Strong electron-phonon coupling and bipolarons in Sb2S3

Antimony sulphide (Sb$_2$S$_3$) is an Earth-abundant and non-toxic material that is under investigation for solar energy conversion applications. However, it still suffers from poor power conversion efficiency and a large open circuit voltage loss that have usually been attributed to point or interfacial defects and trap states. More recently, a self-trapped exciton has been suggested as the microscopic origin for the performance loss. By using first-principles methods, we demonstrate that Sb$_2$S$_3$ exhibits strong electron-phonon coupling, which results in a large renormalization of 200 meV of the absorption edge when temperature increases from 10K to 300K, and in a quasi-1D electron polaron that is delocalized in the ribbon direction of the crystal structure, but localized in the inter-ribbon directions. The calculated polaron formation energy of 67 meV agrees well with experimental measurements, suggesting that self-trapped excitons are likely to form with the mediation of an electron polaron. Our results demonstrate the importance of systematically investigating electron-phonon coupling and polaron formation in the antimony chalcogenide family of semiconductors for optoelectronic applications.