Understanding Detrimental and Beneficial Grain Boundary Effects in Halide Perovskites

Grain boundariesplay a key role in the performance of thin‐film optoelectronic devices and yet their effect in halideperovskite materials is still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Noncrystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard – electron backscatter diffraction(EBSD) – destroys halideperovskite thin films. Here, this problem is solved by using a solid‐state EBSD detector with 6000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length, and mobility shows that grain boundariesare not benign but have a recombination velocity of 1670 cm s−1, comparable to that of crystalline silicon. Amorphous grain boundariesare also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundarycharacter offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halideperovskite thin films. It also suggests a new approach for passivating grain boundaries, independent of surface passivation, to lead to even better performance in optoelectronic devices.