Role of Spontaneous Orientational Polarization in Organic Donor-Acceptor Blends for Exciton Binding

Electron-hole pairs at the interface between electron-donating and electron-accepting molecules form charge-transfer excitons (CTEs) via Coulomb attraction. Generally, the attraction energy of the CTE is weaker than that of the Frenkel exciton because of spatial separation of the charge pair; thus, the binding of the CTE is expected to be sensitive to an electric field. Here, the shielding of the binding energy of the CTEs by an internal electric field induced by spontaneous orientation polarization (SOP) of the solid-state donor-acceptor blend film is reported. When the blend film forms a large SOP, the photogenerated CTEs spontaneously dissociate without an external electric field, resulting in carrier diffusion and carrier lifetimes that are in the milliseconds or longer. In the absence of a large SOP, the CTEs preferentially form exciplexes that quickly release their energy as light rather than dissociation. Hence, the control of SOP in the donor-acceptor blend films can provide new insights into exciton binding and facilitate the development of tailored high-performance organic semiconductor devices.