We present the first measurements of forward- (FET) and back-electron transfer (BET) dynamics at the single molecule level performed under highly controlled, well-characterized ultrahigh vacuum conditions. FET from an excited perylene bisimide sensitizer to the conduction band of a single crystalline acceptor (GaN or Al2O3) and BET to the dye ground state were monitored by single molecule fluorescence intermittency. In the case of GaN, the surface was covered with a heteroepitaxial insulating layer of Sc2O3 which serves as a tunable electron injection barrier between sensitizer and semiconductor. The Sc2O3 spacer layer allowed isolation of the sensitizer/semiconductor distance dependence on FET and BET. The observed charge transfer dynamics was correlated with surface structure characterized by AFM, UPS and XPS. Our results point to the origin of non-exponential charge transfer kinetics in dye-sensitized solar cells that persist even on single crystalline surfaces.