Experimental discrimination of geminate vs. nongeminate recombination in hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) has become one of the most widely used materials for photovoltaic applications and other electronic thin film devices ranging from thin film transistors to radiation detectors. However, in spite of these technical achievements, there still remain open questions regarding the fundamental properties of recombination in a-Si:H. One of these questions is how to experimentally distinguish geminate recombination of electrons and holes which were produced by the same photon from nongeminate recombination of unrelated charge carriers. This question is of interest, as nongeminate recombination is the determining factor for many electronic properties. Here, a study of charge carrier recombination in intrinsic a-Si:H using pulsed electrically and pulsed optically detected magnetic resonance spectroscopies is presented in order to measure the influence of spin-dependent recombination on photoluminescence (PL) and photoconductivity (PC). The results show that band tail state recombination influences the PL but not the PC. This indicates that these recombination channels are geminate and to not contribute to charge transport. Recombination through the silicon dangling bond is shown to influence both PL and PC constituting nongeminate recombination. These experiments provide a direct and unambiguous observation of nongeminate and geminate recombination in a-Si:H. Also, the observation of spin coherence in a-Si:H at room temperature is made using pEDMR spectroscopy on a-Si:H solar cell devices.