Metastable defect migration under high carrier injection in hydrogenated amorphous silicon p-i-n solar cells

The evolution of excess defects in hydrogenated amorphous silicon p-i-n solar cells, induced by a forward current in the dark, has been studied by modeling their measured dark and illuminated current-voltage and quantum efficiency characteristics at different stages of degradation. Our electrical-optical model is based on the solution of Poisson's and continuity equations. Modeling reveals that metastable defects are mainly produced in regions where tail-to-tail recombination of injected electrons and holes is high. These regions are characterized by either high defect density or low electric field. Simulation of experimental characteristics after 1 h of current injection indicates that the spatial generation of current-induced defects is highly nonuniform, with the main defect formation occurring near the pi interface, and to a lesser extent towards the ni interface. Few defects are generated over the bulk intrinsic layer. Modeling of the characteristics after a longer duration of current injection indicates a broadening of the current-induced defect zone from the interfaces to the bulk intrinsic layer. After prolonged current injection, the density of excess dangling-bond defects in the bulk intrinsic layer increases significantly, while the defect density near the pi interface actually decreases, resulting in a more uniform distribution of excess metastable defects. Evidence from modeling suggests that some metastable defects have migrated from the interfaces towards the bulk. We thus conclude that prolonged current injection not only produces excess metastable defects, but also causes these defects to migrate to regions of lower defect density.