Deposition sequence and substrate work function in controlling the interface energy level alignment in organic photovoltaic (OPV) devices with copper phthalocyanine (CuPc) as the donor and fullerene ( C ₆₀) as the acceptor were studied using ultraviolet photoelectron spectroscopy. We found that the energy offset at the highest occupied molecular orbital of donor ( HOMO _D ) and the lowest unoccupied orbital of acceptor ( LUMO _A ) , which limits the maximum open-circuit voltage of heterojunction OPV, can be changed from 0.64 ( C ₆₀ on CuPc) to 0.86 eV (CuPc on C ₆₀ ) by reversing the deposition sequence. Furthermore, by controlling the substrate work function from 2.81 to 5.07 eV, the LUMO _A -HOMO _D offset can be effectively tuned from 0.86 to 1.27 eV. The results suggest that electrodes in OPV devices can have significant influences on the electronic structures and energy levels of the donor/acceptor interface, and thus provide a viable means for performance enhancement.