Strong Electron-Phonon Coupling and Bipolarons in Sb₂S₃

Sb₂S₃ 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. More recently, there has been some discussion in the literature about the role of carrier trapping in the optical properties of Sb₂S₃, with some reporting self-trapped exciton as the microscopic origin for the performance loss, while others have found no evidence of carrier trapping with only large polarons existing in Sb₂S₃. By using first-principles methods, we demonstrate that Sb₂S₃ exhibits strong electron-phonon coupling, a prerequisite for carrier self-trapping in semiconductors, which results in a large renormalization of 200 meV of the absorption edge for the temperature range of 10 K to 300 K. When two electrons or holes are added to the system, corresponding to a carrier density of 1.6 x10²⁰ cm^-³, we find wavefunction localization consistent with the presence of bipolarons accompanying a significant lattice distortion with the formation of Sb and S dimers. The formation energies of the electron and hole bipolarons are -330 meV and -280 meV per carrier, respectively.<br/>Our results reconcile some of the controversy in the literature regarding carrier trapping in Sb₂S₃ , and demonstrate the existence of large electron-phonon coupling and carrier self-trapping that might place a fundamental limit on the open circuit voltage and consequently the maximum efficiency of the photovoltaic cells[1].<br/><br/>[1] Yun Liu, Bartomeu Monserrat, Julia Wiktor, Physical Review Materials, 7, 085401 (2023)