Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-05-09
2001-11-13
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S249000, C136S256000, C136S262000, C257S437000, C257S449000, C257S461000, C438S074000, C438S092000, C438S093000
Reexamination Certificate
active
06316716
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a solar cell, having an integral protective diode which has a polarity opposite the solar cell and includes at least one diode semiconductor layer, and the solar cell has at least one back contact, a substrate as a carrier for photoactive layers that include at least one front layer and one layer toward the substrate of different polarities, and a front contact, and the protective diode is integrated in the solar cell and disposed on the front side of the solar cell. The invention also relates to a method for producing a solar cell, having an integral protective diode which has a polarity opposite the solar cell and includes at least one diode semiconductor layer, and the solar cell has at least one back contact, a substrate as a carrier for photoactive layers that include at least one front layer and one layer toward the substrate of different polarities, and a front contact, and layers are formed in particular by epitaxial. growth.
In large-area diodes with areas of a plurality of square centimeters of the kind that solar cells of semiconductor material with at least one p-n junction form, micro-short circuits, that is, local, small-sized electrical (ohmic) connections via the p-n junction of the semiconductor material, are often unavoidable. They arise for instance from surface damage during substrate production or from dopant accumulations, for instance at crystal defects such as dislocations, in particular in the formation of epitaxially formed p-n junctions, for instance in solar cells made from the elements of groups III-IV of the periodic system.
The thus-created micro-short circuits admittedly impede the function of the diode as a solar cell to only a slight extent if at all in the flow direction. However, when the cell is operated in the depletion direction, the defects can cause the destruction of the cell. For instance, if a plurality of solar cells or solar generators are connected in series in a so-called string to form a solar array, then when the p-n junction of a generator is blocking—which can happen if it is in shadow—the solar current is forced through the ohmic micro-short circuits by the high string voltage of the remaining solar cells or generators that are still in the light. This can lead to severe local heating, redoping that produces low impedance, or in other words local severe denaturing of the semiconductor, and finally the destruction of the cell itself.
To avoid such local severe heating or so-called hot spots, it is known in series-connected solar cells to dispose protective diodes parallel to the solar cells, with a flow direction extending counter to that of the solar cells.
In solar cell strings, that is, a group of series-connected solar cells or solar cell modules, the individual solar cells can be protected with integral protective diodes for the sake of achieving greater reliability and avoiding the failure of entire strings (see Lippold, Trogisch, Friedrich: Solartechnik [Solar Technology], Berlin, Ernst, Verlag für Architektur u. Techn. Wiss. [Ernst, Publishing House for Architecture and Engineering] 1984, pages 265, 266). However, the additional expense for internesting to form strings of suitable integral protective diodes that are provided on the backsides of the individual solar cells is considerable, since in addition electrically conductive connectors must be applied to the front side of the solar cells.
A solar cell of the type defined at the outset can be found in German Patent Disclosure DE 38 26 721 A1. For forming the protective diode, it is necessary first to apply an additional layer of semiconducting material to the photoactive layer; this additional layer is then regionally etched away.
From the United States professional journal article J. M., Olson et al, “A 27.3% efficient Ga
0.5
In
0.5
P/GaAs tandem solar cell”, Appl. Phys. Lett. 56 (7), 1990, pages 623-625, a cascade solar cell is known in which a tunnel diode extends between pairs of photoactive layers that form photoactive cells.
A p-i-n solar cell with a Schottky protective diode can be found in European Patent Disclosure EP 0 327 023 A1.
SUMMARY OF THE INVENTION
The object of the present invention is to refine a solar cell with an integral protective diode, and a method for producing such a cell, in such a way that the effort and expense of production remains low while high reliability is simultaneously attained, and in which the internesting to form strings requires no additional expense. At the same time, the formation of the protective diode should occur in the production of the solar cell itself.
According to the invention, this object is attained by a solar cell in which a tunnel diode extends between the current-generating photoelectrically active layers and the region of the substrate toward the front, the tunnel diode being recessed in the immediate region of the protective diode, and regionally, the region of the substrate toward the front, or a layer toward the front applied to it or formed by it, with a photoactive layer of corresponding polarity toward the front, is the at least one diode semiconductor layer of the protective diode.
According to the invention, the diode semiconductor layer, is regionally formed by a layer of the solar cell itself, and the photoactive layers extend in spaced-apart fashion from the region of the solar cell that forms the diode semiconductor layer. It is also provided that the protective diode is in electrically conductive contact, by its metallizing, with the front contact.
In a refinement of the invention, the protective diode is separated from the photoactive layers of solar cell by an insulating layer such as an antireflection layer, along which the metallizing of the protective diode can extend as far as the front contact. Alternatively, the possibility exists that the protective diode is connected to an electrically conductive connector, which is connected to the electrically conductive connector that originates at the front contact of the solar cell. The possibility also exists that on the one hand the metallizing of the protective diode is connected to the front contact of the solar cell, and on the other, that a connector originates at the metallizing itself and is in turn electrically conductively connected to a further solar cell.
The protective diode itself can be formed as a Schottky diode, MIS contact diode, p-n contact diode or metal-alloy diode.
Schottky diodes are semiconductor metal contacts that behave like a diode. If a metal is put in contact with an undenatured, or in other words low-doped, for instance n-conductive semiconductor, then electrical conduction can take place in only one direction. The metal of a boundary layer to the semiconductor acts in this case like a p-conductive semiconductor, which together with the n-semiconductor forms a a Schottky diode.
MIS contact diodes are similar to the Schottky contacts; the semiconductor and the metal are separated by a thin insulating layer, through which electrons can tunnel. The advantage of such MIS contact diodes is better replicability in production and improved temperature stability.
A diode with a p-n junction is created by redoping the polarization of the semiconductor layer in the surface region, for instance by implantation and/or inward diffusion of a suitable dopant to form a local junction.
Metal-alloy diode contacts are diodes that are created by inward diffusion of a dopant opposite the semiconductor doping from the metal contact during the sintering or the alloying, thus creating a p-n junction in the semiconductor.
According to the invention, it is provided that layers forming the tunnel diode extend between the photoactive layers and the region of the substrate toward the front or the substrate layer having the polarity of the photoactive layer toward the front. Such a tunnel diode is additionally provided whenever the region of the substrate toward the front is embodied as a photoactive region, so that the photoactive region formed on the substrate side is se
Angewandte Solarenergie - ASE GmbH
Dennison, Scheiner Schultz & Wakeman
Diamond Alan
LandOfFree
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