Engineering Diffusion of Charge-Transfer States at Organic Semiconductor Heterojunctions

Interfacial excited states formed at electron donor-acceptor heterojunctions, called charge-transfer (CT) states, are essential intermediates for organic photovoltaic and light-emitting devices. While CT states are known to diffuse, questions remain around the underlying transport mechanism and the materials parameters that determine the diffusion length. In this work, we examine CT state diffusion in donor-acceptor mixtures via photoluminescence quenching methods. The diffusion length is extracted across donor and acceptor type as well as relative composition. In general, we find that CT states are at least as mobile as their bulk counterparts, despite being confined to an interface. Further, the energy of the CT state, and the presence of low-lying triplet states do not strongly determine the scale of the diffusion length. We instead find a correlation between the diffusion length and the mobility of the slowest component charge carrier. The connection between charge carrier transport and CT state diffusion suggests strategies to engineer CT state migration in various devices. These conclusions may also elucidate CT state transport at hybrid interfaces of organic and inorganic semiconductors.