Opto-Electrical Characteristics of Halide Perovskites

Halide perovskites have shown rapid development in opto-electrical applications, such as solar cells, light-emitting diodes, and photodetectors owing to their superior optical and electrical properties. Particularly, halide perovskites surpassed the state-of-art efficiency of semiconductor compounds such as CdTe and copper indium gallium selenide (CIGS) used in solar cells in just about a decade. Perovskite's ease of synthesis in ambient environment and its simple sandwich device structures indicate the potential of this thin-film solar cell technology to become a low-cost alternative to the commercially available photovoltaic technologies. However, the halide perovskites suffer from moisture-induced degradation, which holds up the commercialization. Long-term operation in the ambient environment for halide perovskites without costly encapsulation is in great demand. Recent research in optoelectronics has diverted from the use of the original 3-dimensional (3D) halide perovskites to 2-dimensional (2D) halide perovskite materials. Besides the inherited superior optical and electrical properties from their 3D counterparts, the 2D halide perovskites that are composed of the hydrophobic organic ligands feature satisfying stability against moisture. In this dissertation, the desirable properties of 2-dimensional (2D) halide perovskite for photovoltaics were discovered via electrical transport study and numerical simulation. Probing with electrical and optical stimuli, the transport process including injection, drift, diffusion, recombination, and extraction of photoexcited charge carriers were investigated using a scanning probe technique called scanning photocurrent microscopy (SPCM), and then numerically simulated via the finite element analysis. Below-bandgap energy states were discovered at the crystal edge of the 2D halide perovskites. The desorption of the long-chain organic ligands that served as spacers between the 2D inorganic sheets caused the formation of 3D halide perovskite-like crystalline near the edge. Using the combination of SPCM measurement and finite element analysis simulation, we proved that type-II heterojunctions were formed at the interfaces between the 2D perovskite and 3D perovskite, and these rectified junctions could efficiently facilitate electron-hole dissociation and suppress the energy loss through charge recombination. As our most important finding, this type-II heterojunction is rooted in the boosted photovoltaic efficiency reported previously. Adopting the bulk solar cell structure, we further proposed the design idea for utilizing such rectified heterojunction in 2D perovskite-based photovoltaic devices, promising higher stability and efficiency in the future perovskite-based solar cell. Significant developments in the performance and stability of 3D perovskite-based opto-electrical devices were also demonstrated in this dissertation by embedding 3D halide perovskite nanoparticles in a more stable compound. Light-emitting diodes based on host-guest structure of inorganic 3D halide perovskite CsPbBr3 and non-perovskite material Cs4PbBr6 showed highly enhanced device stability and power-conversion-efficiency. The inorganic composition of the emitting material and the self-assembled encapsulation of 3D halide perovskite nanoparticles led to improved device stability under continuous operation. The shallow trap-states generated at the CsPbBr3/Cs4PbBr6 interfaces resulted in strongly enhanced photoluminescence and quantum yield that led to improved device efficiency. The studies in this dissertation indicated great potential of halide perovskite towards more stable and efficient optoelectronic devices.