Recently 12&percent; efficient indium tin oxide (ITO) on silicon solar cells have been reported. Experiments indicate the presence of a thin interfacial insulating layer. Thus, these devices appear to belong to a class of semiconductor‐insulator‐semiconductor (SIS) solar cells where one of the semiconductors is a degenerate wide‐band‐gap oxide. We have developed a theory in terms of minority‐carrier tunnel current transport through the interfacial layer where one semiconductor is in a nonequilibrium mode. The wide‐band‐gap semiconductor serves to block band‐to‐band majority‐carrier current and thus, in principle, give better device performance than with an MIS solar cell. The effects of interfacial layer thickness, substrate doping level, surface states and interface charge, temperature on the performance of SIS solar cells have been calculated. These indicate that real‐world ITO on silicon cells should be able to achieve 20&percent; efficiency under AMl illumination. Other combinations of semiconductors would yield even better performance.