Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1996-06-14
1997-12-30
Weisstuch, Aaron
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
117 1, 117 13, 117 35, 117902, 257 64, 437 4, H01L 3104, H01L 310368, H01L 3118
Patent
active
057025380
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
For the photovoltaic generation of energy in the power range, highly efficient, inexpensive, large-area solar cells with long-term stability are necessary which are composed of environmentally compatible materials which are adequately available. These requirements are not fulfilled simultaneously at present by any solar-cell type. Monocrystalline silicon solar cells (c--Si) which already fulfill all the other requirements except inexpensiveness come closest to the requirements at present.
The achievement of high efficiencies (>20%) requires, in addition to a good surface passivation (surface recombination rate Sr<100 cm/s for the rear side and Sf<1000 cm/s for the front side or emitter side), the use of silicon crystals whose diffusion length L of the minority charge carriers is about three times greater than the thickness d of the initial wafer used. However, for reasons of mechanical robustness, the <100>-oriented c--Si wafers used at present still require a thickness of over 300 .mu.m. The diffusion lengths of approximately 900 .mu.m necessary for 20% efficiency can therefore be achieved only by the use of high-purity, low-oxygen and low-carbon crystals which are consequently very expensive.
The optimum physical thickness of c--Si solar cells is, however, not the 300 .mu.m used at present, but between 60 .mu.m and 90 .mu.m. This is due to the fact that, on the one hand, the open-circuit voltage Voc of solar cells rises with decreasing thickness d, while, on the other hand, only a silicon layer thickness of approximately 100 .mu.m is sufficient to absorb the usable sunlight (AM1.5) completely and convert it into current. In addition, a cell thickness d of about 60 .mu.m-90 .mu.m necessitates a substantially lower material requirement. As a result of the lower requirements imposed on the diffusion length (180 .mu.m-270 .mu.m), the requirements imposed on the material quality would also be lower, so that inexpensive crucible-pulled material (Cz--Si) could also be used as starting material for highly efficient silicon solar cells.
SUMMARY OF THE INVENTION
The Si monocrystals at present chiefly used as starting material are <100>-oriented and can be processed to form 60 .mu.m-90 .mu.m thick solar cells only with very high sawing and yield losses. An economical production is therefore not possible on this basis.
One solution route for arriving at sufficiently thin, but fracture-resistant absorbers has been taken with polycrystalline silicon thin-layer solar cells on foreign substrates. The inexpensiveness, large-area producibility and productive capacity of this process have not yet, however, been demonstrated.
The object of the present invention is to provide a solar cell made of crystalline silicon, which has, in a cost- and material-saving way, a high efficiency of 20 percent and over.
In general terms, the present invention is a solar cell constructed on a mechanically robust 60 to 90 .mu.m thick silicon semiconductor wafer as a substrate. It has three mutually inclined monocrystalline regions which form three circular sectors whose interfaces and boundary lines extend radially with respect to one another and form angles of less than 180.degree. with one another. Two of the interfaces are first-order twin grain boundaries between two <111> crystal planes in each case. A light p-doping is in the wafer with a shallow, n+-doped emitter 0.2-2 .mu.m deep on a front side on the wafer and a first passivation layer on the front side of the wafer.
A second passivation layer or a back surface field is on the rear side of the wafer. Current-collecting contacts are on the front and rear sides.
Advantageous developments of the present invention are as follows.
The solar cell is produced from crucible-drawn silicon (Cz--Si), with an efficiency of more than 20 percent.
The interfaces are approximately perpendicular to the plane of the wafer and form the angles W6, W7, and W8 with respect to one another, where:
The surfaces of the three monocrystalline regions ar
REFERENCES:
patent: 5579388 (1996-11-01), Endroes et al.
G. Martinelli, Solid State Phenomena, vol. 32-33 (Aug. 1993), pp. 21-26.
Applied Physics Letters, vol. 62, No. 25, 21 Jun. 1993, G. Martinelli et al, "Growth Of Stable Dislocation-Free 3-Grain Silicon Ingots For Thinner Slicing,"pp. 3262-3263.
23rd IEEE Photovoltaic Specialists Conference, 10 May 1993, D.S. Ruby et al, "Simplified Process For 23% Efficient Silicon Concentrator Solar Cells, "pp. 172-177.
10th E.C. Photovoltaic Solar Energy Conference, 8 Apr. 1991, M.A. Green et al, "Recent Advances In Silicon Solar Cell Performance,"M.A. Green et al, pp. 250-253.
Journal of the Electrochemical Society, vol. 110, No. 1, Jan. 1993, H.J. Queisser, "Properties Of Twin Boundaries in Silicon", pp. 52-56.
Solar Cells, vol. 31, No. 3, Jun. 1991, D. Palmeri et al, "A Reverse Silicon Solar Cell", pp. 217-222.
12th European Photovoltaic Solar Energy Conference, 11 Apr. 1994, A. Benati et al, "Evaluation of Very Thin Crystalline Silicon Solar Cells For Large Scale Application", pp. 1804-1806.
Endros Arthur
Martinelli Giuliano
Siemens Solar GmbH
Weisstuch Aaron
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