Mannino Giovanni, Deretzis Ioannis, Smecca Emanuele, La Magna Antonino, Alberti Alessandra, Ceratti Davide, Cahen David
CNR-IMM, Zona Industriale Strada VIII No. 5, 95121 Catania, Italy.
Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
J Phys Chem Lett. 2020 Apr 2;11(7):2490-2496. doi: 10.1021/acs.jpclett.0c00295. Epub 2020 Mar 16.
Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr, CsPbBr and for the first time FAPbBr) by spectroscopic ellipsometry in the range of 1-5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic-tetragonal and tetragonal-cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.
单晶是理解卤化物钙钛矿体相性质的基准。我们确实通过光谱椭偏仪在1-5电子伏特范围内研究了溴化铅钙钛矿单晶（MAPbBr、CsPbBr以及首次研究的FAPbBr）的介电函数，同时将温度从183 K变化到440 K。在子带隙区域发现了极低的吸收系数，表明这三种晶体都具有很高的光学质量。我们通过临界点分析提取了带隙值，结果表明基于陶克方法得到的值被系统地低估了。两个结构相变，即正交晶系-四方晶系和四方晶系-立方晶系，表现出不同的光学行为，前者具有不连续的特征。光学数据与密度泛函理论（DFT）计算的互相关表明，在给定温度下，八面体倾斜在调整带隙值方面发挥了作用，而热膨胀的差异影响了带隙随温度变化趋势的斜率。
