高压下有机-无机杂化钙钛矿CH_3NH_3PbI_3的结构及光学性质研究

High-pressure structural and optical properties of organic-inorganic hybrid perovskite CH_3NH_3PbI_3

近年来,随着有机-无机杂化钙钛矿太阳能电池的飞速发展,对此类材料基本物性的探索引起了科学家们的广泛关注.本文利用金刚石对顶砧装置对甲胺基碘化铅(CH_3NH_3PbI_3)进行高压实验,研究了室温下压力诱导CH_3NH_3PbI_3的结构变化以及压力对其光学性质的调控,实验最高压力为7 GPa.原位高压同步辐射X射线衍射实验结果显示,CH_3NH_3PbI_3样品在0.3 GPa由四方相转变为正交相,在4 GPa左右开始非晶化.结合原位高压吸收和荧光光谱,分析了压力对CH_3NH_3PbI_3带隙大小的调控作用.进一步利用原位高压拉曼光谱和红外光谱实验研究了CH_3NH_3PbI_3晶体中有机阳离子(CH_3NH_3^+)在高压下的行为.完全卸压后,样品恢复到加压前的初始状态.研究结果可为深入了解有机-无机杂化钙钛矿的光学性质和结构稳定性提供一些信息.

Recent advance in highly efficient solar cells based on organic-inorganic hybrid perovskites has triggered intense research efforts to ascertain the fundamental properties of these materials. In this work, we utilize diamond anvil cell to investigate the pressure-induced structural and optical transformations in methylammonium lead iodide（CH_3NH_3PbI_3） at pressures ranging from atmospheric pressure to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment shows that the sample transforms from tetragonal（space group I4cm） to orthorhombic（space group Imm2）phase at 0.3 GPa and amorphizes above 4 GPa. Pressure dependence of the unit cell volume of CH_3NH_3PbI_3 shows that the unit cell volume undergoes a sudden reduction at 0.3 GPa, which can prove the observed phase transition. We provide the high-pressure optical micrographs obtained from a diamond anvil cell. Upon compression, we can visually observe that the opaque black sample gradually transforms into a transparent red one above 4 GPa. We analyze the pressure dependence of the band gap energy based on the optical absorption and photoluminescence（PL） results. As pressure increases up to 0.25 GPa, the absorption edge and PL peak move to the longer wavelength region of 9 nm.However, abrupt blueshifts of the absorption edge and PL peak occur at 0.3 GPa, followed by a gradual blueshift up to 1 GPa, these phenomena correspond to the previously observed phase transitions. Phase transition increases the band gap energy of CH_3NH_3PbI_3 as a result of reductions in symmetry and tilting of the [PbI_6]^（4-） octahedral. Upon further compression, the sample exhibits pressure-induced amorphization at about 4 GPa, which significantly affects its optical properties. Further high pressure Raman and infrared spectroscopy experiments illustrate the high pressure behavior of organic CH_3NH_3~＋ cations. Owing to the presence of hydrogen bonding between organic cations and the inorganic framework, all of the bending and rocking modes of CH3 and NH3 groups are gradually red-shifted with increasing pressure. The transition of N—H stretching mode from blueshift to redshift as a result of the attractive interactions between hydrogen atoms and iodine atoms is gradually strengthened. Moreover, all the observed changes are fully reversible when the pressure is completely released. In situ high pressure studies provide essential information about the intrinsic properties and stabilities of organic-inorganic hybrid perovskites, which significantly affect the performances of perovskite solar cells.