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Abstract Domain features and domain walls in lead halide perovskites (LHPs) have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of solution‐processable semiconductors. Usingnonpolarizedlight and simple imaging configurations, ferroelastic twin domains and their switchings through multiple consecutive phase transitions are directly visualized. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, optically anisotropic domains inside the material bulk. The findings show these domain walls serve as internal reflectors and steer energy transport inside halide perovskitesoptically. First‐principles calculations show universal low domain‐wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile moving observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of nonvolatile on–off switching and tunable positioning endowed by their universal ferroelasticity. 

It is shown that in the formamidinium (FA) lead iodide/titania heterostructure α‐HC(NH₂)₂PbI₃/TiO₂the organic layer‐mediated interface, i.e., FAI/TiO₂, can induce photovoltaic diode effect via positive bias poling. The band gap of the heterostructure is reduced to zero upon the positive poling due to combined effects of ion diffusion, rotation of organic moieties, and ferroelectric redistribution. The perovskite part in the organic layer‐mediated interface FAI/TiO₂gives rise to a strong polarization of 18.69 μC cm⁻², compared to that (0.89 μC cm⁻²) in the inorganic layer‐mediated interface PbI₂/TiO₂. The strong polarization of the organic layer‐mediated interface is closely related to the diode effect associated with the reordering of the ferroelectric polarization and charge distribution, as a consequence of the mobility and rotation of organic moieties in FAI/TiO₂upon the positive bias poling. The latter effect also provides an explanation on why the FAPbI₃‐based devices can largely reduce the scanning hysteresis in theJ–Vcurves (Yang et al.,Science2015,348, 1234) and why the organic layer‐mediated halide perovskite heterostructure is one of the most promising candidates for the fabrication of highly efficient solar cells or optoelectronic devices. 

Abstract Theoretical and experimental investigations of various exfoliated samples taken from layered In₄Se₃crystals are performed. In spite of the ionic character of interlayer interactions in In₄Se₃and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few‐nanometer‐thick flakes of In₄Se₃by mechanical exfoliation of its bulk crystals. The In₄Se₃flakes exfoliated on Si/SiO₂have anisotropic electronic properties and exhibit field‐effect electron mobilities of about 50 cm² V⁻¹ s⁻¹at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS₂and TiS₃. In₄Se₃devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry‐dependent band structure calculations for the most expectedaccleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In₄Se₃crystals is possible, while the fast anisotropic photoresponse makes In₄Se₃a competitive electronic material, in the TMC family, for emerging optoelectronic device applications. 

Abstract The surface composition of perovskite films is very sensitive to film processing and can deviate from the optimal, which generates unfavorable defects and results in efficiency loss in solar cells and slow response speed in photodetectors. An argon plasma treatment is introduced to modify the surface composition by tuning the ratio of organic and inorganic components as well as defect type before deposition of the passivating layer. It can efficiently enhance the charge collection across the perovskite–electrode interface by suppressing charge recombination. Therefore, perovskite solar cells with argon plasma treatment yield enhanced efficiency to 20.4% and perovskite photodetectors can reach their fastest respond speed, which is solely limited by the carrier mobility. 

Abstract Formamidinium (FA)‐based lead iodide perovskites have emerged as the most promising light‐absorber materials in the prevailing perovskite solar cells (PSCs). However, they suffer from the phase‐instability issue in the ambient atmosphere, which is holding back the realization of the full potential of FA‐based PSCs in the context of high efficiency and stability. Herein, the tetraethylorthosilicate hydrolysis process is integrated with the solution crystallization of FA‐based perovskites, forming a new film structure with individual perovskite grains encapsulated by amorphous silica layers that are in situ formed at the nanoscale. The silica not only protects perovskite grains from the degradation but also enhances the charge‐carrier dynamics of perovskite films. The underlying mechanism is discussed using a joint experiment‐theory approach. Through this in situ grain encapsulation method, PSCs show an efficiency close to 20% with an impressive 97% retention after 1000‐h storage under ambient conditions.