Cations Substitution Tune Structure And Properties In Lead Halide Perovskites

Organic-lead trihalide hybrid perovskites have been widely investigated for cells,photodetectors,lasing,light-emitting diodes,and hydrogen production,owing to their superior characteristics including direct bandgap,highly balanced hole and electron mobility,strong absorption coefficient and long carrier lifetime etc.Currently,the most-studied lead halide perovskites usually have methylammonium（MA）at the A site.In fact,there are three types of MAPbI₃ crystals,orthorhombic,tetragonal and cubic structures.MAPb I₃ crystals have a tetragonal structure at room temperature,which suffers from lattice distortion and strain.The optical response range of the MAPb I₃ perovskite is not wide enough for perfect light absorption by solar cells.Also,a MAPb I₃ crystal is unstable,with a decomposition temperature of 500 K.Composition engineering could be a potential solution to solve these problems.A mixed cation perovskite has been shown to be one of the most practical and successful strategies for improving the properties of lead halide perovskite.However,it is still not clear what role organic cations play,nor how they influence the materials properties in MAPbI₃ perovskite.In this work,the conditions of perovskite-type organic lead-iodine single crystals grown by inverse temperature crystallization process were explored.A series of high quality room temperature stable single crystals were synthesized by adjusting the cation concentration and size.In order to investigate the effect of cations concentration and sizes on the of perovskite organic lead halides crystal,the effect of structure,optical features,thermal stability,and electrical transport properties are investigated.The main contents and conclusions of the study are as follows:1.High-quality methylamine lead iodine single crystals were grown by an inverse temperature crystallization method.The effects of concentration of growth liquid,growth temperature,and growth time on crystal crystallization were studied.The crystal structure,thermal properties,optical properties,and electrical properties were also investigated.2.We report a route to synthesis cubic phase crystal EA@MAPb I₃ and FA@MAPb I₃ by using inverse temperature reactive crystallization process.Big size organic cation EA⁺and FA⁺cations incorporated into MAPbI₃ could obtain stable cubic single crystal via altering the PbI₆ octahedral cage and relaxed strain.The large radius of EA⁺causes lattice dilation and adjusts the tolerance factor toward 1,favourable to stabilize the cubic perovskite.MA⁺cation incorporated has reduced the lattice volume and relaxed the strain in lattice and thereby prevent the phase transition from the cubic phase to δ-phase.Both EA@MAPbI₃ and FA@MAPb I₃ single crystals show remarkable thermal stability with no endothermic peak at range 50-170℃.Direct dark I-V measurement of cubic phase crystals indicates low trap state density.Both EA@MAPb I₃ and FA@MAPbI₃ have superior light-absorbing capability which holds potential to be a suitable photoelectric material.3.In order to study the relationship between crystal performance and A-site cations,the optical properties,thermal stability,and electron transport properties of this series of single crystals were tested.Our findings suggest that:1)Precise band gap engineering is achieved by controlling bond angles;2)Thermal stability is related to the probability of HI formation,which is directly related to the acidity of the organic species;and 3)Organic cations affect transport property of crystal via affecting single structure.The crystals have a cubic phase and low lattice strain possessing low trap density.These findings provide possibilities to tune the structures and properties of the existing well-studied 3D perovskites and to design new materials.