Takaaki Agui

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.

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.

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.

Improved junction formation procedure for low temperature deposited CdSCdTe solar cells

Thin film CdSCdTe solar cells with high efficiencies above 15% were produced by vacuum evaporation at substrate temperatures lower than 400°C. The junction quality was found to be improved by introducing an In (or Sn)-oxide buffer layer on the transparent conducting oxide film, and Voc greater than 0.84 V and FF greater than 75% could be obtained. Furthermore, the two-step CdS formation method was found to be effective for obtaining high-efficiency cells on a sodalime glass.

Characteristics of GaAs-based concentrator cells

GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5% at 20-suns of AM1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1.5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region. It is possible to improve it by reducing contact resistance and using uniform concentrated illumination.

High efficiency InGaP/InGaAs tandem solar cells on Ge substrates

Over 30% AM1.5G efficiency was achieved by adding a small quantity of indium into a GaAs bottom cell in the conventional tandem cell on a Ge substrate. Characteristics of InGaAs cells on Ge were investigated by varying In-composition. The maximum efficiency was obtained for the cell with 0.01 In-composition, which was lattice-matched to Ga and produced no misfit-dislocations. Relatively high efficiencies were obtained for the cells with In-compositions less than 0.1, which did not produce cracks but misfit-dislocations. InGaP/In/sub x/Ga/sub 1-x/As tandem cells with In-composition x between 0.01 and 0.07 demonstrated higher efficiency than the conventional InGaP/GaAs cells, that was attributed to an increase in photo-currents both in the top and bottom cells. Remarkably, an In/sub 0.49/Ga/sub 0.51/P/In/sub 0.01/Ga/sub 0.99/As tandem cell lattice-matched to Ga showed an improvement in Voc, which was attributed to an elimination of misfit-dislocations in thick GaAs layers. Also, those lnGaP/In/sub x/Ga/sub 1-x/As cells with low In-compositions were found to be favorable for improving efficiency of triple junction cells using Ga cells. Over 31% AM1.5G efficiency was demonstrated for the lnGaP/In/sub x/Ga/sub 1-x/As/Ge triple-junction cells with In-composition x of 0.01 and 0.06, at present.

Paper-Thin InGaP/ GaAs Solar Cells

A paper-thin, lightweight InGaP/GaAs solar cell with high efficiency and flexibility has been developed. A high-efficiency thin-film cell can be obtained for cell fabrication both before and after removing the substrate. Introducing a tunnel junction as the contact layer between the cell and metal film improves cell characteristics (Fill Factor (FF) and open-circuit voltage (Voc)). A highly doped n-type layer is necessary for good ohmic contact at the metal film interface. High radiation resistance of a thin-film cell was confirmed for a GaAs cell with a one-micron base layer. The thin-film cell was laminated for better handling. The laminated cell efficiency was about 22%. Anti-reflective coating is necessary on the laminate film to improve cell efficiency. A prototype unit panel using the laminated cells was developed for space application. An output power per weight of over 1W/g is possible for the unit panel. However, development of a bypass diode with thin-film structure is currently a problem, and reliability tests need to be performed for the unit panel

High-efficiency radiation-resistant InGaP/GaAs tandem solar cells

A world-record efficiency of 26.9% (AMO, 28/spl deg/C) has been obtained for InGaP/GaAs tandem solar cells fabricated by the MOCVD method. The radiation resistance of the InGaP/GaAs tandem solar cells has also been evaluated following 1 MeV electron irradiation. Degradation in tandem cell performance has been confirmed to be mainly attributed to large degradation in the GaAs bottom cell, which features a highly doped base layer. Similar radiation-resistance with GaAs-on-Ge cells has been observed for the InGaP/GaAs tandem cell. However, some recovery of the tandem cell performance has been found due to minority-carrier injection under light illumination or forward bias, which causes defect annealing in InGaP cells. The optimal design of the InGaP base layer thickness for current matching at end of life (EOL) (after irradiation with 10/sup 15/ electrons cm/sup -2/) has been examined.