Abstract :
The record solar cell efficiency based on hybrid organic-inorganic perovskites (HOIPs) has reached 22% which rivals that of conventional silicon solar cells. Combined with its inexpensive solution based processing and all earth abundant compositions, HOIPs are among the most promising next generation solar cell materials. A challenge for hybrid perovskite solar cells is device instability, which calls for understanding of the perovskite structural stability and phase transitions. In this talk, I will present our recent elastic and inelastic neutron scattering studies on two HOIPs, methylammonium lead iodide and formamidinium lead iodide, to understand the perovskite structural stability and phase transitions. We show by quasi-elastic neutron scattering that rotational modes of the organic cations are intimately linked to the photovoltaic effects. We also discuss our recent elastic neutron scattering studies on the nature of the structural phase transitions.