Light-induced cell performance and lifetime degradation are investigated using blue and red light illuminations in order to clarify the responsible factor for the metastable defect activation. The evaluation of the cell-performance decay curves with exponential decay fittings results in the same decay time constants for the degradation. Spectral response degradation is investigated under blue-light illumination. Only the decrease in red response indicates deep distribution of the activated metastable defect into the bulk. Injected carriers are responsible for the defect activation. Carrier diffusion lengths of more than 100 mum in a cell at a degraded state is considered to make blue light possible to degrade the bulk. (C) 2002 Published by Elsevier Science B.V.