Photon-enhanced thermionic emission (PETE) devices applied to solar energy may be considered as high-temperature solar cells. A cathode structure for PETE was designed for high temperature energy conversion in solar concentrating systems. Surface-hydrogenated diamond is one of the few semiconductors showing negative electron affinity and a work function as low as 1.7 eV if nitrogen doped, that is connected to a significant thermionic emission at moderate temperatures (up to 800 °C, where hydrogen desorbs inducing a positive electron affinity). But diamond is transparent to solar radiation, consequently advanced techniques for preparing an efficient sunlight absorbing diamond are necessary.