Carrier Diffusion Lengths Exceeding 1 μm Despite Trap-Limited Transport in Halide Double Perovskites

We image charge carrier transport over nanometers–micrometers and picoseconds–microseconds in halide double perovskites Cs2AgBiBr6 and Cs2AgTlBr6 single crystals using stroboscopic scattering microscopy. Both materials exhibit long, microsecond carrier lifetimes because of their indirect or symmetry-forbidden direct bandgaps. We extract free-charge and trap-limited mobilities near the surface and in the crystal bulk. The free-charge mobilities for both materials (∼10–50 cm2/(V s)) can reach those reported for archetypal lead halide perovskites. We measure trap densities exceeding 1017 cm–3 within ∼20 nm of the crystal surface. Measurements on freshly cleaved or thermally annealed crystals suggest the traps are primarily halide vacancies that likely form through surface bromine degassing. Although these traps considerably slow charge transport, they are energetically shallow, enabling thermally induced detrapping and mobile carriers over microseconds at room temperature. This defect tolerance yields carrier diffusion lengths exceeding 1 μm even in the presence of large trap densities and under solar excitation conditions where traps are not saturated. These results suggest that halide double perovskites could rival the best lead-based perovskites for photovoltaic and optoelectronic applications.