Atomic-Scale Insights into Flexoelectricity and the Enhanced Photovoltaic Effect at the Grain Boundary in Halide Perovskites.

Grain boundaries (GBs) generally exist in halide perovskites and are often accompanied by structural distortions or composition segregation, significantly altering their optoelectronic properties. However, the atomic-scale mechanisms underpinning these effects remain elusive due to the inherent complexity of the GB structures. By employing aberration-corrected transmission electron microscopy, we directly visualize the atomic structures of GBs in halide perovskites, uncovering the emergence of flexoelectricity and associated polarization-induced shift-currents. We demonstrate that a large strain gradient at 52° GBs induces significant flexoelectric polarization. This flexoelectricity is observed across GBs with different compositions and misorientation angles. First-principles calculations confirm that such flexoelectric polarization can enhance the photovoltaic effect, resulting in a shift-current of ∼15 μA V-2. These findings uncover a previously unrecognized role of GBs in halide perovskites and provide new insights into leveraging GB engineering to achieve flexoelectricity and regulate optoelectrical properties in halide perovskites.