Local Crystal Misorientation Influences Non-radiative Recombination in Halide Perovskites

We use ultrasensitive electron backscatter diffraction (EBSD) to map the local crystal orientations, grains, and grain boundaries in CH3NH3PbI3 (MAPI) perovskite thin films. Although the true grain structure is broadly consistent with the morphology visible in scanning electron microscopy (SEM), the inverse pole figure maps taken with EBSD reveal grain structure and internal misorientation that is otherwise hidden. Local crystal misorientation is consistent with the presence of local strain, which varies from one grain to the next. We acquire co-aligned confocal optical photoluminescence (PL) microscopy images on the same MAPI samples used for EBSD. We correlate optical and EBSD data, showing that PL is anticorrelated with the local grain orientation spread, suggesting that grains with higher degrees of crystalline orientational heterogeneity (local strain) exhibit more non-radiative recombination. We find that larger grains tend to have larger grain orientation spread, consistent with higher degrees of strain and non-radiative recombination. Polycrystalline halide perovskite thin films have achieved high photovoltaic power conversion efficiencies but still have room to improve. Grain boundaries are an obvious possible challenge, but the role of grain boundaries has remained confusing: surprisingly, to date there is little correlation between grain size and solar cell efficiency. One challenge has been the prevalent use of non-crystallographic techniques to identify grain size. Using a state-of-the-art electron backscatter diffraction detector, we image the grain and sub-grain structure in perovskites. These measurements indicate the presence of local strain within grains, which further leads to non-radiative recombination and efficiency losses. These findings suggest that growing large grains without significant intragrain strain will enable more efficient devices and indicate that intragrain strain will be useful to characterize when scaling up perovskite thin film deposition for a various optoelectronic device applications. Understanding the origins of non-radiative recombination centers is critical to improving photovoltaic performance. This study probes structural causes of non-radiative recombination in prototypical halide perovskite semiconductors using electron backscatter diffraction (EBSD). Multimodal microscopy correlating the EBSD with confocal photoluminescence shows that higher local strain leads to higher non-radiative recombination in the studied compositions. Furthermore, larger grains show higher strain. These results help explain why performance in perovskite photovoltaics has not tracked with grain size, pointing the way toward further improvements.