Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1990-06-15
1992-01-14
Weisstuch, Aaron
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
156647, 156662, 357 30, 437 2, 136259, H01L 3106, H01L 3118
Patent
active
050807250
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to improvements in the fabrication of solar cells whereby the structure of the top surface of the cell facilitates the trapping within the cell of light that is weakly absorbed by the cell material.
Ideally, solar cells should be designed to maximise the area of the cell intercepting the sun's rays while minimizing the volume of cell material; that is, the cells should be as thin as practical Not only does this minimize the cost of cell material, it also can give a performance advantage by minimising the loss of light generated carriers by recombination in the cell volume.
However, if the cell is made too thin, an increasingly large portion of the light will pass through the cell without being absorbed and be wasted. Due to these conflicting constraints, there will be an optimum thickness for highest energy conversion efficiency for each cell material. For crystalline silicon solar cells, this optimum thickness is about 100 microns, reducing to thinner values if the cell is used with focussed (concentrated) sunlight. This assumes that the cell incorporates a technique for trapping weakly absorbed light into the cell for as long as possible.
A common method for controlling reflection in crystalline solar cells is to form geometrical features on the cell surface by anisotropic etching to expose intersecting (111) crystallographic planes. Pyramids or grooves formed in this way reduce reflection by giving light two chances of being coupled into the cell. If the fraction of light reflected at the first point of incidence is R, the overall fraction of light reflected with this "double bounce" approach will be approximately R.sup.2, a much smaller number.
The light trapping properties of such pyramids and grooves have been investigated both theoretically and experimentally. Theoretically, it has been with these features as normally implemented, the best light trapping performance is obtained with geometrical features on both top and rear cell surfaces leg, see FIG. 5) . Experimentally, this can be inconvenient as in the case of rear-contacted cells, or it can give poorer results than expected due to decreased effective rear reflectance because of the multiple "bounces" of light on the rear surface.
A simply implemented improvement is described based on "tilting" the previous geometrical features. This gives excellent light trapping properties while allowing planar rear surfaces to be retained. Perhaps of more general importance is the fact that it can also be used to significantly reduce reflection loss from the cell/encapsulant interface, without the need for anti-reflection coatings.
According to a first aspect, the present invention consists in a solar cell comprising a substrate having a top, or light receiving, surface in which is formed a geometric structure of ridges or pyramids and a bottom surface which is substantially flat, the geometric structure being tilted relative to the bottom surface of the substrate.
According to a second aspect the present invention consists in a method of manufacturing a solar cell comprising the steps of: its (100) plane; and substrate thereby producing a geometric structure of ridges or pyramids in said top surface, the geometric structure being tilted relative to a bottom surface of the substrate by said angle .alpha. as a result of the selective etch.
In preferred embodiments of the present invention tilt angles in the range of 2.degree.-33.degree. will typically be used, with angles of about 7.degree.-8.degree. being preferred for unencapsulated cells and angles of 15.degree.-24.degree. being preferred for encapsulated cells.
In embodiments in which a ridged top surface is employed on a crystalline substrate the structure will preferably be tilted towards a (111) plane with a component of tilt towards the (110) plane, while in embodiments employing a pyramid structure on a crystalline substrate the structure will preferably be tilted symmetrically towards the (110) plane. In either case, a second tilt towards another plane might also optiona
REFERENCES:
patent: 4137123 (1979-01-01), Bailey et al.
patent: 4360701 (1982-11-01), Evans, Jr.
patent: 4379944 (1983-04-01), Borden et al.
N. M. Bordina et al., Geliotekhnika, vol. 18, No. 3, pp. 6-11 (1982).
Green Martin A.
Wenham Stuart R.
Unisearch Limited
Weisstuch Aaron
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