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.
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.
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.
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