Elucidating the Spatial Dynamics of Charge Carriers in Quasi-Two-Dimensional Perovskites.

Quasi-two dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) have shown better ambient stability with decent solar cell performances. However, the power conversion efficiency of quasi-2D OIHPs is still below that of 3D polycrystalline perovskites. To understand the limitation of quasi-2D OIHPs, we explore charge carrier properties in 3D and quasi-2D perovskites using advanced scanning probe microscopy techniques. Kelvin probe force microscopy (KPFM) identifies slow degradation in quasi-2D perovskites by measuring photovoltage variations under thermal and humid conditions. Bias-driven photocurrent maps obtained by conductive-atomic force microscopy (c-AFM) measurements reveal local inhomogeneous conduction and hysteretic currents in quasi-2D perovskites while relatively uniform conductivity is observed on individual grains in the 3D perovskite counterparts. In addition, bias-driven KPFM and I-V measurements in the lateral Au electrode devices show higher charge carrier dynamics with stronger potential drop at the interfaces in the 3D perovskite than those of the quasi-2D perovskite devices. The combination of c-AFM and KPFM results confirm less ionic conduction in the quasi-2D perovskites as compared to the 3D perovskites. Our study elucidates underlying mechanisms behind the lower efficiency of quasi-2D perovskites, which is necessary for further development of efficient and stable perovskite-based devices.