A high-efficiency solar thermophotovoltaic (STPV) system has been demonstrated using spectrally selective planar absorber/emitter systems and a GaSb TPV cell. In this study, a novel approach for designing the STPV system based on the efficiency of unidirectional radiative heat transfer has been introduced. To achieve high extraction and photovoltaic conversion efficiencies, the spectrally selective absorber/emitter based on a coherent perfect absorber composed of a thin molybdenum layer sandwiched between hafnium layers was applied. The extraction efficiency was further investigated with respect to the absorber/emitter area ratio. The experimental efficiency of STPV reached 5.1% with the area ratio of 2.3.

We have measured the absorption, luminescence, luminescence excitation spectra, and time response of the luminescence intensity of poly(di-n-hexylsilane) (PDHS) film and of low-temperature glass of dilute PDHS solution at 2 K. It is found that the luminescence peak energies shift with the excitation energy below $3.385\pm 0.005\mathrm{eV}$ for the film and below $3.463\pm 0.005\mathrm{eV}$ for the glass, and that the rise and decay times of luminescence intensity at the peak energies are 70 and 300 ps, respectively, for the film, and 130 and 260 ps, respectively, for the glass. In order to explain these results, numerical diagonalization of the one-dimensional Frenkel exciton Hamiltonian with disorder is carried out, and the luminescent process is formulated with the assumption that the exciton does not change the site by phonon scattering. We estimate the transition-dipole moment to be ${\mu }_0=3.0\mathrm{}\times {10}^{-18}\mathrm{esu}\mathrm{}\mathrm{cm}$ for the film and ${\mu }_0=4.2\mathrm{}\times {10}^{-18}\mathrm{esu}\mathrm{}\mathrm{cm}$ for the glass and the transition constant to be $c_{\mathrm{tr}}=1.9\mathrm{}\times {10}^{12}{\mathrm{s}}^{-1}$ for the film and $c_{\mathrm{tr}}=0.4\mathrm{}\times {10}^{12}{\mathrm{s}}^{-1}$ for the glass. We discuss the role of hidden structures at the low-lying exciton states that appear in the site representation of exciton wave functions. The hidden structure correlates the microscopic picture with the phenomenological segment model. It is found that the long-range dipole-dipole interaction contributes to the long luminescence rise time.

Optical rectennas are expected to be applied as power sources for energy harvesting because they can convert a wide range of electromagnetic waves, from visible light to infrared. The critical element in these systems is a diode, which can respond to the changes in electrical polarity in the optical frequency. By considering trade-off relationship between current density and asymmetry of IV characteristic, we reveal the efficiency limitations of MIM diodes for the optical rectenna and suggest a novel tunnel diode using a double insulator with an oxygen-non-stoichiometry controlled homointerface structure (MO_x/MO_x−y). A double-insulator diode composed of Pt/TiO₂/TiO_1.4/Ti, in which a natural oxide layer of TiO_1.4 is formed by annealing under atmosphere. The diode has as high-current-density of 4.6 × 10⁶ A/m², which is 400 times higher than the theoretical one obtained using Pt/TiO₂/Ti MIM diodes. In addition, a high-asymmetry of 7.3 is realized simultaneously. These are expected to increase the optical rectenna efficiency by more than 1,000 times, compared to the state-of-the art system. Further, by optimizing the thickness of the double insulator layer, it is demonstrated that this diode can attain a current density of 10⁸ A/m² and asymmetry of 9.0, which are expected to increase the optical rectenna efficiency by 10,000.

For effective photon conversion in solar-thermophotovoltaic (TPV) systems, an enclosed-space confined emitter system is proposed, and its power generation potential is demonstrated. A cuboid monolithic absorber/emitter is applied for higher extraction efficiency without dead areas. An analysis using an enclosed space shows a 4.1% higher absolute system efficiency than that using a planar absorber/emitter system. In the experiment, system efficiency reaches 7.0%, which is obtained after multiplying the power measured from one cell by five. A system efficiency more than 20% is achievable by further improvement with a back surface reflecting TPV cells and a perfectly enclosed space.