We have reported on the theory of semiconductor‐insulator‐semiconductor (SIS) solar cells and the experimental characteristics of indium tin oxide (ITO) ‐SiOx‐silicon cells in previous publications. There appear to be no reports in the literature on SIS devices of any type with conversion efficiencies greater than about 12%. Theoretically, 20% efficiency should be possible using 0.2 &OHgr; cmp‐type silicon with a 12‐A˚ interfacial layer. This paper seeks to identify the various possible loss mechanisms (and the range of efficiency loss associated with each of them) that seem to be impeding the achievement of the theoretical limit. To determine the losses, the darkI‐Vcharacteristics as a function of temperature and performance parameters such asVoc,Jsc, FF, and &eegr; as a function of the intensity of illumination (up to 6 suns) have been examined. The intensity measurements reveal in a very distinct way the presence of an interfacial layer and serve to confirm the applicability of the theoretical model.