Disorder vs Delocalization: Which Is More Advantageous for High-Efficiency Organic Solar Cells?

We investigate the combined influence of energetic disorder and delocalization on electron-hole charge-transfer state separation efficiency in donor-acceptor organic photovoltaic systems using an analytical hopping model and Monte Carlo calculations, coupled with an effective mass model. Whereas energetic disorder increases the separation yield at intermediate and low electric fields for low-efficiency blends with strongly localized carriers, we find that it reduces dramatically the fill factors and power conversion efficiencies in high-efficiency solar cells that require high carrier delocalization within the conjugated segment and high mobility-lifetime product. We further demonstrate that the initial electron-hole distance and thermalization processes play only a minor role in the separation dynamics.