Patent Application
20070144577
1. Field of Invention
This invention relates to solar cells and more particularly to semiconductor photovoltaic cells and a process for forming electrical contacts in a solar cell structure.
2. Description of Related Art
It is well-known that under light illumination, photovoltaic (PV) solar cells comprising semiconductor wafers generate electric current. This electric current may be collected from the cell by means of front and back side metallization on the wafer which acts as electrical contacts on front and back sides of the solar cell. A partially electrically conductive paste, which typically contains silver and/or aluminum, is screen printed onto front and back surfaces of the cell through a mask. For the front (active) side of the solar cell structure, the mask typically has openings through which the paste contacts the surface to be metallized. The configuration of the openings determines the shape of a pattern that the paste will form on the surface of the cell and the ultimate shape of the electrical contacts. The front side mask is typically configured to produce a plurality of thin parallel line contacts and two or more thicker lines that are connected to and extend generally perpendicular to the parallel line contacts.
After spreading paste on the mask, the mask is removed and the wafer bearing the partially conductive paste is initially heated such that the paste dries. Later, the wafer is “fired” in an oven and the paste enters a metallic phase and at least part of it diffuses through the front side surface of the solar cell and into the cell structure while a portion is left solidified on the front side surface. The multiple thin parallel lines thus form thin parallel linear electrical contacts referred to as “fingers”, intersected by thicker perpendicular lines referred to as “bus bars”. The purpose of the fingers is to collect the electrical current from the front side of the PV cell. The purpose of the bus-bars is to receive the current from the fingers and transfer it away from the cell.
Typically, the width and the height of each finger is approximately 120 microns and 10 micron respectively. Inherent technical limitations of screen printing technology further introduce 1-10 micron fluctuations in finger height and 10-30 micron or greater fluctuations in width. While the fingers are sufficient to harvest small electric currents, the bus-bars are required to collect a much greater current from the plurality of fingers and therefore have a substantially larger cross section and width.
Back side metallization involves a layer of partially conductive paste containing aluminum over the entire back surface of the cell except for a few small areas. During the initial heating, the paste dries. Then silver/aluminum paste is screen printed in certain areas that have not been printed with aluminum paste and is further dried. Then, when the wafer is subjected to “firing”, wherein the aluminum paste forms a passivation layer called a Back Surface Field (BSF) and aluminum contacting layer and the silver/aluminum paste forms silver/aluminum pads. The aluminum contacting layer collects the electrical current from the PV cell itself and passes it to the silver pads. The silver/aluminum pads are used to take the electric current away from the PV cell.
The area that is occupied by the fingers and bus bars on the front side of the solar cell is known as the shading area and prevents solar radiation from reaching the solar cell surface. This shading area decreases solar cell conversion efficiency. Modern solar cell shading occupies 6-10% of the available solar cell surface area.
In addition, the presence of metallization on the front side and the silver/aluminum pads on the back side results in a decrease of voltage generated by the PV cell proportionate to the metallization area. Therefore, in order to achieve maximum conversion efficiency of the PV cell, it is desirable to minimize the area occupied by front side metallization. In addition, it is also desirable to minimize the area of silver metallization on the back side, in particular, to reduce the amount of silver/aluminum paste required. This will increase cell efficiency and will substantially decrease the cost of solar cell fabrication because silver/aluminum paste can be expensive.
The use of modern screen printing technology for front side metallization achieves a certain minimal level of metallization by optimizing widths and thicknesses of fingers and bus-bars for the solar cell being produced. However, there are principle limitations that prevent further decreases of the metallization area. Firstly, the cross sectional dimensions of the fingers cannot be less than certain dimensions in order to avoid excessive resistive losses due to electric current flow through the fingers during solar cell operation. In addition, bus bars are required to have minimum cross-sectional dimensions also to avoid resistive losses during operation. In addition, conventional technology does not allow eliminating the silver/aluminum pads on the rear side of the solar cell because PV module production requires the solar cells to be interconnected in-series via tinned copper tabs soldered to the silver/aluminum pads.
Several papers describe methods for printing very narrow fingers of ≦70 micron width (B. Raabe, F. Huster, M. McCann, P. Fath, HIGH ASPECT RATIO SCREEN PRINTED FINGERS, Proc. of the 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain; Jaap Hoornstra, Arthur W. Weeber, Hugo H. C. de Moor, Wim C. Sinke, THE IMPORTANCE OF PASTE RHEOLOGY IN IMPROVING FINE LINE, THICK FILM SCREEN PRINTING OF FRONT SIDE METALLIZATION, Proc. of the 14th European Photovoltaic Solar Energy Conference, 30.06-04.07 1997, Barcelona, Spain; and A. R. Burgers, H. H. C. de Moor, W. C. Sinke, P. P. Michiels, INTERRUPTION TOLERANCE OF METALLIZATION PATTERNS, Proc. of the 12th European Photovoltaic Solar Energy Conference, 11-15 Apr. 1994, Amsterdam, The Netherlands). Unfortunately, conventional fingers of ≦70 microns have narrow cross sections that are too small to handle the necessary level of electric current capable of being produced by the solar cell without excessive resistive losses. In order to achieve adequate finger conductivity it may be necessary to either apply a second layer of screen printed paste on top of the first one or to apply a layer of metal on top of an initial screen printed metallization, using galvanic technology. The resulting cost and complexity of these methods add a prohibitively high expense to the production of photocells.
Heretofore, there appears to be no simple way to produce a photovoltaic solar cell having reduced front side shading and no conventional screen printed silver/aluminum pads on the back side.
In accordance with one aspect of the invention, there is provided a photovoltaic apparatus. The apparatus includes a semiconductor photovoltaic cell structure having a front side surface and a back side surface provided by respectively doped portions of semiconductor material forming a photovoltaic junction. The apparatus further includes a plurality of electrical contacts embedded in the front surface of a respective one of the portions of semiconductor material, the electrical contacts being distributed in two dimensions across the surface and separated from each other and in electrical contact with the respective one of the portions of semiconductor material. The apparatus further includes a back side electrical contact on the back surface of the other of the respective portions of semiconductor material and in electrical contact therewith.
The electrical contacts may be distributed in two orthogonal directions across the surface.
The electrical contacts may be distributed evenly in the two orthogonal directions.
The electrical contacts may be arranged in an array.
The electrical contacts may be arranged in rows and columns.
Contacts of alternate rows may be arranged to lie in positions adjacent spaces between contacts in adjacent rows.
Generally each of the electrical contacts may have a contact surface facing generally normal to the front side surface and operable to be connected to a conductor.
The contact surface may have a generally rectangular shape.
The contact surface may have a generally circular shape.
The contact surface may have a star shape.
A solar cell apparatus may be made from the photovoltaic apparatus and may further include a first electrode for contacting the electrical contacts. The first electrode may include an electrically insulating optically transparent film having a surface, an adhesive layer on the surface of the film, at least one electrical conductor embedded into the adhesive layer, a conductor surface of the electrical conductor protruding from the adhesive layer, and an alloy bonding the electrical conductor to at least some of the electrical contacts such that current collected from the solar cell by the electrical contacts is gathered by the electrical conductor.
The electrical conductor may be connected to a common bus.
The electrical contacts may be arranged in rows and columns. The electrode may include a plurality of electrical conductors arranged in parallel spaced apart relation and the electrical conductors may be in contact with a plurality of the electrical contacts in a respective row or column.
Each of the electrical conductors may be connected to a bus.
The solar cell apparatus may further include a second electrode for contacting the back side electrical contact. The second electrode may include a second electrically insulating film having a second surface, a second adhesive layer on the second surface of the second film, at least one second electrical conductor embedded into the second adhesive layer, a second conductor surface of the second electrical conductor protruding from the second adhesive layer, and a second alloy bonding the second electrical conductor to the back side electrical contact such that current received at the solar cell from the back side electrical contact is provided by the electrical conductor.
In accordance with another aspect of the invention, there is provided a process for forming contacts in a semiconductor photovoltaic cell structure. The process includes distributing a plurality of individual portions of electrical contact paste in two dimensions across a front side surface of a semiconductor photovoltaic cell structure comprising respective doped portions of semiconductor material forming a photovoltaic junction; causing the individual portions of electrical contact paste to become embedded in the front side surface such that the individual portions of electrical contact paste form respective separate electrical contacts in the front side surface, the separate electrical contacts being in electrical contact with a corresponding doped portion of semiconductor material; and forming a back side electrical contact on a back side surface provided by the other of the respective portions of semiconductor material and in electrical contact therewith.
Distributing may include printing the individual portions of electrical contact paste on the front side surface.
Printing may include screen printing.
Distributing may include distributing the individual portions of electrical contact paste in two orthogonal directions across the surface.
Distributing may include distributing the individual portions of electrical contact paste evenly in the two orthogonal directions.
Distributing may include distributing the individual portions of electrical contact paste in an array.
Distributing may include distributing the individual portions of electrical contact paste in rows and columns.
Distributing may include causing the individual portions of electrical contact paste in alternate rows to lie in positions adjacent spaces between contacts in adjacent rows.
Causing the individual portions of electrical contact paste to become embedded in the front side surface may include heating the semiconductor photovoltaic cell structure with the portions of electrical contact paste thereon for a sufficient time and at a sufficient temperature to permit at least some of the electrical contact paste of each individual portion of electrical contact paste to enter a metallic phase and diffuse through the front side surface and into the portion of semiconductor material below the front side surface while leaving a sufficient portion of electrical contact paste in the metallic phase at the front side surface to act as an electrical contact surface of the separate electrical contact so formed.
The process may further include laying on the front side surface an electrode comprising an electrically insulating optically transparent film having an adhesive layer in which at least one electrical conductor is embedded such that a conducting surface thereof bearing a coating comprising a low melting point alloy protrudes from the adhesive layer, such that the conducting surface contacts a plurality of the electrical contacts formed in the semiconductor photovoltaic cell structure front side surface, and causing the low melting point alloy to melt to bond the conducting surface to the plurality of electrical contacts to electrically connect the electrical contacts to the electrical conductor to permit the electrical conductor to draw current from the solar cell through the electrical contacts.
The process may further include connecting the at least one electrical conductor to a bus.
The electrical contacts may be arranged in rows and columns and the electrode may include a plurality of electrical conductors arranged in parallel spaced apart relation. The electrode may be laid on the front side surface such that each electrical conductor is in contact with a plurality of the electrical contacts in a respective row or column.
The process may further involve connecting each of the electrical conductors to a common bus.
The process may further involve laying on the back side surface an electrode made of a second electrically insulating film having a second adhesive layer in which at least one second electrical conductor is embedded such that a second conducting surface thereof, bearing a second coating comprising a second low melting point alloy protrudes from the second adhesive layer, such that the second conducting surface contacts the back side electrical contact formed on the semiconductor photovoltaic cell structure back side surface and causing the second low melting point alloy to melt to bond the second conducting surface to the back side electrical contact to electrically connect the back side electrical contact to the second electrical conductor to permit the electrical conductor to supply current to the solar cell through the back side electrical contact.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In drawings which illustrate embodiments of the invention,
Referring to
Semiconductor Photovoltaic Cell Structure
Referring to
Process for Forming Electrical Contacts
Referring back to
The process may begin by printing the individual portions of electrical contact paste 157 on the front side surface 14 such as by screen printing. Printing may involve screen printing wherein a mask 150 having a plurality of openings 152 arranged in a desired distribution, such as in an array of rows and columns 154 and 156, for example, is made to receive an amount of electrical contact paste 157 containing aluminum, silver, adhesive and silicon, in a solvent. A spreader 158 is then drawn across the mask 150 such that the paste 157 is distributed in two dimensions across the front side surface 14 through the openings 152 in the mask 150.
The spreader 158 may be moved in two orthogonal directions at successive points in time, for example, to distribute the electrical contact paste 157 in the two orthogonal directions across the front side surface 14. Automated machinery may be used to cause the electrical contact paste 157 to be distributed across the front side surface 14, through the openings 152 in the mask 150.
Various opening shapes and arrangements may be employed in the mask 150 to distribute the electrical contact paste in any desired distribution such as evenly in the two orthogonal directions, unevenly in the two orthogonal directions, in an array, in rows and columns, in staggered rows in which alternate rows lie in positions adjacent spaces between openings in adjacent rows, in gaussian distributions in one/or two directions, in distributions providing an increasing density of openings toward one side and/or end of the mask or any other distribution.
After the electrical contact paste has been distributed, the mask 150 may be separated from the surface, leaving the distributed electrical contact paste in separate isolated islands as shown at 160, for example, in the desired pattern of distribution, i.e., rows and columns, even rows and columns, uneven rows and columns, staggered rows and columns, etc.
Then, the electrical contact paste 160 is heated until dry. When the paste 160 is dry, back side metallization paste 15 is applied to an entire back side surface 13 of the structure 11 and is heated until dry. When both the electrical contact paste 160 and the back side metallization paste 15 have dried, the individual portions of electrical contact paste 160 are caused to become embedded in the front side surface 14 such that the individual portions of electrical contact paste form respective separate electrical contacts in the front side surface 14 and the back side metallization paste 15 is fused into the back side surface 13. In the embodiment shown, this action is shown generally at 162 in which the semiconductor cell structure 11 with the distributed electrical contact paste 160 and back side metallization paste 15 thereon is passed through an oven 164 where it is heated for a sufficient time and at a sufficient temperature to permit a small portion of the electrical contact paste of each individual portion of electrical contact paste to enter a metallic phase and diffuse through the front side surface 14 and into the semiconductor photovoltaic cell structure below, while leaving a sufficient portion (nearly all) of electrical contact paste 160 in the metallic phase exposed at the front side surface 14.
The electrical contact paste 160 forms electrical contacts 16 in the front side surface 14, the electrical contacts being in electrical contact with the n-type semiconductor material beneath the active side surface, but separate from other contacts. Each electrical contact 16 has an electrical contact surface 37 formed by the portion of electrical contact paste 160 in the metallic phase left on the front side surface 14. The electrical contacts 16 are thus intermittently positioned across the front side surface 14.
Similarly, the back side metallization paste 15 is fused to a back side surface 13 of the semiconductor photovoltaic cell structure 11 thereby creating a back surface field and provides a back side electrical contact 17.
In the embodiment shown, the oven 164 has an outlet 166 through which a completed semiconductor photovoltaic cell apparatus 12, having a front side surface 14 with a plurality of separate electrical contacts 16 embedded therein and a back side electrical contact 17 comprising a single large contact fused therein is provided.
Semiconductor Photovoltaic Cell Apparatus
As a result of the process shown in
Referring to
In the embodiment shown the electrical contacts 16 are distributed in two orthogonal directions, shown generally at 30 and 32 and, in this embodiment, they are distributed evenly in these two directions. In other words, the spacing between the contacts in the first direction 30 is uniform and the spacing between the contacts in the second direction 32 is also uniform. In the embodiment shown, the contacts are arranged in rows and columns, a first row being shown generally at 34 and a first column being shown generally at 36. The contacts are thus arranged in an array in this embodiment.
Alternatively, other distributions of contacts may have been laid by the mask 150 shown in
In the embodiment shown, the electrical contacts 16 have an electrical contact surface 37 having an elongated rectangular shape, having a length 38 of between approximately 0.5 mm to approximately 2 mm and a width 40 of between approximately 0.1 mm to 1 mm. In the embodiment shown, each contact surface 37 has generally the same length and width dimensions and is oriented in generally the same direction, i.e., aligned in the first orthogonal direction 30. It will be appreciated that each contact 16 is physically isolated in that it is set apart from each other electrical contact. However, each contact 16 is also in electrical contact with the n-type material under the front side surface 14 to make electrical connection with the semiconductor photovoltaic cell structure 11. Therefore, while the electrical contacts 16 appear physically separate when viewed from the front side surface 14 of the solar cell structure, they are in fact electrically connected to the semiconductor photovoltaic cell structure beneath the front side surface 14. In one sense, the contacts 16 appear to be intermittent “fingers” across the front side surface 14 rather than continuous linear fingers as in the prior art.
Referring to
Referring back to
These distances may be equal or different. Again, alternatively, the contacts 52 may be distributed across the front side surface 14 with increasing density in the first and/or second directions 30 and 32 or more generally with constant or changing density in these two directions.
As stated, each electrical contact 52 has a circular contact surface 53, having a diameter 62 of approximately 1 millimetre. Again, each electrical contact 52 is embedded in the front side surface 14 and into the n-type layer 20 of the semiconductor photovoltaic cell structure 11. Circular openings in the mask 150 described in
Referring to
Referring to
Referring to
Solar Cell Unit
Referring to
In the embodiment shown in
In the embodiment shown, the alloy bonding the electrical conductor 100 to at least some of the electrical contacts may include a material that may be heated to solidify and electrically bond and connect the electrical conductor 100 to a plurality of electrical contacts 72 in a row. The alloy may be a coating on the conductor surface 102, for example.
As shown in
Initially, the first electrode 92 may be curled as shown in
Alternatively, the rear edge 106 of the first electrode 92 may be aligned with a right hand side edge 124 of the semiconductor cell apparatus 12 and rolled out across the front side surface 14 of the semiconductor cell apparatus in a manner such that the conductors 100, 112, 114 and 116 contact a plurality of electrical contacts 72 in a respective row of electrical contacts 72 on the front side surface 14 of the semiconductor cell apparatus 12.
In the embodiment shown, the electrical conductors 100, 112, 114 and 116 extend beyond the optically transparent film 94 and are terminated in contact with a common bus 107, which may be formed of metallic foil, such as copper, for example.
Further details of general and alternate constructions of the first electrode 92 may be obtained from applicant's International Patent Application published under International Publication Number WO 2004/021455A1, which is incorporated herein by reference.
The second electrode 93 is similar to the first electrode 92 in all respects and in fact a plurality of the above described first electrodes may be pre-manufactured and individual ones applied to the front side surface 14 or back side electrical contact 17 as desired. It should be noted however that the second electrode 93 need not be optically transparent like the first electrode since the back side is not intended to receive light.
The back side electrical contact 17 has no rows of contacts, but rather is a single flat planar contact extending across the entire back side surface 13 of the semiconductor cell structure. The conductors 100, 112, 114 and 116 of the second electrode 93 are prepared with the low melting point alloy paste and the electrode 93 is adhesively secured to the back side electrical contact 17 such that the low melting point alloy is operable to bond the conductors to the back side electrical contact 17 when sufficiently heated.
As shown in
After the first electrode 92 is laid on top of the front side surface 14 such that the conductors 100, 112, 114 and 116 contact respective columns 36, 118, 120 and 122 of contacts 72, for example, and the second electrode 93 is laid on the back side electrical contact 17, the resulting apparatus may be regarded as an assembly. The assembly is then heated such that the low melting point alloy associated with the first electrode 92 is caused to melt to bond the conducting surfaces of respective conductors 100, 112, 114 and 116 of the first electrode 92 to contact surfaces of respective rows of electrical contacts 72 to electrically connect the electrical contacts to the electrical conductors and to cause the low melting point alloy associated with the second electrode 93 to bond the conducting surfaces of respective conductors to the back side electrical contact 17, to permit the electrical conductors to pass current through the solar cell through the electrical contacts. Once the low melting point alloy has completed this bonding, a completed solar cell as shown at 10 in
A solar cell produced as described above may provide several advantages. Due to the reduced area occupied by the electrical contacts in the front side surface, there is less shading of the p-n junction which can cause as much as 5-10% more electric current to pass through the solar cell. In addition, as there is less area occupied by metallization and the back surface field area is not interrupted by silver/aluminum fingers, the cell can generate a voltage of up to 3% more than conventional cells. Overall these two effects may increase the efficiency of the solar cell by up to 10-15%. Furthermore, the production costs of solar cells of the type described are lower than with conventional solar cells because a substantially less amount of silver is used in forming contacts.
While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.
