Competing Crystallization in Multi-ion Perovskites

To reach their impressive power conversion efficiencies, perovskite solar cells have benefited from extensive empirical optimization. Major progress came from combinatorial optimization of perovskite compositions that now contain fully six or more components, e.g. Cs, MA, FA, Pb, I, Br, and others [1]. Unfortunately, the lack of understanding of the precise role of each component limits further progress in this now-exponentially-growing combinatorial space. Using ultrafast spatio-temporal imaging of carrier diffusion, we discovered that the carrier diffusivity is independent of composition in perovskite single crystals [2]. It is exclusively in polycrystalline thin films that different compositions play a crucial role in influencing carrier diffusivity and lifetime. Specifically, we found that in the stable cesium-formamidinium perovskite, perovskite films crystallize inhomogeneously: they produce grains whose cores have a lower bandgap, and whose shells have a higher bandgap. We then use this knowledge and find that the incorporation of a small amount of methylammonium homogenizes crystallization. This flattens the energetic landscape for carriers to move among grains. The proposed mechanism, through which the perovskite grain formation governs carrier transport, clarifies the widely-observed - but previously-unexplained - beneficial role of mixing.