Concentrating solar technologies represent economically viable alternatives to distributed PV since the optics production cost per unit surface is far lower than that of PV cells. The reduction of the active conversion area allows for expensive multijunction cells with efficiency as high as 45% to be used in concentrating PV (CPV). [1] However, CPV is struggling commercially because high-density arrangements are prevented by issues in excess heat removal. On the other hand, concentrating solar power (CSP) technologies, [2,3] based on transfer of high-temperature thermal energy through a fluid toward thermomechanical engines, are promising for large plants characterized by thermal-to-electric efficiency around 35%, with the important capabilities of energy cogeneration and storage.The solar thermionic-thermoelectric generator (ST 2 G) explored here takes the advantages of CSP and CPV to provide an effective alternative concept for future solar technologies by combining the hightemperature operations, typical of CSP, to the compactness, scalability, reliability, and long operating lifetime of the solid-state converters, typically used in CPV systems. By combining a thermionic energy converter (TEC), operating at high but practical temperatures (<1100 °C) that can be achieved with point-focus solar concentrators and the development of low-work-function materials, with a thermoelectric energy generator (TEG) thermally connected in series, the ST 2 G technology can meet requirements of economic cost-effectiveness guaranteed by high-temperature solid-state converters. [4] 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 lowwork-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-toelectric 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.

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Statistical theory and applications of lock-in carrierographic image pixel brightness dependence on multicrystalline Si solar cell efficiency and photovoltage J. Appl. Phys. 112, 054505 (2012) Signal contrast in coherent Raman scattering: Optical phonons versus biomolecules J. Appl. Phys. 112, 053101 (2012) Indirect optical absorption in silicon via thin-film surface plasmon J. Appl. Phys. 112, 043103 (2012) Strong photoluminescence from diameter-modulated single-walled carbon nanotubes Secondary electron emission from diamond films is studied as a function of the primary electron beam energy and bulk material properties. A formulation of a simple model of the secondary electron emission coefficient, as a function of the primary electron beam energy, has been found to be helpful in defining physical criteria able to guide the optimization of the diamond film electron emission performance. The secondary electron mean escape depth deduced from the model is indeed related to the density of defects in the material and represents the main factor in determining the low energy secondary electron yield. These results are supported by Raman spectroscopy measurements, indicating a lower graphitic content and a higher crystalline quality of the diamond phase in films showing better secondary electron and photoemission yields. We demonstrate that a diamond film, acting as a stable and proportional electron multiplier, can be used as a converter of backscattered electrons into secondary electrons in scanning electron microscopy. It will be shown that the use of a diamond film converter is suitable to improve the signal to noise ratio of images providing an enhanced compositional contrast.

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