The mobility‐lifetime products ( &mgr;&tgr;) and interface property have been examined through the photovoltaic studies in actual hydrogenated amorphous silicon (a–Si:H)p–i–njunction solar cells. A small amount of boron atoms included ina–Si:H enhances the &mgr;&tgr; products of both electrons and holes up to the order of 10−7cm2/V, which corresponds to the carrier diffusion length in excess of 5000 A˚. The doped window layer possessing inferior photoelectric property works as the recombination region for photocarriers generated in the activeilayer, and practically dominates the interface property together with the surface recombination velocityS0at the electrode/doped layer interface. TheS0at the SnO2/pa–Si:H interface is estimated to be about 3×102cm/s with an assumption of the electron mobility at 0.1 cm2/Vs. Prolonged light exposure causes a reversible change of the &mgr;&tgr; products in every layer composing thep–i–njunction. These experimental results are discussed in connection with photovoltaic performances.