Molecular factors controlling charge pair generation in organic photovoltaic materials

Through remarkable advances in materials design, the efficiency of photovoltaic energy conversion in molecular materials has risen from 1% to over 20% within 2 decades. Some recent reports argue that charge photogeneration can occur directly in neat films of the best-performing molecular materials, and that this process may assist current generation in heterojunction devices. Here we address this assertion by combining experimental measurements of charge generation in single-component and heterojunction devices with a computational model of the generation and evolution of delocalized excited states in such systems. We identify key molecular parameters that are likely to assist charge generation in high-performance materials, including the exciton binding energy, reorganization energy, energetic disorder, electronic coupling and the molecular packing motif. We show that including state delocalization is critical to the results. While we find that charge generation in single domains is unlikely to drive photocurrent generation in low-offset heterojunctions, the same molecular parameters favour charge generation in both device architectures.