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.