Hybrid organic-inorganic perovskites (HOIPs) are revolutionizing the solar cell research field - within only five years since the first report in 2009, the record power conversion efficiency of HOIPs based solar cells has reached 20.1%. This represents the highest efficiency among all solution processable materials and the fastest rate of efficiency improvement in the history of all photovoltaic materials. Based on this trend, HOIPs have been called the “next big thing in photovoltaics” and worldwide research efforts have recently experienced explosive growth. Among various HOIPs being studied, methylammonium lead iodide (CH3NH3PbI3) is currently the champion solar cell material and is being most actively studied. Despite the impressive solar cell performance, the microscopic mechanism of the high performance is yet to be fully understood. Also, several peculiar observations such as ferroelectric-like behavior and positive temperature coefficient of bandgap (whereas most other semiconductors have negative temperature coefficient) which have important implications for solar cell performance are not yet explained. To understand and ultimately control these behaviors, it is critically important to characterize the atomic structure and dynamics in CH3NH3PbI3 to obtain deeper insights on the structure-property-performance relationships.
In this talk, I will present our recent work that combined temperature dependent X-ray scattering, elastic and quasi-elastic neutron scattering, ultraviolet photoelectron spectroscopy, optical spectroscopy and density functional theory calculations to probe the impact of temperature dependent atomic structure and dynamics of CH3NH3PbI3 on its optoelectronic properties. Our results show that the rotation of CH3NH3+ and the associated dipole impact the dielectric permittivity, exciton binding energy and charge recombination. Our results also show that the energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) shift in the same direction as CH3NH3PbI3 as temperature increases. Our experimental results are corroborated by density functional theory calculations which show that the lattice expansion and bond angle distortion cause different degree of orbital overlap between the Pb and I atoms and the anti-bonding orbital nature of both HOMO and LUMO results in the same direction of their shift. Implications of our findings on understanding the superb photovoltaic performance, ferroelectric-like behavior and positive temperature coefficient of bandgap will be discussed.