Electronic structures of planar and mixed C₇₀/CuPc heterojunctions in organic photovoltaic devices

Understanding the electronic structures of organic donor-acceptor heterojunction is of pronounced importance for the optimization of organic photovoltaic cells. Here, the detailed electronic structures of a planar fullerene (C₇₀)/copper phthalocyanine (CuPc) bilayer and a mixed C₇₀:CuPc bulk heterojunction (BHJ) have been studied via in situ photoemission spectroscopy. The results show that the energy level alignment by lining up separately observed energy levels of individual organic materials is not valid for these organic heterojunctions. The energy offset between the highest occupied molecular orbital of donor-like CuPc and the lowest unoccupied molecular orbital of acceptor-like C₇₀, which is regarded as the origin of open-circuit voltage (V_OC), is found to increase from 0.55 eV in the bilayer structure to 0.8 eV in the BHJ, which is possibly associated with the polarizability changes of C₇₀ and CuPc molecules in the BHJs. This change is confirmed by the V_OC variation in devices, where the V_OC dramatically increased from 0.35 to 0.46 V by replacing the C₇₀/CuPc bilayer with C₇₀:CuPc BHJ. The thermal annealing effect on the mixed C₇₀:CuPc BHJ reveals vertical phase separation, resulting in inhomogeneous concentration distribution in profile.