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Small-Band-Gap Halide Double Perovskites.

Adam H SlavneyLinn LeppertAbraham Saldivar ValdesDavide BartesaghiTom J SavenijeJeffrey B NeatonHemamala I Karunadasa
Published in: Angewandte Chemie (International ed. in English) (2018)
Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs2 AgTlX6 (X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in AI BII X3 perovskites, the band-gap transition in AI2 BB'X6 double perovskites can show substantial metal-to-metal charge-transfer character. This band-edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B- and B'-site metal frontier orbitals. Calculations reveal a shallow, symmetry-forbidden region at the band edges for 1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br2 loss at ambient conditions.
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