■ Title : Ion transport in lead halide perovskites from bulk to interface
■ Speaker : Dr. Gee Yeong Kim
Max Planck Institute for Solid State Research
■ Date and time : 12/23(Mon) 1:30 ~ 2:30 PM
■ Venue : Applied Engineering Building (W1-1), Lecture Room #1318
■ Host : Prof. WooChul Jung
■ Abstract : Organic-inorganic lead halide perovskites are considered as the most promising photovoltaic materials. Lead halide perovskite has some anomalies properties such as a huge apparent low frequency dielectric constant and severe hysteretic current-voltage behavior. In order to investigate the key features of its performance, not only electronic, but also ionic transport properties need to be considered. This talk will provide the finding that methylammonium lead iodide (MAPI) is mixed conductor, result from stoichiometric polarization measurement. Moreover, the mobile charge carriers are iodine vacancies, holes and electrons. Indeed, offering the surprising finding that light enhances not only electronic but also ionic conductivities of MAPI by several orders of magnitude. Here I will also show this photo-ionic effect for perovskites with mixed anions and cations. Unlike the iodide, the bromide perovskite does not show this effect. Furthermore, we also proposed a mechanism of photo-ionic effect and its impact on photo-decomposition. The mechanism of ion conduction enhancement by light is thus based on the interaction of electronic into ionic defects. Additionally, we also applied “nano-ionics” (heterogeneous doping) to investigate the space charge effect in lead halide perovskite interfaces. The results give indications of ionically-driven equilibrium space charge potential forming at the lead halide perovskite with oxide (Al2O3 and TiO2) contacts. These studies could lead to new insights of interfacial phenomena in photovoltaics, implications for photo-decomposition and also new optoionic devices.