Ronald P. Gale

Lightweight tandem GaAs/CuInSe/sub 2/ solar cells

High-efficiency, ultralightweight, mechanically stacked 4-cm/sup 2/ thin-film tandem solar cells are discussed. The tandem stack consists of a single-crystal, thin-film Ga(Al)As cell fabricated by the cleavage of lateral epitaxy for transfer (CLEFT) process and adhesively bonded to the top of a CdZnS/CuInSe/sub 2/ polycrystalline thin-film cell deposited on glass. Maximum tandem efficiency in a four-terminal configuration of 21.6% AM0 have been demonstrated. This represents the highest thin-film cell efficiency reported to date. Individual subcells with efficiencies of 19.5% for CLEFT GaAs and 3.0% for CuInSe/sub 2/ have also been achieved. Cell specific power as high as 600 W/kg has been achieved with a 4-cm/sup 2/ cell weight of 188 mg without coverglass, at an efficiency of 20.8% AM0. >

High efficiency GaAs/CuInSe/sub 2/ tandem junction solar cells

High-efficiency, mechanically stacked tandem junction solar cells based on a double-heterostructure GaAs single-crystal thin-film top cell and a polycrystalline CuInSe/sub 2/ (CIS) thin-film bottom cell were developed to meet the power needs projected for future spacecraft. The best performance of these tandem cells achieved so far is 21.3% AM0, one sun, for a 1 cm/sup 2/ four-terminal device at 28 degrees C. A GaAs subcell efficiency of 18.8% and a CuInSe/sub 2/ subcell efficiency of 2.5% were measured for this device. Top cell efficiency up to 19.5% and lower cell efficiency up to 2.83% were measured for other tandem cells. A 3.0% CIS cell was achieved using a GaAs filter with a glycerol optical matching medium. This demonstrated efficiency provides for specific powers up to 620 W/kg when 50 mu m thick substrate and cover glasses are incorporated. Favorable results were obtained from thermal cycling experiments conducted to evaluate survivability of thin GaAs films in adhesive/glass sandwich structures.<<ETX>>

High voltage, monolithically interconnected GaAs thin film solar submodules

The successful monolithic interconnection of four thin-film single-crystal GaAs cells, demonstrating one of the key advantages of thin-film cells made by the cleavage of lateral epitaxy for transfer (CLEFT) process, is reported. Monolithically series-interconnected 16 cm/sup 2/ thin-film four-cell strings exhibiting a V/sub oc/ of 4.04 V and a total-area submodule efficiency of 21.0% under AM 1.5 global illumination at 26 degrees C was successfully fabricated. An eight-cell string which had a V/sub oc/ of 7.96 V, an AM 1.5 efficiency of 21.2%, and a total submodule area of 2 cm/sup 2/ was produced. Details of the cell fabrication process and the cell performance are presented.<<ETX>>

High-efficiency GaAs/CuInSe/sub 2/ and AlGaAs/CuInSe/sub 2/ thin-film tandem solar cells

Tandem cell throughput has been increased, and quantities of cells that produce tens of watts of power in total were fabricated. An improved efficiency of 23.1% AM0/one sun at 28 degrees C has been obtained for 4 cm/sup 2/ tandem cells. The mechanically stacked tandem cells consist of an n+:AlGaAs/n:GaAs/p:GaAs/p+:A thin-film top cell and an n+:CdZnS/p:CuInSe/sub 2/ thin-film bottom cell. In addition to being highly efficient, the cells are light in weight and radiation resistant. Large numbers of tandem cells have been completed, and individual cell performances exceeded 20% for the GaAs top cell and 3% for the CuInSe/sub 2/ (CIS) bottom cell. To attain increased radiation resistance and even higher end-of-life efficiencies, the use of an AlGaAs high-bandgap cell for the upper cell was investigated. Large areas of thin-film AlGaAs were produced using the CLEFT process and filters to simulate AlGaAs cell structures to be used over the CIS cells were fabricated. CIS cells have been tested under these filters. Results of these measurements indicate that significantly higher efficiencies can be expected from the lower cell in this configuration, and very high end-of-life efficiencies are possible with this approach.<<ETX>>

High specific power (AlGaAs)GaAs/CuInSe/sub 2/ tandem junction solar cells for space applications

The authors report the fabrication of a high-efficiency, lightweight GaAs/CuInSe/sub 2/ tandem cell on a 2-mil-thick substrate, update performance improvements in thin-film GaAs/CuInSe/sub 2/ tandem cells and discuss their application to space power systems. The efficiency of 4 cm/sup 2/ GaAs/CuInSe/sub 2/ tandem cells has improved to 21.6% AM0. This is the highest efficiency ever reported for a thin-film photovoltaic cell. Among the tandem cells fabricated, the efficiencies of the best individual GaAs and CuInSe/sub 2/ subcells were 19.5% and 3.0%, respectively. Lightweight 4 cm/sup 2/ tandem cells have been successfully fabricated with efficiencies as high as 20.8%. These cells weighed about 180 mg without optimized substrate trimming. The effects of radiation and operating temperature on GaAs/CuInSe/sub 2/ tandem cells is also discussed, and an interconnect scheme for forming a voltage-matched circuit of three GaAs cells in parallel and three CuInSe/sub 2/ cells in series is proposed.<<ETX>>

High-efficiency thin-film GaAs bifacial solar cells

Thin-film GaAs bifacial solar cells, which consist of only 5 mu m of GaAs fabricated by the cleavage of lateral epitaxy for transfer (CLEFT) process, are designed to produce power from light entering both the front and back sides of the cells. In terrestrial applications, these cells can efficiently convert scattered light from the ground below and adjacent to the array such as from white sand or white paint. For applications in space, the Earths albedo would provide back illumination to boost the cell output. A 4 cm/sup 2/ cell was found to be 22.6% and 12.9% efficient for 1-sun AM 1.5 global illumination from the front and back, respectively. With an improved response from the back of the cell and an optimized geometry for the array, effective cell efficiencies of over 30% are achievable for thin-film GaAs bifacial cells.<<ETX>>

Applications of circuit transfer technology to displays and optoelectronic devices

This paper reviews the use of circuit transfer processes for optoelectronic applications. In this process, a circuit comprising either Si or GaAs devices is removed from its original substrate and is transferred to a second substrate. For active matrix displays, CMOS circuits are transferred in this way to glass substrates, and active matrix liquid crystal displays with pixel format of 1280 by 1024 (with 1000 lines per inch) have been successfully formed. Photovoltaic circuits have also been transferred to glass and to other photovoltaic devices to further the formation of multi-bandgap tandem structures. LED arrays have been formed successfully by this technique as well. This work shows the potential for combining CMOS and III-V circuits to form integrated optical input and output devices, as well as optical power delivery to silicon circuits. We review progress in these areas and suggest new applications of the technology.

Voltage-matched, two-terminal, GaAs (AlGaAs)/CuInSe/sub 2/ tandem solar cells

Mechanically stacked tandem solar cells based on GaAs thin-film single-crystal top cells and CuInSe/sub 2/ (CIS) bottom cells were developed to meet the weight and radiation-resistance requirements of advanced spacecraft power systems. Efficiencies over 23% have been demonstrated with these cells in a four-terminal configuration. In order to use the device as a direct replacement for conventional single-junction cells with existing array designs. a two-terminal, voltage-matched tandem cell with 8 cm/sup 2/ area has been designed and fabricated. Tandem cells with efficiencies over 20% have been demonstrated with this design. AlGaAs high-bandgap cells have been demonstrated with efficiency over 17% for future AlGaAs/ClS tandem cells capable of higher end-of-life efficiency. The performance of the two-terminal tandem cells and the AlGaAs high-bandgap cell are described.<<ETX>>

P-93: Process and Performance Improvements for a Simple Low-Cost Multi-Domain Vertical-Alignment LCD

Process and performance improvements have been made in Kopins simple, low-cost MVA liquid crystal display to bring the fabrication process closer to manufacturing. Key results include striation-free displays with spun-on polyimide alignment layers, elimination of boundary stick via pixel design, overall viewing angle improvement with an MVA-matched wide viewing polarizer, and transmission improvements with high delta n LC, cell gap, driving mode, LED and BEF backlight combination.

23.5% thin-film space concentrator cells

Thin-film AlGaAs-GaAs double-heterostructure concentrator cells were fabricated which exhibit total-area conversion efficiencies as high as 23.5% AM0 at 100 suns, 25 degrees C. This is one of the best space concentrators measured to date at NASA and is designed for a thin-film cell without a prismatic coverglass. This solar cell structure consists of a GaAs/AlGaAs film less than 5 mu m thick mounted to a glass cover/superstrate, with coplanar back-side contacts. The coverglass is not prismatic. The CLEFT process, a method for mechanically separating epitaxial. layers from their substrate, is used to process these cells into thin films. The advantages of single-crystal GaAs are thereby retained, while reducing weight and cutting cost by allowing for substrate reuse. Thin-film cells also have better thermal management capabilities and can be stacked for use in tandem structures. Cell fabrication and performance are described, and directions for further improvements are identified.<<ETX>>