Diamond-Based Structures for Photon-Enhanced Thermionic Emission

Photon-enhanced thermionic emission (PETE) [1] represents a promising method for the efficient conversion of concentrated solar energy. In PETE devices, absorbed photons create photoexcited electrons in the conduction band. These electrons then thermalize and are thermionically emitted from the hot cathode surface. This process therefore enables the efficient conversion of both photons and heat generated from the absorption of concentrated sunlight.<br/>Cathodic materials in PETE devices typically consist of heterostructures with layers designed for light absorption and electron emission. Hydrogen terminated diamond emitters are particularly interesting because of their negative electron affinity, which persists at temperatures up to 700°C. This makes silicon/diamond PETE cathodes [2] especially promising for highly concentrated, point-focus solar concentrating systems.<br/>A study of the evolution and etching of detonation nanodiamond (DND) seeds has been performed to assess the optimal plasma assisted CVD parameters which optimize the diamond growth on the heavily doped silicon substrate. The temporal evolution of DND seed has been studied [3], highlighting their etching during the initial CVD diamond growth phase. This is fundamental to determine the minimum diamond film thickness that can be achieved given the initial DND seed density [4].<br/>A detailed analysis of the PETE performance as a function of NCD film thickness, ranging from 40 nm to 1 µm, reveals the significant impact of grain boundaries within the diamond emitting layer. Raman spectroscopy and Kelvin-Probe Force Microscopy (KPFM) [5] indicate that grain boundaries serve as preferential paths for electron transport and emission, enhancing the overall emission properties. Specifically, an 80 nm-thick diamond emitter exhibits the highest emission current density, attributed to the optimized grain boundary distribution. The plasma enhanced CVD deposition parameters can therefore be varied in order to optimize the grain boundary density while minimizing the surface’s electron affinity in order to maximize electron emission.<br/>To further enhance photon absorption, the silicon substrate was nano-structured using femtosecond pulsed laser treatments, resulting in a tenfold increase in emission current density. These results indicate that the increase in photon absorption exceeds the recombination of charge carriers due to the introduction of additional defect centers.<br/><br/>[1] J. W. Schwede <i>et al.</i>, "Photon-Enhanced Thermionic Emission for Solar Concentrator Systems”, <i>Nature Materials</i>, vol. 9, 762–767, <b>2010</b>, doi: 10.1038/nmat2814.<br/>[2] T. Sun <i>et al.</i>, "Thermally Enhanced Photoinduced Electron Emission from Nitrogen-Doped Diamond Films on Silicon Substrates”, <i>Physical Review B</i>, vol. 90, no. 12, 121302-121307, <b>2014</b>, doi: 10.1103/physrevb.90.121302.<br/>[3] R. Salerno <i>et al.</i>, “Etching Kinetics of Nanodiamond Seeds in the Early Stages of CVD Diamond Growth”, <i>ACS omega</i>, vol. 8, no. 28, 25496–25505, <b>2023</b>, doi: 10.1021/acsomega.3c03080.<br/>[4] M. Tomellini <i>et al.</i>, “Impact of Seed Density on Continuous Ultrathin Nanodiamond Film Formation”, <i>Diamond and Related Materials</i>, vol. 133, 109700, <b>2023</b>, doi: 10.1016/j.diamond.2023.109700.<br/>[5] R. Salerno <i>et al.</i>, “Low Electron Affinity Silicon/Nanocrystalline Diamond Heterostructures for Photon-Enhanced Thermionic Emission”, <i>ACS Applied Energy Materials</i>, vol. 7, no. 3, 868–873, <b>2024</b>, doi: 10.1021/acsaem.3c02735.