Open-circuit voltage of organic solar cells: Effect of energetically and spatially nonuniform distribution of molecular energy levels in the photoactive layer

Here, the open-circuit voltage (V-OC) of organic solar cells (OSCs) in which the energy levels of the frontier molecular orbitals of the photoactive materials vary depending on the position within the active layer is investigated. The V-OC in these devices is critically affected by the energy level of the frontier molecular orbitals that are spatially located near the electrodes (i.e., the highest occupied molecular orbital of the donor near the anode and the lowest unoccupied molecular orbital of the acceptor near the cathode). Other parameters such as the charge generation/recombination and the energy distribution of charge-transfer states appear to exert less influence on the V-OC. This work demonstrates that, to maximize the V-OC, the spatial distribution of molecular energy levels should be controlled, especially in OSCs that exhibit spatial variation of the energy levels of organic materials within their active layer.