The Structure Distortion And Function Change Of Bi-based Organic-inorganic Hybrid Perovskites Caused By Mixed Halide

The photovoltaics,as one of the solutions to the future energy challenges,have draw much attentions during the past decades.The halide perovskite as a new material for solar cells has seen an increase in power conversion efficiency from 9.7%to-22%in just few years.One major disadvantage of MAPbX₃ perovskite（the most popular kind of perovskite）is that it contains lead（Pb）,which is rather toxic and makes the material unstable.An important task is therefore to replace lead with other less toxic element.Bi³⁺ and Pb²⁺ have the same 6s26p0 electronic confguration,what’s more,Bi has low toxicity and Bi-based perovskite has excellent stability.Thus as a new material,Bi-based perovskites have been the subject of increased attention.For the hybrid halide perovskites ABX₃,X stands for the halogens.When mixed halogens were used,the materials are called mixed halide perovskites.Mixed halide in the hybrid perovskites effectively tunes their optical bandgaps,photoluminescence abilities,carrier diffusion length,and subsequently the photovoltaic performances.However,most of studies of the mixed halide perovskites focus on the Pb-based perovskites and a deep understanding about the specific effect of the doping halogens on the structural features and the functionalities of the mixed halide perovskites remain one of the most uncertain questions.Thus,in this paper,we investigate a series of of Bi-based halide perovskite as a mixed halide model:[NH₂（CH₂CH₃）₂]₃Bi(Cl_1-xBr_x)₆（x = 0,0.135,0.255,0.385,0.847,1）single crystals.The detailed exploration revealed that there is a systematic structural and functional regulation over the ratio of Cl/Br.The results shed light on the hidden structural mechanisms of the mixed halide perovskites,and are promising useful directions for the precise chemical methods needed to modulate the properties of materials via mixed halide resources.The main contents are as fellows:（1）Using six kinds of mixed solvents with the molar atio of HCl/HBr = 1/0,0.8/0.2,0.6/0.4,0.4/0.6,0.2/0.8,and 0/1,we sysnthesized a series of[NH₂（CH₂CH₃）₂]₃Bi(Cl_1-xBr_x)₆（x = 0,0.135,0.255,0.385,0.847,and 1）crystals.And the inclusion ratio of the second halogen into a mixed halide crystal is found to be not linearly correlated with the ratio in the mixed halide resources.Normally,the minority ions are more likely to be excluded into the crystals.The different Br/Cl ratios in the single crystals from the precursor solutions remind us to be cautious about the halogen concentration of the in situ formed films of mixed halide perovskites.（2）We studied the crystal structures of this series of[NH₂（CH₂CH₃）₂]₃Bi(Cl_1-xBr_x)₆ crystals.In the Bi-X coordination,the Cl and Br share the same position,which means that the two different halogens can appear on one site,with an occupancy sum being equal to unity.And this fractional occupancy is also dominant in the lead coordination of the well-known mixed halide perovskites systems.（3）In the crystal structures,the unit cell parameters a and c increase linearly with the ratio of bromide.And by determining the statistical local dipole moments of each {BX₆} octahedron,we found there are relatively large differences of the dipole moments with different halogen ratios.This data means that the mixed halides could make an obvious contribution to the distortion of {BX₆} octahedron.The distortion of the BiX₆ octahedron may be caused by the variation of the bend length and angle of X₁-Bi-X₂,and the interactions between BiX₆ anions and the diethylammonium cations.（4）Optical absorption,thermal properties,and the SHG measurements have confirmed that,with increased Br inclusion,[NH₂（CH₂CH₃）₂]₃Bi(Cl_1-xBr_x)₆ crystals exhibit a regulated effect on their bandgaps,thermal stabilities,and SHG capacities.And because of the strong SHG efficiency,bulk[NH₂（CH₂CH₃）₂]₃BiBr₆（TDBB）single-crystals were grown and were proven to be a kind of potential nolinear optical crystals.