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.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)National Supercomputing Centre (NSCC) SingaporePublished versionThis research was supported by Nanyang Technological University under its start-up grant (M4080514), the Ministry of Education under its AcRF Tier 2 grants (MOE2016-T2-1-034), the Science and Technology Program of Guangzhou (no. 2019050001), the National Natural Science Foundation of China (NFSC) (grant no. 51802331), the National Research Foundation (NRF) Singapore under its Competitive Research Programme (NRF-CRP14-2014-03), the NRF Investigatorship (NRF-NRFI-2018-04), the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (grant no. 2017B030301007), the National Center for International Research on Green Optoelectronics, and the 111 Project. F.G. acknowledges the financial support from the Swedish Energy Agency (2018-004357) and Knut och Alice Wallenbergs Stiftelse (Dnr KAW 2019.0082). F.G. is a Wallenberg Academy Fellow. The computational work for this article was (fully/partially) performed on resources of the National Supercomputing Centre, Singapore (www.nscc.sg). R.S. and M.M. acknowledge the funding support from Ministry of Education, Singapore grant numbers AcRF Tier 1, RG191/17, and MOE2017