Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1998-03-11
2000-11-14
Nguyen, Nam
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
438 71, H01L 310236, H01L 21302
Patent
active
061472972
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of German patent document 195 22 539.2, the disclosure of which is expressly incorporated by reference herein.
1. Field of the Invention
The present invention relates to a solar cell having an emitter layer disposed on a base material, and a surface texture distinguished by rows of crisscrossing pointed ribs, and to a process for the fabrication of such a solar cell.
2. Background of the Related Art
Solar cells are components which convert sunlight or other light into electric energy. A solar cell usually comprises a semiconductor material, which is provided with a p-n junction. In the semiconductor, incident light generates positive-charged and negative-charged carriers which are separated by the p-n junction. The electric power generated in this matter is tapped via metal contacts on the n- and p-sides.
From the many known embodiments of solar cells, the subject matter of the present invention is confined to solar cells with a high degree of efficiency of >20%. These electric components, also called "high-efficiency" solar cells, are preferably made of silicon. "High-efficiency" solar cells are distinguished, in particular, by the fact that losses due to the conversion of light into electricity are minimal. This is made possible by a number of measures, described in detail, for instance, by M. A. Green, "High-efficiency Silicon Solar Cells", TransTech Publications, Aedermannsdorf, Switzerland (1987).
The solar cell with the hitherto highest attainable efficiency is the so-called PERL ("Passivated emitter and rear, locally diffused") solar cell described, for example in Appl. Phys. Lett. 57(6), Jun. 8, 1990, pp. 602-604. The construction of [the, as] such known, "high performance solar cell" can be seen in FIG. 1. The PERL cell is provided with a textured surface S, which is coated with a layer of silicon dioxide S10.sub.2, due to its electric passivation and antireflex effect. Under the silicon dioxide layer is an emitter layer E, which is provided with regions of high n.sup.++ doping and of normal n.sup.+ doping. The emitter layer is formed in a three-dimensional configuration by selective etching in such a manner that the contour of its surface corresponds to the impression of many adjacently disposed square pyramids. The emitter layer is provided deeper and with higher doping only at sites where the emitter layer comes into contact with a metal contact strip M.
Due to the typical design of the characteristic surface texture of the emitter layer, on the one hand, the light penetrating through the emitter layer into the solar cell enters the cell with less losses and, on the other hand it cannot so easily leave the interior of the solar cell again. In addition to the antireflex effect of the top silicon dioxide layer, this layer also contributes to the electric passivation of the surface.
An essential aspect of this textured surface of the PERL cell is the two-step n-doping of the emitter, which in the area under the contacts of the metal bridges is high doped (and therefore low-ohmic) and is deeper than in the areas under the "inverted pyramids", where the emitter layer is doped weaker and runs flatter. (For the sake of completeness, it is noted that base contact electrodes BE are provided on the rear side of the solar cell facing the emitter layer.)
Two photomasking steps are required for the fabrication of PERL solar cells: an etching step to produce the riblike surface texture and a local diffusion step for placing the deep n.sup.++ diffusion areas at sites over which the metal contact bridges are disposed. In a third process step, the surface contour of the solar cell is coated with a full-surface n.sup.+ diffusion layer.
Despite its previously unattainable degree of efficiency, the two previously mentioned masking steps make fabrication of the PERL solar cell complicated and expensive.
Furthermore, the elongated metal contact bridges reduce the effective solar cell surface which absorbs light energy, thereby limiting
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Glunz Stefan
Wettling Wolfram
Fraunhofer Gesellschaft zur Foerderung der angewandten Forschung
Miggins Michael C.
Nguyen Nam
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