Strong electron-phonon coupling and bipolarons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mtext>Sb</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mtext>S</mml:mtext><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>
Yun Liu, Bartomeu Monserrat, Julia Wiktor
Abstract
Antimony sulfide $({\mathrm{Sb}}_{2}{\mathrm{S}}_{3})$ is an Earth-abundant and nontoxic 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 ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$, with some reporting self-trapped excitons as the microscopic origin for the performance loss, while others have found no evidence of carrier trapping with only large polarons existing in ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$. By using first-principles methods, we demonstrate that ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ exhibits strong electron-phonon coupling, a prerequisite for carrier self-trapping in semiconductors, which results in a large renormalization of $200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ of the absorption edge when temperature increases from 10 to $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. When two electrons or holes are added to the system, corresponding to a carrier density of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, we find wave function 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 $\ensuremath{-}330$ and $\ensuremath{-}280$ meV per carrier, respectively. Our results reconcile some of the controversy in the literature regarding carrier trapping in ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ 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.