We investigate the performance of amorphous Si (a-Si) solar cells fabricated with Inductively Coupled Plasma (ICP) deposition technique. The ICP system produces a-Si films with low defect density (3×10 15cm- 3), resulting in a conversion efficiency of 9.6%. Deep level transient spectroscopy (DLTS) reveals that hole carriers trapped at defects near the valence band edge delocalize at 130 K; while trapped electrons can only be emitted into the conduction band near room temperature. Spectrally resolved DLTS study further indicates that light soaking enhances the emission rate of the tapped electrons near the conduction band edge while reduces the transition moments from the hole-trapping defect levels to the conduction band. The combined effects and light soaking-induced defects are responsible for the degradation of a thin film solar cell by light soaking.