Nonadiabatic Exciton and Charge Separation Dynamics at Interfaces of Zinc Phthalocyanine and Fullerene: Orientation Does Matter.

Interface orientation between zinc phthalocyanine (ZnPc) and fullerene (C60) affects their interfacial charge separation dynamics; however, the underlying physical origin is still elusive. In this work, we have employed the time-dependent density functional theory (TDDFT) method to explore excited-state properties of ZnPc and C60 heterojunctions with both face-on and edge-on configurations. Spectroscopically bright absorption is from locally excited (LE) singlet excitons within ZnPc. In the face-on configuration, LE excitons are much higher in energy than charge-transfer (CT) excitons, thereby making charge separation process favorable. However, in the edge-on configuration, LE excitons are the lowest ones and CT ones are higher in energy; thus, charge separation is not efficient. Subsequently, we have carried out TDDFT-based nonadiabatic dynamics method to simulate photoinduced exciton and charge separation dynamics of ZnPc and C60 heterojunctions with both edge-on and face-on configurations. In the former, there are no exciton transfer and charge separation processes observed within 300 fs simulation time; while, in the latter, fragment-based electronic transition density matrix analysis reveals that only LE excitons |C60ZnPC*⟩ and CT excitons |C60-ZnPC+⟩ are involved. The exciton transfer from |C60ZnPC*⟩ to |C60-ZnPC+⟩ is completed within about 100 fs in which charge separation takes place with electron-hole distances increasing from 1.0 to 4.5 Å. This exciton transfer process is essentially in company with electron transfer from ZnPc to C60 but almost not involving hole transfer. These gained insights not only rationalize experiments but also enrich our knowledge to design donor-acceptor orientations to optimize organic photovoltaic performance.