Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1996-08-28
1998-08-25
Nguyen, Nam
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
257465, 438 57, 438 95, H01L 3105
Patent
active
057979987
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of solar cell technology and in particular to improvements in thin film devices.
The Buried Contact Solar Cell, the subject of earlier U.S. Pat. Nos. 4,726,850 and 4,748,130, has been widely used by photovoltaic manufacturers for implementation with thick substrate (greater than 120 microns) devices.
The present invention is associated with a new solar cell structure and its implementation, whereby multiple interleaved n and p-type silicon horizontal layers are used to greatly increase the collection probability for carriers generated by the absorption of light in poor quality material. Such structures are not feasible with conventional approaches where bulk substrates are formed from ingots which are subsequently sawn into the individual wafers or substrates. However, new thin film approaches for layer formation such as chemical vapour deposition, solution growth, liquid phase epitaxy, plasma deposition and recrystallization or plasma deposition of amorphous material and subsequent crystallization, etc., make it feasible to form structures comprising multiple horizontal layers of interleaved n- and p-type material. Furthermore, prior to another recent invention by the present inventors, entitled "Buried Contact Interconnected Thin Film and Bulk Photovoltaic Cells" U.S. Pat. No. 5,595,607 (PCT/AU92/00658), such a structure would never have been considered as suitable for photovoltaic device fabrication, due to the difficulty in contacting all the layers of a given polarity which need to be connected in parallel. In the invention described in U.S. Pat. No. 5,595,607 PCT/AU92/00658, the Buried Contact Solar Cell approach is used to provide grooves through the entire thickness of all the horizontal doped silicon layers to enable contact to be made to each respective layer of the appropriate polarity by using the corresponding dopant type to heavily diffuse the groove walls as shown in FIG. 1. These grooves can be of any dimension provided they are deep enough to contact all the active silicon layers, and narrow enough to avoid excessive losses due to the reduction in useful surface area, where light falling on the contact area is prevented from entering the silicon material.
SUMMARY OF THE INVENTION
The present invention consists in a solar cell having at least three layers of alternate polarity material defining p-n junctions between each pair of alternate layers, the at least three layers having a maximum dopant concentration above 10.sup.17 atoms/cm.sup.3, and the at least three layers having a thickness which is not substantially greater than a minority carrier diffusion length for the dopant concentration of the material in the respective layer.
Typically the thickness of the doped layers will be in the range of 0.05 times to 5 times the minority carrier diffusion length and preferably in the range of 0.2 to 2 times the minority carrier diffusion length for the respective doped material. A device with a spatially non-uniform diffusion length and/or thickness will fall within the scope of this invention if any region of the layers falls within the above ranges. A demonstratable measure of these condition being satisfied would be the achievement of high average collection probabilities for carriers.
Cells may be of doped single crystalline or polycrystalline silicon, amorphous silicon and its alloys, cadmium telluride, cadmium sulphide, copper indium diselenide, an alloy of the form CuIn.sub.x Ga.sub.1-x S.sub.y Se.sub.1-y or other semiconductor material. In cells comprising a stack of silicon layers, one or more relatively thin layers of silicon/germanium alloy could also be included. Layers of insulating material such as silicon oxide, nitride or oxy-nitride could also be intermittently interleaved within the stack.
Embodiments of the invention may also include layers of intrinsic material or lightly doped material between pairs of alternate polarity layers, such that the junctions are p-Intrinsic-n junctions.
In embodim
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patent: 4891074 (1990-01-01), Oushinsky et al.
patent: 5595607 (1997-01-01), Wenham et al.
Green Martin Andrew
Wenham Stuart Ross
Nguyen Nam
Pacific Solar Pty. Limited
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