up to 70% of the initial efficiency after working for 100 h. The results of this study can help in preparing electron transport layers to construct long-term stable planar PSCs, which are favorable for fabricating large-area PSCs and modules in future researches.

Since 2009, organic-inorganic halide perovskites have been widely studied in the field of optoelectric materials due to their unique optical and electrical properties. Pb-based halide perovskite solar cells (PSCs), in particular, currently have a record efficiency of 25.2%, thus showing strong potential in commercialization. However, the market prospects of PSCs have been hampered by the toxicity of lead-based materials. Therefore the seeking of less toxic and environmentally friendly elements that can replace Pb is of great interest. Tin-based perovskites are the most promising choice at present due to its similar electronic configuration as Pb, and can even have more superior semiconductor properties. As a rising star of lead-free perovskite solar cells, tin-based PSCs have drawn much attention and made promising progress during the past few years. However, it is still challenging to obtain efficient and stable tin-based PSCs because of the low defects formation energy and the oxidation of bivalent tin. Among all Pb-free perovskite materials that show photovoltaic performance, formamidinium tin tri-iodide (FASnI₃) based PSCs are the most promising because of the suitable band gap, low exciton bind energy, and high carrier mobility. The main drawbacks of tin-based perovskite material are its instability because of the easy oxidation of Sn²⁺ into Sn⁴⁺ and high dark current which arises from high p-type carrier concentration. The latter originates from the low formation energy of Sn vacancies. Many strategies have been developed to overcome these problems and promote the performance of tin-based PSCs. On one type of pursuit to avoid the oxidation of Sn²⁺, reduction additives (e.g., SnF₂, pyrazine, hydrazine vapor, hydroxybenzene sulfonic acid or its salt, and π-conjugated polymer) and solvent-free processing have been introduced and shown to be effective up to a point. In another type, Cs or Br alloying and construction of low-dimensional structures in tin-based perovskite have also been shown to be promising. In this review, the optical and electrical properties of tin-based perovskite are systematically discussed. And then, the film fabrication methods and different device architectures of tin-based PSCs are summarized. Finally, the current challenges and a future outlook for tin-based PSCs are discussed.

In recent years, hybrid lead halide perovskites have attracted significant research interest in the optoelectronic fields owing to their exceptional physical and chemical properties. However, their commercialization process is limited largely because of the sensitive nature of perovskite materials towards external stresses, such as heat, UV irradiance, oxygen, and moisture. Among various perovskite-stabilization methods, deposition of a protective layer over the vulnerable perovskite film via simple atomic layer deposition (ALD) technology is of great potential. However, the corrosive effect of H₂O or O₃ on perovskites, which is used as the oxygen source during ALD process, is one of the main obstacles in the application of regular ALD technology for coating compact and highly conformal layer directly onto the perovskite film. In this study, by introducing bifunctional 5-aminovaleric acid (AVA) crosslinking into the layers of CH₃NH₃PbBr₃ (MAPbBr₃) units, we propose a simple yet effective strategy to prevent the degradation of sensitive perovskite structure during the ALD process when H₂O is used as the oxygen source. The formed crosslinked 2D/3D structure of AVA(MAPbBr₃)₂ perovskite film was extremely dense and ultra-smooth compared to the coarse MAPbBr₃ film. With the passivation and protection of AVA, the AVA(MAPbBr₃)₂ perovskite film exhibited high moisture resistance, thereby leading to the successful deposition of dense and conformal Al₂O₃ protective layer onto the perovskite surface. The deposition of Al₂O₃ layer with different thicknesses had a negligible effect on the crystalline phase and morphology of AVA(MAPbBr₃)₂ film, as confirmed by X-ray diffraction, UV-Vis absorption spectroscopy, and scanning electron microscopy characterizations. The steady-state photoluminescence (PL) intensity and time-resolved PL lifetime of AVA(MAPbBr₃)₂ film was kept almost unchanged before and after the coating of Al₂O₃ layer, suggesting that the thin Al₂O₃ layer did not significantly alter the optical properties of the perovskite material, thereby enabling the potential usages in optical and optoelectronic devices. The thermal stability and water resistance ability of Al₂O₃-coated AVA(MAPbBr₃)₂ film was proven to have greatly improved in accelerated circumstances. No impurities or decomposition were detected for Al₂O₃-coated AVA(MAPbBr₃)₂ film after the long-time annealing at high temperature (150 ℃ for 2 h), whereas the crosslinked 2D/3D structure of bare MAPbBr₃ film quickly broke down at the elevated temperature. Intriguingly, the AVA(MAPbBr₃)₂ film with 15-nm-thick Al₂O₃ coating layer could endure strong water corrosion for at least 10 min when immersed in water. Overall, the proposed strategy could not only give a good reference for successfully depositing metal oxides onto the perovskite films with preservation of the materials' intrinsic properties, but also provide a method of introducing amino acid to passivate and protect the perovskite materials from H₂O corrosion during the ALD process. Therefore, the proposed work has practical potential in improving the device stability against various external stresses under different operating conditions, thereby paving way for various applicational advances.

Organic-inorganic hybrid lead halide perovskites have emerged as the most promising materials in the field of optoelectronics due to their unique electronic and optical properties. However, the poor long-term material and device stabilities of these materials have limited their practical application. Compared to organic-inorganic hybrid perovskites, all-inorganic halide perovskites like CsPbX₃ (X = Cl, Br, I) show enhanced thermal stability and the potential to resolve the issue of instability. Nevertheless, the structural and physical properties of all-inorganic CsPbX₃ halide perovskites with multiple structural polymorphs are still under debate. A recent research article on CsPbI₃ reported the wrongly indexed the XRD pattern of γ-CsPbI₃ as α-CsPbI₃. Consequently, the band gap of γ-CsPbI₃ (1.73 eV) was erroneously designated for α-CsPbI₃. Therefore, there is a need for systematic research on the relationship between the structural features and electronic properties of CsPbX₃. Here, we present a comprehensive theoretical study of the structural, thermodynamical and electronic properties of three polymorphic phases, α-, β-, and γ-CsPbX₃. The space group of α-, β-, and γ-CsPbX₃ are Pm${\rm{\bar 3}}$m, P4/mbm, and Pnma, respectively. First-principles calculations indicate that the phase transition from the high-temperature α-phase to the low-temperature β-phase and then to the γ phase is accompanied by an increase in the degree of PbX₆ octahedral distortion. The zero-temperature energetic calculations reveal that the γ-phase is the most stable. This is consistent with the fact that experimentally, the γ-phase is stabilized at a relatively low temperature. Analysis of the electronic properties indicates that all the CsPbX₃ perovskites exhibit a direct-gap nature and the band gap values increase from α to β, and then to the γ phase. From the analysis of the orbital hybridization near the band gap edges, the increase can be explained by the downshift of the valence band edges caused by the gradual weakening of the Pb-X chemical bond. Among all the phases, the strongest Pb-X interaction in the α-phase leads to the most dispersive band-edge states and thus the smallest carrier effective masses, which are beneficial for carrier transport. Additionally, the band gaps decreased by changing the halogen type from Cl to Br and I under the same phase. this is a consequence of the increased X np orbital energies from Cl 3p to Br 4p and then to I 3p that leads to a high valence band edge for CsPbI₃ and results in the smallest band gap. Our results provide deep understanding on the relationship between the physical properties and structural features of all-inorganic lead halide perovskites.

Inorganic halide CsPbI₃ perovskite colloidal quantum dots (QDs) possess remarkable potential in photovoltaics and light-emitting devices owing to their excellent optoelectronic performance. However, the poor stability of CsPbI₃ limits its practical applications. The ionic radius of SCN⁻ (217 pm) is comparable to that of I⁻ (220 pm), whereas it is marginally larger than that of Br⁻ (196 pm), which increases the Goldschmidt tolerance factor of CsPbI₃ and improves its structural stability. Recent studies have shown that adding SCN⁻ in the precursor solution can enhance the crystallinity and moisture resistance of perovskite film solar cells; however, the photoelectric properties of the material post SCN⁻ doping remain unconfirmed. To date, it has not been clarified whether SCN⁻ doping occurs solely on the perovskite surfaces, or if it advances within their structures. In this study, we synthesized inorganic perovskite CsPbI₃ QDs via a hot-injection method. Pb(SCN)₂ was added to the precursor for obtaining SCN⁻-doped CsPbI₃ (SCN-CsPbI₃). X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were conducted to demonstrate the doping of SCN⁻ ions within the perovskite structures. XRD and TEM indicated a lattice expansion within the perovskite, stemming from the large steric hindrance of the SCN⁻ ions, along with an enhancement in the lattice stability due to the strong bonding forces between SCN⁻ and Pb²⁺. Through XPS, we confirmed the existence of the S peak, and further affirmed that the bonding energy between Pb²⁺ and SCN⁻ was stronger than that between Pb²⁺ and I⁻. The space charge limited current and time-resolved photoluminescence results demonstrated a decrease in the trap density of the perovskite after being doped with SCN⁻; therefore, the doping process mitigated the defects of QDs, thereby increasing their optical performance, and further enhanced the bonding energy of Pb－X and crystal quality of QDs, thereby improving the stability of perovskite structure. Therefore, the photoluminescence quantum yield (PLQY) of the SCN-CsPbI₃ QDs exceeded 90%, which was significantly higher than that of pristine QDs (68%). The high PLQY indicates low trap density of QDs, which is attributed to a decrease in the defects. Furthermore, the SCN-CsPbI₃ QDs exhibited remarkable water-resistance performance, while maintaining 85% of their initial photoluminescence intensity under water for 4 h, whereas the undoped samples suffered complete fluorescence loss due to the phase transformations caused by water molecules. The SCN-CsPbI₃ QDs photodetector measurements demonstrated a broad band range of 400–700 nm, along with a responsivity of 90 mA∙W⁻¹ and detectivity exceeding 10¹¹ Jones, which were considerably higher than the corresponding values of the control device (responsivity: 60 mA∙W⁻¹ and detectivity: 10¹⁰ Jones). Finally, extending the doping of SCN⁻ into CsPbCl₃ and CsPbBr₃ QDs further enhanced their optical properties on a significant scale.

Since organic-inorganic halide perovskites were first used in the field of solar cells in 2009, they have emerged as the most promising high-efficiency and low-cost next-generation solar cells. However, even though conventional lead perovskite halide perovskite solar cells have achieved a record efficiency of 25.2%, there is scope for improvement in terms of the detrimental properties of their constituent heavy metals. In addition, their theoretic efficiencies are limited by the large bandgap. Tin perovskite has received considerable attention in recent years, due to its heavy-metal-free character and superior semiconductor properties, such as a suitable bandgap and a high carrier mobility. In order to fabricate tin perovskite solar cells (TPSCs) of high-efficiency, the major obstacles have to be overcome, including fast crystallization of tin perovskites, high p-type carrier concentration, and high defect density. Even if Sn²⁺ has similar electronic configuration as Pb²⁺, Sn²⁺ has two more active electrons, which render tin perovskite less stable. To deal with these problems many strategies are developed. Lewis bases, such as dimethyl sulfoxide, are widely used to slow down the crystallization rate of tin perovskite, while oxide protective layer and plentiful additives (e.g., SnF₂, liquid formic acid, and hydrazine vapor) have been found to reduce their oxidation. Furthermore, low-dimension structure and device engineering have been verified effectively promote TPSCs performance. Owing to the aforementioned strategies, the efficiency and stabilities of TPSCs were improving rapidly over the past few years, which indicates that TPSCs are the most promising candidate of lead-free perovskite solar cells. Recently, the certified efficiency of TPSCs reached over 12%, which is the maximum value for lead-free perovskite solar cells. Herein, we discuss the crystal and band structures, as well as the optoelectronic properties of tin perovskites. Furthermore, recent representative studies on tin perovskite are introduced, along with the strategies employed to improve the conversion efficiency, including the achievements based on component modification, dimension control, crystallization engineering and device structure design. Finally, we highlight the challenges presented by tin perovskites and the possible paths to improve device performance.

In less than a decade, metal halide perovskites (MHPs) have been demonstrated as promising solar cell materials because the photoelectric conversion efficiency (PCE) of the representative material CH₃NH₃PbI₃ rapidly increased from 3.8% in 2009 to 25.2% in 2009. However, defects play crucial roles in the rapid development of perovskite solar cells (PSCs) because they can influence the photovoltaic parameters of PSCs, such as the open circuit voltage, short-circuit current density, fill factor, and PCE. Among a series of superior optoelectronic properties, defect tolerance, i.e., the dominate defects are shallow and do not act as strong nonradiative recombination centers, is considered to be a unique property of MHPs, which is responsible for its surprisingly high PCE. Currently, the growth of PCE has gradually slowed, which is due to low concentrations of deep detrimental defects that can influence the performances of PSCs. To further improve the PCE and stability of PSCs, it is necessary to eliminate the impact of these minor detrimental defects in perovskites, including point defects, grain boundaries (GBs), surfaces, and interfaces, because nonradiative recombination centers seriously affect device performance, such as carrier generation and transport. Owing to its defect tolerance, most intrinsic point defects, such as V_I and V_MA, form shallow level traps in CH₃NH₃PbI₃. The structural and electronic characteristics of the charged point defect V_I- are similar to those of the unknown donor center in a tetrahedral semiconductor. It is a harmful defect caused by a large atomic displacement and can be passivated to strengthen chemical bonds and prevent atom migration by the addition of Br atoms. Owing to the ionic nature of MHPs and high ion migration speed, there are a large number of deep detrimental defects that can migrate to the interfaces under an electric field and influence the performance of PSCs. In addition, the ionic nature of MHPs results in surface/interface dangling bonds terminated with cations or anions; thus, deep defects can be passivated through Coulomb interactions between charged ions and passivators. Hence, the de-active deep-level traps resulting from charged defects can be passivated via coordinate bonding or ionic bonding. Usually, surface-terminated anions or cations can be passivated by corresponding cations or anions through ionic bonding, and Lewis acids or bases can be passivated through coordinated bonding. In this review, we not only briefly summarize recent research progress in defect tolerance, including the soft phonon mode and polaron effect, but also strategies for defect passivation, including ionic bonding with cations or anions and coordinated bonding with Lewis acids or bases.

In recent years, lead-halide perovskites, one of the most competitive material types in the field of semiconductors, has attracted widespread attention because of its easy preparation, low cost, and high performance. Lead-halide perovskites are a type of material with an ABX₃ structure, in which A is an organic or inorganic monovalent cation, B is a divalent cation, and X is a halogen ion. Among them, the B-site ion and X-site ion form an octahedron, with the B-site ion occupying the center and the X-site ion located at the apex of the octahedron. This type of octahedron can undergo lattice changes such as rotation or tilt through the replacement of different halogen anions, which affects the material band gap. The octahedron is located in the center of a cube, which is composed of A-site ions. These structures constitute the basic unit of the perovskite. Compared with the widely used Ⅱ-Ⅵ or Ⅲ-Ⅴ semiconductor nanocrystalline materials, perovskite nanocrystals have great application potential owing to their superior optoelectronic performance. However, their stability problem restricts further development, making them unable to compete in commercial applications. Studies on the stability of perovskite materials began in 2009. It was discovered through experiments that perovskite materials would undergo irreversible degradation under the action of liquid polar solvents, which confirmed that humidity and air are important factors in perovskite degradation. With further research, the problem of illumination has also come to the surface. It was found through experiment that, when oxygen and humidity were excluded, the light condition could also have a certain negative impact on perovskite materials, and subsequently perform a certain repair effect. Research in this area can lay a foundation for the preparation of high-stability perovskite materials and devices, adjust the structure and performance of perovskite by lighting technology (especially laser irradiation), and expand its comprehensive application in the field of optoelectronics. This article focuses on the changes in perovskites under laser irradiation and the related applications. First, it reviews the unstable changes and micro-mechanisms that laser-irradiation induces in lead-halide perovskites, including accelerated degradation, repair of defects, segregation, phase transitions, and changes in the grain size. Second, based on these mechanisms, it explains how researchers have recently used laser-irradiation technology to control the performance of perovskite films and devices. In addition, it also introduces the application of the laser direct writing process in the fields of perovskite patterning and photoelectric detection. Finally, this paper summarizes the changes induced by the laser-irradiation illumination and applications of laser-irradiated lead-halide perovskites.

Halide perovskites are ionic semiconductors with outstanding features, such as high defect tolerance, long carrier diffusion length, strong photoluminescence, narrow emission line width, solution processability, and low cost of fabrication. These advantages render them promising candidates for photovoltaics, lasers, displays, and photodetectors. Theoretical and experimental studies have demonstrated that the optical properties of perovskite materials can be strongly affected by their crystal size and dimension. Owing to their ionic nature and low formation energy, perovskites can be synthesized via precipitation. This process typically involves in situ transformation of the precursors with solvent evaporation and/or solvent mixing. It is well known that the physical/chemical properties of solvents play a vital role in determining the size and dimension of the resultant products. Therefore, elucidating the effects of the solvent on perovskite crystallization is crucial for improving the performance of perovskite-based devices. Moreover, the coordination effects between perovskite precursors and solvents are a dominant parameter that influence the crystallization process because the dissolution of perovskite precursors is strongly correlated with the coordination between the perovskite precursors and solvents. Herein, this minireview summarizes recent research advances in comprehending the perovskite precursor-solvent interactions with a focus on the coordination effects. In particular, we have endeavored to discuss the influence of coordination effects on the formation of polycrystalline thin films, quantum dots, and single crystals. It was found that the formation of perovskite-solvent intermediates in coordinated solvents retard the nucleation and growth of perovskite crystals; this proves beneficial for the fabrication of high-quality micrometer-sized perovskite polycrystalline films. Meanwhile, the preformed intermediates contribute to undesired impurities and defects in single crystals and quantum dots. These insights are exceedingly helpful for the crystallization control of perovskites, thus enabling better device performance and enhanced stability. Finally, the minireview discusses the challenges facing perovskite crystallization, along with a short perspective for future studies.

Planar p-i-n perovskite solar cells (pero-SCs) with solution-processed fabrication, low cost, flexible device fabrication, and negligible hysteresis have attracted significant interest. Hole transport material (HTM) plays a crucial role in improving the performance of p-i-n planar pero-SCs by facilitating hole extraction and then reducing surface recombination. Two types of HTMs have been used in p-i-n pero-SCs. p-Type inorganic semiconductors, such as NiO, CuI, Cu₂O, CuSCN, and graphene oxide, have shown good efficiency and stability; however, organic semiconductor-based HTMs (e.g., poly[bis(4-phenyl) (2, 4, 6-trimethylphenyl)amine] (PTAA), polyarylamine (poly-TPD), poly(N-9-heptadecanyl-2, 7-carbazole-alt-5, 5-(4, 7-di(thien-2-yl)-2, 1, 3-benzothiadiazole)) (PCDTBT), and triphenylamine or thiophene derivative) have outstanding processability for simple one-step solution process at low temperatures; therefore, they should be investigated further. However, their electrical properties are usually inferior than those of inorganic semiconductors, and additives are required to improve their mobility and conductivity, which complicates device processing and results in hysteresis and poor device stability. Therefore, for the organic semiconductor-based HTMs, new hole transport layer (HTL) materials should be developed with easy synthesis process, highly reproducible photovoltaic performance, and better understanding of the structure–property relationship between the HTL materials and the device performance. Herein, an X-type HTM 4, 4, 4'', 4'''-(pyrazine-2, 3, 5, 6-tetrayl)tetrakis(N, N-bis(4-methoxyphenyl)aniline) (PT-TPA) containing pyrazine as the molecular core and triphenylamine (TPA) derivative as branches was designed and synthesized. We introduce an electron-deficient para-diazine as the core and electron-donating methoxytriphenylamine as the peripheral unit to enhance the dipole moment of PT-TPA, which could induce an intermolecular charge transfer. Compared with 4, 4'', 4'', 4'''-silanetetrayltetrakis(N, N-bis(4-methoxyphenyl)aniline) (Si-OMeTPA), the pyrazine core not only endows PT-TPA with good crystallinity but also improves the charge transfer property and plane conjugation of the molecular center, which significantly enhances the hole mobility of PT-TPA. The p-i-n planar pero-SCs based on dopant-free PT-TPA HTL showed a power conversion efficiency of 17.52%, which is approximately 15% higher than that of the device with a Si-OMeTPA HTL.

Perovskite solar cell is a star of the new generation of photovoltaic technology with the greatest application potential due to its simple preparation process and rapid efficiency improvement. At present, the mainstream perovskite solar cell adopts p-i-n structure, using carrier transport materials to extract electrons and holes respectively, so as to realize electric energy output. However, the dependence of traditional p-i-n perovskite solar cell on electron transport layer and hole transport layer makes it not a cost-effective cell, and greatly increases the risk of device stability. Therefore, the design and preparation of perovskite p-n homojunction to realize carrier separation and transmission is considered as an important direction of structural innovation. In recent years, it has been reported frequently that perovskite photoelectric materials exhibit flexible conductivity from p-type, intrinsic to n-type depending on self-doping or external impurities doping. Furthermore, the perovskite p-n homojunction has been developed by a combined deposition method, which provide the possibility for designing and preparing perovskite homojunction solar cells (PHSCs). PHSCs abandon the traditional electron transport layer and hole transport layer, simplifying the device structure. It can not only improve the working stability and reduce the production cost, but also further release the application potential of perovskite solar cells in the field of flexibility and translucency, which can promote the practical popularization of perovskite solar cells. Nevertheless, the PHSCs is still in its infancy, and there are many technical problems to be solved which restrict its efficiency and stability improvement as well as its scale and industrial production. Firstly, the doping degree of perovskite materials should be further increased for high efficiency perovskite homojunction. It means that more accurate self-doping method and exogenous doping processes for heavy doping perovskite need to be developed. Secondly, the stability of the perovskite homojunction should be enhanced to promote the practical application, which requires us to start with the three aspects of inhibiting perovskite decomposition, blocking ion migration, and developing the supporting encapsulation technology to carry out relevant research programs. Thirdly, it is an important task for the industrialization of PHSCs to realize the large-scale preparation through combined deposition method, preservation transfer of perovskite films or superficial doping technology. In this paper, the research progress of PHSCs is reviewed in terms of p-type/n-type doping process and perovskite homojunction. The basic structure, working principle and existing technical problems of PHSCs are discussed in detail. This work has wide ranging impacts beyond solar cells, including emerging applications in light emission, photoelectric detector and neuromorphic computing.

Metal halide perovskites are considered as promising candidates for lighting applications owing to their excellent optoelectronic properties, such as high electron/hole mobility, high photoluminescence quantum yield, high color purity, and facile color tunability. In recent years, perovskite light-emitting diodes (LEDs) have developed rapidly, and their external quantum efficiencies (EQEs) have exceeded 20% for green and red emissions. However, the EQEs and stabilities of blue (particularly deep-blue) perovskite LEDs are still inferior to the green and red counterparts, which severely restricts the application of perovskite LEDs in high-performance and wide color gamut displays as well as white light illumination. Therefore, summarizing the development of blue perovskite LEDs and discussing the opportunities and challenges associated with their future applications will help to guide the further development of the entire perovskite LED field. In this review, according to the emission color, we divide the blue perovskite LEDs into three parts for a better discussion, i.e., the emissions in the sky-blue, pure-blue, and deep-blue regions. We introduce their developed history and discuss the basic strategies to achieve blue emission. There are three typical methods to obtain perovskite emitters with blue emission, i.e., (1) composition engineering, (2) dimensional engineering, and (3) synthesis of perovskite nanocrystals and quantum dots. For composition engineering, changing ions in perovskite ABX₃ structure can easily tune the perovskite emission color, particularly while changing the anions in "X" position. Therefore, modulating the ratio between the X-site anions of Br^- and Cl^- can cause perovskites to emit blue photons ranging from 420 to 490 nm, which almost covers the entire blue spectrum. For dimensional engineering, perovskite materials can form a series of low-dimensional structures (layered structures) with the insertion of organic ligands between the perovskite frameworks. This type of low-dimensional perovskite material typically exhibits better lighting properties than those exhibited by its three-dimensional counterpart owing to its unique charge or energy transfer process of charge carriers. Blue perovskite nanocrystals and quantum dots with high photoluminescence quantum yields are excellent candidates for realizing high-performance pure-blue and deep-blue devices because they can easily incorporate Cl^- in their crystals, which is considerably limited in perovskite thin films owing to the poor solubility of inorganic chloride sources in polar solvents. Furthermore, we discuss several challenges associated with blue perovskite LEDs, such as the inferior device performance in the pure-blue and deep-blue regions, difficulty in hole injection, electroluminescence (EL) instability of mixed halide perovskite systems, and lagged operation lifetime, and introduce potential solutions accordingly. Note that the challenges faced by blue perovskite LEDs are also the opportunities for research in this area. Therefore, this review is of a great reference value for the next evolution of blue perovskite LEDs.

Chemical components of perovskite layers play a key role in improving the efficiency and stability of perovskite solar cells. Pure inorganic perovskites exhibit good thermal and light stabilities; however, the smaller radius of Cs⁺ leads to a poor perovskite phase stability. In this case, the Cs-rich (CH(NH₂)₂)_xCs_1−xPbI₃ ((CH(NH₂)₂⁺=FA⁺) perovskite seems more promising because it simultaneously offers the above-mentioned properties, while not forming an unstable perovskite phase. Thus far, the synthesis of Cs-rich FA_xCs_1−xPbI₃ perovskite has been realized by introducing excess formamidinium iodide (FAI) as an additive. However, FAI sublimates at a high temperature and excessive FAI sublimation necessitates even greater temperatures. Therefore, it is difficult to precisely control the ratio of the sublimated FAI from the perovskite film. Herein, the precise synthesis of Cs-rich FA_xCs_1−xPbI₃ perovskites at relatively low sublimation temperatures using amine additives, such as methylammonium iodide (MAI), dimethylamine iodide (DMAI), ethylamine iodide (EAI), ammonium iodide (NH₄I), and formamidine acetate (FAAC), was studied. The reaction temperature was reduced when utilizing these additives. Moreover, the window period for the preparation has been widened, which is particularly important for the preparation of pure phase Cs-rich FA_xCs_1−xPbI₃ perovskite films for large devices. In the experiment, perovskite FA_0.15Cs_0.85PbI₃ was selected because of its good stability. The reaction process of the additive that assisted perovskite preparation was studied. Firstly, 0.85 mmol of MAI, DMAI, EAI, FAAC, and NH₄I each were added to 1 mmol of FA_0.15Cs_0.85PbI₃ solution. Then, the precursor solution was spin-coated and thermally annealed. The FA_0.15Cs_0.85PbI₃ films were formed by sublimation of the additives during thermal annealing. The influence of different additives on the film formation process was traced using X-ray diffraction (XRD) measurements and UV-visible absorbance spectra (UV-Vis abs). The results showed that MAI and DMAI could be used as additives in the preparation of FA_0.15Cs_0.85PbI₃ films. The strong intermolecular interaction between these additives and PbI₂ could benefit the formation of Cs₄PbI₆ and prevent the formation of δ-CsPbI₃. Cs⁺ is easier to migrate in Cs₄PbI₆ than in δ-CsPbI₃, which provides a necessary condition for the ion exchange reaction. Simultaneously, the mild sublimation temperature of the additives ensured that the films maintain their perovskite phase. Finally, pure phase Cs-rich FA_xCs_1−xPbI₃ perovskites were prepared using this method at a relatively lower temperature of 200 ℃. The XRD and UV-Vis absorption results confirmed the precise synthesis of FA_0.15Cs_0.85PbI₃. The FA_0.15Cs_0.85 PbI₃ solar cells synthesized with MAI and DMAI achieved the maximum power conversion efficiencies of 15.6% and 15.1%, respectively.

Organic-inorganic metal halide perovskite-based tandem solar cells have attracted significant research attention in recent years. The power conversion efficiency of perovskite-based tandem can efficiently meet the requirements of practical applications; however, their instability limits their commercialization. The most commonly used wide-bandgap perovskites suitable for top sub-cells, which are based on I/Br alloying at X site, often suffer from severe phase segregation. When exposed to light illumination, a smaller bandgap phase appears and acts as a carrier trap, leading to a reduction in the quasi-Fermi level splitting and large V_OC deficit. The narrow-bandgap perovskites suitable for bottom sub-cells, which are based on Sn/Pb alloying at B sites, always face atmospheric instability. When exposed to air, Sn²⁺ is rapidly oxidized to Sn⁴⁺, which can shorten the carrier diffusion length and result in a drop in efficiency. Herein, we summarize the recent advances in perovskite-based tandem solar cells from the viewpoint of stability. We analyzed the stability data of highly efficient perovskite-based tandems reported so far, such as perovskite/silicon, perovskite/perovskite, and perovskite/copper indium gallium selenide (CIGS) tandems. We found that the key to improve the perovskite-based tandems is to improve the stability of the perovskite sub-cells. Then, we systematically analyzed the phase and atmospheric instability of wide- and narrow-bandgap perovskite, respectively, providing some reasonable strategies to tackle the instability. Compositional engineering, crystallinity optimization, and employing other perovskites with wide bandgaps are effective means to avoid phase instability of the I/Br alloying perovskite. Introducing the reducing additives, improving the film morphology, and forming a 2D/3D structure can help in improving the atmospheric stability of Sn-Pb narrow bandgap perovskites. Furthermore, we review the intrinsic instability of perovskite and corresponding improvement methods, which are inevitable in future tandem solar cells. By reducing the methylamine (MA) content in perovskite component and suppressing ion migration, the long-term operational stability is greatly enhanced. Finally, we briefly summarize the instability issues related to the interconnecting layer. In addition to the optimization of perovskite-based tandem devices, encapsulation also plays a crucial role in improving stability against environmental stressors. Studies based on improving the stability of perovskite-based tandems are still in the early stage. However, with a deeper understanding of the stability of perovskite sub-cells and the interconnecting layer, the commercialization of perovskite-based tandems, especially perovskite/silicon tandem devices, is promising to be achieved in the near future.