To investigate the relation between interface passivation and the performance of an a-Si:H/c-Si solar cell, we monitored the glow discharge deposition process of n-doped a-Si:H films on p-doped crystalline silicon in situ by transient photoconductivity measurements in the microwave frequency range (TRMC measurements). After the end of the deposition process, we also performed ex situ TRMC measurements of the heterojunctions. From three representative samples, we prepared solar cells and their performances were compared to the TRMC measurements.