METHOD AND APPARATUS FOR APPLYING A METALLIZATION PATTERN TO A SUBSTRATE FOR A PHOTOVOLTAIC CELL

Abstract: Method for applying a metallization to at least one of the outer surfaces of a substrate for a photovoltaic element in accordance with a predetermined pattern of electrical conductors which comprises a set of relatively thin lines (fingers) and a set of relatively wide strips (busbars) connected thereto for transporting electrical charge carriers, which method comprises the steps of: (i) applying a metal-containing conductive paste to the relevant surface in accordance with said determined pattern, and (ii) drying the paste applied to the surface, wherein in the first step (i) a first paste for the set of the finger lines is applied using a stencil and a second paste for the set of the busbar strips is subsequently applied using an apparatus for contact-free application; apparatus for performing this method and photovoltaic element with substrate manufactured according to this method.

Description: METHOD AND APPARATUS FOR APPLYING A METALLIZATION PATTERN TO A SUBSTRATE FOR A PHOTOVOLTAIC CELL The invention relates to a method for applying a metallization to at least one of the outer surfaces of a substrate for a photovoltaic element in accordance with a predetermined pattern of electrical conductors which comprises a set of relatively thin lines and a set of relatively wide strips connected thereto for transporting electrical charge carriers, which method comprises the steps of (i) providing said substrate, at least one of the outer surfaces of which is adapted for applying of a metallization thereto, (ii) applying a metal -containing conductive paste to the relevant surface in accordance with said determined pattern, and (iii) drying the paste applied to the surface. It is known to apply a metallization pattern to the front side of a photovoltaic element, for instance a silicon solar cell, using a screen-printing technique. According to this known method an emulsion layer suitable for this purpose is applied to a stainless steel mesh clamped in a frame, into which layer is recessed the pattern of the metallization to be applied. The thus obtained screen is arranged over the front side of a silicon substrate, whereafter the metal -containing conductive paste is applied to the emulsion layer, for instance a paste of silver particles, frit, a binder and a solvent. The paste is skimmed off using a squeegee and pressed via the mesh through the apertures in the emulsion layer onto the substrate. The thus created paste pattern on the substrate is then dried in a furnace, wherein the solvent evaporates, heated while air or oxygen is added to burn organic binders, and sintered to adhere the metal particles to each other and to the substrate. It is an important advantage of the known method that a substrate to which a paste is applied by screen-printing can be dried, further heated and sintered in one furnace cycle so as to obtain the intended metallization pattern. An inherent drawback of the known method is that the minimum width of the metallization lines to be formed on the substrate is determined by the mesh width of the screen used. A screen of width mesh 325 or higher (i.e. 325 or more meshes per inch) is used for instance to print very fine lines . When the line width is in the order of magnitude of the pitch of the screen (for a screen of width mesh 400 for instance the pitch amounts to 65 μm) , the opening in the emulsion layer is critical: the percentage of open surface fluctuates significantly, which results in a correspondingly fluctuating line width of the applied paste. The use of thinner wires for the screen or increasing the distance between the wires thereof increases the uniformity of deposited metal paste lines but reduces the strength of the screen and thereby reduces the lifespan to a level which may be economically unacceptable. During the design of a metallization pattern of a solar cell, the objective is to create the thinnest possible lines in order to keep shadow losses resulting from the metallization as low as possible and thereby the efficiency as high as possible. In order to hold resistance losses in thin lines below determined values, a determined minimum height of these lines is however required. It is another drawback of the screen-printing technique that this imposes an upper limit on the viscosity of a paste for use such that the line height to be realized with this paste is limited to an undesirably low value. It is an object of the invention to provide a method for applying a metallization pattern with relatively thin and well-defined lines which have a height such that resistance losses in a solar cell with such a pattern are negligible, or at least remain below an acceptable level. It is a further object to provide such a method wherein a paste applied to a substrate can be dried, further heated and sintered in one furnace cycle, so as to obtain the intended metallization pattern. It is another object to provide a method which can be performed in rapid and cost-saving manner. These objects are achieved, and other advantages gained, with the method stated in the preamble, wherein according to the invention in the second step (ii) a first paste for the set of relatively thin lines is applied using a stencil and a second paste for the set of relatively wide strips is subsequently applied using an apparatus for contact-free application. A stencil is a foil, usually of a metal, for instance nickel or stainless steel, which is placed instead of a screen over the substrate for a solar cell, wherein apertures in the foil are formed in accordance with a line pattern for arranging on the substrate. It has been found that a pattern for very thin, smooth and relatively high lines of a first paste can be applied to the substrate in one print run by means of a stencil. The pattern of relatively wide strips forming the busbars in a solar cell can be applied immediately thereafter in contact-free manner by means of an apparatus suitable for the purpose without intervening drying of the line pattern. A second print run with a second stencil or with a screen, which would be necessary to apply the pattern with the busbars intersecting the thin lines, is hereby dispensed with. The advantage of the screen- printing technique, that it is possible to suffice with only one furnace cycle after printing, is hereby retained, while the drawbacks thereof do not occur. The paste for the wide strips is applied for instance using an arrangement for contact-free dispensing or other technique of contact-free application. In a first embodiment of the method according to the invention the first paste and the second paste are identical. In a subsequent embodiment the second paste has a lower viscosity than the first paste, which provides the advantage that the second paste has rheological properties such that it can be more easily transported through an apparatus for contact- free dispensing than the first paste, the viscosity of which is chosen such that very thin and relatively high lines can be printed with this paste. The method is particularly suitable for embodiment with a stencil comprising a foil in which are arranged slots which correspond with the set of relatively thin lines and which have a width smaller than about 100 μm, in particular smaller than about 50 μm, wherein the foil has for instance a thickness smaller than about 50 μm. The method according to the invention is for instance performed using an apparatus for contact-free application containing a nozzle placeable above the substrate, wherein the second paste is applied by moving the nozzle and the substrate relative to each other such that the nozzle follows the predetermined pattern of the set of relatively wide strips. The nozzle has for instance a cross -section with an aspect ratio having a value not equal to 1, preferably a value greater than 2, more preferably a value at least equal to 5. The nozzle preferably extends with a long axis of the cross-section parallel to the surface of the substrate and perpendicularly of the direction of the relative movement of nozzle and substrate, wherein the nozzle more preferably has a cross -section with a long axis with a length of about 1.5 mm and a short axis with a length of about 300 μm. The invention further relates to an apparatus for performing the above described method. The invention will be elucidated hereinbelow on the basis of embodiments and with reference to the drawings. In the drawings : Fig. 1 shows in top view a substrate of a square solar cell to which according to the invention a first paste is applied in a pattern of thin lines in a first step, Fig. 2 shows in top view the substrate of fig. 1 to which according to the invention a second paste is applied in a pattern of relatively wide strips in a second step, Fig. 3 shows in perspective view a simplified illustration of an embodiment of an arrangement for contact-free dispensing of paste for the busbars to a substrate for a solar cell, Fig. 4 shows the arrangement of fig. 3 at a later point in time, Fig. 5 shows in side view a cross -section through the arrangement of fig. 4, and Fig. 6 shows in side view a detail of an alternative embodiment of an arrangement for contact-free dispensing. Corresponding components in the figures are designated with the same reference numerals. Fig. 1 shows a substrate 1 for a solar cell with a part of a so-called H-pattern. A full H-pattern consists of relatively thin lines or fingers and relatively wide strips or busbars. The fingers have the purpose of carrying the current to be generated by the solar cell over the whole surface to the busbars, which in turn serve as central discharge for the current and for connecting in series a subsequent solar cell by means of conductors for mounting on the busbars. By means of a stencil formed by a 50 μm thick foil of a suitable metal into which are recessed slots with a width of 50 μm, a pattern is printed on substrate 1 of lines 2 of a conductive paste which contains inter alia about 70% by weight of silver in the form of very small spherical particles with a diameter of about 1-2 μm and a small fraction of flocculent particles or platelets with a largest dimension of about 5 μm. The lines 2 printed with the stencil in question have a width of about 55 μm and a height of about 20 μm in the dried and annealed state. Fig. 2 shows substrate 1 of fig. 1 to which two strips 3 of a second suitable and per se known conductive paste are applied using an arrangement for contact-free dispensing. The strips 3 applied with the arrangement in question have a width of about 1.5 mm and a height of about 300 μm in the dried and annealed state, and form the busbars of the solar cell. Fig. 3 shows a dispensing apparatus 4 for contact- free application of paste for the busbars to a substrate 1 for a solar cell at a moment in time t=t 0 . Shown are a container 5 for holding paste under pressure with for instance compressed air, a housing 7 with a drive for a conveyor screw 8 and a nozzle 9 which debouches above a substrate 1 provided with lines 2. The dispensing apparatus is movable relative to substrate 1 along two mutually perpendicular suspension arms 10, 11 and a vertical guide (not shown) , wherein the relative movement of substrate 1 and nozzle 9 can be regulated using a control circuit (not shown) such that the nozzle follows the predetermined pattern of the set of busbars for applying. The figure further shows a reference frame for the three mutually perpendicular directions of movement X, Y, Z for nozzle 9. Nozzle 9 has a cross -section with a long axis of about 1.5 mm in the Y-direction and a short axis with a length of about 300 μm in the X-direction (so that the aspect ratio amounts to 5) . Fig. 4 shows dispensing apparatus 4 of fig. 3 at a moment in time t=t ι , at which the apparatus is displaced along arm 10 over a determined distance in the direction of arrow X. During the displacement paste is carried out of container 5 via conduit 6 to the rotating (rotation symbolized by curved arrow ω) conveyor screw 8, and using this conveyor screw 8 applied in contact-free manner to substrate 1 via nozzle 9 in accordance with a straight wide strip 3 extending transversely over lines 2. Once the whole strip 3 has been applied, successively the feed of paste is interrupted, apparatus 4 is displaced along arm 11 over a distance such that nozzle 9 debouches precisely above the location of a following busbar to be applied, whereafter the feed of paste and the movement along arm 10 can be resumed in (opposing) X-direction in order to apply a subsequent busbar 3 to substrate 1. Fig. 5 shows a cross -section along line V-V through the arrangement shown in fig. 4. In the shown embodiment the distance Z 0 between the underside of nozzle 9 and the surface of substrate 1 is adjustable between 0.020 mm and 5 mm, whereby the thickness d of the paste strip to be applied to substrate 1 can be adjusted in the range of about 10-300 μm. Fig. 6 shows a detail of an alternative embodiment of the dispensing apparatus, wherein the housing 7 with nozzle 9 shown in fig. 5 can be tilted in the X,Z plane so that in the tilted position (drawn in dashed lines) of housing 7 and nozzle 9 at an angle to the perpendicular the paste 3 can be applied to the surface of substrate 1, whereby when the feed of paste ceases the outer end of strip 3 is prevented from protruding above the level of the remaining part of this strip.

Claims: CLAIMS 1. Method for applying a metallization to at least one of the outer surfaces of a substrate (1) for a photovoltaic element in accordance with a predetermined pattern of electrical conductors which comprises a set of relatively thin lines (2) and a set of relatively wide strips (3) connected thereto for transporting electrical charge carriers, which method comprises the steps of (i) providing said substrate (1) , at least one of the outer surfaces of which is adapted for applying of a metallization thereto, (ii) applying a metal -containing conductive paste to the relevant surface in accordance with said determined pattern (2, 3), and (iii) drying the paste applied to the surface, characterized in that in the second step (ii) a first paste for the set of relatively thin lines (2) is applied using a stencil and a second paste for the set of relatively wide strips (3) is subsequently applied using an apparatus (4) for contact- free application. 2. Method as claimed in claim 1, characterized in that the first paste and the second paste are identical. 3. Method as claimed in claim 1, characterized in that the second paste has a lower viscosity than the first paste. 4. Method as claimed in any of the claims 1-3, characterized in that the stencil comprises a foil in which are arranged slots which correspond with the set of relatively thin lines (2) and which have a width smaller than about 100 μm. 5. Method as claimed in claim 4, characterized in that the width is smaller than about 50 μm. 6. Method as claimed in either of the claims 4 or 5, characterized in that the foil has a thickness smaller than about 50 μm. 7. Method as claimed in any of the foregoing claims, wherein the apparatus (4) for contact-free application comprises a nozzle (9) placeable above the substrate (1), characterized in that the second paste is applied by moving the nozzle (9) and the substrate (1) relative to each other such that the nozzle (9) follows the predetermined pattern of the set of relatively wide strips (3) . 8. Method as claimed in claim 7, characterized in that the nozzle (9) has a cross -section with an aspect ratio having a value not equal to 1. 9. Method as claimed in claim 8, characterized in that the aspect ratio has a value greater than 2. 10. Method as claimed in claim 9, characterized in that the aspect ratio has a value at least equal to 5. 11. Method as claimed in any of the claims 8-10, characterized in that the nozzle (9) extends with a long axis of the cross -section parallel to the surface of the substrate (1) and perpendicularly of the direction of the relative movement of nozzle (9) and substrate (1) . 12. Method as claimed in any of the claims 8-11, wherein the nozzle (9) has a cross -section with a long axis with a length of about 1.5 mm and a short axis with a length of about 300 μm. 13. Apparatus (4) for contact-free application of a second paste for a set of relatively wide strips (3) to at least one of the outer surfaces of a substrate (1) for a photovoltaic element in accordance with a method as claimed in any of the claims 1-6, characterized in that it comprises a nozzle (9) placeable above the substrate, wherein the substrate (1) and this nozzle (9) are movable relative to each other such that during a relative movement of substrate (1) and nozzle (9) the nozzle (9) follows the predetermined pattern of the set of relatively wide strips (3) . 14. Apparatus (4) as claimed in claim 13 for performing a method as claimed in claim 7, characterized in that the nozzle (9) has a cross -section with an aspect ratio having a value not equal to 1. 15. Apparatus (4) as claimed in claim 14 for performing a method as claimed in claim 8, characterized in that the aspect ratio has a value greater than 2. 16. Apparatus (4) as claimed in claim 15 for performing a method as claimed in claim 9, characterized in that the aspect ratio has a value at least equal to 5. 17. Apparatus (4) as claimed in any of the claims 14- 16 for performing a method as claimed respectively in any of the claims 8-10, characterized in that the nozzle (9) extends with a long axis of the cross -section parallel to the surface of the substrate (1) and perpendicularly of the direction of the relative movement of nozzle (9) and substrate (1) . 18. Apparatus (4) as claimed in any of the claims 14- 17 for performing a method as claimed respectively in any of the claims 8-11, characterized in that the nozzle (9) has a cross -section with a long axis with a length of about 1.5 mm and a short axis with a length of about 300 μ . 19. Photovoltaic element provided with a metallization pattern (2, 3) applied in accordance with a method as claimed in any of the claims 1-12.

Inventors: WEEBER ARTHUR WOUTER, WEEBER, ARTHUR, WOUTER
Applicants: STICHTING ENERGIE, WEEBER ARTHUR WOUTER, STICHTING ENERGIEONDERZOEK CENTRUM NEDERLAND, WEEBER, ARTHUR, WOUTER
Application year: 2000
Main classification: H01L31
IPC codes: H01L31, H05K3