Mixed-Domain Charge Transport in the S-Se System from First-Principles

Electronic mobility and conductivity of S1-xSex are computed from first-principles throughout the compositional spectrum. This system spans both the band transport domain and the hopping domain due to the inherently semicrystalline, pseudomolecular atomic structure in the form of the packed eight-atom rings. By and large, mobility decreases initially with Se alloying, before eventually increasing toward the Se-end. Even dilute addition of Se into S triggers strong polaronic localization of electrons and a rapid transition from band transport to hopping. Up to x = 0.50, electron transport exhibits a mixture of quantum motion ("tunneling") and temperature-activated hopping behavior, whereby the former dominates at 200 K and below while the latter dominates at higher temperatures. Electron transport turns completely temperature activated around x = 0.50, before returning to the mixed behavior by x = 0.75, and recovering band transport at pure Se. Intrinsic electron conductivity monotonically increases with x due to the increased intrinsic carrier concentration as the band gap decreases, though the benefit is at most by an order of magnitude up to x = 0.25. Our work provides insights into intricate charge transport properties of the S-Se system while demonstrating a holistic first-principles blueprint for treating mixed-domain charge transport in complex crystals spanning band transport and hopping.