Elucidation of photovoltage origin and charge transport in Cu2O heterojunctions for solar energy conversion

Abstract

Heterojunctions between p-type cuprous oxide (Cu₂O) and suitable n-type layers stand out as some of the best performing and abundant oxide photoabsorbers currently available for the generation of hydrogen with photoelectrochemical cells or conversion of solar energy to electricity. In this contribution, we used drift-diffusion semiconductor modeling to investigate the mechanism governing the charge transport in TiO₂/Ga₂O₃/Cu₂O and TiO₂/Al:ZnO/Cu₂O heterojunctions. The simulated photovoltage 0.9 V for TiO₂/Ga₂O₃/Cu₂O agrees well with the measured value of 1.0 V and the governing mechanism is identified to be thermionic emission of electrons across the Ga₂O₃/TiO₂ interface. By modeling an optimized increase in Ga₂O₃ donor concentration, a photovoltage improvement of only 0.1 V is achievable, whereas further optimizing the electron affinity of Ga₂O₃ may lead to more significant improvement approaching 0.7 V. The optimized electron affinity of Ga₂O₃ enabled a simulated photovoltage of 1.6 V, close to the theoretical limit for Cu₂O with a 2.17 eV bandgap energy. Additionally, we find that simulations can reproduce the measured photovoltage of TiO₂/Al:ZnO/Cu₂O only when an interface recombination layer at the Al:ZnO/Cu₂O interface is included in the model. Our findings enable detailed understanding of the charge transport mechanism in Cu₂O heterojunctions and offer various design directions for further photovoltage improvement.