Photovoltaic devices based on hybrid perovskite materials have exceeded 22% efficiency due to high charge carrier mobilities and lifetimes. Properties such as photocurrent generation and open circuit voltage are influenced by the microscopic structure and orientation of the perovskite crystals, but are difficult to quantify on the intra-grain length scale and are often treated as homogenous within the active layer. Here, we map the topography, local short circuit photocurrent, open circuit photovoltage, and local powerconversion efficiency in state-of-the-art methylammonium lead iodide perovskite solar cells using photoconductive atomic force microscopy. We find, within individual grains, spatially-correlated heterogeneity in short circuit current and open circuit voltage up to 0.6 V. These variations are related to different crystal facet surfaces and have a direct impact on the macroscopic power conversion efficiency. We attribute this heterogeneity to a facet-dependent density of trap states at the surface [1]. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of polycrystalline and single crystal devices for photovoltaic and lighting applications.

[1] Nature Energy 1, Article number: 16093 (2016)