Strong self-trapping by deformation potential limits photovoltaic performance in bismuth double perovskite

Bismuth-based double perovskite Cs₂AgBiBr₆ is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10⁻¹¹ cm³ s⁻¹) and low charge carrier mobility (around 0.05 cm² s⁻¹ V⁻¹). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs₂AgBiBr₆, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs₂AgBiBr₆ could impose intrinsic limitations for its application in photovoltaics.