Title of Invention

METHOD OF PATTERNING OF SOLAR CELL

Abstract

A method of patterning of solar cell modules comprising the steps of forming of a first electrode by deposition a layer (2) of transparent conducting oxide (TCO) on a substrate (1) screen printing grid lines (3) of silver paste on the TCO layer, (2) followed by laser scribing (4) alongside the grid lines to obtain isolated cells, deposition of amorphous silicon cells on the surface of the isolated cells followed by deposition of a second electrode of a metal layer on the entire surface, screen printing parallel strips of silver paste (3) or acid etch resist paste (7) on the metal layer in a manner such that the parallel strips overlap with the grid lines and leaving small gaps there between, etching away the metal layer from the gaps (8) followed by connecting the electrical contacts to end bus-bars.

Full Text

FIELD OF THE INVENTION: This invention relates to a method of patterning of metal contacts for solar cells and the solar cells produced thereby. BACKGROUND OF THE INVENTION: In order to be able to satisfy the application-related demands for voltage ranges for 12 volts or higher voltages, it is necessary to connect solar cells in series Compared with the connection of discrete crystalline silicon wafers, thin-film solar cells offer a significant advantage, namely, the possibility of production by depositing large-area films onto substrates by spraying, evaporation, sputtering and chemical vapor deposition and plasma discharge processes. These films are then divided into individual solar cell areas and electrically connected. For the fabrication of a full module, an area of or the full substrate is initially completely coated with a film of transparent, electrically conductive material of tin oxide doped with fluorine or antimony, zinc oxide or indium oxide on tne top or which is added a second film with two or more layers of at least one semi conductive material, hereafter sections of these films are selectively removed to form on the substrate a plurality of regularly spaced photovoltaic cells. Subsequently applied is a large area covering film to serve as a second electrode layer. To permit the necessary series connection, this covering layer is interrupted in the direct neighborhood of those strips at which material has been selectively removed from the first electrode layer and from the semiconductor layers above it. Known method of achieving separation between individual areas referred to as "structuring or patterning" the layers-include, for example, material removal processes of a thermal, mechanical, laser scribing or those employing structuring bar or patterning" the layers-include, for example, material removal processes of a thermal mechanical, laser scribing or those employing structuring • bar or lift-off technology. However, such removal methods have considerable disadvantages. For instance, the formation of condensate on a cool point on the substrate when removing material by thermal means such as laser scribing; scattered flaked-off material and fissures in the remaining layer in the case of mechanical removal; and failure to totally remove all residues of said agent when removing material by chemical means in lift off technique. The present methods for patterning the second electrode in a thin film module provide lower yield due to remnants of the traces of the etched material or its modified form in the area between the strips and this results in poor yields. Further, screen-printing of a pattern is quite simple and the technique is suitable for providing high throughput. The printing may be done by a conducting paste such as silver paste, which can behave as a mask during the acid etching and can be left in place during lamination of the modules. This helps in protecting the underlined metal layer during further processes of edge sandblasting, soldering and lamination. However, a conductive paste is found to give poor yields due to shorting of the cells through defects of the films. Also silver paste is very expensive and is not economical to be used in terrestrial low efficiency solar cells. OBJECTS OF THE INVENTION: It is therefore an object of this invention to propose a method of patterning of metal contacts for solar cells, which gives high yields and better performance. ,lt is a further object of this invention to propose a method of patterning of metal contacts for solar cells, which provides additional protection to the cell during outdoor deployment of the solar cells. Another object of this invention to propose a method of patterning of metal contacts for solar cells, which is cost effective and simple. DETAILED DESCRIPTION OF THE INVENTION: Thus according to this invention is provided a process for the production of amorphous silicon solar cell modules comprising the steps of depositing of a first electrode of a layer of transparent conducting oxide (TCO) on a substrate, screen printing grid line soft silver paste on the TCO layer, followed by laser scribing alongside the grid lines to obtain isolated cells, deposition of amorphous silicon cells on the surface of isolated cells followed by deposition of. a. second electrode or a metal layer on the entire surface, screen printing parallel strips silver paste or acid etch resist paste on the metal layer in a manner such that the parallel strips overlap with the grid line and leave small gaps there between, etching away the metal layer from the gaps followed by connecting the electrical contacts to end bus-bars. According to this invention is further provided an amorphous silicon solar cell module comprising a substrate; parallel strips of a first electrode of transparent conducting oxide (TCO) on the substrate; grid lines on the TCO layer; amorphous silicon cells deposited on TCO layer and the grid lines; a second electrode of a metal layer; disposed over the silicon cells and connected in series to the first electrode by said grid lines interposed between the first electrode and the amorphous silicon cells and, a layer of acid etch resist provided on the second electrode, leaving gaps there between and the second electrode being interrupted in gaps alongside the strips. In accordance with this invention a method for patterning of the second electrode is provided, which employs a chemical process for which the second electrode requires masking from etchants, which are usually acid or salts which remove deposited metal by metal displacement reaction such as ferric nitrate for silver layer etching. In the present invention, acid etch resist has been used as a masking material. The said material can be dispensed with screen-printing process in the required pattern usually parallel strips with desired pattern. The paste can be used on a thin second electrode layer where the said electrode layer can be 200-nano meter thick silver layer deposited by vacuum evaporation or sputtering.The paste can be left or removed on the second electrode after etching the metal layer from the in between space of the printed strips. The process essentially involves two-screen printing and one laser-scribing step for the cell patterning and integration. The first step involves cleaning of the glass, followed by TCO deposition, screen printing ot grid pattern will silver, laser scribing, silicon deposition by plasma CVD process and finally silver deposition by DC magnetron sputtering. This is then followed by screen printing of silver for masking the layer for patterning and interconnects or alternately printing by acid etch resist, etching of silver from in between spaces of strips and consequently soldering and lamination. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS: The invention will now be illustrated with the help of the accompanying drawings where Fig 1 shows the amorphous silicon solar cell module according to the invention. The module comprises a substrate ((1) usually made of glass, on which the TCO layer (2) is deposited. Grid lines 3 of silver paste are printed on the TCO layer (2) followed by laser scribing the TCO layer at positions (4) alongside the grid lines to lead to individual sub cells. Amorphous silicon cells (5) of single junction or double junction type are deposited on the grid lines (3) followed by sputter deposition of a metal layer, such as for example, a silver layer on the entire- surface. A layer (7) of acid etch resist is screen-printed on the metal layer, leaving small gaps (8) for metal etching. The metal is etched away from the gaps (8) leaving individual cells. The positive and negative contacts are taken from the end grid lines (bus bars). The solar module thus consists of series-connected solar sub cells. The solar sub cells consist of glass substrate as base for all the thin film solar strips and consist a first electrode of transparent conducting oxide and the second electrode of a metal such as silver as described herein before. The series connection between the sub cells is made through screen printed silver lines which are thick enough and have roughness so that the semi conducting layer is not able to offer resistance between the first and the second electrodes. The pattering of the Sencod electrode is done by masking it from the acidic etchants using acid etch resist. This resist has been found to provide better yields than using a conducting paste for masking and did not show environmental degradation in the performance of the modules even though it was not removed after the patterning. The invention will now be explained in greater detail with the help of the following non-limiting example Example: Double junction amorphous silicon solar modules were fabricated utilizing the acid etch resist for the patterning of the second electrode. Glass substrates of 1 mm and 3mm thickness and 300 mm by 900 mm were cleaned in an online detergent scrubbing clean station. Layers of 70 to 150 nanometer thick of silicon dioxide and 600 to 800 nanometer thick layer of transparent conducting oxide such as fluorine doped tin oxide known as transparent conducting electrode, were deposited by chemical vapor deposition technique. The transparent conducting layer has a sheet resistance of less thar10 ohm/square and transmission of 78 to 82% with haze 4 to 15%. Parallel 400-micrometer wide lines of silver paste at a separation of 2 centimeter were screen printed and fired at 550 degree Celsius for 10 minutes. The transparent electrode was patterned alongside these lines by using a laser. A set of photovoltaic layers P,l, N,P,I,N of amorphous silicon where P layers are silicon carbide and N layer of first junction is microcrystalline nature. The second electrode of-Silver-metal with 150-to 400- nanometer thickness was deposited on the entire-area. 2 centimeter wide parallel strips of silver paste or acid etch resist paste were screen printed on the second electrode such that these overlap the screen-printed silver lines and leave a 500 to 800 micrometer wide exposed area of this second electrode to be removed by etchant such as aqueous solution of ferric nitrate. After patterning of the second electrode, the electrical contacts were connected to the end bus bars and the module was laminated and packaged. These modules were tested under a solar simulator and yielded an efficiency of 6.5 to 7% as produced. These modules were mounted in streetlights and other application and were found to stabilize at 5.0 to 5.5%. The solar cells patterned with acid etch resist provides better yields and performance than the modules patterned with silver paste. The modules patterned with acid etch resist do not show environmental degradation in the field. The acid etch resist provides additional protection from moisture. The acid resist need not be removed after patterning as it helps in protecting the solar cell while it is deployed in the field for solar power generation. The acid etch resist provides significant cost advantage over silver printed contact. The printing of the acid etch resist can be adopted for mass production of thin film modules using screen- printing. WE CLAIM 1. A method of patterning of solar cell modules comprising the steps of forming of a first electrode by depositing a layer (2) of transparent conducting oxide by chemical vapour deposition technique (CVD) on a substrate such as 1 to 3 mm thick glass sheet (1), screen printed grid lines (3) of silver paste on TCO layer (2) followed by laser scribing (4) alongside of the gridlines to obtain isolated cells (5) and on the surface of the isolated TCO strips followed by deposition of amorphous silicon by suitable techniques such as plasma CVD on the entire surface followed by deposition of the second electrode of a metal layer such as silver by sputtering or vacuum evaporation on the entire surface, screen printing parallel strips of low temperature silver paste or acrid etch resist paste (7) on the metal layer in a manner such that the parallel strips overlap with the grid lines (3) and leaving small gaps (8) there between, etching away the rectal layer from the gap (8) in an acidic solution like ferric nitrate followed by connecting the electrical contacts to end bus bars characterized in that a solar module consists of series - connected solar subcells by screen printed lines, each subcell consist of glass substrate as base for all the thin film solar strips and consist a first electrode of transparent conducting oxide and the second electrode of a metal and further characterized in that the acid etch resist provides protection from acidic solution during separation and from moisture during outdoor deployment. 2. The method as claimed in claim 1, wherein the layer of TCO (2) is deposited by chemical vapor deposition. 3. The method as claimed in claim 1, wherein said TCO layer is 600-800 mm thick. 4. The method as claimed in claim 1, wherein the substrate is a glass substrate having thickness 1 to 3 mm. 5. The method as claimed in claim i, wherein the silver paste is deposited on screen prints. 6. The method as claimed in claim 1, wherein the amorphous silicon solar cells are single junction or double function. 7. The method as claimed in claim 1 & 6 wherein the silicon cells are deposition by plasma chemical vapor deposition. 8. The method as claimed in claim 1, wherein the second electrode is silver layer 150mm to 400 thick. 9. The method as claimed in claim 1 and 8, wherein etching is conducted using an etching as acid or acidic salt such as ferric nitrate. 10. The method as claimed in claim 1 wherein a layer of silicon dioxide is optionally deposition on the glass prior to deposition of TCO layer. A method of patterning of solar cell modules comprising the steps of forming of a first electrode by deposition a layer (2) of transparent conducting oxide (TCO) on a substrate (1) screen printing grid lines (3) of silver paste on the TCO layer, (2) followed by laser scribing (4) alongside the grid lines to obtain isolated cells, deposition of amorphous silicon cells on the surface of the isolated cells followed by deposition of a second electrode of a metal layer on the entire surface, screen printing parallel strips of silver paste (3) or acid etch resist paste (7) on the metal layer in a manner such that the parallel strips overlap with the grid lines and leaving small gaps there between, etching away the metal layer from the gaps (8) followed by connecting the electrical contacts to end bus-bars.

Documents:

00257-kol-2005-abstract-1.1.pdf

00257-kol-2005-abstract.pdf

00257-kol-2005-claims-1.1.pdf

00257-kol-2005-claims.pdf

00257-kol-2005-correspondence-1.1.pdf

00257-kol-2005-correspondence-1.2.pdf

00257-kol-2005-correspondence.pdf

00257-kol-2005-description(complete)-1.1.pdf

00257-kol-2005-description(complete)-1.2.pdf

00257-kol-2005-description(complete).pdf

00257-kol-2005-drawings.pdf

00257-kol-2005-form-1.pdf

00257-kol-2005-form-18.pdf

00257-kol-2005-form-2-1.1.pdf

00257-kol-2005-form-2-1.2.pdf

00257-kol-2005-form-2.pdf

00257-kol-2005-form-3.pdf

00257-kol-2005-form-5.pdf

257-KOL-2005-FORM-27-1.1.pdf

257-KOL-2005-FORM-27.pdf

257-kol-2005-granted-abstract.pdf

257-kol-2005-granted-claims.pdf

257-kol-2005-granted-correspondence.pdf

257-kol-2005-granted-description (complete).pdf

257-kol-2005-granted-drawings.pdf

257-kol-2005-granted-examination report.pdf

257-kol-2005-granted-form 1.pdf

257-kol-2005-granted-form 18.pdf

257-kol-2005-granted-form 2.pdf

257-kol-2005-granted-form 3.pdf

257-kol-2005-granted-form 5.pdf

257-kol-2005-granted-gpa.pdf

257-kol-2005-granted-reply to examination report.pdf

257-kol-2005-granted-specification.pdf