Origin of a large difference of power conversion efficiency between the hexagonal LuMnO3 and the hexagonal LuFeO3 ferroelectric photovoltaics

Abstract Hexagonal LuMnO3 (h-LMO) and LuFeO3 (h-LFO) compounds, in their ferroelectric phases (space group P63cm), are promising ferroelectric photovoltaic materials for converting sunlight into electricity. However, the recent experimental studies demonstrate that the h-LMO exhibits much higher solar to electric power conversion efficiency (PCE) than the h-LFO. In this study, we explain the origin of this difference, basing our analysis on the electronic structure of both compounds, determined from the first-principles calculations at the density functional theory level. Our results demonstrate that the h-LMO PCE is higher than the h-LFO PCE because of the two facts: (1) the effective mass of the photogenerated charges in the h-LFO is larger than in the h-LMO, (2) the binding energy of the photogenerated electron-hole pairs (excitons) is higher in the h-LFO than in the h-LFO. We also report a high anisotropy of the electronic photocurrent in both compounds: due to much larger electron effective mass along the c-axis direction, the photocurrent flow along any direction within the hexagonal basal plane is much more efficient.