Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu₂O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu₂O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu₂O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu₂O-based photovoltaics. In the case of Cu₂O, the free carrier model underestimates the efficiency of a Cu₂O solar cell by as much as 1.9 absolute percent at room temperature