Toshimitsu Mochizuki

Characterizations of radiation damages in multi-junction solar cells focused on subcell internal luminescence quantum yields via absolute electroluminescence measurements

Via absolute electroluminescence (EL) measurements, we characterized degradations of internal luminescence quantum yields in respective subcells in GaInP/GaAs/Ge triple-junction solar cells after radiation damages by proton irradiations with different energy and fluence. Compared with typical open-circuit-voltage characterizations, the internal luminescence quantum yield turned out to be a sensitive, high-dynamic-range, quantitative, and fair indicator of radiation damage, since it purely represents material-quality change due to radiation damage, independently from small differences in band-gap energy due to alloy composition fluctuations and in other cell structures.

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. We found that the conversion-efficiency limit ηsc significantly drops when the geometric mean ηint* of all subcell ηint in the stack reduces from 1 to 0.1, and that ηsc degrades linearly to logηint* for ηint* below 0.1. For ηint*<0.1 differences in ηsc due to additional intermediate reflectors became very small if all subcells are optically thick for sun light. We obtained characteristic optimized bandgap energies, which reflect both ηint* decrease and AM1.5 spectral gaps. These results provide realistic efficiency targets and design principles.

Subcycle Optical Response Caused by a Terahertz Dressed State with Phase-Locked Wave Functions

The coherent interaction of light with matter imprints the phase information of the light field on the wave function of the photon-dressed electronic state. A driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a subcycle time scale in a GaAs quantum well in the presence of a multicycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of the excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.

Solar-cell radiance standard for absolute electroluminescence measurements and open-circuit voltage mapping of silicon solar modules

In this work, we propose and demonstrate a durable and distributable Lambertian light-emitter secondary standard using the electroluminescence (EL) of a Si solar cell. This standard is useful for calibration of the absolute sensitivity of an EL-imaging infrared camera used to acquire quick on-site measurements of the absolute EL efficiencies of individual Si solar cells in modules and arrays. The developed method enables the realization of quantitative open-circuit voltage mapping.

Absolute electroluminescence imaging of multi-junction solar cells and calibration standards

We developed absolute electroluminescence (EL) calibration standards to evaluate absolute radiative-emission rates from subcells in multi-junction (MJ) solar cells. The absolute-EL-measurement system consists of an EL imaging setup and an emission-intensity-calibrated planar light-emitting diode with a circular open aperture as an emission-intensity standard. We applied this system to the measurements of the absolute EL imaging of a monolithic satellite-use InGaP/GaAs/Ge MJ solar cell. From the observed absolute EL images, we characterized external EL quantum efficiencies and internal open-circuit voltages of InGaP and GaAs subcells.

High-efficiency III–V//Si tandem solar cells enabled by the Pd nanoparticle array-mediated “smart stack” approach

Smart stack is a handy technique to produce two-terminal tandem structures from various photovoltaic materials using Pd nanoparticle arrays as bonding mediators. Because of the increasing interest in III–V/Si integration, we herein demonstrated smart stack-based triple-junction cells consisting of InGaP/GaAs and crystalline Si subcells. Despite the use of classic Al-back surface field-type Si subcells, current matching with the InGaP/GaAs subcells was realized, and the promising efficiency of 25.1% was successfully achieved. The potential and versatility of the smart stack approach as a fabrication tool for high-efficiency multi-junction cells were further enhanced through this study.

Characterization and modeling of radiation damages via internal radiative efficiency in multi-junction solar cells

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.

Calibration standards and measurement accuracy of absolute electroluminescence and internal properties in multi-junction and arrayed 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.

Multi-junction-solar-cell designs and characterizations based on detailed-balance principle and luminescence yields

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.

Conversion efficiency limits and optimized designs for tandem solar cells with realistic sub-cell material quality

To examine practical efficiency limits of tandem solar cells, we calculated conversion efficiency (η_sc) and optimized sub-cell band-gap energies (E_g) in 1-, 2-, 3-, 4-, and 5-junction cells via a detailed-balance theory, taking account of the realistic internal luminescence quantum yields (y_int) of the sub-cell materials. We found that η_sc drastically decreases as the geometric mean y_int* of all sub-cells y_int drops from 1 to 0.1, and decreases proportionally to logy_int * in the low-y_int * region. Even with moderate material quality with y_int * = 0.001, η_sc above 30% (35%) should be still available in 2-junction (3-junction) tandem cells, though we need optimized designs with much higher E_g than those known for the ideal cases with y_int * = 1.