Interest in InP-based solar cells has recently been renewed due to the demonstration of excellent radiation resistance and the achievement of device efficiencies in excess of 20% (AM 1.5). Previous analysis of the indium tin oxide (ITO)/InP cell show it to be an extremely shallow homojunction which could, in principle, offer an even greater resistance to radiation than the deliberately grown organometallic chemical vapor deposition homojunctions. In this work, we have fabricated efficient solar cells of ITO/InP using dc magnetron deposition of the ITO into single-crystal InP substrates. The aim was to optimize the efficiency by varying the proximity of the substrate to the target and the input power to the plasma. Slight variations in efficiency have been observed but correlation with the fabrication parameters has been weak. Nevertheless, efficiencies over 16.5% have been achieved and verified at the Solar Energy Research Institute. This is the highest ever achieved for devices of this construction. Detailed studies of the annealing behavior of the cells have been made and interfacial changes observed using Raman spectroscopy and secondary ion mass spectroscopy. These observations, together with measurements of light and dark current/voltage, and quantum efficiency characteristics, have been used to model the behavior of the cells and explain their lack of sensitivity to the fabrication conditions.