Defect-Induced Band-Edge Reconstruction of a Bismuth-Halide Double Perovskite for Visible-Light Absorption

Halide double perovskites have recently been developed as less toxic analogs of the lead perovskite solar-cell absorbers APbX3 (A = monovalent cation; X = Br or I). However, all known halide double perovskites have large bandgaps that afford weak visible-light absorption. The first halide double perovskite evaluated as an absorber, Cs2AgBiBr6 (1), has a bandgap of 1.95 eV. Here, we show that dilute alloying decreases 1’s bandgap by ca. 0.5 eV. Importantly, time-resolved photoconductivity measurements reveal long-lived carriers with microsecond lifetimes in the alloyed material, which is very promising for photovoltaic applications. The alloyed perovskite described herein is the first double perovskite to show comparable bandgap energy and carrier lifetime to those of (CH3NH3)­PbI3. By describing how energy- and symmetry-matched impurity orbitals, at low concentrations, dramatically alter 1’s band edges, we open a potential pathway for the large and diverse family of halide double perovskites to compete with APbX3 absorbers.卤化物双钙钛矿（halide double perovskites）近年来被开发为铅基钙钛矿太阳能电池吸光材料APbX3（其中A为一价阳离子，X为溴或碘）的低毒性替代材料。然而，目前已知的所有卤化物双钙钛矿均具有较大的带隙（bandgap），导致其可见光吸收能力较弱。首个被作为吸光材料研究的卤化物双钙钛矿Cs₂AgBiBr₆（编号1）的带隙为1.95 eV。本研究表明，微量合金化可将该材料的带隙降低约0.5 eV。重要的是，时间分辨光电导率（time-resolved photoconductivity）测试结果显示，该合金化材料中的载流子（carrier）具有微秒量级的长寿命，这在光伏应用中极具潜力。本研究中报道的合金化钙钛矿，是首款带隙与载流子寿命可与(CH₃NH₃)PbI₃相媲美的双钙钛矿材料。本研究阐明了低浓度下能量与对称性匹配的杂质轨道如何显著改变该材料的能带边，从而为庞大且种类丰富的卤化物双钙钛矿家族赶超APbX3类吸光材料提供了一条可行的研究路径。