2, Northern Illinois University, DeKalb, Illinois, United States
3, Northwestern University, Evanston, Illinois, United States
4, Northwestern University, Evanston, Illinois, United States
Solution processable hybrid organic-inorganic perovskites (HOIPs) such as methylammonium lead iodide (MAPbI3) represent a research forefront owing to prospects of enhanced performance in solar energy conversion, solid-state lighting, and information processing applications. In contrast to traditional inorganic semiconductors, the interactions between the organic and inorganic sub-lattices in HOIPs likely give rise to their extraordinary optoelectronic properties, such as long carrier lifetimes (possibly via a direct-indirect bandgap character) and hot carrier protection. Better understanding of the fate of non-equilibrium phonons is hence crucial for the further improvement of HOIP-based technologies. Direct probing of lattice temperature can provide insights into the mechanisms and timescales of electron-phonon and phonon-phonon interactions. Here we employ infrared transient absorption spectroscopy to investigate lattice heating in MAPbI3 and formamidinium lead iodide (FAPbI3). The strong temperature sensitivity of the absorbance of organic vibration modes permit probing of lattice thermalization with tens-of-fs time resolution. We observe long thermal equilibration time (hundreds-of-picosecond) that is one to two orders-of-magnitude slower than those observed for fully inorganic semiconductors. The slow thermal equilibration in HOIPs arise from a widely separated (weakly overlapping) phonon density of states (PDOS), which can impact the electronic and heat transport properties of HOIPs, and provide insights to manipulate such properties for the broader class of organic-inorganic hybrid materials.