A buried boron (B) doped layer has been fabricated by ion implantation into the bulk structure of a chemical vapor deposition (CVD) diamond film engineered to act as a photo-thermionic cathode for high-temperature solar cells. The boron layer implantation is a fundamental step in order to obtain an efficient diamond-based solar cell. Implantation of boron ions has been performed by fixing the ion dose to a value of 1×10^15 at/cm2, while varying the ion beam kinetic energy from 40 to 250 keV which localizes the doped layer at different depths (from tens to a few hundreds of nanometers) below the absorbing surface of the cathode. Characterization of the optical and photoelectronic properties of the different implanted layers has been carried out so as to evaluate their effectiveness within the cathode structure. An ion beam kinetic energy of 40 keV is found to significantly increase the quantum efficiency of the CVD diamond plate. This will be useful for the development of the diamond-based photo-thermionic cathode.