We present a concept of nonequilibrium solar cell, heat recovery solar cell (HERC cell), with its theoretical efficiency exceeding the detailed-balance limit. HERC cell uses an absorber hotter than electrodes, in terms of the lattice temperatures, and carrier-energy selecting layers in front of the electrodes. As being different from hot carrier solar cells, HERC cell does not require fast carrier extraction within the thermalization time and therefore the concept can be used to improve Si solar cells. Thermoelectric voltage produced by the temperature difference recovers the reduction due to the internal voltage drop in the hot absorber and improves the open-circuit voltage as a whole.
Transport and recombination of excess carriers in an InGaAs solar cell are investigated by using time-resolved photoemission spectroscopy. We found that photovoltage rises and decays over 820 ps and 980 ps, respectively, at the pump fluence of 0.16 μJ/cm2. This result shows that charge separation and recombination occur in a close time scale while charge separation is substantially faster than recombination in a GaAs solar cell which we studied in the previous study. This implies that the InGaAs cell suffers from higher non-radiative recombination loss. We also analyze the limiting factor of the temporal resolution for the present technique. The temporal resolution can be improved by employing a light source with a higher photon energy while its benefit is not drastic. Alternative methods for the improvement are discussed. In addition, time-resolved photoluminescence spectroscopy was performed in order to compare the two time-resolved techniques. The photoluminescence decay of a GaAs cell shows a fast decay at a weak photo-injection level, which becomes slower at higher injection levels as observed in previous studies.
We performed optical pump-THz probe spectroscopy on bulk GaAs to investigate the nature of exciton Mott transition. The behavior of excitonic correlation in the proximity of the Mott transition density is elucidated through the resonant excitation of 1s excitons with using a nonlinear terahertz spectroscopy technique. We discuss the anomalous charge carrier dynamics of the metallic phase on the verge of Mott transition that appears only at low temperatures.
We developed methodologies and calibration standards for absolute electroluminescence (EL) measurements for CONTACT-LESS evaluation of various internal properties of multi-junction and arrayed solar cells, such as open-circuit voltages, external and internal radiative efficiencies, and luminescence-coupling efficiency. Several independent calibration methods were compared that used: 1) a calibrated EL imaging system, 2) proximity measurement with a large-area photodiode, 3) an integrating-sphere system, and 4) planar light-emitting diodes with a circular aperture. The comparison clarified the advantages and disadvantages of each method, and showed consistency within 30% uncertainty, resulting in a 7-meV uncertainty in open-circuit voltage measurements.
In order to understand the radiation effects in space-used multi-junction solar cells, we characterized degradations of internal radiative efficiency (ηint i ) in respective subcells in InGaP/GaAs double-junction solar cells after 1-MeV electron irradiations with different electrons fluences (Φ) via absolute electroluminescence (EL) measurements, because ηint i purely represents material-quality change due to radiation damage, independently from cell structures. We analyzed the degradation of ηint i under different Φ and found that the data of ηint i versus Φ in moderate and high Φ regions are very similar and almost independent of subcell materials, while the difference in beginning-of-life qualities of InGaP and GaAs materials causes dominant difference in sub-cell sensitivity to the low radiation damages. Finally, a simple model was proposed to explain the mechanism in degradation of ηint i, and also well explained the degradation behavior in open-circuit voltage for these multi-junction solar cells.
KEYWORDS: Luminescence, Quantum efficiency, Electroluminescence, Solar energy, Solar cells, Multijunction solar cells, Tandem solar cells, Light emitting diodes, External quantum efficiency, Satellites
We developed a straightforward method based on detailed balance relations to analyze individual subcells in multi-junction solar cells via measuring absolute electroluminescence quantum yields. This method was applied to characterization of a InGaP/GaAs/Ge 3-junction solar cell for satellite use. In addition to subcell I-V characteristics and internal luminescence yields, we derived balance sheets of energy and carriers, which revealed respective subcell contributions of radiative and nonradiative recombination losses, junction loss, and luminescence coupling. These results provide important diagnosis and feedback to fabrications. We calculated conversion-efficiency limit and optimized bandgap energy in 2-, 3-, and 4-junction tandem solar cells, including finite values of sub-cell internal luminescence quantum yields to account for realistic material qualities in sub-cells. With reference to the measured internal luminescence quantum yields, the theoretical results provide realistic targets of efficiency limits and improved design principles of practical tandem solar cells.
By means of THz pump and optical probe spectroscopy, we observed that the incident THz pulse induces a strong spectral modulation of the 1s heavy-hole exciton peak of GaAs quantum wells due to Rabi splitting. Our precise measurements in the time domain show that the Rabi splitting follows the instantaneous THz electric pulse at extremely strong fields but persists up to a negative delay time of ~1 ps at weak fields. This field dependent dynamics of the Rabi splitting indicates that the excitonic field ionization governs a nonperturbative nonlinear dynamics of excitons by causing a reduction of the dephasing time
Intense terahetz (THz) pulses induce a photoluminescence (PL) flashes from undoped GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10000-fold from that of the steady state. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes trap states during the one-picosecond period of the THz pulse and release carriers existing in a giant reservoir containing many trap states in the AlGaAs layers.
KEYWORDS: Green fluorescent protein, Proteins, Bioluminescence, Luminescence, Receptors, Resonance energy transfer, Gallium arsenide, Energy transfer, Simulation of CCA and DLA aggregates, Dynamical systems
Luciferases are used as the reporter gene for promoter activity, whereas a green fluorescent protein (GFP) is used as marker for cellular function and localization. Recently, bioluminescence resonance energy transfer (BRET) between luciferase and YFP is used for analysis of inter-molecular reaction such as ligand-receptor in the living cells. The neuropeptides nocistatin (NST) and nociceptin/orphanin FQ (Noc/OFQ) are derived from the same precursor protein, while NST exhibits antagonism against Noc/OFQ-actions. In this study, we attempt an intra-molecular BRET system for monitoring dynamic biological process of the production of NST and Noc/OFQ in the living cells. At first, we constructed a fusion protein (Rluc-GFP) covalently linking luciferase (Renilla luciferase; Rluc) to Aequorea GFP as an intra-molecular BRET partner. Furthermore, we inserted constructs of mouse NST and Noc/OFQ (Rluc-m-GFP) or bovine NST and Noc/OFQ (Rluc-b-GFP) containing a proteolytic cleavage motif (Lys-Arg) within Rluc-GFP. When these constructions were transfected into Cos7 cells, all fusion proteins had luciferase activity and specific fluorescence. Luminescence spectra of Rluc-GFP, Rluc-m-GFP and Rluc-b-GFP fusion proteins with DeepBlueC as a substrate showed two peaks centered at 400 nm and 510 nm, whereas Rluc showed one peak centered at 400 nm. These results indicate that the proteolytic cleavage motif inserted fusion proteins between luciferase and GFP are available for intra-molecular BRET systems at first step.
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