Abstract Hybrid thermionic-photovoltaics (TIPV) are solid-state thermal-to-electric energy converters that rely on the non-isothermal transport of photons and electrons through a vacuum gap. In contrast to pure thermionic converters, the absorption of photons in a photovoltaic anode produces an electrochemical potential that can be delivered as electricity, ultimately boosting the power generation capacity of the device. In this work, the proof of concept of a three-terminal TIPV converter where thermionic and photogenerated currents are collected independently is reported. Thermionic electrons are injected in the conduction band of a semiconducting anode (n-type InP), from where they are directly extracted. Photogenerated electrons are also extracted from the conduction band of the anode, but they are then reinjected in the valence band through an independent hole-selective contact (p-type InGaAs). By using a low workfunction engineered anode (BaFx/InP) and cathode (ScxOy/W) a maximum power generation capacity of 125.6 and 0.35 mW cm-2 for PV and thermionic sub-devices, respectively, is demonstrated, operating at 1400?C. This proof of concept paves the way for the development of efficient hybrid thermionic and photovoltaic converters for the direct conversion of heat into electricity, and subsequently contributes to finding an efficient alternative to thermoelectric generators.

An array of 2500 vertical graphitic microwires was fabricated within a single-crystal diamond plate with the purpose of creating distributed electrically conductive structures intended for the development of electronic devices operating at high temperatures. To this end, the structural, morphological, and electrical properties of the diamond/graphite system were investigated as a function of temperature up to 550?C, by means of optical and secondary electron microscopy (SEM), micro-Raman spectroscopy and current-voltage measurements. The vertical microstructuring of the diamond bulk was obtained by a laser-induced phase transition from diamond to graphite by means of ultra-short laser pulses (100?fs duration, 800?nm wavelength, 1?kHz repetition rate). As inferred from SEM micrographs, the graphitic wires display a high-aspect-ratio with length of approximately 200??m and diameter of about 10??m. The electrical resistivity of the single microwire is estimated to be 0.49?0.15???cm at room temperature, then decreasing linearly with temperature with a coefficient of approximately -1??10-2?K-1. Raman spectroscopy results point out the absence of structural alterations after high-temperature operations.

The following technical report discloses the results of the thermal test carried out on the single crystal diamond matrix detector prototype for the ITER Radial Neutron Camera (RNC) diagnostics system.

Black diamond is an emerging material for solar applications. The femtosecond laser surface treatment of pristine transparent diamond allows the solar absorptance to be increased to values greater than 90% from semi-transparency conditions. In addition, the defects introduced by fs-laser treatment strongly increase the diamond surface electrical conductivity and a very-low activation energy is observed at room temperature. In this work, the investigation of electronic transport mechanisms of a fs-laser nanotextured diamond surface is reported. The charge transport was studied down to cryogenic temperatures, in the 30-300 K range. The samples show an activation energy of a few tens of meV in the highest temperature interval and for T < 50 K, the activation energy diminishes to a few meV. Moreover, thanks to fast cycles of measurement, we noticed that the black-diamond samples also seem to show a behavior close to ferromagnetic materials, suggesting electron spin influence over the transport properties. The mentioned properties open a new perspective in designing novel diamond-based biosensors and a deep knowledge of the charge-carrier transport in black diamond becomes fundamental.

Thermionic-photovoltaic (TIPV) converters exploit both electrons and photons emitted by a hot cathode to produce electric power. In this work, a TIPV converter structured with a tungsten cathode and an In0.53Ga0.47As photovoltaic (PV) anode (0.75 eV bandgap) is demonstrated to provide an increased output voltage with respect to the reference thermionic energy converter made of the same materials and operating under similar conditions. The higher voltage, measured to be from 0.3 to 0.5 V in the explored 1150-1450 degrees C temperature range, is explained by the additional contribution of the PV anode. Another work function engineered TIPV converter, obtained by coating the W cathode with LaB6 and the In0.53Ga0.47As PV anode with BaFx, has led to an output power 2 orders of magnitude higher than the uncoated TIPV converter at 1200 degrees C cathode temperature. This opens the route towards future optimized TIPV electrodes.

With the aim of presenting the processes governing the Laser-Induced Periodic Surface Structures (LIPSS), its main theoretical models have been reported. More emphasis is given to those suitable for clarifying the experimental structures observed on the surface of wide bandgap semiconductors (WBS) and dielectric materials. The role played by radiation surface electromagnetic waves as well as Surface Plasmon Polaritons in determining both Low and High Spatial Frequency LIPSS is briefly discussed, together with some experimental evidence. Non-conventional techniques for LIPSS formation are concisely introduced to point out the high technical possibility of enhancing the homogeneity of surface structures as well as tuning the electronic properties driven by point defects induced in WBS. Among these, double- or multiple-fs-pulse irradiations are shown to be suitable for providing further insight into the LIPSS process together with fine control on the formed surface structures. Modifications occurring by LIPSS on surfaces of WBS and dielectrics display high potentialities for their cross-cutting technological features and wide applications in which the main surface and electronic properties can be engineered. By these assessments, the employment of such nanostructured materials in innovative devices could be envisaged.

Polycrystalline lanthanum hexaboride (LaB) discs were sintered by hot pressing of commercial powders and successively polished, in order to investigate their thermionic emission capability. LaB samples presented values of work function in the range 2.55-2.61 eV. Discs with polished surface, independently from the composition and density, demonstrated to provide a Richardson constant around 45 A cm2K2. These results are promising for the use of sintered LaB discs in thermionic energy converters operating at temperatures in the 1400-1700 °C range. Moreover, since the properties of solar absorption and spectral selectivity of LaB make them appealing for the conversion of concentrated solar radiation, a modelling study to estimate the energy conversion efficiency was performed, aimed at identifying the proper engineering strategies able to make the resulting devices competitive for electrical conversion of concentrated solar radiation.

Two-dimensional laser-induced periodic surface structures with a deep-subwavelength periodicity (80 nm ? ?/10) are obtained for the first time on diamond surfaces. The distinctive surface nanotexturing is achieved by employing a single step technique that relies on irradiation with two temporally delayed and cross-polarized femtosecond-laser pulses (100 fs duration, 800 nm wavelength, 1 kHz repetition rate) generated with a Michelson-like interferometer configuration, followed by chemical etching of surface debris. In this Letter, we demonstrate that, if the delay between two consecutive pulses is <=2 ps, the 2D periodicity of nanostructures can be tuned by controlling the number of pulses irradiating the surface. Under scanning mode, the method is effective in treating uniformly large areas of diamond, so to induce remarkable antireflection properties able to enhance the absorptance in the visible up to 50 times and to pave the route toward the creation of metasurfaces for future diamond-based optoelectronic devices.

The homogeneity of hydrogen termination on single-crystal CVD diamond surfaces exposed to air was studied with a new approach that encloses a combination of sheet resistance measurements, performed using four-point-probe method in Van Der Pauw configuration, and electron reflectivity measurements performed at intermediate kinetic energy to derive surface sensitive information on the chemical terminations (inelastic mean free path <0.8 nm). The coherence between the 2-dimensional spatial distributions of sheet resistance and electron reflectivity maps, as well as their similar dynamic range and consistency with an electron energy loss analysis, demonstrates that the proposed combined approach allows for a reliable and easy evaluation of both the effectiveness and homogeneity of the hydrogen-termination process of a diamond surface.

The thermal and electrical performance of the innovative solar thermionic-thermoelectric generator (STG) has been carefully analyzed by using a high-flux solar simulator. Specific technological solutions have been integrated with respect to the first prototype for the operational testing at the moderate temperature range of 700-1000 K. The influence of spacers with different thickness has been studied to quantify the reduction of output current due to space-charge effect, whereas a BaF-based coating has been deposited on the anode to make its work function (~2.1 eV) approach the cathode one, on which a hydrogen-terminated nitrogen doped nano-diamond film acts as emitter (work function of ~2.0 eV). The engineered ceramic absorber has demonstrated to be a reliable selective solar absorber with a high solar-to-thermal efficiency of 84% for solar concentration ratio of 200 suns, extrapolated to 83% at 500 suns. As expected, the thermionic energy converter still provides a small converted power. However, the minimum inter-electrode spacing used (50 ?m) and the engineered anode allow to better figure out the present limitations in the electrical power generation and how to surpass them in the near future.

During the last years, innovative concepts of solid-state devices, such as multi-junction solar cells and thermophotovoltaic converters, have emerged as efficient means of direct electricity production. The accurate estimation of the conversion efficiency of such devices using simple yet effective numerical tools is a necessity to optimize their performance. This work presents an in-house code, which is based on the Drift Diffusion Model to simulate p-n junction diodes, under equilibrium (bias voltage application) and non-equilibrium (bias voltage and device illumination) conditions. Under non-equilibrium, illumination can originate from either solar radiation (conventional photovoltaic cell) or a thermally heated emitter (thermophotovoltaic operation). The drift-diffusion and Poisson's equations are solved using a one-dimensional (1D) finite element Petrov-Galerkin method based on piecewise nonlinear interpolants of second-order accuracy, while the total current is evaluated in a post-process manner using the Scharfetter-Gummel scheme. Initially, the model is verified against the freeware SimWindows. Later on, a parametric analysis on the photovoltaic cell design and operating conditions reveals that its efficiency is highly affected by its total length, the n-type sub-region width, the doping levels of both p and n regions, the semiconductor material type, and, the device's operating temperature. In contrast to other solvers, this one takes into account the model parameters' dependence on temperature and electromagnetic spectrum, while it can be extended to incorporate the thermally stimulated electron emission in thermionic-based devices and 2D spatial effects. Finally, the calculated conversion efficiencies can be used to build a Reduced Order Model that can be further coupled with a computational fluid dynamics model to evaluate the overall thermo-electric performance of a solid-state device.

Dielectric microspacers (DMS) are important components in thermal energy converters. Engineered DMS are fabricated and characterized on different substrates by depositing patterned ceramic thin films of alumina (Al2O3) and zirconia (ZrO2) with a thickness ranging from 0.3 to 3 mu m. Both Al2O3 and ZrO2 films are electrically and thermally optimized, finding zirconia more suitable as a thermal and electrical insulating material at high temperature, whereas the developed DMS are morphologically analyzed by scanning electron microscopy. The analysis of thermal simulations carried out with COMSOL Multiphysics allows identifying the best geometrical constraints for each single structure, whereas simulations carried out by the Fluent software allow identifying the best arrangement for DMS, leading to a solution with optimized pattern in terms of amount and spatial distribution so to achieve the required electrical and thermal insulation for practical applications. DMS are integrated within thermionic-photovoltaic devices to be validated experimentally, and enhanced electron emission measurements are successfully performed at a cathode temperature up to 1350 degrees C to verify the operational feasibility and potential of this technology.

Thermal and concentrated solar solid-state converters are devices with no moving parts, corresponding to long lifetimes, limited necessity of maintenance, and scalability. Among the solid-state converters, the thermionic-based devices are attracting an increasing interest in the specific growing sector of energy conversion performed at high-temperature. During the last 10 years, hybrid thermionic-based concepts, conceived to cover operating temperatures up to 2000 degrees C, have been intensively developed. In this review, the thermionic-thermoelectric, photon-enhanced thermionic emission, thermionic-photovoltaic energy converters are extensively discussed. The design and development processes as well as the tailoring of the properties of nanostructured materials performed by the authors are comprehensively described and compared with the advances achieved by the international scientific community.

Starting in January 2017, AMADEUS (www.amadeus-project.eu) is the first project funded by the European Commission to research on a new generation of materials and solid-state devices for ultra-high temperature energy storage and conversion. By exploring storage temperatures well beyond 1000 ºC, one of the main objectives of the project is to create new PCMs (phase change materials) with latent heat in the range of 1000-2000 kWh/m, an order of magnitude greater than that of typical salt-based PCMs used in concentrated solar power (CSP), along with developing advanced thermal insulation, PCM casing designs, and novel solid-state thermal-to-electric energy conversion devices able to operate at temperatures in the range of 1000-2000 ºC. In particular, the project is investigating silicon-boron based alloys as PCMs and hybrid thermionic-photovoltaic (TIPV) devices for energy conversion. This paper describes the main project R&D activities and the results that have been attained during the first two years of the project. This includes the thermophysical characterization of Si-B alloys, the wettability and solubility analysis of said alloys with solid refractory materials, the numerical simulation of phase-change and heat losses through thermal insulation cover, as well as the realization of the two main proof-of-concept experiments: the TIPV converter, and the full latent heat energy storage system.

Irradiation of diamond with femtosecond (fs) laser pulses in ultra-high vacuum (UHV) conditions results in the formation of surface periodic nanostructures able to strongly interact with visible and infrared light. As a result, native transparent diamond turns into a completely different material, namely "black" diamond, with outstanding absorptance properties in the solar radiation wavelength range, which can be efficiently exploited in innovative solar energy converters. Of course, even if extremely effective, the use of UHV strongly complicates the fabrication process. In this work, in order to pave the way to an easier and more cost-effective manufacturing workflow of black diamond, we demonstrate that it is possible to ensure the same optical properties as those of UHV-fabricated films by performing an fs-laser nanostructuring at ambient conditions (i.e., room temperature and atmospheric pressure) under a constant He flow, as inferred from the combined use of scanning electron microscopy, Raman spectroscopy, and spectrophotometry analysis. Conversely, if the laser treatment is performed under a compressed air flow, or a N2 flow, the optical properties of black diamond films are not comparable to those of their UHV-fabricated counterparts.

Low-cost carbon-conductive films were screen-printed on a Plexiglas(R)substrate, and then, after a standard annealing procedure, subjected to femtosecond (fs) laser treatments at different values of total accumulated laser fluence phi(A). Four-point probe measurements showed that, if phi(A)> 0.3 kJ/cm(2), the sheet resistance of laser-treated films can be reduced down to about 15 omega/sq, which is a value more than 20% lower than that measured on as-annealed untreated films. Furthermore, as pointed out by a comprehensive Raman spectroscopy analysis, it was found that sheet resistance decreases linearly with phi(A), due to a progressively higher degree of crystallinity and stacking order of the graphitic phase. Results therefore highlight that fs-laser treatment can be profitably used as an additional process for improving the performance of printable carbon electrodes, which have been recently proposed as a valid alternative to metal electrodes for stable and up-scalable perovskite solar cells.

The use of high power pulsed lasers is an effective tool for microstructuring material surfaces. It appears particularly useful when the material has some characteristics making difficult using other procedures (e.g. a high hardness). The present work reports on the femtosecond-laser treatment on tantalum diboride ultra-high temperature ceramics with different starting porosity fractions. The interaction with the laser beam creates a pattern with a complex multi-scale structure on the ceramic surface, whose characteristics depend on accumulated laser fluence and pristine porosity. Optical properties are significantly changed, allowing to separately optimize the interaction of the material with electromagnetic radiation spectrally located in different regions. As a case study, we apply the proposed strategy considering high-temperature solar thermal absorber applications, where the independent management of UV-Visible-Near IR radiation (sunlight) and Mid-IR (thermal radiation at the operating temperatures) are required. The correlation between the typical sizes of the realized multi-scale structures and the optical parameters (solar absorptance and thermal emittance in our example application) is discussed using an original predictive approach. The method here shown can be extended to every situation where materials are required to simultaneously interact with electromagnetic radiation in various spectral ranges.

Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic-photovoltaic (TIPV) devices. The chemical composition and work function ? of the films with different thicknesses were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The lowest value of ? = 2.1 eV was obtained for the film with a thickness of ~2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function.

Surface treatments were performed on single-crystal semi-insulating 6H-SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films' optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm as well as detecting fine (160 nm) and coarse (450-550 nm) ripples according to different values of the laser pulse fluence (?) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ? exceeds 2.15 J cm, whereas no compositional variations occur for lower values of ?. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H-SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Thermionic energy converters are heat engines based on the direct emission of electrons from a hot cathode toward a colder anode. Because the thermionic emission is unavoidably accompanied by photonic emission, radiative energy transfer is a significant source of losses in these devices. In this Letter, we provide the experimental demonstration of a hybrid thermionic-photovoltaic device that is able to produce electricity not only from the electrons but also from the photons that are emitted by the cathode. Thermionic electrons are injected in the valence band of a gallium arsenide semiconducting anode, then pumped to the conduction band by the photovoltaic effect, and finally extracted from the conduction band to produce useful energy before they are reinjected in the cathode. We show that such a hybrid device produces a voltage boost of similar to 1 V with respect to a reference thermionic device made of the same materials and operating under the same conditions. This proof of concept paves the way to the development of efficient thermionic and photovoltaic devices for the direct conversion of heat into electricity.