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Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites

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Science  08 Feb 2019:
Vol. 363, Issue 6427, pp. 627-631
DOI: 10.1126/science.aah5065

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Advantages of adding just enough alkalis

For organic-inorganic hybrid solar cells, addition of cesium or rubidium cations can improve power conversion efficiency. Correa-Baena et al. used nanoscale x-ray fluorescence imaging to show that the addition of metal cations at low concentrations homogenized the distribution of bromide and iodide anions and also increased charge-carrier lifetimes. However, at high concentrations, aggregation and cluster formation led to increased charge recombination.

Science, this issue p. 627

Abstract

The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano–x-ray fluorescence and complementary measurements, we found that the halide distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.

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