Previously unachieved values are obtained for the open-circuit voltage (0.7 to 0.8 V) and the fill-factor (76%) at the illuminated p-InP/V3+/2+-HCl semiconductor/electrolyte contact (< 100> face) coated with a submonolayer amount of silver. Photoelectrochemical solar cell efficiency is 11%, despite significant optical reflection and electrolyte absorption losses. Differential capacitance measurements demonstrate a semiconductor/surface energy barrier height of up to phi(B) = 1.2 eV, independent of the Ag treatment. This value is considerably larger than typically found at solid-state p-InP/Ag contacts. Barrier heights derived from Mott-Schottky data are compared to photovoltages; results indicate that interface recombination and a slow charge-transfer (bare p-InP) and thermionic emission from the metal (Ag-treated p-InP) are important factors limiting the cell efficiency. Flatband potential shifts owing to a slow charge-transfer under photocurrent flow are reduced by the Ag treatment. In a finite potential range of about 400 mV (0 to -400 mV/SCE), both photovoltages and Mott-Schottky data reveal an unpinned Fermi level at bare and Ag-treated electrodes. The unpinned barrier follows the ideal Schottky barrier model for an absolute SCE potential of -5.0 eV/SCE. Reference to related work at InP and GaAs is made.