(20e) Understanding and Tuning Charge-Transfer State Diffusion at Organic Semiconductor Heterojunctions

Charge transfer (CT) states are bound electron-hole pairs spanning electron donor-acceptor heterojunctions. These states are key intermediates in organic photovoltaic cells and light-emitting devices. Although the mobility of CT states has been demonstrated, direct methods to tune the diffusion length have not been determined, precluding further studies of the impact of migration on device performance. In addition, there remain questions regarding the underlying transport mechanism and the materials parameters that set the scale for the diffusion length. In this work, we measure CT state diffusion in organic donor-acceptor mixtures via steady-state photoluminescence quenching methods. The diffusion lengths are compared for several donor-acceptor pairings and mixture compositions. We observe that the energy of the CT state, and the presence of low-energy triplet states do not strongly determine the diffusion length. We instead find a correlation between the diffusion length and the mobility of the slowest component charge carrier. Additionally, we find that CT states are at least as mobile as their bulk counterparts, despite being confined to an interface. The observed connection between charge carrier transport and CT state diffusion provides a strategy to manipulate CT state migration in various devices. Further, these conclusions may also explain differences in CT state diffusion lengths measured among various material systems, including hybrid interfaces of organic and inorganic semiconductors.