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.