Geminate and non-geminate recombination in a-Si:H:: Old questions and new experiments

Radiative recombination in a-Si:H at 1.4 eV is associated with recombination between electrons and holes trapped in the respective bandtails. According to the widely accepted radiative tunneling model two recombination regimes are to be expected with different kinetics: Geminate recombination at low and moderate generation rates and non-geminate distant pair recombination at high generation rates. However, both at high- and low generation rates the observations appear to be in conflict with the radiative tunneling model. The present models are discussed in view of results from frequency resolved spectroscopy (FRS) and recent investigations of pulsed optically detected magnetic resonance (PODMR). These studies show that geminate and non-geminate recombination coexist in a-Si:H. The radiative recombination channels have been identified by PODMR as exchange coupled pairs (excitons) and strongly dipolar coupled electron hole pairs.