Charge Generation and Transfer Mechanism of Bilayer Organic Photovoltaics with Unconventional Energy Alignment

The generation and transport processes of charge carriers in bilayer organic photovoltaics (OPVs), which rely on a suitable interfacial energy alignment between the donor and acceptor molecules, were previously described. The consensus is that the open-circuit voltage (Voc) of bilayer OPVs is generally smaller than the difference between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor. In this study, the unconventional energy alignment of OPVs that disobey the above classical theory is clarified using a bilayer heterojunction model. In the device, the charge is transmitted via the recombination of positive and negative carriers at the interface of the heterojunction, and the built-in electric field at the interface also promotes the generation of free carriers. This unconventional energy level structure potentially endows OPVs with a higher theoretical Voc to further improve device performance.