It is known that emission from Er ions implanted into a silicon wafer can be excited by illumination of the non-implanted side of the sample. In such a configuration, energy has to be transferred across the entire thickness of the sample (300-500μm), which exceeds by two orders of magnitude absorption depth of a 514.5nm line of an Ar ion laser. We have shown that for the non-implanted side illumination configuration, the energy transfer process leads to a delay in the onset of Er photoluminescence signal, whose magnitude depends on the excitation power. In the present contribution, we investigate the microscopic mechanism responsible for this delay. We postulate that it can be related to exciton dissociation at a p-n junction created by Er doping at a Si/Si:Er interface. We confirm this hypothesis by showing that the actual value of the delay time can be tuned by a bias voltage applied to the junction.