Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)(2)(MA)(n-1)PbnI3n+1 (BA = CH3(CH2)(3)NH3+, MA = CH3NH3+) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as n = 4, are found to be mixture of multiple perovskite phases, with n = 2, 3, 4 and approximate to infinity, that naturally align in the order of n along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of similar to 358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices.