A New Approach to the Analysis of Forward Bias Dark Current Characteristics of a-Si:H Solar Cells

Generally the dark forward bias current voltage (JD-V) characteristics of a-Si:H solar cells are analyzed without clearly separating their contributions due to carrier recombination in the bulk from that at the p/i interface regions nor those imposed by carrier injection from the p and n contacts. Furthermore their exponential regimes are interpreted and fitted with constant diode quality factor n with modeling which is based on many fitting parameters that have not been reliably established. A new approach has been developed for the analysis of JD-V characteristics for the entire voltage range relevant to the operation of solar cells, which has allowed the three contributions to be identified and characterized. It is based on the analysis of their bias dependent differential diode quality factors, n(V), from which important information on the energy distribution of the defect states in the i-layers has been obtained. Results are presented and discussed here for p-i-n a-Si:H solar cells having sufficiently low p/i interface recombination so that the limitations by the bulk recombination on open circuit voltage, Voc, can be identified. These results are then correlated with the defect state distributions previously obtained from the analysis on differential diode quality factor n(V) characteristics for hydrogen diluted and undiluted intrinsic layers both in the annealed state as well as after introducing light induced defects.