A metastable evolution of a-Si:H p-i-n diodes submitted to a high reverse bias for a long period of time was observed. This forming process leads to a significant decrease of the reverse current and increase of the breakdown voltage. The mechanisms underlying this forming process have been investigated using the constant photocurrent method experiment. The effects of annealing and current-induced defect creation have been studied. The forming process is found to be most consistent with a metastable increase in the dopant activation, leading to an improvement of the p-layer blocking efficiency.