High-Quality CH3NH3PbI3 Films Obtained via a Pressure-Assisted Space-Confined Solvent-Engineering Strategy for Ultrasensitive Photodetectors

Vapor Phase Growth of Air-Stable Hybrid Perovskite FAPbBr3 Single-Crystalline Nanosheets

Organic-inorganic hybrid perovskites have attracted tremendous attention owing to their fascinating optoelectronic properties. However, their poor air stability seriously hinders practical applications, which becomes more serious with thickness down to the nanoscale. Here we report a one-step vapor phase growth of HC(NH2)2PbBr3 (FAPbBr3) single-crystalline nanosheets of tunable size up to 50 μm and thickness down to 20 nm. The FAPbBr3 nanosheets demonstrate high stability for over months of exposure to air with no degradation in surface roughness and photoluminescence efficiency. Besides, the FAPbBr3 photodetectors exhibit superior overall performance as compared to previous devices based on nonlayered perovskite nanosheets, such as an ultralow dark current of 24 pA, an ultrahigh responsivity of 1033 A/W, an external quantum efficiency over 3000%, a rapid response time around 25 ms, and a high on/off ratio of 104. This work provides a strategy to tackle the challenges of hybrid perovskites toward integrated optoelectronics with requirements of nanoscale thickness, high stability, and excellent performance.

Formamidinium Lead Iodide Perovskite Thin Films Formed by Two-Step Sequential Method: Solvent-Morphology Relationship

Thin films made of formamidinium lead iodide (FAPbI₃) perovskites prepared by a two-step sequential deposition method using various solvents for formamidinium iodide (FAI) - isopropanol, n-butanol and tert-butanol, were studied with the aim of finding a correlation between morphology and solvent properties to improve film quality. They were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and their photophysical properties were studied by means of absorption and photoluminescence (PL) spectroscopies. XRD patterns, absorption and PL spectra proved α-phase formation for all selected solvents. An excessive amount of PbI₂ found in perovskite films prepared with n-butanol indicates incomplete conversion. Thin film morphology, such as grain and crystallite size, depended on the solvent. Using tert-butanol, thin films with a very large grain size of up to several micrometers and with preferred crystallite orientation were fabricated. The grain size increased as follows: 0.2-0.5, 0.2-1 and 2-5 µm for isopropanol, n-butanol and tert-butanol, respectively. A correlation between the grain size and viscosity, electric permittivity and polarizability of the solvent could be considered. Our results, including fabrication of perovskite films with large grains and fewer grain boundaries, are important and of interest for many optoelectronic applications.

Suppression of Phase Transitions in Perovskite Thin Films through Cryogenic Electron Beam Irradiation

Organic-inorganic hybrid perovskites (OIHPs) with superior optoelectronic properties have emerged as revolutionary semiconductor materials for diverse applications. A fundamental understanding of the interplay between the microscopic molecular-level structure and the macroscopic optoelectronic properties is essential to boost device performance toward theoretical limits. Here, we reveal the critical role of CH₃NH₃⁺ (MA) in the regulation of the physicochemical and optoelectronic properties of a MAPbI₃ film irradiated by an electron beam at 130 K. The order-to-disorder transformation of the MA cation not only leads to a notably enhanced photoluminescence emission but also results in the suppression of the orthorhombic phase down to 85 K. Taking advantage of the regulation of MA cation dynamics, we demonstrate a perovskite photodetector with 100% photocurrent enhancement and long-term stability exceeding one month. Our study provides a powerful tool for regulating the optoelectronic properties and stabilities of perovskites and highlights potential opportunities related to the organic cation in OIHPs.

Surface Energy-Driven Preferential Grain Growth of Metal Halide Perovskites: Effects of Nanoimprint Lithography Beyond Direct Patterning

Hybrid organic-inorganic lead halide perovskites have attracted much attention in the field of optoelectronic devices because of their desirable properties such as high crystallinity, smooth morphology, and well-oriented grains. Recently, it was shown that thermal nanoimprint lithography (NIL) is an effective method not only to directly pattern but also to improve the morphology, crystallinity, and crystallographic orientations of annealed perovskite films. However, the underlining mechanisms behind the positive effects of NIL on perovskite material properties have not been understood. In this work, we study the kinetics of perovskite grain growth with surface energy calculations by first-principles density functional theory (DFT) and reveal that the surface energy-driven preferential grain growth during NIL, which involves multiplex processes of restricted grain growth in the surface-normal direction, abnormal grain growth, crystallographic reorientation, and grain boundary migration, is the enabler of the material quality enhancement. Moreover, we develop an optimized NIL process and prove its effectiveness by employing it in a perovskite light-emitting electrochemical cell (PeLEC) architecture, in which we observe a fourfold enhancement of maximum current efficiency and twofold enhancement of luminance compared to a PeLEC without NIL, reaching a maximum current efficiency of 0.07598 cd/A at 3.5 V and luminance of 1084 cd/m² at 4 V.

VLS Homoepitaxy of Lead Iodide Nanowires for Hybrid Perovskite Conversion

Controlled fabrication of lead halide-based perovskite (LHP) nanostructures provides a new methodology for exploiting the excellent optoelectronic properties of the material. Here, we report the vapor-liquid-solid (VLS) growth of a highly uniform and dense array of [0001]-oriented PbI₂ nanowires using PbI₂ thin film as the epitaxial substrate layer. We show that reducing the lattice mismatch of the van der Waals epitaxial PbI₂ substrate layer is necessary to accommodate the aligned nanowire growth. Our proposed layer growth model suggests that the nanowire growth is stabilized by maintaining the {0001} liquid-solid interface, which stems from the nucleation on the PbI₂ substrate layer. We also demonstrate that the strain-induced nanowire deflection after conversion into CH₃NH₃PbI₃ depends on the transfer sequence and conversion time. These findings provide a general opportunity to design and fabricate nanostructures, such as heterojunctions or superstructures for future device applications, rationally based on lead halide or LHP nanowires.

Nanomechanical Approach for Flexibility of Organic-Inorganic Hybrid Perovskite Solar Cells

The mechanical flexibility of perovskite solar cells as well as high power conversion efficiency is attracting increasing attention. In addition to existing empirical approaches, such as cyclic bending tests, in this study we report the tensile properties of the perovskite materials themselves. Measuring the tensile properties of free-standing perovskite materials is critical because (1) tensile properties represent the realistic mechanical properties of the film-type perovskite layer in the solar cells including the effects of various defects, and (2) deformation behavior of the perovskite layer at any deformed state of the solar cells can be analyzed using solid mechanics with the tensile properties as input. Critical bending radius of MAPbI₃-based flexible solar cells is found to be between 0.5 and 1.0 mm by the decrease in power conversion efficiency during cyclic bending deformation. This finding agrees well with the critical bending radius of 0.66 mm determined based on the elastic deformation limit of 1.17% for MAPbI₃ found by in situ tensile testing. Scanning electron microscopy observations and hole-nanoindentation tests suggest that the formation of coarse cracks in the perovskite layers is the primary cause of the decrease in power conversion efficiency observed in flexible perovskite solar cells.

Pressure-Assisted Annealing Strategy for High-Performance Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors

Owing to their low trap-state density, high carrier mobility, and high thermal stability, CsPbBr₃ perovskite microcrystals (MCs) have attracted significant attention for applications as photodetectors (PDs). However, solution synthesis processes lead to MC films with high void density, seriously limiting the performance of the PDs. Here, a pressure-assisted annealing strategy is introduced to significantly reduce the void density and decrease the surface roughness. The resulting self-powered all-inorganic CsPbBr₃ perovskite MC thick-film PDs show improved performance characteristics, with responsivities and detectivities of up to 0.206 A W^-1 and 7.23 × 10¹² Jones, respectively. Moreover, the on/off ratios of the devices are up to 10⁶, and the highest linear dynamic range reaches 123.5 dB. These improved results indicate that the pressure-assisted annealing method is an effective strategy to enhance the performance of solution-synthesized perovskite MC PDs.