Simulations of Frenkel to Wannier-Mott Exciton Transitions in a Nanohybrid System

Excitation energy transfer at a prototypical organic/inorganic interface is described theoretically. The nanohybrid system to be investigated is built up by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocrystal of 3903 atoms. To determine the time scale of excitation energy transfer all relevant electronic excitations of the organic and inorganic part are computed together with the related excitation energy transfer couplings. Values of the coupling lie in the millielectronvolt range or less. This motivates a Golden Rule description of the excitation energy transfer. Different Frenkel excitons are chosen as excitation energy donor levels. Due to the H-aggregate configuration of the organic part, the number of exciton wave function nodes increases with decreasing exciton energy. As a result, the couplings of the individual molecules to a certain ZnO electron-hole pair cancel each other more intensively and the overall transfer rate gets smaller. The highest exciton levels decay most rapidly and are characterized by lifetimes in the picosecond region. The lower part of the exciton band, however, has lifetimes in the nanosecond region. The Golden Rule description is finally compared to a direct solution of the time-dependent Schrodinger equation. The obtained transfer dynamics confirm those of the rate equation approach when the higher part of the Frenkel exciton band is considered. In the lower part, the reduced number of final electron hole pair states in the inorganic part blocks the Frenkel exciton decay.