The Effect of Band-Bending on Hybrid Bulk Heterojunction Solar Cell Performance

Hydrid bulk heterojunction solar cells (HBHJs) consist of nanostructured metal oxide (TiO₂ and ZnO) acceptors and polymer light absorbing donors. To date, these promising architectures have only achieved moderate efficiency due to surprisingly low short circuit photocurrents. To elucidate the underlying properties that result in this low current, a detailed understanding of interfacial properties is required. Herein, we quantify a vacuum level offset of 0.8 eV between the polymer and the TiO₂ created by spontaneous charge transfer upon heterojunction formation. This offset imparts a barrier to charge separation at the TiO₂-polymer interface and explains the poor performance of cells to date. New materials for improved photocurrent are proposed by tuning the energetics of the heterojunction.