R.W. McClelland

Eyeglass-based systems for wearable computing

This paper reports on progress in the area of hardware for wearable computing. Our specific interest is in reducing conventional head-mounted display systems to eyeglass scale. We report here the successful demonstration of an ergonomic eyeglasses-based display. We present some of the general considerations in the design of such eyeglasses and briefly review our approach and initial findings.

Monolithic integration of a light‐emitting diode array and a silicon circuit using transfer processes

This letter reports the attainment of a monolithically integrated light‐emitting diode array on a silicon integrated circuit. The emitters are first formed epitaxially on a lattice‐matched substrate and are subsequently transferred to the silicon. Interconnections are made using thin‐film techniques between the 128 separately addressable light‐emitting diodes and the driver circuit. This work demonstrates attainment of a high level of optoelectronic/logic integration.

Monolithic series-connected gallium arsenide converter development

The development of monolithic GaAs photovoltaic devices intended to convert light generated by a laser or other bright source to electricity is reported. The converters described can provide higher operating voltage than is possible using a single-junction converter, owing to the use of a monolithic circuit that forms a planar series-connected string of single-junction sub-cells. This planar monolithic circuit is arranged to deliver the desired voltage and current during operation at the maximum power point. A description is presented of two, six, and twelve-junction converters intended for illumination by a laser diode with a wavelength of 0.8 mu m. Design and characterization data are presented for optical power in the range of 100 mW to 1 W. The best conversion efficiency exceeds 50%.<<ETX>>

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-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>>

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>>

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>>

Portable Human/Computer Interface Mounted in Eyewear

This paper presents results on the development of an eyeglass based human/computer interface. The interface comprises a display mounted within the eyeglasses, and a lens for relaying information inconspicuously to the wearers eye. The paper will discuss eyeglass interface systems that utilize miniature displays and magnifying optics to provide a field of view of up to 10 degrees, with a resolution of approximately .03 degrees per pixel. Details of the design and construction of such systems, including methods of addressing the need for prescriptive correction will be presented. The paper concludes with comments on adding other new features to the interface system.