Tatsuya Takamoto

Over 30% efficient InGaP/GaAs tandem solar cells

A two-terminal monolithic InGaP/GaAs tandem solar cell with a new efficiency record of 30.28% is realized with a practical large area of 4 cm2 under one-sun air-mass 1.5 global illumination. We report improvements of the tandem cell performance by introducing a double-hetero (hereafter DH) structure InGaP tunnel junction, in which the InGaP layers are surrounded by high band gap AlInP barriers. The DH structure by AlInP barriers increase the peak current of InGaP tunnel junction. The AlInP barrier directly below the InGaP top cell, which takes the part of a back surface field (hereafter BSF) layer, is found to be considerably effective in reflecting minority carriers in the top cell. The AlInP BSF layer does not only form a high potential barrier but also prevents the diffusion of zinc from a high doped tunnel junction toward the top cell during epitaxial growth. Furthermore, an InGaP tunnel junction reduces the absorption loss, which exists in a GaAs tunnel junction, and increases the photogenerated curr...

Mechanism of Zn and Si diffusion from a highly doped tunnel junction for InGaP/GaAs tandem solar cells

Diffusion of impurities (Zn and Si) from a tunnel junction during epitaxial growth and the effects of impurity diffusion on InGaP/GaAs tandem cell properties have been investigated. Zn diffusion from the tunnel junction has been found to deteriorate the effect of the back-surface field layer on minority carrier reflectance in the InGaP top cell and degrade the quantum efficiency of the top cell. Furthermore, Zn diffusion has been found to be enhanced around the threading dislocations from a GaAs substrate and creates shunt paths only in the top cell region. Si diffusion, which degrades the quantum efficiency of the GaAs bottom cell, has also been observed when a different substrate with high etch pit density was used. Such anomalous diffusion of Zn has been found to be suppressed by using a double-hetero structure InGaP tunnel junction sandwiched by AlInP layers. It has been found that the Zn diffusion occurs as a layer highly doped with Si being formed nearby and Zn diffuses in the opposite direction fro...

Development of InGaP/GaAs/InGaAs inverted triple junction concentrator solar cells

We have been developing InGaP/GaAs/InGaAs inverted triple junction solar cells for a concentrator application with a target efficiency of 45%. We reduced the series resistance in the cells. As a result we improved the maximum concentration ratio up to around 300-suns and obtained an efficiency of 43.5% as an official value measured by Fraunhofer Institute for Solar Energy Systems. Recently we achieved an efficiency of around 44% in house measurement by improving short current density (Jsc) in a cell by reduction of shadow loss. We performed reliability tests on receivers with inverted triple junction solar cells. No obvious degradation was observed in open circuit voltage and fill factor measured under 150-suns after 1,500h exposure to 85 degree Celsius and 85% humidity atmosphere. In addition improvement of 1-sun efficiency up to 37.7% was confirmed in a cell with 1.047cm2 aperture area.

Evaluation of InGaP/InGaAs/Ge triple-junction solar cell under concentrated light by Simulation Program with integrated Circuit Emphasis

The characteristics of a multi-junction solar cell under concentrated light were evaluated by Simulation Program with Integrated Circuit Emphasis (SPICE). We developed the multi-unit model and analyzed the affects of the chromatic aberration and intensity distribution for the multi-junction cells. In the multi-unit model, the same numbers of units as grid numbers are installed for every electrode, and the units were connected to each other via lateral resistances. In order to obtain the generation current from each diode, we measured the intensity of concentrated light through the pinhole using single-junction solar cells consisting of InGaP, GaAs and Ge as detectors. By using the multi-unit model, we could successfully calculate the electrical cell performances taking the chromatic aberration and intensity distribution into account, and the calculated value agreed well with the experimental value. The multi-unit model will be very useful for cell designs and performance analysis of the concentrator cells.

High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells

The radiation response of 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs solar cells was measured and analyzed in comparison with those of InP and GaAs. The degradation of the minority-carrier diffusion length was estimated from the spectral response data. The damage coefficient KL for the 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs was also determined. The radiation resistance increases with an increase in the fraction of In–P bonds in InGaP, InGaAsP and InGaAs. Differences in the radiation resistance of InGaP, InGaAs and InGaAs materials are discussed. Minority-carrier injection under forward bias is found to cause partial recovery of the degradation on irradiated InGaP and InGaAsP cells.

Superior radiation-resistant properties of InGaP/GaAs tandem solar cells

The observation of minority-carrier injection-enhanced annealing of radiation damage to InGa0.5P0.5/GaAs tandem solar cells is reported. Radiation resistance of InGaP/GaAs tandem solar cells as is similar with GaAs-on-Ge cells have been confirmed with 1 MeV electron irradiations. Moreover, minority-carrier injection under light illumination and forward bias conditions is shown to enhance defect annealing in InGaP and to result in the recovery of InGaP/GaAs tandem solar cell properties. These results suggest that the InGaP/GaAs(/Ge) multijunction solar cells and InGaP-based devices have great potential for space applications.The observation of minority-carrier injection-enhanced annealing of radiation damage to InGa0.5P0.5/GaAs tandem solar cells is reported. Radiation resistance of InGaP/GaAs tandem solar cells as is similar with GaAs-on-Ge cells have been confirmed with 1 MeV electron irradiations. Moreover, minority-carrier injection under light illumination and forward bias conditions is shown to enhance defect annealing in InGaP and to result in the recovery of InGaP/GaAs tandem solar cell properties. These results suggest that the InGaP/GaAs(/Ge) multijunction solar cells and InGaP-based devices have great potential for space applications.

Two-Terminal Monolithic In0.5Ga0.5P/GaAs Tandem Solar Cells with a High Conversion Efficiency of Over 30%

A world-record efficiency of 30.28% has been attained for two-terminal monolithic In0.5Ga0.5P/GaAs tandem solar cells under one-sun air-mass 1.5 global illumination. The cell area has a practical size of 4 cm2. At first, high efficiency In0.5Ga0.5P single junction cells had been developed by improving the minority carrier lifetime. Second, the GaAs single junction cells had been investigated to obtain higher open-circuit voltage. Third, the tandem cell performance had been improved by using an InGaP tunnel junction with AlInP barriers which constitute a double-hetero structure and increase the peak current of the tunnel junction. In addition, the AlInP barrier located beneath the InGaP top cell have been found to be effective for reflecting minority carriers in the top cell and for suppressing the diffusion of zinc from the highly doped tunnel junction toward the top cell during epitaxial growth.

Future development of InGaP/(In)GaAs based multijunction solar cells

Although technologies for the InGaP/InGaAs/Ge cell have been matured, there is still room for improvement of the InGaP/(In)GaAs/Ge cell in practical level. Band gap of the top cell should be increased a little to get higher V/sub OC/. Thinning the Ge substrate is thought to be effective to increase a power per weight even for rigid panel. For concentrator application, grid pitch, cell size and current matching design should be optimized with taking account of the spectrum of concentrated light. The InGaP/(In)GaAs based solar cells shall be cornerstone in high efficiency multijunction solar cells in future. High efficiency cell consisted of 1 eV lattice-match material such as InGaAsN is strongly desired for high efficiency 4-junction or 6-junction cell. Wafer bonding and layer transfer techniques might be sophisticated to make solar cells. Paper-like InGaP/GaAs solar cells with efficiency of 29.4% on flexible metal film developed by SHARP Corp. are newly reported. Material cost of the cell is basically very low, because it has only very thin layers of III-V compounds and cheap metal film. Thin film technology shall be a hint for future cells.

Thermal annealing study of 1 MeV electron-irradiation-induced defects in n+p InGaP diodes and solar cells

The study presents detailed isothermal and isochronal annealing recovery of photovoltaic parameters in n+/p InGaP solar cells after 1 MeV electron irradiation. Correlation of the solar cells characteristics with changes in the deep level transient spectroscopy data observed in irradiated and annealed n+/p InGaP diodes and solar cells shows that the H2 (Ev+0.50 eV) and H3 (Ev+0.76 eV) defects have a dominant role in governing the minority-carrier lifetime as well as carrier removal. However, capacitance–voltage measurements indicate that other defects must also play a role in the carrier removal process. In addition, the concentration of the H2 defect is found to decay significantly as a result of room temperature storage for 40 days, suggesting that InGaP-based solar cells will display superior radiation tolerance in space. Finally, the deep donor-like-defect H2 is tentatively identified as a phosphorus Frenkel pair.

High-efficiency InGaP/In0.01Ga0.99As tandem solar cells lattice-matched to Ge substrates

Conversion efficiency (AM1.5G) of more than 30% was achieved by adding a small quantity of Indium into a GaAs bottom cell in the conventional tandem solar cell on Ge substrate. It was found that the lattice-mismatch between GaAs and Ge caused misfit-dislocations in thick GaAs layers and reduced an open-circuit voltage (Voc) of the cell. An In0.49Ga0.51P/In0.01Ga0.99As tandem cell lattice-matched to Ge showed a great improvement in efficiency, which was attributed to an increase in the Voc of the bottom cell and increases in the photocurrents both in the top and bottom cells due to reductions in band-gap energy.