Previous studies have shown that the characteristics of the semiconductor‐insulator‐semiconductor (SIS) solar cell can be controlled by the thin interfacial layer. In this paper, we present the results of a theoretical investigation of the role of interface states at the SiO2‐Si interface in the ITO‐SiO2‐Si SIS solar cell. A numerical solution of the governing transport equations yields the dependence of efficiency, short‐circuit current, and open‐circuit voltage on interface state density. It is shown theoretically that occupied acceptorlike interface state lower the barrier height and thus raise the dark saturation current while lowering the open‐circuit voltage. Experimental results are presented which relate the saturation current to the process step which forms the interfacial layer and which support the model.