Dark current‐voltage (J‐V) characteristics ofp‐i‐na‐Si solar cells are studied to clarify the effect of thep/iinterface layer on the open‐circuit voltage (Voc). It is shown that the recombination current in thep/iinterface region is predominant in thea‐Si solar cells with ani‐layer thickness of less than 200 nm and has a great effect on the dark‐current transport in the solar cells. It is also shown that the current transport of the cells with thep/iinterface layer of constant band gap of a thickness of more than 14 nm is effectively the same as that of a cell in which the band gap of the wholeilayer is equal to that of the interface layer. A model in which the dark current of the cell consists of the recombination current at thep/iinterface and the recombination current in the bulk region of theilayer is proposed. Using this model, theVocdegradation for thea‐Si solar cells with thep/iinterface layer is shown to be due to the defects generated in theilayer.