钙钛矿光伏材料原子动力学的中子散射研究

Organic-inorganic hybrid perovskite photovoltaic materials have been attracting the worldwide attention for their rocketing efficiencies and lower cost. It is believed that the reorientation rotation of CH3NH3 in the prototype CH3NH3PbI3 is responsible for the screening of carriers and the formations of large polarons for its ultrafast polar fluctuations in the picosecond scale. However, the cubic phase of all-inorganic perovskite CsPbI3 processes a decent photovoltaic efficiency as well. As a result, it is necessary to perform a systematic atomic-dynamics study to understand how the carriers are protected from recombinaitons. Based on our previous study on CH3NH3PbI3, which has been published, we are proposing to extend our atomic dynamics studies to the related HC(NH2)2PbI3 and CsPbI3, using the similar inelastic (quasi-elastic) neutron scattering technologies. We will determine the phonon spectra of both compounds and the reorientation rotational mode of HC(NH2)2 molecules as well. These complete atomic dynamics data allow us to establish the universal carrier protection mechanism for all perovskites. Our study is expected to provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized.

快速提高的转换效率和低廉的成本使钙钛矿有机-无机杂化光伏材料备受关注。目前普遍认为CH3NH3PbI3中有机分子的再取向旋转导致了皮秒尺度的极化涨落，起到屏蔽载流子而形成大极化子的作用。然而，进一步研究发现全无机钙钛矿CsPbI3的立方相也具有较好的光伏性质。因此，系统研究钙钛矿体系的原子动力学对理解载流子屏蔽机制具有重要意义。基于之前对CH3NH3PbI3的声子与分子再取向旋转的研究，申请人拟继续以非（准）弹性中子散射为主要技术手段研究HC(NH2)2PbI3和CsPbI3，获得声子谱和HC(NH2)2分子的再取向旋转模式等原子动力学数据。综合分析CH3NH3PbI3、HC(NH2)2PbI3和CsPbI3，从原子动力学角度来探索钙钛矿光伏材料载流子屏蔽的普适规律，深化对该类材料的光伏效应的物理认识，为实验上进一步优化甚至提高材料的性能提供明确的物理依据。

快速提高的转换效率和低廉的成本使钙钛矿有机-无机杂化光伏材料备受关注。该类材料的组成兼具无机刚性骨架与有机集团的丰富分子构型，且二者之间具有氢键相互作用。这些“杂化”特点使得该类材料具有与传统无机半导体材料完全不同的性质。要完整地理解这一类材料，需要同时了解材料的声子（集体动力学）与分子再取向旋转模式（单粒子动力学），而非弹性中子散射与准弹性中子散射是研究这一问题的最佳技术手段。本项目正是以该方法研究有机-无机杂化钙钛矿材料的原子结构与动力学。我们在日本散裂中子源NOVA谱仪测量了CH(NH2)2PbI3（缩写为FAPbI3）粉末样品，获得了不同温度下材料的全散射数据；在日本散裂中子源AMATERAS谱仪上获得了样品不同温度下的准弹性散射和非弹性散射数据。综合分析表明该材料在140K和290K先后发生了从四方I相到四方 II相，再到立方相的结构相变；在四方相FA分子具有C2/C3再取向旋转特征，而在立方相具有各向同性再取向旋转模式。这一结构相变可归结为FA分子热运动的冻结，通过氢键相互作用使得 无机晶格发生畸变，从而诱发整体的结构相变。这一结果为准确理解FAPbI3的优异光伏性能提供了结构与动力学基础。我们也发现FAI在室温附近具有巨压卡效应，原位拉曼散射研究表明压力对氢键的抑制作用是产生该效应的物理根源。同时在相关的有机塑晶材料新戊二醇、碳硼烷等体系中发现了庞压卡效应，利用中子散射技术证实了该效应来源于压力对分子再取向旋转的抑制。