Observation of Internal Photoinduced Electron and Hole Separation in Hybrid 2-Dimentional Perovskite Films

【摘要】：Two-dimensional(2 D) organolead halide perovskites have recently emerged as an attractive material for applications in photovoltaics and other optoelectronic devices. The structural formula of the 2 D perovskite is generally given as(A)_2(CH_3NH_3)_(n-1)M_nX_(3n+1), where A is a large aliphatic or aromatic alkylammonium cation working as an insulating layer, M is the metal cation, and X is the halide anion. Recent reports have demonstrated that the 2 D multi-layered perovskite films actually comprised multiple perovskite phases(with various n values from 1, 2, 3 and 4 to near ∞), even though the films were intended to be prepared as a single-phase. This hybrid feature seems to be ineluctable in fabricating 2 D films. However, two important questions remain yet-to-be-answered: first, how the different perovskite phases align in the hybrid films; second, whether the band alignment between different phases induces energy funneling or instead charge separation. The latter is especially important because it dictates the application of these hybrid 2 D perovskite films: energy funneling is useful for light-emitting applications, whereas charge separation would be more beneficial for light conversion or detection. Here we report a unique spontaneous charge(electron/hole) separation property in multi-layered 2 D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. We find that indeed multiple perovskite phases with various n values co-existed in the 2 D perovskite films, and more interestingly, these perovskite phases were naturally aligned in the order of n along the growth direction perpendicular to the substrate. Driven by the built-in band alignment between different perovskite phases, consecutive internal electron transfer from small-n to large-n perovskite phases and hole transfer in the opposite direction were observed in a film of ~358 nm thickness. This unique self-charge-separation property of the 2 D perovskite films can facilitate their applications in photovoltaics and other optoelectronics devices.