Experimental Evidence of Localized Shallow States in Orthorhombic Phase of CH3NH3PbI3 Perovskite Thin Films Revealed by Photocurrent Beat Spectroscopy

We demonstrate evidence of localized states near the band edge of lead halide perovskite semiconductor CH3NH3PbI3 thin films. In these films, the high-temperature tetragonal and the low-temperature orthorhombic phases coexist at low temperatures. We determine the exact energy level of the localized states in the orthorhombic phase via photocurrent (PC) beat spectroscopy. The lack of observation of localized states in photoluminescence and absorption measurements indicates that the density of the localized states is fairly low even in solution-processed polycrystalline films. Ultrafast PC spectroscopy enables us to monitor low-density localized states in inhomogeneous polycrystalline films.