Substrate dependence of energy level alignment at the donor–acceptor interface in organic photovoltaic devices

The interface energy level alignment between copper phthalocyanine (CuPC) and fullerene (C60), the widely studied donor–acceptor pair in organic photovoltaics (OPVs), on indium–tin oxide (ITO) and Mg substrate was investigated. The CuPC/C60 interface formed on ITO shows a nearly common vacuum level, but a dipole and band bending exist, resulting in a 0.8 eV band offset at the same interface on Mg. This observation indicates that the energy difference between the highest occupied molecular orbital of CuPC and the lowest unoccupied molecular orbital of C60, which dictates the open circuit voltage of the CuPC/C60 OPV, can be tuned by the work function of the substrate. Furthermore, the substrate effect on the energy alignment at the donor/acceptor interface can satisfactorily explain that a device with an anode of a smaller work function can provide a higher open circuit voltage.