We present the first measurements of forward- (FET) and back-electron transfer (BET) dynamics in a model dye-sensitized solar cell performed under highly controlled, well-characterized ultrahigh vacuum conditions. FET from a perylene bisimide (PTCDI) sensitizer excited state to the conduction band (CB) of single crystalline GaN and BET to the dye ground state were monitored by single molecule fluorescence intermittency. The GaN 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 while slowing the charge transfer (CT) reactions to single-molecule fluorescence measurement time scales. The GaN/Sc2O3 substrate and GaN/Sc2O3/PTCDI system were extensively characterized by AFM, XPS, UPS and LEED to correlate the observed distribution of CT kinetics with surface morphology and electronic structure.