Integration of Si Heterojunction Solar Cells with III-V Solar Cells by the Pd Nanoparticle Array-Mediated "Smart Stack" Approach.

This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality.

Anomalous Metal Phase Emergent on the Verge of an Exciton Mott Transition.

We investigate the exciton Mott transition (EMT) by using optical pump-terahertz probe spectroscopy on GaAs, with realizing the condition of Mott's gedanken experiment by the resonant excitation of 1s excitons. We show that an anomalous metallic phase emerges on the verge of the EMT as manifested by a peculiar enhancement of the quasiparticle mass and scattering rate. From the temperature and density dependence, the observed anomaly is shown to originate from the electron-hole (e-h) correlation which becomes prominent at low temperatures, possibly suggesting a precursor of e-h Cooper pairing.

Conversion efficiency limits and bandgap designs for multi-junction solar cells with internal radiative efficiencies below unity.

We calculated the conversion-efficiency limit ηsc and the optimized subcell bandgap energies of 1 to 5 junction solar cells without and with intermediate reflectors under 1-sun AM1.5G and 1000-sun AM1.5D irradiations, particularly including the impact of internal radiative efficiency (ηint) below unity for realistic subcell materials on the basis of an extended detailed-balance theory.

Thorough subcells diagnosis in a multi-junction solar cell via absolute electroluminescence-efficiency measurements.

World-wide studies on multi-junction (tandem) solar cells have led to record-breaking improvements in conversion efficiencies year after year. To obtain detailed and proper feedback for solar-cell design and fabrication, it is necessary to establish standard methods for diagnosing subcells in fabricated tandem devices. Here, we propose a potential standard method to quantify the detailed subcell properties of multi-junction solar cells based on absolute measurements of electroluminescence (EL) external quantum efficiency in addition to the conventional solar-cell external-quantum-efficiency measurements.

Analysis of oxyluciferin photoluminescence pathways in aqueous solutions.

We evaluated the pK(a) values of oxyluciferin and its conjugate acids and bases theoretically with the help of experimental correction values, from which free energies for the first excited and the ground states of all the species were estimated. On the basis of these results, we calculated pH-dependent absorption spectra, where the relative absorption intensities of various species strongly depend on photoexcitation energy, and we further analyzed the photoluminescence pathways of oxyluciferin in aqueous solutions with various pH. In the case of 350 nm photoexcitation, in particular, experiments have shown that dominant emission color is green and it attenuates with pH decreasing, while blue (3 < pH < 8) and red (pH < 3) emissions appear.

Mode imaging and loss evaluation of semiconductor waveguides.

An imaging and loss evaluation method for semiconductor waveguides coupled with non-doped quantum wells is presented. Using the internal emission of the wells as a probe light source, the numbers and widths of the modes of waveguides with various ridge sizes were evaluated by CCD imaging, and the obtained values were consistent with effective index method calculation. Waveguide internal losses were obtained from analyses of the Fabry-Pérot fringes of waveguide emission spectra.

Analysis of photoexcitation energy dependence in the photoluminescence of firefly luciferin.

The whole pathways for photoluminescence, which include absorption, relaxation and emission, of firefly luciferin in aqueous solutions of different pH values with different photoexcitation energies were theoretically investigated by considering protonation/deprotonation. It is experimentally known that the color of fluorescence changes from green to red with a decrease in the photoexcitation energy. We confirmed with the theoretical analysis that the peak energy shift in the fluorescence spectra with varying photoenergies is due to a change in photoluminescence pathway.

Intrinsic radiative lifetime derived via absorption cross section of one-dimensional excitons.

Intrinsic radiative lifetime is an essential physical property of low-dimensional excitons that represents their optical transition rate and wavefunction, which directly measures the probability of finding an electron and a hole at the same position in an exciton. However, the conventional method that is used to determine this property via measuring the temperature-dependent photoluminescence (PL) decay time involves uncertainty due to various extrinsic contributions at high temperatures. Here, we propose an alternative method to derive the intrinsic radiative lifetime via temperature-independent measurement of the absorption cross section and transformation using Einstein's A-B-coefficient equations derived for low-dimensional excitons.

Gain-switched pulses from InGaAs ridge-quantum-well lasers limited by intrinsic dynamical gain suppression.

Gain-switched pulses of InGaAs double-quantum-well lasers fabricated from identical epitaxial laser wafers were measured under both current injection and optical pumping conditions. The shortest output pulse widths were nearly identical (about 40 ps) both for current injection and optical pumping; this result attributed the dominant pulse-width limitation factor to the intrinsic gain properties of the lasers. We quantitatively compared the experimental results with theoretical calculations based on rate equations incorporating gain nonlinearities.

Evidence for two-dimensional spin-glass ordering in submonolayer Fe films on cleaved InAs surfaces.

Magnetotransport measurements have been performed on two-dimensional electron gases formed at InAs(110) surfaces covered with a submonolayer of Fe. Hysteresis in the magnetoresistance, a difference in remanent magnetoresistance between zero-field-cooling procedures and field-cooling procedures, and logarithmic time-dependent relaxation after magnetic field sweep are clearly observed at 1.7 K for a coverage of 0.