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Maksym Kovalenko1 2

1, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, , Switzerland
2, Empa–Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, , Switzerland

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. We present fully inorganic, perovskite-derived zero-dimensional SnII material Cs4SnBr6 that exhibits room-temperature broad-band photoluminescence centered at 540 nm with a quantum yield (QY) of 10-20 % [1]. A series of analogous compositions following the general formula Cs4-xAxSn(Br1-yIy)6 (A=Rb, K; x≤1, y≤1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands.
We also present the synthesis, the structure as well as electronic and optical properties of a family of hybrid tin (II) bromide compounds comprising guanidinium [G, C(NH2)3+] and mixed cesium-guanidinium cations: G2SnBr4, CsGSnBr4, and Cs2GSn2Br7. G2SnBr4 has a one-dimensional structure that consists of chains of corner-shared [SnBr5]2- square pyramids and G cations situated in-between the chains. G2SnBr4 is a luminescent phase with a broad emission band resulting from trapped excitonic states. Cs+ exhibits pronounced structure-directing effect: with a mixture of Cs+ and G cations mono and bilayer two-dimensional perovskites, CsGSnBr4 and Cs2GSn2Br7 are formed. The dimensionalities of the crystallographic structures have a direct impact on the electronic structures and the experimental optical band gaps are consistent with quantum confinement effects predicted by first principle simulations. Moreover, the flat shape of guanidinium cations induces anisotropic out-of-plane tilts of the [SnBr6]4- octahedra in the CsGSnBr4 and Cs2GSn2Br7 compounds. The related strong anisotropies of the halide perovskite lattice distortions have in turn a direct influence on their electronic and optical properties.

References:
1. B. Benin et al. Angew. Chem. 2018, 57, 11329–11333
2. O. Nazarenko et al. submitted.

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