Local Crystal Misorientation Influences Non-radiative Recombination in Halide Perovskites
Sarthak Jariwala a b, Hongyu Sun c, Gede W.P. Adhyaksa c, Andries Lof c, Loreta Muscarella c, Bruno Ehrler c, Erik C. Garnett c, David S. Ginger a
a University of Washington, US, Seattle, United States
b University of Washington, US, Seattle, United States
c Center for Nanophotonics, AMOLF, The Netherlands, Science Park, 104, Amsterdam, Netherlands
Oral, Sarthak Jariwala, presentation 002
DOI: https://doi.org/10.29363/nanoge.nipho.2020.002
Publication date: 25th November 2019

Understanding how grain structure and grain boundaries affect non-radiative recombination is a key challenge facing the use of halide perovskites, indeed any semiconductor, for photovoltaic applications. We use electron backscatter diffraction (EBSD) images to map the local crystal orientations in thin films of CH3NH3PbI3 (MAPI), the archetypal halide perovskite for photovoltaics.  These EBSD images allows the direct identification of grains and grain boundaries in MAPI films. Although this grain structure is broadly consistent with the structures visible in conventional scanning electron microscopy (SEM) and optical microscopy data, the inverse pole figure (IPF) maps taken with EBSD reveal subtle internal crystal orientation variations of the grain structure. This local crystal misorientation leads to orientation spread within grains indicating the presence of local strain which varies from one grain to the next. Furthermore, we use crystallographic identification to demonstrate the presence of sub-grain boundaries and their location within grains. In solar cells, non-radiative recombination is a key figure of merit, which ultimately controls the power conversion efficiency of a given material. To quantify the impact of local grain structure on non-radiative recombination, and hence photovoltaic performance, we also acquire co-aligned confocal optical photoluminescence (PL) microscopy images on the same MAPI samples used for EBSD. By correlating the optical and EBSD data, we find that the PL is anticorrelated with the local grain orientation spread taken near the film surface, suggesting that grains with higher degrees of crystalline orientational heterogeneity exhibit more non-radiative recombination. These results provide critical insight into the interplay between local crystal orientation heterogeneity and local non-radiative recombination in halide perovskite thin films and may explain why the expected correlation between grain size and photovoltaic performance has been difficult to observe in halide perovskite solar cells.

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