Varian Associates, Inc.

Ronald L Bell

Ronald L Moon

George A Antypas

Peter G Borden

Syed B Hyder

Solar cells with different bandgaps are stacked to form a multijunction photovoltaic converter with a high conversion efficiency. By stacking the cells mechanically rather than by growing them all at one time, the most convenient combination of materials may be selected for the cells. The stacking is completed by means of a glass sealing layer with moats containing inter-cell contacts which are bonded to the cells.

Method of making a transparent and electrically conductive bond

A grooved solar cell for deployment at a set angle with respect to incoming radiation has reduced contact obscuration and series resistance. A regular array of walls formed by grooves in a crystalline semiconductor substrate is exposed to incoming radiation. Metallic contacts are formed on those walls of the grooves which are not to be exposed to sunlight. The solar cell is mounted in an apparatus at a set angle with respect to incoming radiation. The array of walls is exposed to incoming radiation while the walls with metal contacts are substantially shadowed. In a preferred embodiment the grooves are formed by using orientation specific etches.

Grooved solar cell for deployment at set angle

Stacked multijunction photovoltaic converters

An improved collector is provided for a thermionic energy converter. The collector comprises a p-type layer of a semiconductor material formed on an n-type layer of a semiconductor material. The p-n junction is maintained in a forward biased condition. The electron affinity of the exposed surface of the p-type layer is effectively lowered to a low level near zero by the presence of a work function lowering activator. The dissipation of energy during collection is reduced by the passage of electrons through the p-type layer in the metastable conduction band state. A significant portion of the electron current remains at the potential of the Fermi level of the n-type layer rather than dropping to the Fermi level of the p-type layer. Less energy is therefore dissipated as heat and a higher net power output is delivered from a thermionic energy converter incorporating the collector.

Collector for thermionic energy converter

A lens or mirror is employed to concentrate sunlight onto a photovoltaic cell. The cell employs a broad conductor around the periphery of its active surface, and narrow higher resistance conductor strips over the inner part of the surface. The cell and mirror are sized, positioned and shaped such that the image of the sun on the cell's surface is non-uniform, with a proportionately higher concentration of light falling near the periphery of the cell adjacent the peripheral conductors. Thus, the generated current is greater at the periphery, so that less energy is lost due to series resistance of the surface conductors, for more efficiency compared to a uniformly illuminated cell. The non-uniformity can conveniently be obtained by utilizing the natural spherical aberrations of a simple spherical mirror or lens. Also the cell can be positioned relatively close to the mirror outside the focal plane, so that the illuminated area on the cell is larger than the focused image of the sun in the focal plane. Thus, for given diameters of cell and mirror, the arrangement is more compact, and much less sensitive to inaccuracies in mirror curvature.

Concentrator arrangement for photovoltaic cell

A solar energy converter uses gallium arsenide photovoltaic cells to convert light to direct current. Optical concentrators reduce the needed area of cells. Gallium arsenide retains high conversion efficiency up to several hundred degrees, so the waste heat may be used to produce mechanical power in a Rankine cycle engine.

Solar energy converter with waste heat engine

A long wavelength photoemitter, for example a III-V semiconductor, having a work function reduction activation layer thereon, with means for overcoming the energy barrier between the semiconductor conduction band edge and the vacuum comprising means for thermally energizing the photoexcited electrons in the conduction band from a lower energy level therein to a higher 'metastable' energy level in which they may reside for a sufficient time such that the electrons can pass with high probability from the elevated energy level into the vacuum over the energy barrier. In one embodiment, promotion of electrons to this higher energy level in the conduction band results from proper selection of the semiconductor alloy with conduction band levels favoring such room temperature thermal excitation. In another embodiment, a Schottky barrier is formed between the semiconductor emitter surface and the activation layer, by means of which an internal electric field is applied to the cathode resulting in high effective electron temperature for energy level transfer analogous to the intervalley electron transfer process of the Gunn effect. In yet other embodiments, composite semiconductor bodies are fabricated in which one region may advantageously be designed for efficient absorption of long-wavelength photons, and another for efficient operation of the promotion mechanism, which together assure a high quantum efficiency. Other properties of the biased promotion layer may be used to minimize emission of electrons which have been excited by purely thermal means, thus providing a low dark current, usually considered to be incompatible with long-wavelength infrared response.

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Woodside, CA, United States of America

Ronald L Bell