Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2000-10-23
2002-08-27
Diamond, Alan (Department: 1754)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S244000, C136S256000, C136S261000, C257S443000, C257S466000, C257S465000, C438S066000, C438S080000, C438S068000, C438S081000, C439S894000, C439S883000, C439S954000, C439S890000
Reexamination Certificate
active
06441297
ABSTRACT:
The invention relates to a method and a device according to the respective characterizing portions of the independent claims. Accordingly, the invention relates to a solar cell arrangement.
Solar cell arrangements find a wide variety of applications. Large-surfaced solar cell arrangements are used in photo-voltaic systems, for example, which can provide sufficient energy for consumers with a higher demand. In this case, the costs of the cells often play a subordinated role because they are not significant compared to the costs required for a connection to a public supply system, or because there is no useful alternative, as in the case of aerospace applications.
Solar cell arrangements are also used in a variety of small devices, currently having a low output, such as pocket calculators and wristwatches. However, in principle, it is also feasible to use solar cell arrangements in electronic consumer goods appliances having a slightly higher energy requirement than a pocket calculator or a watch. For example, solar cell arrangements could be considered for charging and operating portable computers and cell phones independent of the network where the operating voltage usually ranges between 6 and 12 V.
Technically, this is feasible, because a cell phone battery, for example, which provides approx. 2 Watts for a call, requires a charging voltage of approx. 7 Volt and a total charge of approx. 550 mAh to 1200 mAh. In order to keep the cell phone functional merely by means of insolation a surface of approx. 50 cm
2
is available on the rear.
With solar cell arrangements based on silicon technology having said surface and assuming an efficiency of only approx. 14% and small series connection losses a voltage of more than approx. 7 Volt and currents of more than 100 mA at the maximum power point are achievable so that it is possible, in principle, to achieve a charge by means of insolation of sufficient duration.
The cells to be installed, however, should not affect the size or compactness, nor should they be too expensive in order to be used in consumer products. To meet the first requirement relatively small solar cells of a few square centimeters have to be connected which requires very small modules instead of the large-surfaced solar cell modules commonly used in photo-voltaic systems. These must be mounted so as to be cost-effective.
Previously, the use of solar cell arrangements often failed because the costs were too high. This was also due to the high costs of series connection. In principle, in series connection the base of the first cell has to be connected conductive with the emitter of the second cell. Typically, the base is arranged on one solar cell surface, the emitter on the opposite surface.
When solar cells on the basis of crystalline silicon wafers are used for solar cell arrangements there are several options, in principle, according to the state of the art to achieve the series connection. For example, the cell front surface of a solar cell can be connected electrically conductive with the rear surface of the following cell via tin-coated copper strips or the like. Further, it is known in the art from “Direct Conversion of Energy” by K. J. Euler, Karl Thiemig KG Publishers, Munich, 1967, page 55, to arrange the individual solar cells slightly overlapping in the manner of shingles or roof tiles and then to electrically connect the base rear contact of one cell with the emitter front contact on the following cell. From the article “Emitter Wrap-Through Solar Cell” by J. M. Gee et al., Proc. 23
rd
IEEE PVSC, Louisville, 1993, pages 265-270, it is known to provide a solar cell with front and rear emitter of the n+type which encompasses a p-doped base material. While the emitter on the front has to be provided only with an anti-reflex coating two contact grids are provided on the rear, one of the p-type for contacting the base and one of the n-type for contacting the emitter, respectively. Both grids form a pattern of interlinked fingers connected at one end via so-called bus bars. In order to connect the emitter on the front with the emitter on the rear or with the emitter contact grid provided there, holes are made in the substrate by means of laser pulses, which are then doped and filled with metal using a selective method so as to produce a conductive connection. While the publication discusses the series resistance of an individual element, it does not specify how multiple cells can easily be connected.
It was further proposed in an article published in the Internet entitled “The crystalline-silicon photo-voltaic R&D Project at NREL and SNL” by J. M. Gee and T. F. Ciszek at http://www.sandia.gov, to provide a module assembly concept where all cells of a module are encapsulated and electrically connected in one single step. For this purpose, the cells are contacted on the rear where a rear module surface plane includes both the electrical circuit and the encapsulation material in one single piece, and a one step method is provided for arranging said components into a module.
For this purpose, the cells contacted on the rear are placed on a plane base having a pre-formed electric connecting pattern and deposited on the rear surface beyond said base. This requires handling individual cells, which is problematic in the above mentioned applications because of the size of the cells.
The article “An industrial multi-crystalline ewt solar cell with screen printed metallization” by A. Schönecker et al., 14
th
European Photo-voltaic Solar Energy Conference, Barcelona, 1997, pages 796 and following, specifies connecting a number of cells in that they are contacted via bus bars, for example using the above described emitter-wrap-through technology, and subsequently connecting the bus bars of the individual modules via electrical conductors. The article does not specify how the individual modules should be handled.
The article “Advances in thin film PV technologies” by H. A. Aulich, 13
th
European Photo-voltaic Solar Energy Conference, Nice, France, Oct. 23 to 27, 1995, pages 1441 and following specifies a number of different thin film solar modules consisting primarily of materials other than silicon. Also, the use of amorphous silicon is described where reference is made to the so-called photo degradation, i.e. a deterioration in the effectiveness over time.
A further method of contacting is known from the article “High efficiency (19.2%) silicon thin-film solar cells with interdigitated emitter and base front-contacts” by C. Hebling et al., 14
th
European Photo-voltaic Solar Energy Conference, Barcelona, Spain, Jun. 30 to Jul. 4, 1997.
It proposes a so-called SOI structure, i.e. “silicon on insulator structure” where the actual cell is applied to an insulating layer. The described method is comparatively expensive, however.
From the book “Silicon solar cells—advanced principles and practice” by Martin A. Green, ISBN 0 7334 09946, published by the Center for photo-voltaic devices and systems, University of New South Wales, Sydney, NSW 2052 in March 1995, solar cells with a so-called buried structure are known, where grooves are made in the material by means of a laser, mechanical cutting wheels or other mechanical or chemical means, and the grooves are chemically cleaned. Subsequently, a strong doping agent is applied and the grooves are filled with metal. Contacts are obtained thus after taking further steps. The book also mentions the above mentioned finger-like cells contacted on the rear as well as point contacted solar cells where, instead of a p-conductive bus bar, p-conductive contact points are distributed over the cell so as to better determine certain cell properties.
From the U.S. Pat. No. 4,612,408, an interconnected solar cell array is known which is produced in that a number of the arrangements are formed on the semiconductor surface, grooves are formed in the surface which, in part, extend into the substrate, an oxide layer is formed on selected sections of one surface and on the surfaces of the grooves, the grooves are filled with an insulating ma
Fath Peter
Keller Steffen
Willeke Gerhard
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