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.
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.
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
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