Energy Levels and Open-Circuit Voltages in Organic Solar Cells

The open-circuit voltage (VOC) of the organic photovoltaic (OPV) cell is one of the key parameters to determine the power-conversion efficiency of the OPV cell. In this work, the energy offset (EDA) of several donor-acceptor heterojunctions is determined by using the in-situ ultra-violet photoemission spectroscopy (UPS). It was found the substrate workfunction has negligible impact on the energy level alignment between the donor and the acceptor. The planar OPV cell is then fabricated to relate the VOC to the experimentally measured EDA. A surprising transition has been observed in the VOC-EDA plot. A simple mathematical model is developed, based on the exciton dissociation process inside the OPV cell. The model successfully explains the observed transition.