On the Relationship Between Donor/Acceptor Interface Energy Levels and Open‐Circuit Voltages

Energy offset (EDA) from a number of donor/acceptor heterojunctions is measured using ultraviolet photoemission spectroscopy. It is found that substrate work functions have little impact on the energy level alignments at donor/acceptor heterojunctions. Planar‐heterojunction organic photovoltaic cells are made to test the relationship between energy offset and open‐circuit voltage (VOC). VOC is found to increase linearly as a function of EDA. The VOC, however, takes a surprising turn at EDA = 1.5 eV and starts to decrease as a function of donor–acceptor energy levels. To explain this experimental observation, a theoretical model to quantify the relationship between VOC and EDA is developed. The proposed model well explains the experimental data and, in particular, the reverse trend of VOC on EDA. By grouping several material constants into one variable, a simple universal plot that well describes the experimental data for both planar‐heterojunction and bulk‐heterojunction cells is generated. Donor/acceptor energy offset (EDA) and open circuit voltage (VOC) for donor/fullerene (C70) systems are concurrently measured by using ultraviolet photoemission spectroscopy and organic solar cells. VOC is found to first increase and then decrease as a function of EDA. Based on a two‐step exciton dissociation process, a theoretical model is developed to quantify the observed relationship between VOC and EDA.