Small grains as recombination hot spots in perovskite solar cells

•Role of grain boundaries in perovskite solar cells is systematically investigated•Small grains lead to high recombination losses, limiting the open-circuit voltage•The complete distribution sizes must be considered for future optimization Unlike interfacial recombination, much remains unknown about bulk polycrystalline metal halide materials. In this work, we investigate how changing size films influences their properties and photovoltaic device performance. We demonstrate that small introduce significant losses propose an intuitive heuristic model explains observation by taking into account geometry dynamics charge diffusion, which dominates transport near at voltage conditions. Our results allow us simulate effects on performance, highlighting need improve microstructure thin reducing prevalence act as hot spots due very density defects. Non-radiative its interfaces prohibits performance from reaching Shockley-Queisser limit. 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