Migration of Charge-Transfer States at Organic Semiconductor Heterojunctions.

Charge-transfer (CT) states formed at organic donor-acceptor (D-A) semiconductor heterojunctions play a critical role in optoelectronic devices. While mobile, their migration has not been extensively characterized. In addition, the factors impacting the CT state diffusion length (LD) have not been elucidated. Here, CT state LD is measured by using photoluminescence quenching for several D-A mixtures, with migration occurring along the bulk heterojunction. All D-A pairings considered yield a similar LD ∼ 5 nm in equal mixtures despite variations in the CT state energy and the constituent molecular structures. The CT state LD varies strongly with mixture composition and is well-correlated to the slowest charge carrier mobility, suggesting a direct method to tune CT state transport. These findings may be applied to elucidate the role of CT state migration in organic photovoltaic and light-emitting devices as well as to broadly explain the transport of interfacial excited states along inorganic and hybrid organic-inorganic heterojunctions.