Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu 2 O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu 2 O; 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 Cu 2 O 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 Cu 2 O-based photovoltaics. In the case of Cu 2 O, the free carrier model underestimates the efficiency of a Cu 2 O solar cell by as much as 1.9 absolute percent at room temperature.