REEL-TO-REEL PLATING OF CONDUCTIVE GRIDS FOR FLEXIBLE THIN FILM SOLAR CELLS
The present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells. In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece.
Latest SOLOPOWER, INC. Patents:
- Roll-to-roll processing method and tools for electroless deposition of thin layers
- ROOF INTEGRATED SOLAR MODULE ASSEMBLY
- INTEGRATED STRUCTURAL SOLAR MODULE AND CHASSIS
- Electroplating methods and chemistries for deposition of group IIIA-group via thin films
- Electroplating Solutions and Methods For Deposition of Group IIIA-VIA Films
The present application claims priority to U.S. Provisional Application No: 61/169673 filed Apr. 15, 2009 entitled “Reel to Reel Plating of Conductive Grids for Flexible Thin Film Solar Cells”, the entirety of which is incorporated herein by reference.
BACKGROUND1. Field of the Inventions
The present inventions generally relate to solar cell fabrication and, more particularly, to fabrication of flexible thin film solar cells.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1−xGax (SySe1−y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. It should be noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in
If the substrate 11 of the CIGS(S) type cell shown in
After fabrication, individual solar cells are typically assembled into solar cell strings or circuits by interconnecting them in series electrically, i.e. by connecting the (+) terminal of one cell to the (−) terminal of a neighboring cell. This way the total voltage of the solar cell circuit is increased. The solar cell circuit is then laminated into a protective package to form a photovoltaic module.
As shown in
Although the low electrical resistivity of such materials plays an important role in their choice, in operation, there is a trade off relationship between their size, i.e. height and width, and their electrical resistance, which critically depends on the cross sectional area of the fingers and the busbars. Since the fingers are spread over the illuminated surface, in order to reduce the shadowing effect caused by their presence on the illuminated surface, their width needs to be minimized while their height needs to be maximized to keep the cross sectional area high and therefore the resistance low. However, in ink deposition or screen printing approaches, when the width of the finger is reduced to minimize the shadowing loss, the height of the finger also gets reduced due to the nature of these processes and the nature of the inks and pastes used. Therefore, for narrow fingers the cross sectional area gets reduced and the resistance of the finger increases causing the overall efficiency of the solar cell to go down despite the fact that more light enters the device. It should be noted that resistivity and bulk resistivity mean the same in this application and they have the units of “ohm-cm”. Sheet resistance of a layer is defined as the resistivity of the material making up the layer divided by the thickness of the layer and has the units of “ohms per square”. The resistance of a conductive line, which has the units of “ohms” is equal to the resistivity of the material making up the line times the length divided by the cross sectional area of the line.
From the foregoing, there is a need in the thin film solar cell industry for improved grid structures and manufacturing methods that allows fabrication of narrow fingers with low resistance so that the conversion efficiency of the solar cells may be improved.
SUMMARYThe present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells.
In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in a adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece.
In a preferred aspect there is described a method of roll to roll manufacturing low electrical resistivity conductive grids having reduced shading effect for solar cells, comprising: providing a flexible continuous workpiece, the flexible continuous workpiece comprising a continuous flexible substrate, a bottom contact layer disposed atop the continuous flexible substrate, an absorber layer disposed atop the bottom contact layer, a transparent conductive layer disposed atop the absorber layer, and a first conductive film having a first resistivity disposed atop predetermined areas of a top surface of the transparent conductive layer and in electrical communication with the transparent conductive layer to form a raised grid pattern along a length of the flexible continuous workpiece, wherein the raised grid pattern includes a plurality of adjacent grids, with each grid having a predetermined grid width, a predetermined grid length, and a predetermined spacing between adjacent grids along the length direction of the flexible continuous workpiece, wherein a sheet resistance of the first conductive film is less than the sheet resistance of the transparent conductive layer, and wherein the top surface of the transparent conductive layer and the raised grid pattern disposed thereon form a front surface of the flexible continuous workpiece; applying an electrodeposition solution onto an effective plating region established on a portion of the front surface, including a part of the first conductive film, and onto an anode placed across from the portion of the front surface, the effective plating region having a length that is substantially the same as a width of the workpiece and a predetermined width that is at least longer than the predetermined spacing between adjacent grids; applying a voltage between the anode and the part of the first conductive film; selectively electrodepositing a conductive material from the electrodeposition solution onto the first conductive film and not the transparent conductive layer to form a second conductive film having a second resistivity atop the first conductive film, thereby forming the low electrical resistivity conductive grids having reduced shading effect, wherein the first resistivity is greater than the second resistivity; and moving the front surface, including the part of the first conductive film, through the effective plating region, during the steps of applying the electrodeposition solution, applying the voltage, and selectively electrodepositing.
These and other aspects and advantages are described further herein
Described herein are methods and apparatus to form low electrical resistance grid patterns over illuminated side of photovoltaic cells or solar cells. In one embodiment, initially a conductive grid pattern is formed, preferably by a screen printing or ink deposition technique, over a transparent conductive layer of a solar cell structure. In the following step, a conductive material is selectively electroplated over the conductive grid pattern using the electroplating apparatus. The electroplated conductive material increases the height of the conductive grid pattern and reduces its electrical resistance. It should be noted that the resistivity of an electroplated conductor such as electroplated Cu or Ag is lower than the resistivity of screen printed or ink deposited conductors such as Ag pastes.
As shown in
The width ‘W’ of the effective plating region is greater than the distance ‘d’ between the grid patterns of the first conductive film 102. This way it is assured that a portion of the first conductive film 102 or a portion of the already plated grid pattern is always in the effective plating region 120. Since the resistances of the first and second conductive films 102 and 103 are much lower than that of the transparent conductive layer 112, the plating current preferentially passes through the fingers 124 and/or the raised fingers 125, depositing material there rather than on the transparent conductive layer. It should be noted that the bulk resistivity of the Ag-based material forming the first conductive film 102 is in the range of 10-30 micro-ohm-cm, whereas the resistivity of materials forming the transparent conductive layer 112 (
As the workpiece 105 is advanced through the electrodeposition cell 132, the electrodeposition electrolyte 146 flows towards the front side 101A of the workpiece 105, contacts it and flows out of both the entrance opening 149A and the exit opening 149B. The electrolyte 146 is pumped into the chamber 140 from an electrolyte supply tank (not shown) and the used electrolyte leaves the cell through the entrance opening 149A and the exit opening 149B. This used electrolyte may be flowed into a recycling tank (not shown) to filter and replenish it. The replenished electrolyte is then redirected into the electrodeposition cell 132 or the electrolyte supply tank (not shown). In this embodiment, the side walls 142A and 142B of the rectangular chamber 140 and the edges of workpiece as they pass through the plating chamber define the effective plating region 120.
The surface contacts 134 may be made of conductive rollers or brushes which negatively polarize the surface 104A and the first conductive film 102 which is shown as the finger 124 in
As can be seen in
Therefore, in one embodiment a finger plating or grid plating method comprises the steps of: i) providing a continuous flexible workpiece with two edges and a width, the workpiece comprising multiple solar cell structures on its front surface, each solar cell structure having a conductive grid pattern with fingers which are parallel to the two edges of the workpiece, ii) applying an electrodeposition solution onto an effective plating region on the front surface of the workpiece and onto an anode placed across from the front surface of the workpiece, the effective plating region having a length that is substantially the same as the width of the workpiece and a predetermined width that is larger than a distance between the grid patterns of adjacent solar cell structures, iii) applying a voltage between the anode and two contacts that touch the front surface of the workpiece while moving the workpiece and the effective plating region with respect to each other and in a direction that is substantially parallel to the fingers of the grid patterns thus causing electrodeposition of a conductive material from the electrodeposition solution onto the conductive grid patterns of solar cell structures, wherein the two contacts are provided on two sides of the effective plating region and a distance between the two contacts is less than or equal to the total length of each of the fingers of the grid patterns.
Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
Claims
1. A method of roll to roll manufacturing low electrical resistivity conductive grids having reduced shading effect for solar cells, comprising:
- providing a flexible continuous workpiece, the flexible continuous workpiece comprising a continuous flexible substrate, a bottom contact layer disposed atop the continuous flexible substrate, an absorber layer disposed atop the bottom contact layer, a transparent conductive layer disposed atop the absorber layer, and a first conductive film having a first resistivity disposed atop predetermined areas of a top surface of the transparent conductive layer and in electrical communication with the transparent conductive layer to form a raised grid pattern along a length of the flexible continuous workpiece, wherein the raised grid pattern includes a plurality of adjacent grids, with each grid having a predetermined grid width, a predetermined grid length, and a predetermined spacing between adjacent grids along the length direction of the flexible continuous workpiece, wherein a sheet resistance of the first conductive film is less than the sheet resistance of the transparent conductive layer, and wherein the top surface of the transparent conductive layer and the raised grid pattern disposed thereon form a front surface of the flexible continuous workpiece;
- applying an electrodeposition solution onto an effective plating region established on a portion of the front surface, including a part of the first conductive film, and onto an anode placed across from the portion of the front surface, the effective plating region having a length that is substantially the same as a width of the workpiece and a predetermined width that is at least longer than the predetermined spacing between adjacent grids;
- applying a voltage between the anode and the part of the first conductive film;
- selectively electrodepositing a conductive material from the electrodeposition solution onto the first conductive film and not the transparent conductive layer to form a second conductive film having a second resistivity atop the first conductive film, thereby forming the low electrical resistivity conductive grids having reduced shading effect, wherein the first resistivity is greater than the second resistivity; and
- moving the front surface, including the part of the first conductive film, through the effective plating region, during the steps of applying the electrodeposition solution, applying the voltage, and selectively electrodepositing.
2. The method of claim 1, wherein each grid includes fingers disposed parallel to the flexible continuous workpiece edges extending along the length of the flexible continuous workpiece.
3. The method of claim 2, wherein in the step of moving, portions of the flexible continuous workpiece are continuously advanced into a front side of the effective plating region and through the effective plating region to form the second conductive film and then continuously advanced out of the effective plating region through a back side of the effective plating region after forming the second conductive film.
4. The method of claim 3 wherein the step of selectively electrodepositing includes applying a first electrical contact adjacent the front side of the effective plating region and a second electrical contact adjacent the back side of the effective plating region, wherein a distance between the first and second contacts is less than or equal to the length of each of the fingers.
5. The method of claim 4, wherein in the step of moving the portions of the flexible continuous workpiece are released from a supply roll of the flexible continuous workpiece and wound as a receiving roll when advanced out of the -effective plating region.
6. The method of claim 4, wherein the first and the second contacts are conductive roll contacts rolling on the front surface as the flexible continuous workpiece is advanced.
7. The method of claim 4, wherein the first and the second contacts are conductive brush contacts sweeping the front surface as the flexible continuous workpiece is advanced.
8. The method of claim 1, wherein the first resistivity of the first conductive film is in the range of 10-30 micro ohm-cm, the second resistivity of the second conductor is in the range of 2-10 micro ohm-cm, and the resistivity of the transparent conductive layer is in the range of 200-500 micro ohm-cm.
9. The method of claim 1, wherein the first conductive film includes a silver (Ag) based conductive material formed using one of a screen printing process and an ink jet printing process.
10. The method of claim 1, wherein the second conductive film includes one of copper, silver, a copper alloy and a silver alloy.
11. The method of claim 1, wherein the transparent conductive layer includes a stack including a transparent buffer layer deposited over the absorber layer and a transparent conductive oxide (TCO) layer deposited over the transparent buffer layer, and wherein the transparent buffer layer includes one of CdS and ZnS, and the TCO layer includes one of ZnO and indium tin oxide (ITO).
12. The method of claim 1, wherein the absorber layer includes a group IBIIIAVIA compound semiconductor.
13. The method of claim 1, wherein the substrate includes one of a stainless steel foil and an aluminum foil.
14. The method of claim 1, wherein the bottom contact layer includes at least one of Mo, W, Ta, Ti, Cr and Ru materials.
15. The method of claim 1, wherein the effective plating region is defined by an enclosure including a front wall and a back wall extending along the length of the effective plating region and two side walls extending along the width of the effective plating region.
16. The method of claim 15, wherein the flexible continuous workpiece enters the effective plating region through an entrance opening the in the front wall and exits the effective plating region through an exit opening in the back wall.
17. The method of claim 16, wherein the electrodeposition solution is delivered through a top opening of the enclosure and used electrodeposition solution flows out of the enclosure through at least one of the entrance and exit openings.
18. The method of claim 5, wherein the effective plating region is defined by an enclosure including a front wall and a back wall extending along the length of the effective plating region and two side walls extending along the width of the effective plating region, wherein the front and the back walls forms the front and back sides of the effective plating region, respectively.
19. The method of claim 18, wherein the electrodeposition solution is delivered through a top opening of the enclosure and used electrodeposition solution flows out of the enclosure through at least one of the entrance and exit openings.
20. The method of claim 19, wherein the first and the second contacts are conductive roll contacts rolling on the front surface as the flexible continuous workpiece is advanced.
21. The method of claim 19, wherein the first and the second contacts are conductive brush contacts sweeping the front surface as the flexible continuous workpiece is advanced.
22. The method of claim 1, wherein the thickness of the first conductive film is in the range of 1-10 microns.
23. The method of claim 1, wherein the thickness of the second conductive film is in the range of 1-5 microns.
24. The method of claim 1, wherein the thickness of the transparent conductive layer is in the range of 0.1-0.5 microns.
Type: Application
Filed: Apr 15, 2010
Publication Date: Oct 21, 2010
Applicant: SOLOPOWER, INC. (San Jose, CA)
Inventor: Bulent M. Basol (Manhattan Beach, CA)
Application Number: 12/761,240
International Classification: C25D 5/02 (20060101); C25D 7/00 (20060101);