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 (C-70)/copper phthalocyanine (CuPc) bilayer and a mixed C-70: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 C-70, 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 C-70 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 C-70/CuPc bilayer with C-70:CuPc BHJ. The thermal annealing effect on the mixed C-70:CuPc BHJ reveals vertical phase separation, resulting in inhomogeneous concentration distribution in profile. (C) 2011 Elsevier B.V. All rights reserved.