Improvements in the performance of indium tin oxide/indium phosphide (ITO/InP) solar cells have been achieved by using dc magnetron sputter deposited n‐ITO onto an epitaxial p/p+ structure grown on good quality commercial p+ bulk substrates. The composition of the sputtering gas has been investigated and the highest efficiency cells resulted when the surface of the epilayer was exposed to an Ar/H2 plasma before depositing the bulk of the ITO in a more typical Ar/O2 plasma. With H2 processing, record efficiencies of 18.9% global, 1000 W m−2, 25 °C (17.0% air mass zero) were achieved. Without H2 processing, the devices exhibited lower efficiencies and were unstable. Type conversion of the InP was shown to occur and was established as being associated with the ITO (possibly due to Sn donors) rather than sputter damage. These improvements in performance have resulted from the optimization of the doping, thickness, transport, and surface properties of the p‐type base, as well as from better control over the ITO deposition procedure.