Solar cell with reduced shading and method of producing the same

Details Solar cell with reduced shading and method of producing the same Solar cell with reduced shading and method of producing the same

1999-07-19

2000-11-07

Chapman, Mark

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

H01L 3100

Patent

active

061439762

DESCRIPTION:

BRIEF SUMMARY
Reduced shading can, for example, be achieved in a solar cell in which both the n and the p contacts are located on the rear side. In this way the front side is not shaded by any contact and is therefore available without restriction for the irradiation of light.
A solar cell without front-side metallization is known, for example, from R. A. Sinton, P. J. Verlinden, R. A. Crane, R. M. Swanson, C. Tilford, J. Perkins and K. Garrison, "Large-Area 21% Efficient Si Solar Cells", Proc. of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, 1993, pages 157 to 161. To produce same, varyingly doped areas are generated side by side in a plurality of masking steps and are metallized or contacted by applying a multilayer metal structure on top thereof. The metal structures are applied by thin-film techniques.
One drawback is that the method needs a plurality of masking steps and is complex as a result. All the charge carriers also have to reach the rear side of the solar cell by way of diffusion, there being a greater probability of charge carrier recombination which in turn reduces the solar cell's collection efficiency.
Another idea for a solar cell without front-side metallization is known from the article entitled "Emitter Wrap-Through Solar Cell" by James M. Gee et al. in a paper for the 23rd Photovoltaic Specialists Conference 1993, Louisville, pages 265 to 270. The solar cell described there comprises an emitter layer placed close to the front side with a pn junction adjacent thereto. Contact holes drilled and metallized by means of a laser connect the emitter layer to metallized contacts positioned on the rear side. The rear-side contacts are also disposed on the rear side interdigital to the "front-side contacts". This solar cell suffers from the disadvantage of a high number of contact holes that have to be drilled with a laser, this large number requiring about 10,000 contact holes per solar cell for a typical solar cell 100 cm.sup.2 in size and a typical gap of 1 mm between the contact holes. This reduces the throughput in automated production. Furthermore, the contact holes and the associated contacts disposed on the rear side have to be adjusted in relation to one another. Undesired structural transformations in the silicon may also be produced in the contact holes drilled with a laser, thereby making it possible to create additional recombination centers for pairs of charge carriers which further reduce collection efficiency. The reduced mechanical strength may lead to rupture in these solar cells.
The present invention's object is to design a solar cell without front-side contacts which create shade; such a solar cell is simple and inexpensive to produce and satisfies other requirements for a high-output solar cell.
In accordance with the invention, this object is solved by a solar cell according to claim 1. Preferred embodiments of the invention and a method of producing same can be gathered from further claims.
The solar cell according to the invention is built up from a (110)-orientation crystalline silicon substrate. This material enjoys the advantage that it exhibits (111) planes aligned vertical to the (110) surface. Anisotropic etching oriented toward the crystal structure makes it possible to generate depressions, holes or openings with a high aspect ratio and two vertical side walls in the (110) substrate. The solar cell according to the invention has a plurality of elongated slots aligned parallel to (111) planes and extending through the entire thickness of the silicon substrate or breaking through this substrate. The inner surfaces of the slots have a high doping corresponding to the conductivity type of the flat emitter layer generated at least on the front side. A grid-like first contact pattern is located on the rear side of the solar cell for electrically connecting the bulk material. Interdigital thereto, a second grid-like contact pattern which overlaps with the slots at least in part and thus ensures the emitter layer's electrical connection is disposed on the rear side.


REFERENCES:
patent: 5067985 (1991-11-01), Carver et al.
patent: 5468652 (1995-11-01), Gee
Martinelli, G., et al.: "Growth of Stable Dislocation-Free 3-Grain Silicon Ingots for Thinner Slicing", Applied Physics Letters, vol. 62, No. 25, Jun. 21, 1993, pp. 3262-3263.
Gee, J.M. et al.: "Emitter Wrap-Through Solar Cell", Proceedings of the Photovoltaic Specialists, Louisville, May 10-14, 1993, No. Conf. 23, May 10, 1993, IEEE, pp. 265-270.

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