Advanced Materials for High-Temperature Solid-State Converters of Thermal and Concentrated Solar Energy

High-temperature solar cells and thermal energy converters are possible by exploiting hybrid mechanisms, such as thermionic-thermoelectric generation [1], thermionic-photovoltaic conversion [2, 3], and photon-enhanced thermionic emission (PETE) concept [4], which represent novel and promisingly efficient (&gt;50%) mechanisms for the exploitation of concentrated sunlight.<br/>Ultrashort laser pulses can tailor the optical properties of concentrated sunlight absorbers by maximizing solar absorption and selectivity thanks to surface periodic nanostructures [5]. Solar thermionic energy converters with surface nanotextured surfaces have already been demonstrated to enhance the selective absorption in prototypes based on nanodiamond emitters. More advanced PETE converters rely on the concept that engineered semiconductor photocathodes can provide an efficient electron emission, obtained by a synergistic combination of photogeneration and thermionic emission. Surface nanotexturing induced by ultrashort laser treatments can also tailor the electronic properties of semiconductors so that PETE cathodes can be drastically enhanced in terms of photosensitivity even to sub-bandgap radiation.<br/>Specifically, in the case of black diamond, surface nanotexturing combined with surface-hydrogenation on the opposite film side is proposed as a radically new and potentially effective PETE cathode up to temperatures of 700 °C. CVD diamond is transparent to solar radiation due to its wide bandgap, consequently, black diamond technology was developed to drastically increase its absorption coefficient (solar absorptance even &gt;99% in the double-textured samples [6]) and photogeneration capability under sunlight irradiation. A final p-type/intrinsic structure merges the technologies of surface texturing by fs-laser, boron-implantation for the formation of a buried p-type layer, and laser-induced graphitic microchannels, to form an innovative defect-engineered black diamond cathode for the conversion of concentrated sunlight. Results under a high-flux solar simulator is reported and discussed by demonstrating for the first time the PETE effect at temperatures from 300 to 500 °C.<br/>But there is more. Ultrathin nanocrystalline diamond [7,8] emitters deposited on surface-nanotextured silicon can be a viable and cost-effective solution for PETE converters [9], as well as perovskite-based PETE cathodes, currently under development for linear-focus concentrators.<br/><br/>Bibliography<br/>[1] D. M. Trucchi et al., "Solar Thermionic-Thermoelectric Generator (ST2G): Concept, Materials Engineering, and Prototype Demonstration," Advanced Energy Materials, vol. 8, no. 32, 2018.<br/>[2] A. Bellucci et al., "A Three-Terminal Hybrid Thermionic-Photovoltaic Energy Converter", Advanced Energy Materials, vol. 12, no. 20, 2022.<br/>[3] A. Bellucci et al., "Photovoltaic Anodes for Enhanced Thermionic Energy Conversion," ACS Energy Letters, vol. 5, no. 5, pp. 1364-1370, 2020.<br/>[4] J. W. Schwede et al., "Photon-enhanced thermionic emission for solar concentrator systems," Nature Materials, vol. 9, pp. 762-767, 2010.<br/>[5] P. Calvani et al., "Black diamond for solar energy conversion," Carbon, vol. 105, pp. 401-407, 2016 2016.<br/>[6] M. Girolami et al., "Optical characterization of double-nanotextured black diamond films," Carbon, vol. 138, pp. 384-389, 2018.<br/>[7] M. Tomellini and R. Polini, "Impact of seed density on continuous ultrathin nanodiamond film formation", Diamond Relat. Mater., vol. 153, 109700, 2023.<br/>[8] R. Salerno et al., "Etching Kinetics of Nanodiamond Seeds in the Early Stages of CVD Diamond Growth", ACS Omega, vol. 8, pp. 25496-25505, 2023.<br/>[9] R. Salerno et al., "Low Electron Affinity Silicon/Nanocrystalline Diamond Heterostructures for Photon-Enhanced Thermionic Emission," ACS Applied Energy Materials, vol. 7, no. 3, pp. 868-873, 2024.