Grain boundaries play a key role in the performance of thin‐film optoelectronic devices and yet their effect in halide perovskite 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 halide perovskite 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 boundaries are not benign but have a recombination velocity of 1670 cm s−1, comparable to that of crystalline silicon. Amorphous grain boundaries are also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halide perovskite 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.

A detailed understanding of charge transport is vital to maximize the efficiencies of optoelectronic devices. Using a back-contact architecture, the authors probe transport of electrons and holes separately in polycrystalline hybrid perovskite thin films. Isolating photoexcited charge carriers in separate regions of the device leads to long diffusion ranges of carriers. The authors demonstrate a back-contact perovskite solar cell that operates on majority-carrier diffusion. These results highlight electrode interfaces as limiting aspects of current back-contact architectures, indicating opportunities for improvement.

Nanophotonic patterns are imprinted into halide perovskite films using polymer stamps and a solvent mixture to control the perovskite's crystallization.