Titolo della tesi: Photon-Enhanced Thermionic Emission cathodes for concentrated solar energy conversion
The research activities at the basis of this doctoral project were focused on the design, characterization and fabrication of innovative high-temperature solar cells based on the concept of photon-enhanced thermionic emission (PETE). PETE is a conversion mechanism that exploits both photo-electric and thermionic emission concepts, and that allows to overcome the limitations of standard junction cells when operating at high temperatures. The proposed solar cell concept, which implies a direct application in conversion of concentrated solar radiation and energy recovery from high-temperature thermal sources, is made entirely of highly-engineered synthetic diamond, can operate up to about 800 °C and allows the combination of an additional conversion stage or energy storage system. High potential conversion efficiencies (> 40%), scalability typical of solid-state devices, non-toxicity for human health represent the main key advantages with respect to conventional technologies. The core of this doctoral project is represented by the development of single-crystal synthetic diamond PETE photocathodes (including also attempts with 6H-SiC), on which an innovative scientific concept is applied: the defect engineering of a semiconductor that does not natively interact with solar radiation (i.e. diamond, with an energy bandgap of 5.5 eV, is known to be solar-blind). The project foresaw the employment of technologies able to completely modify the optical and photoelectronic properties, turning the material from transparent to highly absorbing, but maintaining the excellent charge transport capability. To this end, this doctorate thesis reports on a variety of experimental investigations aimed mainly at advancing the understanding of fundamental processes of surface defect engineering, performed by inducing the formation of periodic surface nanotexturing through the use of ultra-short pulsed lasers. Furthermore, since the ultimate aim is to develop an original all-diamond PETE photo-cathode, efforts were also directed at the optimization of the thermionic emission capabilities of diamond.
In conclusion, the proof-of-concept of high-temperature solar cell proposed in this thesis should be considered exemplificative of the controlled and effective defect engineering strategy, which can be extendable for future development of a new class of ultra-wide bandgap optoelectronic devices.