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The thermionic-thermoelectric solid-state technology, characterized by solar-to-electric conversion efficiency feasibly > 40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide-based radiation absorbers, surface-textured at the nanoscale to obtain a solar absorptance > 90%, and chemical vapor deposition diamond films, acting as low- work-function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar-to-electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal-to-electric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid-state conversion alternative for future concentrating solar plants

Thin films of borides (LaB6 and CeB6) and nitrides (CNx and AlN) were grown by ns (ArF, 193 nm) pulsed laser deposition (PLD) on refractory substrates selected by considering the thermal matching with the thin films in terms of thermal expansion and thermal and electric conductivities. Thin layers of these materials can act as efficient electron emitters for thermionic cathodes operating at high temperature (up to 2000 °C) due to their low work function, typically < 3 eV, and the high melting point. The electron emission was found to depend strongly on the film thicknesses. The physical properties of the thin films were optimized by varying the deposition temperature of the substrates, the target-to-substrate, the atmosphere (vacuum or Ar gas flow). X-ray spectroscopy (XPS) was used to investigate the properties of the deposited thin films. Ultraviolet PhotoSpectroscopy (UPS) and thermionic emission measurements were performed to evaluate the work functions of the emitting layers deposited on the selected substrates.

In the last years, a hybrid thermionic-photovoltaic (TIPV) direct converter has been proposed and developed for the conversion of concentrated solar energy in the frame of European H2020 project AMADEUS. In order to improve the device performance, the active materials must be characterized by a low work function. Not only, for the thermionic anode must be selected a material with specific characteristics: 1) work function lower than that of the cathode; 2) optical transparency to the blackbody radiation emitted by the emitter. A good strategy is represented by the deposition of a functional coating on the TPV surface. For this reason, BaF2 thin films have been deposited on GaAs substrates by electron-beam thermal evaporation. The thickness of the BaF2 coating has been varied from hundreds of nm down to 1 Å, in order to select the sample with the best performance. The influence of the thickness on the chemical composition and on the work function has been investigated by XPS and UPS, respectively. In particular, the work function values have been calculated from the signal cut-off in acquired He I valence band spectra. In order to avoid the low kinetic energy cut-off caused by the spectrometer, a negative bias V has been applied to the samples. The obtained results revealed that a low work function of 2.1 eV can be reached, when the thickness of the film is of ≈ 1 nm, which is a promising value for a practical application in hybrid TIPV devices. A very interesting effect appeared in the ultra-thin layers of BaF2 with a thickness of only few Å:a new peak positioned at low kinetic energy, near the signal cut-off, appeared in the valence band spectra. This result could be interpreted by considering that conductive band maximum moves above the vacuum level, resulting in negative electron affinity.

Lanthanum hexaboride (LaB6) is a well-known material for electron emission applications. Many works concerning the production of LaB6 thin-films were reported in the last two decades. In particular, PLD demonstrated to be one of the most feasible deposition technique for the synthesis of LaB6 thin films, with optimal results in terms of crystallinity and values of work function lower than 2.8 eV. However, such technique is characterized by a low deposition rate at substrate temperatures higher than 500 °C, required for the proper crystallinity and structure quality. Previous works published in the literature report results obtained by the exclusive use of nanosecond laser assisted PLD. Conversely, a high growth-rate of the films may be correlates to the use of ultrashort pulse lasers as well as deposition at low temperatures. Hence, we propose for the first time PLD assisted by femtosecond pulse laser as an effective method for the development of LaB6 thin films with the desired bulk and surface physical properties to act as excellent thermionic emitters, prepared at room temperature (RT) and with extremely high deposition rates.&nbsp

In the last years, a hybrid thermionic-photovoltaic (TIPV) direct converter has been proposed and developed for the conversion of concentrated solar energy in the frame of European H2020 project AMADEUS. In order to improve the device performance, the active materials must be characterized by a low work function. Not only, for the thermionic anode must be selected a material with specific characteristics: 1) work function lower than that of the cathode; 2) optical transparency to the blackbody radiation emitted by the emitter. A good strategy is represented by the deposition of a functional coating on the TPV surface. For this reason, BaF2 thin films have been deposited on GaAs substrates by electron-beam thermal evaporation. The thickness of the BaF2 coating has been varied from hundreds of nm down to 1 Å, in order to select the sample with the best performance. The influence of the thickness on the chemical composition and on the work function has been investigated by XPS and UPS, respectively. In particular, the work function values have been calculated from the signal cut-off in acquired He I valence band spectra. In order to avoid the low kinetic energy cut-off caused by the spectrometer, a negative bias V has been applied to the samples. The obtained results revealed that a low work function of 2.1 eV can be reached, when the thickness of the film is of ≈ 1 nm, which is a promising value for a practical application in hybrid TIPV devices. A very interesting effect appeared in the ultra-thin layers of BaF2 with a thickness of only few Å:a new peak positioned at low kinetic energy, near the signal cut-off, appeared in the valence band spectra. This result could be interpreted by considering that conductive band maximum moves above the vacuum level, resulting in negative electron affinity.

Lanthanum hexaboride (LaB6) is a well-known material for electron emission applications. Many works concerning the production of LaB6 thin-films were reported in the last two decades. In particular, PLD demonstrated to be one of the most feasible deposition technique for the synthesis of LaB6 thin films, with optimal results in terms of crystallinity and values of work function lower than 2.8 eV. However, such technique is characterized by a low deposition rate at substrate temperatures higher than 500 °C, required for the proper crystallinity and structure quality. Previous works published in the literature report results obtained by the exclusive use of nanosecond laser assisted PLD. Conversely, a high growth-rate of the films may be correlates to the use of ultrashort pulse lasers as well as deposition at low temperatures. Hence, we propose for the first time PLD assisted by femtosecond pulse laser as an effective method for the development of LaB6 thin films with the desired bulk and surface physical properties to act as excellent thermionic emitters, prepared at room temperature (RT) and with extremely high deposition rates.

In the last decades, an increasing interest of the scientific community has been addressed to the renewable energy field [1]. In particular, the solar radiation is considered as an abundant, cheap, clean and sustainable energy source. In this context, promising solid state devices for the conversion of primary energy into electricity like thermionicphotovoltaic (TIPV) converters have been developed. They consist of three elements: electron emitter, collector and photovoltaic (PV) cell [2]. The main characteristics of the emitter are the enhanced capability to emit electrons and the selective thermal emittance, which should be able to satisfy the specific requirements of the PV cell in terms of radiation absorption. Basically, an ideal cathode must be characterized by a low work function, but, for a thermionic energy converter, it has to be higher than that of the anode. In the present work, thin films of different borides and nitrides deposited by pulsed laser deposition (PLD) are proposed in order to improve the performance of TIPV devices. The emitting layers of lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been well established, therefore they are considered good candidates in terms of low work function and high melting point. As it regards nitrides, amorphous carbon nitride (CNx) and hydrogenated aluminium nitride (AlN:H) have been individuated as valid alternatives to borides, because they are characterized by an even lower work function. The films of these materials with a thickness up to 200 nm have been grown on different substrates by PLD. Because the properties of borides and nitrides are strongly dependent on the content of boron or nitrogen, their chemical composition and work function were investigated by XPS and UPS techniques. These analyses were carried out by using an Escalab 250 Xi, equipped with a monochromatized Al Kα and ultraviolet He sources. The work function was determined from the cutoff in the He I spectra. Finally, the homogeneity of the chemical composition along the thickness of the samples has been investigated by XPS depth profiling carried out by means of Ar+ ion sputtering. [1] Fatih Bayrak, Nidal AbuHamdeh, Khaled A. Alnefaie, Hakan F. Öztop, Renewable and Sustainable Energy Reviews, 74 (2017) 755. [2] A. Datas, Appl. Phys. Letters, 108 (2016) 143503.

Lanthanum hexaboride (LaB6) thin-films were grown both by ns (ArF, 193 nm) Pulsed Laser Deposition (PLD). LaB6 layers can act as efficient electron emitters for high temperature (< 2000 °C) thermionic cathodes due to their low work function (around 2.3 eV). The physical properties of the thin-films deposited by PLD were optimized by varying target-substrate working distance (in the range 30-100 mm), bias voltage substrate (with or without -100 V) and process atmosphere (vacuum and Ar pressure). Scanning electron microscopy, and x-ray spectroscopy (XPS) were used to investigate the properties of the deposited thin-films. Ultraviolet Photospetroscopy (UPS) Thermionic emission measurements were performed to evaluate the work functions of LaB6 deposited on metallic substrates.

In the last decades, an increasing interest of the scientific community has been addressed to the renewable energy field [1]. In particular, the solar radiation is considered as an abundant, cheap, clean and sustainable energy source. In this context, promising solid state devices for the conversion of primary energy into electricity like thermionicphotovoltaic (TIPV) converters have been developed. They consist of three elements: electron emitter, collector and photovoltaic (PV) cell [2]. The main characteristics of the emitter are the enhanced capability to emit electrons and the selective thermal emittance, which should be able to satisfy the specific requirements of the PV cell in terms of radiation absorption. Basically, an ideal cathode must be characterized by a low work function, but, for a thermionic energy converter, it has to be higher than that of the anode. In the present work, thin films of different borides and nitrides deposited by pulsed laser deposition (PLD) are proposed in order to improve the performance of TIPV devices. The emitting layers of lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been well established, therefore they are considered good candidates in terms of low work function and high melting point. As it regards nitrides, amorphous carbon nitride (CNx) and hydrogenated aluminium nitride (AlN:H) have been individuated as valid alternatives to borides, because they are characterized by an even lower work function. The films of these materials with a thickness up to 200 nm have been grown on different substrates by PLD. Because the properties of borides and nitrides are strongly dependent on the content of boron or nitrogen, their chemical composition and work function were investigated by XPS and UPS techniques. These analyses were carried out by using an Escalab 250 Xi, equipped with a monochromatized Al Kα and ultraviolet He sources. The work function was determined from the cutoff in the He I spectra. Finally, the homogeneity of the chemical composition along the thickness of the samples has been investigated by XPS depth profiling carried out by means of Ar+ ion sputtering. [1] Fatih Bayrak, Nidal AbuHamdeh, Khaled A. Alnefaie, Hakan F. Öztop, Renewable and Sustainable Energy Reviews, 74 (2017) 755. [2] A. Datas, Appl. Phys. Letters, 108 (2016) 143503.

Lanthanum hexaboride (LaB6) thin-films were grown both by ns (ArF, 193 nm) Pulsed Laser Deposition (PLD). LaB6 layers can act as efficient electron emitters for high temperature (< 2000 °C) thermionic cathodes due to their low work function (around 2.3 eV). The physical properties of the thin-films deposited by PLD were optimized by varying target-substrate working distance (in the range 30-100 mm), bias voltage substrate (with or without -100 V) and process atmosphere (vacuum and Ar pressure). Scanning electron microscopy, and x-ray spectroscopy (XPS) were used to investigate the properties of the deposited thin-films. Ultraviolet Photospetroscopy (UPS) Thermionic emission measurements were performed to evaluate the work functions of LaB6 deposited on metallic substrates.