Photon-enhanced thermionic emission for high-temperature solar energy conversion: materials, devices and emerging opportunities

High-temperature solar energy conversion remains a key challenge for improving the utilization of concentrated solar resources and advancing low-carbon energy systems. Photon-Enhanced Thermionic Emission (PETE) has emerged as a hybrid conversion concept that synergistically combines photonic and thermal excitation of charge carriers in semiconductors to emit electrons across a vacuum gap, thereby generating electrical power. This review offers a detailed report of PETE device architectures, with a particular focus on recent advancements in cathode and anode materials, heterostructure engineering, and design innovations focusing at enhancing device performance. Critical operational parameters are investigated for their impact on power conversion efficiency, including temperature control, space-charge effects, vacuum-gap, intrinsic material properties, and optical characteristics. This review also highlights current research challenges to be addressed to advance PETE technology to practical implementation. These comprises of development of thermally stable cathode materials with lower electron affinity, anodes with low work functions, and robust thermal management strategies to maintain optimal operating conditions. Furthermore, the optimization of optical designs to maximize photon absorption and the capability to operate efficiently under high photon flux conditions are identified as key areas of research for future investigation. By identifying key material bottlenecks, device constraints, and realistic performance limits, this Review establishes material selection and device design guidelines for future research on scalable high-temperature solar energy technologies compatible with emerging net-zero energy pathways.