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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. Image 1 • An important factor that affects the open-circuit voltage (V OC) of organic solar cells is investigated. • The V OC depends significantly on the spatial variation of the molecular energy levels within the photoactive layer. • The energy levels of the photoactive materials near the electrodes are critical in determining the V OC. [ABSTRACT FROM AUTHOR]