In this study it is revealed that the light induced defects (LIDs) responsible for the fast degradation of hydrogenated amorphous silicon (a-Si:H) solar cells under light soaking are located at nanosized voids. This important breakthrough in identifying the local environment of LIDs has been achieved by detailed study of the relation between the nanostructure of a-Si:H and metastability of corresponding solar cell devices under light soaking. We propose that a useful tool to define the nanostructure of a-Si:H is to determine the size distribution of the volume deficiencies, which range from small hydrogenated vacancies up to nanosized voids. The processing window used to vary and control the nanostructure in dense a-Si:H is based on a hydrogen rich plasma at unconventional high processing pressures (~10 mbar). The dense absorber layers with different distributions of volume deficiencies are subsequently incorporated in solar cell devices. For the first time a clear relation between nanostructures of the a-Si:H absorber layer and the fast kinetics of the metastable LIDs of the solar cell during light soaking and thermal annealing is observed. The `fast' degradation (first 10 hours of light soaking) strongly correlates to the density of largest volume deficiencies in the a-Si:H matrix. The “slow” regime (10 tot 1000 hours of light soaking) appears to be independent on the nanostructure of the absorber layer. In addition, the fast metastable defect states are the first ones to be annealed out at relative low annealing temperatures (120-130 C). Although solar cells processed at higher pressure have the same long term degradation kinetics, their FF recovers much faster by thermal annealing when compared to the cells processed at standard low pressure and low hydrogen dilution conditions.@en