WO2011054037A1

WO2011054037A1 - Method and apparatus for light induced plating of solar cells

Info

Publication number: WO2011054037A1
Application number: PCT/AU2010/001461
Authority: WO
Inventors: Alison Joan Lennon; Yu Yao; Kian Fong Chin; Denis Joseph Coyle; Budi Tjahjono; Stuart Ross Wenham
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2009-11-03
Filing date: 2010-11-03
Publication date: 2011-05-12

Abstract

A light induced plating system (LIP) for creating front contacts on a photovoltaic is disclosed. The system involves the use of a patterned metal electrode with a plurality of raised contact areas that are in electrical contact with electropositive material of the rear metallisation of the photovoltaic. The plurality of raised contact areas has a total area less than a total area of the electropositive material when contacted by the patterned metal electrode. The patterned metal electrode may act as either anode or a cathode depending on the arrangement of the LlP system. The patterned electrode may comprise the same metal as the metal that is to being deposited on the front (light incident) side of the solar cell, so as to create a sacrificial anode. A conveying belt system may be used to mount the patterned metal electrode so as to assist in automation of the plating process.

Description

METHOD AND APPARATUS FOR LIGHT INDUCED

PLATING OF SOLAR CELLS

Copyright Notice

A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.

Technical Field of the Invention

The present invention relates generally to the field of device fabrication and, in particular, to the light-induced plating of metal contacts for solar cells.

Background of the Invention

. Solar cells in general require two metal electrodes or contacts to the semiconductor material, one of each polarity, to allow light-generated charge from within the solar cell to be extracted and flow in external electrical wires as electricity. Most solar cells have one polarity of contact on the top surface and the opposite polarity metal contact on the rear surface. For example, in general, silicon solar cells have different metals for the front and rear contacts due to the different requirements when contacting n-type and p-type silicon. In addition, other attributes of the metal such as its electrical conductivity and thermal expansion coefficient as well as its cost affects whether a metal is suitable and/or preferable as a metal contact on one or the other of the solar cell surfaces. In general, most silicon solar cells use different metals on a light receiving surface where shading losses, metal conductivity and contact resistance to the semiconductor are / particularly important. On a non-light receiving surface where higher metal coverage and lower conductivity can be tolerated and where the polarity is opposite to the light receiving surface, other metals are usually preferable. For this reason, most screen-printed silicon solar cells use high conductivity silver, despite its high cost, for the n-type front metal grid contacts and cheaper aluminium to cover most of the p-type rear surface (as the p-type metal contact).

Metal contacts can be formed to solar cells via various metal plating processes, which include electroless plating, electro-plating or light-induced plating (LIP). In LIP the current generated by the solar cell, when exposed to light, is used to drive or contribute to the driving of the plating of metal from metal ions in a solution to exposed n-type material. In the case of silicon solar cells, the n-type silicon is typically exposed at the base of grooves formed in a front-surface dielectric layer that acts as an antireflection coating for the cell. The LIP process typically requires that the rear surface of the cell either acts the anode or is electrically contacted to an anode. The anode is then oxidised to maintain the source of metal ions for the plating process. This process is described in detail by Lawrence Durkee in US Patent 4,144,139 "Method of Plating by Means of Light".

The method described by Durkee requires that rear metal contact of the cell acts as an anode. This restricts the solar cell design to one that utilises the same metal for both the n-type (front surface) and p-type (rear surface) contacts. Most current commercial silicon solar cells use aluminium as the rear surface metal contact of the cell because of its low cost, high tensile strength, high electrical and thermal conductivity. The aluminium is typically applied by screen printing a layer of aluminium paste on the rear of the cell and then briefly firing the resulting cell at temperatures of 700-800°C to create a back surface field.

A second limitation of the method described by Durkee is that it causes corrosion of the rear-surface metal towards the edges of the device due to its closer proximity to where the metal is to be deposited onto the front n-type surface. This corrosion leads to deterioration in the electrical conductivity of the rear metal contact towards the edges of the solar cell. Both of these limitations are unacceptable when fabricating high efficiency solar cells, with virtually all current commercial solar cells requiring different metals for the two polarities of metal contacts.

Other implementations of LIP for solar cells eliminate the problem of corrosion of the rear surface by either preventing the rear surface from entering the plating solution and electrically contacting the rear surface of the solar cell via wires to an external anode. However, such solutions result in complex systems which require significant handling of the solar cell resulting in high system cost and higher than desirable rates of solar cell breakage. Especially for silicon solar cell fabrication, there is a trend towards using thinner silicon wafer substrates in order to reduce the material expense of the cells. This means that plating techniques which require extensive handling in order to keep the rear metal contact of the cell dry and/or electrically connect the rear metal contact to a separate anode are undesirable and result in a high system cost. Summary of the Invention

According to a first aspect of the invention, a method of depositing metal over a region of n-type semiconducting material of a solar cell, the solar cell comprising a p-n junction with an electropositive metal in electrical contact with a p-type semiconductor material of the solar cell, the method comprising:

i. electrically contacting a patterned electrode to the electropositive metal, the ^• patterned electrode comprising a plurality of raised contact areas having a total area less than a total area of the electropositive metal contacted by the patterned electrode; ii. placing the solar cell and patterned electrode in an electrolyte comprising ions of a metal to be deposited; and

iii. illuminating the solar cell, said illumination resulting in the depositing of metal over the region of n-type material.

According to a second aspect of the invention, a patterned electrode comprising a planar metal member and a plurality of raised contact areas arranged to electrically contact an electropositive metal electrical contact of a solar cell, the plurality of raised contact areas having a total area of less than a total area of the electropositive metal when contacted by the patterned electrode.

According to a third aspect of the invention, a plating system comprising:

i. a patterned electrode having a planar metal member and a plurality of raised contact areas arranged to electrically contact an electropositive metal electrical contact of an anode of a solar cell, the plurality of raised contact areas having a total area of less than a total area of the electropositive metal when contacted by the patterned metal anode;

ii. a continuous belt to which the patterned electrode is attached;

iii. a bath comprising an electrolyte including ions of a metal to be deposited onto a p-type surface of the solar cell, the belt passing through the electrolyte to submerge the patterned electrode and the solar cell in the electrolyte;

iv. a light source located to illuminate the solar cell when carried on the patterned electrode through the electrolyte.

Preferably less than 5% of a total area of the electropositive metal is contacted by the patterned electrode..

The raised contact areas of the patterned electrode may comprise a pattern of pins extending generally perpendicularly from a planar surface of the patterned electrode and the pins may have a tip surface area of 0.1 to 0.5 mm². Alternatively the raised contact areas of the patterned electrode may comprise a pattern of ridges projecting from a planar surface of the patterned electrode. The method may also comprise exposing the region of n-type semiconductor material of the solar cell, whereby the illumination results in the depositing of metal onto the region of n-type material. The n-type region may be exposed by making an opening through though an antireflection coating and the metal to be deposited may be selected from one of nickel, copper and silver.

Alternatively the region of n-type semiconductor material of the solar cell may be pre-plated with a metal barrier layer, and the method may comprise depositing a metal layer onto the barrier layer. In this case the metal to be deposited may be copper.

The patterned electrode may be carried on a belt and the belt may pass the patterned metal anode and solar cell through a bath containing the electrolyte and simultaneously under a source of the illumination. The belt will preferably carry a plurality of patterned electrodes each carrying a solar cell.

Each patterned electrode may be shaped so that when a section of the belt carrying one of the patterned electrodes passes through the electrolyte, electrolyte is deflected toward an n-type material surface of a solar cell carried on the patterned electrode from a surface of a previous patterned electrode- exposed to the electrolyte. A leading edge of each patterned electrode may be shaped to deflect the electrolyte by movement of the patterned electrode through the electrolyte, such as by being a wedge shaped.

The electropositive metal surface of the solar cell may be covered by a layer of water resistant polymer, such that the raised areas of the patterned electrode make electrical contact to the solar cell through the water resistant polymer layer.

The metal being deposited may be selected from one of nickel, copper, tin and silver.

The patterned electrode may be carried on a belt such that the belt passes the patterned electrode and solar cell through a bath containing the electrolyte and simultaneously under a source of the illumination. The solar cell is preferably maintained in electrical contact with the raised contact areas of the patterned electrode while immersed in the electrolyte by a mechanism involving downward pressure. The pressure may be applied by removal of electrolyte from below the solar cell and/or flowing electrolyte over the cell and the patterned electrode may contain one or more openings through which electrolyte is removed by a pumping mechanism.

The belt will preferably carries a plurality of patterned electrodes in series, and each patterned electrode may be shaped so that when a section of the belt carrying one of the patterned electrodes passes through the electrolyte, electrolyte is deflected toward an n-type material surface of a solar cell carried on the patterned electrode from a surface of a previous patterned electrode exposed to the electrolyte. This may be achieved by shaping a leading edge of each patterned electrode to deflect the electrolyte by movement of the patterned electrodes through the electrolyte.

One or more regions of n-type semiconductor material of the solar cell are preferably exposed to illumination while immersed in the electrolyte, whereby the illumination results in the depositing of metal onto the region of n-type material. The one or more n-type regions may be exposed to illumination though an openings in an reflection coating. The one or more regions of n-type semiconductor material of the solar cell may also be plated with a metal barrier layer, whereby the illumination results in the depositing of a metal layer onto the barrier layer.

In one embodiment the patterned electrode may operate as a metal anode. In that embodiment the patterned electrode is preferably composed, at least in part, of the metal being deposited to the exposed region of n-type material. The patterned electrode may be formed of a base material which is then plated with a metal to be deposited, which may comprise either a metal or a plastic material such as stainless steel or acrylonitrile butadiene styrene. The metal to be deposited may be selected from one of nickel, silver and copper. The metal may be deposited onto a plurality of solar cells by repeatedly reusing the patterned electrode and the patterned electrode may be periodically replated between uses.

Alternatively, in another embodiment, the patterned electrode may operate as a cathode, which is preferably electrically connected to a separate anode via a voltage source, the separate anode being in contact with the electrolyte. A potential applied between the patterned electrode and the separate anode by the voltage source is preferably arranged to reverse bias the solar cell. All surfaces of the patterned electrode except for the raised areas may in this case be sealed from the electrolyte, such as by being covered with an organic sealing material. The organic sealing material may be applied in liquid form and then cured to form a barrier which is impermeable to the electrolyte.

The electropositive material may be selected from one of aluminium, alloys of aluminium, zinc and alloys of zinc, tin and alloys of tin.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1A is a diagrammatic representation of a solar cell, shown in cross- section, which is used as the plating substrate for the preferred arrangement with silicon exposed in front-surface grooves;

Figure IB is a diagrammatic representation of the arrangement of front-surface grooves on a solar cell substrate for a finger-busbar metal contact pattern;

Figure 2 is a flowchart depicting the process of the preferred arrangement;

Figure 3A is a diagrammatic representation of a section of the plating belt used for the preferred arrangement;

Figure 3B is a diagrammatic representation of the use of side supporting arms to maintain a solar cell in contact with a patterned electrode unit;

Figure 3C is a diagrammatic representation of a patterned electrode unit in a cathode configuration and using a local pumping unit to maintain the solar cell in contact with the raised regions of the patterned electrode;

Figure 3D is a diagrammatic representation of an arrangement of pumping tubes that can be used in conjunction with the apparatus shown in Figure 3C;

Figure 4 is a diagrammatic representation showing how the structure of patterned electrode units on the plating belt of the preferred arrangement can be used to channel the flow of metal ions, dissolved from one electrode, to the cathode surface of the solar cell substrate following on the plating belt;

Figure 5A is a diagrammatic representation surface of the solar cell whilst a hydrated oxide layer protects the remaining rear surface from corrosion by the plating solution;

Figure 5B is a diagrammatic representation showing a variation of the arrangement depicted in Figure 5A in which the patterned electrode includes a sealing perimeter;

Figure 5C is a diagrammatic representation showing another variation of the arrangement depicted in Figure 5A in which the rear surface of the solar cell is further protected by a layer of encapsulating material;

Figure 6 is a diagrammatic representation showing how the plating belt proceeds through a plating bath illuminated by a light source placed above the bath;

Figure 7 is a flowchart depicting an alternative metallisation process;

Figure 8 is a diagram showing the arrangement of anodes when the patterned electrode unit is used in the cathode configuration;

Figure 9 A is a diagram showing the patterned electrode operating as an anode;

Figure 9B is a diagram showing the patterned electrode operating as a cathode; and

Figure 10 is a table showing the molar ratios of plated metal to released aluminium for different patterned electrode configurations. Detailed Description of embodiments

A method and apparatus for the LIP of solar cells is disclosed which enables the plating of metal contact regions on exposed n-type material (metal contact regions) without requiring the p-type electrode of the solar cell to be formed of the same metal. So, for example, conductive metals such as nickel and copper can be used to form metal contacts to n-type semiconductor regions of the solar cell whilst still using low- cost aluminium to contact the p-type semiconductor regions of the cell. For most currently produced silicon solar cells, the n-type metal contact regions are formed on the illuminated side of the solar cell and the p-type contacts are formed on the rear of the cell. Unlike previously described LIP systems for' solar cells having a rear surface metal surface which is different from the metal to be used for the front contacts, the method and apparatus described herein does not require that the rear of the cell be kept out of the solution of metal ions used for plating the front metal contacts of the cell. This enables a significantly simpler plating apparatus to be used and therefore reduces both system cost and costs associated with solar cell breakage due to handling.

The preferred method and apparatus for LIP are described by way of example with respect to the LIP of nickel (as a barrier layer) and then copper to form metal contacts to regions of n-type silicon exposed in grooves patterned in a front-surface dielectric layer of silicon wafer solar cells. The cells being metalised have screen- printed aluminium rear metal electrodes which have been fired at ~ 800 °C for 1-2 minutes. However, it should be apparent that the method and apparatus could also be applied to fabrication of other cells, comprising different semiconductor material(s), different n-type metal electrode materials (e.g., nickel and silver), and other similarly electropositive p-type electrode metals (e.g., aluminium alloys, zinc, tin). The method relies on the formation of a protective metal oxide layer that typically forms spontaneously on the surface of electropositive metals such as aluminium. This protective metal oxide layer can be exploited to provide corrosion protection of the underlying rear metal layer whilst still enabling low-resistance electrical contact to be formed to the solar cell via an array of raised contact regions formed on a conductive electrode. The raised contact regions are preferably constructed as point (or pin) contacts, although other patterns of selective contacting such as raised linear regions (i.e., ridges) can also be employed. Electrodes having such raised regions for selective contacting are referred to hereinafter as patterned electrodes. A patterned electrode can be configured to operate as either the anode or cathode for the LIP process as shown in Figure 9 A and Figure 9B, respectively. For clarity, these schematic drawings do not show the method of ensuring that the solar cell maintains good electrical contact between its rear surface and the patterned electrode.

In the preferred arrangement, n-type metal contacts are formed on the illuminated surface of a silicon solar cell. The solar cell has a layer of metal screen- printed over its entire rear p-type surface. In alternative arrangements, different cell designs can also be used. For example, the rear p-type surface can be passivated with a dielectric layer (e.g., silicon dioxide, silicon nitride, and aluminium oxide), and the rear metal contact may only contact the p-type surface of the solar cell via a pattern of point openings in the dielectric layer. This arrangement is advantageous because it reduces the metal-silicon interface area and therefore reduces the total recombination of carriers at the rear surface. Furthermore, the method can also be adapted for cell designs where both the n-type and p-type contacts are to be formed on the same surface, however in these cases the metal contacting the p-type semiconducting regions must be appropriately patterned and positioned on the surface of the solar cell so that the n-type regions can be illuminated during the LIP process. Typically, for solar cells involving same-surface contacts, the surface which lacks metal contacts is designed to be the illuminated surface and is protected with an antireflection coating.

As mentioned above a patterned electrode which makes electrical contact to the surface of the solar cell's p-type metal contact via small-area contact points can be implemented as an anode or a cathode in a LIP system. In both cases the patterned electrode can be integrated into an inline process thus enabling higher processing throughputs than may otherwise have been possible using a batch process. In the preferred arrangement, a plurality of patterned electrodes are integrated into a conveying unit, or belt, which passes through the metal plating electrolyte. Solar cells can be placed on the patterned electrode units either manually or, more preferably, by automated cell handling equipment, and held in place to ensure low-resistance electrical contact in the small-area raised contact areas whilst the remaining aluminium surface is protected by a barrier oxide layer. Preferably the solar cells can be maintained in electrical contact with the raised areas of the patterned electrode by a suction or pumping mechanism which involves the displacement of electrolyte through small openings made in the patterned electrode in such a way as to create lower pressure regions under the solar cell. Alternatively, other methods which involve the application of downward pressure on the top surface of the wafer can be used. The belt can then move through the plating electrolyte in a plating bath, with the belt speed and length of the plating bath determining the plating time. When the patterned electrode operates as an anode, the electrode surface oxidises (i.e., corrodes) and releases a source of metal ions in the plating electrolyte. Preferably the anode surface is composed of the same metal that is being plated to the n-type surface as this increases the lifetime of the plating electrolyte. However, the patterned anode can either be composed of or at least have a surface layer of one or more other metals that can be oxidised when current flows through the solar cell on illumination. There is no external voltage required when the patterned electrode operates as the anode, because in this case the operating current and voltage of the LIP process is determined by the electrical properties of the solar cell, the electrolyte resistance and interfacial effects associated with the anode and cathode reactions.

The patterned electrode is consumed when it operates in the anode configuration. Rather than having a requirement 'to periodically replace patterned electrodes it is preferable to form the patterned electrode using a base material (such as stainless steel) or a moulding plastic such as acrylonitrile butadiene styrene (ABS) and then electroplate the base material with the required metal (i.e., preferably the metal that is to be plated to the exposed n-type regions of the solar cell). Preferably, the patterned electrodes are readily removed from the plating belt and hence can be periodically re-plated. Electroplating of the anode metal on the patterned electrode ensures a high-purity anode surface.

Furthermore, the patterned electrodes are preferably designed in such a way that they facilitate transfer of the, metal ions from the anode surface to the plating solution which comes in direct contact with the front (cathodic) surface of the solar cell. This facilitated flow of metal ions from the anode surface ensures a uniform metal ion concentration across the solar cell front surface and thus uniform metal plating rates across the cell. In alternative applications, patterned electrode can be designed to provide enhanced plating rates in certain areas of the solar cell (e.g., busbar regions).

In a cathode configuration, the role of the patterned electrode becomes one of providing a low-resistance electrical contact to the rear surface of the solar cell. In this case, the patterned electrode is not oxidised but is in direct electrical connection with an external voltage source and a separate sacrificial anode which must be placed in contact with the electrolyte. The external voltage source can be used to apply a bias to the patterned electrode, the bias being applied in a direction such that it reverse biases the solar cell's pn-junction (diode). This applied potential has the advantage that it can drive the operating point of the solar cell close to short circuit and in doing so increases the cell current and hence plating rate. Furthermore, by reverse-biasing the solar cell the potential over the solar cell is reduced and consequently the tendency for the rear surface aluminium to corrode is reduced. This provides a further mechanism by which undesired contamination of the plating bath with aluminium ions is minimised. For this reason, the preferred arrangement described below- uses the patterned electrode in the cathode configuration.

A preferred arrangement for LIP will now be described with reference to

Figures 1 to 7 and the process of forming front metal contacts to a silicon solar cell. The solar cell to be plated 100 comprises a silicon wafer in which a p-n junction has been formed using a diffusion process. For example, a p-type silicon wafer 105 can be diffused with phosphorous dopants to form an n-type layer 110 on the front surface of the solar cell 100. This doped layer is typically referred to as the emitter layer of the solar cell. Then a dielectric layer 115, composed of silicon nitride or silicon dioxide, is either grown or deposited onto the n-type surface to both passivate the silicon and provide an antireflection coating for the solar cell 100. Silicon nitride is more commonly-used as an antireflection coating for industrial solar cells than silicon dioxide because its deposition can be achieved at lower temperatures (e.g., 400 °C compared to ~ 1000 °C for thermally-grown oxides) and therefore it is more suited to lower-quality silicon substrates such as multi-crystalline wafers. Typically the silicon nitride is deposited by plasma enhanced chemical vapour deposition (PECVD) and the thickness of the deposited layer is ~ 75 nm.

An aluminium paste is screen printed over the rear surface of the solar cell 100 and fired at 800 °C for 1-2 minutes to form a back surface field and a rear aluminium electrode 130 for the solar cell 100. Preferably the thickness of the aluminium layer is 20-25 μιη. Alternatively, the rear electrode can be formed by evaporating 1-5 μηι of aluminium on to the rear surface and then sintering for ~ 30 mins at 400 °C. Various alloys, typically comprising aluminium by part can also be used as the rear electrode material. >

Groove openings (e.g., 120) are then formed in the front-surface dielectric layer 115. These openings can be formed using a laser or by using a dielectric patterning technique such as photolithography or inkjet patterning. In the case of forming the openings 120 by a laser, the front surface can first be covered by a source of phosphorus dopants (e.g., phosphoric acid). The process of laser scribing then locally and transiently melts the dielectric layer 115 and the underlying silicon so that the phosphorus atoms can diffuse into the silicon to form a heavily-doped region under the groove after re-solidification. This laser-patterning approach allows for a lightly diffused n-type layer, or emitter 110 to be used for the solar cell 100 in non-metallised areas, which increases the current collected from the absorption of the short- wavelength (blue) light.

Alternatively groove openings 120 can be etched using photolithography or inkjet patterning of resist, followed by immersion etching of the dielectric layer 115. In this case heavily-doped regions can be formed by subjecting the solar cell 100 to a further diffusion process in which the patterned dielectric layer 115 acts as a mask to restrict the diffusion of dopants to only the silicon areas exposed by the grooves.

Figure IB shows a simple front-grid groove pattern consisting of: (i) a plurality of finger grooves (e.g., 120) which once metallised collect current from the solar cell 100; and (ii) a busbar region 125, from where electrical contact to the front-surface grid contact can be made once the grid pattern is metallised. The optimal spacing between the finger grooves 120 depends on the doping level of the n-type emitter layer 110, with the fingers being spaced more closely for lightly-doped layers (e.g., 1 mm apart for 120 Ω/D emitters) than for more heavily-doped layers (e.g., 2.8 mm apart for 40 Ω/D emitters).

In order to complete the solar cell 100, metal must be deposited in the regions exposed at the base of the grooves 120. Copper is an attractive metal to use because of its high conductivity, low cost and its ability to be readily plated from a solution of copper ions. However, copper atoms can readily diffuse into the silicon material of the solar cell 100 and form recombination centres which severely degrade the performance . of the solar cell 100. Consequently, nickel is commonly used as a barrier metal and is deposited in a layer at the base of the grooves which is thick enough 1-3 μπι to prevent the diffusion of copper ions into the silicon. Preferably, the nickel is sintered for 2 - 30 minutes at ~350 °C to form a nickel silicide alloy which further reduces the contact resistance at the base of the grooves, with the optimal length of the sintering time depending on the depth of the p-n junction at the base of the grooves. Preferably, both the nickel and copper are deposited using the LIP process described below and in reference to Figure 2. In alternative arrangements, the nickel can be deposited using electroless nickel plating or by a printing method such as screen-printing or aerosol jet printing as described elsewhere.

The solar cell 100 depicted schematically in Figure 1A and Figure IB represents the substrate for the LIP process 200 shown in Figure 2. During the LIP process, when the solar cell 100 is illuminated, electrons from the generated electron- hole pairs diffuse to the n-doped silicon regions at the base of the grooves 120 (cathode) and become available for the reduction of metal ions. Similarly, the generated holes will diffuse to the rear aluminium surface of the solar cell 100. Left exposed in a metal plating solution, in the presence of plating additives such as chloride and bromide ions, the rear surface aluminium will oxidise and thus act as the anode reaction. Aluminium is a highly electropositive element and therefore tends to passivate (or oxidise) very readily. So much so that a thin aluminium oxide layer of thickness ~ 4 nm forms immediately on exposure of the aluminium to air or humidity. Although this native oxide layer serves to protect the aluminium, plating additives can act as corrosion agents and initiate pitting corrosion which can ensure that the plating solution maintains access to aluminium metal. The aluminium oxide formed will either remain attached to the rear of the solar cell 100 or dissolve into the plating solution depending on the pH of the solution. In either case, leaving the aluminum rear surface exposed to the solution is undesirable because it degrades the quality of the metal contact and it contaminates the plating solution with aluminium ions which may interfere with the plating of the front-side metal contacts. The described embodiments eliminate these problems by using a patterned electrode to make low-resistance electrical connections to the rear of the cell via small-area raised contacting regions (e.g., pin) whilst using the native oxide layer of aluminium to protect the remaining aluminium surface.

The LIP process of the preferred arrangement 200 will now be described with reference to Figure 2. In step 205, the solar cells 100 are deglazed to remove any native oxides that may have formed in the base of the grooves 120. Preferably, the deglazing is achieved by batch immersion of the solar cells 100 in a solution comprising 1% hydrofluoric acid (HF) in deionised water for 15 to 45 seconds, and more preferably for 20 seconds. This deglaze step also removes any native oxide from the aluminium rear of the solar cell 100.

The solar cells 100 are rinsed for 1 minute in step 210 and then transferred to a plating belt 300 which is depicted in Figure 3A and in step 215 of Figure 2. The plating belt 300 is preferably a meshed structure 305 with criss-crossing metal elements which are protected by being encased in a corrosion-resistant plastic layer, such as teflon, polyethylene, or polypropylene. Alternatively the meshed structure can be formed in one of the corrosion-resistant plastics mentioned above. The meshed structure 305 contains periodically placed slots 325 for the placement of replaceable patterned electrode units 310, with each patterned electrode unit 310 having corner pins 320 which can be inserted into the slots 325. Alternatively, patterned electrode units 310 can be fixed to the meshed structure 305 using corner screws which are then fastened using nuts placed under circular loops in place of the slots 325. All metal components are preferably protected by corrosion resistent (e.g. plastic) materials in order to prevent spurious and undesirable plating of bath components. The plating belt 300 is designed to move through a plating^'bath supporting a number of solar cells 100, each in electrical connection to a separate patterned electrode unit 310.

Each solar cell 100 is placed on the raised contact areas, or pins 315 of a patterned electrode unit 310 as shown in Figure 5 A immediately after the rinse step 210 in order to minimise the thickness of the formed native aluminium oxide layer 505 (as shown in Figure 5A). Even if a thin layer of aluminium oxide does form, the pins 315 can penetrate the thin layer once lower pressure regions are generated under the solar cell 100 as described in more detail below. The pins 315 preferably have a tip contact area of between 0.1 and 0.5 mm², and more preferably 0.1 mm², and a height of between 50 μιη and 2 mm, and more preferably ~ 500 μπι. The optimum spacing of the pins 315 on the patterned electrode unit 310 depends on the characteristics of the solar cell 100 and the tip contact area of the individual pins. Preferably, the pitch of the pins 315 is between 1 to 10 mm, and more preferably, 5 mm. A regular spacing of pins is used, however in alternative arrangements closer spacings of pins 315 can be employed to provide additional protection of the rear surface or collection of non-uniform current from the rear surface of the solar cells 100.

Preferably, the percentage of the total aluminium surface area 130 of the solar cell 100 that is contacted by the pins 315 is between 0.1 and 5% and more preferably ~ 1%. In alternative arrangements, substantially linear contact regions can be used instead of pin (point) regions. This can be achieved by. patterning a sequence of grooves in the patterned electrode units 310 with the ridges between adjacent grooves acting as the contact regions. Preferably, the percentage of the total aluminium electrode area of the solar cell 100 that is contacted by the ridge contact regions is maintained below 5%, and more preferably ~ 1%. Clearly, other contact patterns can also be used as long as they can effectively maintain a low-resistance electrical connection between the patterned electrode unit 310 and the aluminium surface of the solar cell 100 and allow a protective oxide to form on the non-contacted regions.

In further arrangements the pin contacts 315 can be provided by spring-loaded pin units. This arrangement is advantageous when there is significant variation in the thickness of the rear metal layer 130 of the solar cell 100 or in cases where the solar cell 100 may have become slightly bowed or warped due to the aluminium firing process. The spring-loaded pins can vary their height depending on the resulting downward force at each pin location.

Preferably, each patterned electrode unit 310 is formed using a strong, non- deformable and inexpensive metal such as stainless steel or copper. In the case of the preferred cathode configuration, all surfaces except the pins 315 are sealed with a water-impermeable material, such as silicone or polyurethane to eliminate unwanted metal plating , on the electrode unit itself (which can occur when it is operated in the cathode configuration). This is shown in Figure 3C, where the sealing layer 372 is formed over all surfaces except the pins 315, which are exposed to the plating electrolyte 390. Preferably, this sealant is applied in liquid form and then cured to form an impermeable barrier to the electrolyte 390.

Alternatively the patterned electrode can be constructed from numerous plastic materials that demonstrate good resistance to acidic plating electrolytes. Plastics such as Teflon, polypropylene or ABS can be used. The pins 315 can then be inserted through the plastic material in the preferred pin spacing configuration and contact a metal plate at the base of the patterned electrode unit 310. In the cathode configuration, the metal plate is sealed as described above and makes contact to the external voltage supply. In the anode configuration, the metal plate acts as the anode for the LIP process and can be periodically replaced once it has been consumed.

In another variation depicted in Figure 5B, the patterned electrode unit 310 can also comprise perimeter seals 530 which serve to further restrict ingress of plating electrolyte to the region under the rear surface of the solar cell 100. These perimeter seals can be composed of a sealing material such as silicone or be formed from a base metal or plastic material and be subsequently coated using a sealing material. The perimeter seal 530 needs to be formed of a deformable material so that it can accommodate small variations in wafer planarity (e.g., bowing) that can result during the previous aluminium firing, step.

In yet another variation depicted in Figure 5C, before being placed in contact with the pattern electrode unit 310, the rear surface of the solar cell 100 is coated with a layer of the material that will be subsequently used to encapsulate the solar cell 100 in a module. This layer of encapsulating material 540 shown in Figure 5C, can be applied immediately after the wafer is deglazed in step 210 of process 200 depicted in Figure 2, thus resulting in only a thin layer of native oxide 505. The thickness of the encapsulating layer is maintained at less than 1 mm and more preferably less than 0.5 mm, to ensure that the pins 315 can permeate the encapsulating layer 540 and make electrical contact to the solar cell 100 when it is placed in contact with the patterned electrode unit 310.

The encapsulating material 540 may comprise materials such as ethylene vinyl acetate (EVA) which is commonly-used for. encapsulating silicon solar cells, silicones, polyesters, polyurethanes, polyamides, and polyolefins. Preferably, the material selected for the encapsulating layer 540 is the same or very similar to that which will be used to encapsulate the completed solar cells in the module. It is preferable to use a thermoplastic encapsulating material to^' enable cell interconnections to be made after the LIP process. For example, if cell interconnections are to be made by soldering interconnects to the rear surface, then a thermoplastic encapsulating material will flow on application of the soldering heat and thus enable the formation of soldered interconnects. Alternatively the interconnects can be soldered to the rear surface of the solar cell 100 before the encapsulating layer 540 is formed. In this case the interconnects will protrude out from the side of the solar cell 100 as it progresses through the plating bath. This is not a problem if the interconnects can be maintained quite short and do not interfere with either the passage of the patterned electrode units 310 through the solution or the light passing to the front surface of the solar cell 100.

If the encapsulating material used is sufficiently adhesive or sufficiently resilient, once the pins 315 of the pattern electrode unit 310 have made electrical contact to the solar cell 100 via the aluminium layer, then that adhesive force or the resilience can be sufficient grip the pins to maintain the solar cell 100 in electrical contact to the patterned electrode. The initial contact can be made by applying a uniform force to the entire front surface of the solar cell 100 before the patterned electrode unit 310 enters the plating bath. Preferably that force is applied by a large area non-scratching surface that does not create any scratches or defects on the surface of the solar cell 100 that will be exposed to the plating electrolyte, as such defects can initiate spurious plating, which is also known as ghost plating. This arrangement is particularly advantageous in that there is no need to maintain a constant force on the solar cell 100 to keep it in contact with the pins of the patterned electrode unit 310.

Referring to Figures 8 & 9B the sealed patterned electrode units 310 have an electrical connectipn to a voltage source which is maintained adjacent to the plating bath. Electrical connection to this voltage source is made via a fixed electrical conductor 375 which extends along one the side of the plating bath. The patterned electrode units 310 electrically connect to this fixed conductor 375 via a support arm 370 which extends out of the plating electrolyte 390 (as shown in Figure 3C). The support arm 370 is preferably arranged such that it can run in engagement with the fixed electrical conductor 375 which may take the form of metal track. A low resistivity electrical connection between the support arm 370 and the fixed conductor is preferably achieved by providing conductive brushes on the fixed conductor 375 or on the support arm 370 such that the brushes form the electrical interface between the fixed electrical conductor 375 and the moving the support arm 370. Once a solar cell 100 has been placed onto the patterned electrode unit 310 on the plating belt 300 and before the patterned electrode unit 310 and solar cell 100 enter the plating electrolyte 390 the support arm 370 slides into engagement with the fixed conductor 375 and a potential bias can be applied to the patterned electrode unit 310. A small (replaceable) electrolyte-resistant pump unit 360, may also be located at the side of the patterned electrode 310 and used to maintain the solar cell 100 in contact with the pins 315 of the patterned electrode unit 310 by generating lower pressure regions under the rear surface of the solar cell 100 by volumetric displacement of the solution (or air if it is trapped under the solar cell 100) through small openings in the patterned electrode unit.

In the preferred arrangement the pump unit 360 is connected to a series of pumping tubes 365 which extend under the sealed surface of the patterned electrode as depicted in Figure 3D. The pumping tubes contain a plurality of openings 367 which connect through openings in the patterned electrode unit 310. Fluid can be displaced or pumped through these openings to create a differential pressure between the illuminated and rear surfaces of the solar cell 100 and thus provide a uniform downward suction force on the cell. The openings are preferably spaced uniformly in the patterned electrode to provide a uniform downward suction force on the solar cell 100. The suction force can be varied to suit different solar cell substrates and operational conditions by adjusting the ratio between the solar cell surface area, the total area of the suction openings and/or the speed of flow generated by the pump. For a typical commercial solar cell with a silicon wafer thickness of ~ 260 μιη, a differential pressure between 0.05 and 70 kPa, and more preferably 5-10 kPa (notionally ~7 kPa) is required to securely maintain each solar cell 100 on its patterned electrode unit 310 whilst not causing damage due to excessive downward force on the electrode pins 315. Preferably the pump units 360 are triggered to start pumping as soon as the wafer begins to move into the plating bath and are able to tolerate an initial displacement of air before the patterned electrode unit 310 becomes submerged in the plating electrolyte 390.

Alternatively, a central pumping/vacuum unit can be coupled to the patterned electrode units 310 via tubing so that the pump can be maintained external to the plating bath with ducting tubes being connected to each patterned electrode unit. This arrangement can be advantageous because it eliminates the requirement for the pumping unit to be electrolyte resistant.

In a further variation, a manual supporting structure such as depicted in Figure

3B can be used to hold the solar cell 100 in place on the patterned electrode unit 310. Two side support arms 350 may be fixed to the plating belt 300 on both sides of the anode unit 310 as shown in Figure 3B. These side support arms 350 contain teflon pads 355 which contact the edges of each solar cell 100. The area of the Teflon pads is kept small to ensure minimal shading of the solar cell 100 whilst providing a pressure of preferably 1-2 kPa per arm for solar cells 100 based on 125 mm silicon wafers. Clearly, lower pressures are required for smaller solar cell sizes.

The side support arms are preferably designed to lock down on the solar cell 100 after being triggered by a pressure sensor 340 located centrally among the anode pins 315 of the anode unit 310. Preferably, this sensor is integrated into an anode pin 315. The side support arms 350 serve to: (i) ensure that the solar cell remains in contact with the anode unit 310 as it moves through the plating bath; and (ii) provide sufficient pressure to ensure that the anode pins 315 make good electrical contact with the aluminium of. the rear surface. Preferably the side support arms 350 are constructed using the same durable material as the meshed belt 305.

Other methods of holding the solar cells 100 onto the patterned electrode units

310 can also be employed. For example, other methods which employ liquid flows to effectively generate suction forces that hold the solar cells 100 in place against the patterned electrode units 310 can be used. Furthermore, the patterned electrode units 310 can be placed at a slight angle to the motion of the belt and the belt motion relative to the plating solution can be used to generate the downward force on the surface of the solar cell 100 required to maintain low-resistance electrical contact between the solar cell 100 and the patterned electrode 310. ¹

Combinations of these supporting mechanisms can also be employed. For example, a manual supporting structure such as depicted in Figure 3B can be used to support the solar cell 100 on the patterned electrode unit 310 until the unit is submerged in the plating electrolyte 390. The displacement pump can then be triggered to hold the solar cell 100 in place during the plating, thus reducing the possibility of spurious metal plating occurring where the support arms 350 contact the front surface of the solar cell 100.

Figure 6 diagramatically shows the plating belt 300 moving through the plating electrolyte 390 contained in the plating bath 615. A light source housing unit 605 contains an array of compact fluorescent lights (CFLs) having preferably a power of at least 18 W. Other light sources can also be used as long as they provide a uniform light intensity to all plating areas on the plating belt 300. Preferably, the light source is maintained at a distance of 10-20 cm, and more preferably 15 cm, from the surface of the solution. In addition, it is preferable that the height of plating electrolyte 390 in plating bath 615 is 0.5 to 1.0 cm above the surface of the solar cells 100 (each of which is supported on its own patterned electrode unit 310). This is to minimise the amount of light that is absorbed by the plating electrolyte before it reaches the surface of the solar cell 100.

The plating belt 300 preferably moves at a speed between 100 and 500 mm/min, and more preferably 200 mm/min. It enters the plating bath 615 at an angle of approximately 30 degrees to the horizontal. This bath entry angle is typically varied depending on the speed at which the belt is operated with smaller angles being required for faster speeds. Therefore in the preferred arrangement, a plating time of 5 mins requires a plating bath 620 of length 1 m if the belt operates at 200 mm min.

In the preferred arrangement where the patterned electrode unit 310 is operated in the cathode configuration, a voltage source 805 is electrically connected to a pair of anodes 810 which are placed in contact with the plating electrolyte 390 and along each of the side of the plating bath 615 as shown in Figure 8. Alternatively, the anodes can consist of a series of electrically connected small replaceable bar or bag anode units. This arrangement allows for partial replacement of the anodes, with anode bags providing an anode surface with a larger surface area. Although it is preferable to have the electrical field guiding the motion of the metal ions (generated at the anode, surface) in a direction perpendicular to the surface being plated, such anode placement would obstruct the illumination source 605 which is placed directly above the solar cell surfaces to be plated. The anodes 810 are held in place at the edges of the bath by insulating fixtures which enable easy anode removal and replacement. Alternatively, if the anodes are sufficiently thin then they can be placed above the transported solar cells 100 as some shading can be tolerated.

The anodes are composed of the metal being plated, with doped metals being used to reduce passivation effects where possible. For example, nickel anodes are preferably doped with sulphur atoms at a concentration of 50-250 parts per million (ppm) to ensure an anode surface that passivates less easily than a pure nickel layer. Alternatively, the anodes can be structurally composed of a material which can be electroplated (such as the ABS) which can be periodically electroplated to replenish the anode metal.

Preferably, a bias voltage having a magnitude approximately equal to the solar cell's open circuit voltage is selected (e.g., 0.6 V) and applied such that it drives the solar cell's diode in the direction of reverse bias. The ideal operating conditions are selected such that the solar cell 100 is operating at as close as possible to its short circuit current. The bias voltage can be tuned in response to different plating chemistries and electrolyte conductivities. Preferably the bias voltage is applied to the patterned electrode unit 310 before the unit enters the plating electrolyte 390 to ensure maximum protection of the rear metal surface of the solar cell 100.

The patterned electrode units 310 are preferably shaped at the front as shown in Figure 3A to enable non-turbulent flow over the electrode. The shaped edge 330 is also advantageous when the patterned electrode unit 310 is used in the anode configuration in that it assists the flow of metal ions generated on the under surface of the patterned electrode to flow over to the cathodic. (illuminated) surface of the solar cell 100 held to the following patterned electrode unit. This directed flow of metal ions from the patterned electrode (anode) surface towards the cathode surface of the following solar cell 100 is shown in Figure 4. This directed flow is also, in part assisted by the electrical field that forms in the plating electrolyte 390 directing positively-charged ions from the under surface of the pattered electrode unit 3.10 (when acting as an anode) to the illuminated surface of the solar cell 100 in the plating bath 615.

Returning now to the plating process.200 depicted in Figure 2, nickel can first be plated to the exposed Si regions on the illuminated surface of the solar cells 100 placed on the (nickel) plating belt 300 by passing the belt through a plating electrolyte 390 comprising nickel ions in step 220. Preferably, the source of nickel ions is obtained from a nickel sulphamate solution comprising preferably 76 g/L Ni ions and 30 g/L boric acid. Proprietary plating electrolytes can also be used with proprietary plating additives enhancing the properties of the plated metal deposit. Preferably the pH of the nickel plating solution is maintained in the pH range of 3.5 to 4.6 and the temperature of the plating bath is maintained at 35°C. When the solar cells 100 remain in the plating electrolyte 390 for 3 mins with an applied voltage of -0.55 V using the preferred cathode configuration, the thickness of the resulting plated Ni is approximately 1 urn. Using a belt speed of 200 mm/min, a plating bath of length 0.6 m is required.

After the plating belt 300 exits the nickel plating bath it passes into a short deionised water rinse bath in substantially the same way as it entered the nickel plating bath. The solar cells 100 remain attached to their respective patterned electrode units 310 and the length of the water rinse bath is designed such that the effective duration of the water rinse step is 2 minutes.

The plating belt 300 then exits the water rinse bath and passes along first a drying region, and then a collection region where a transponder triggers the release of the solar cells 100 from their respective patterned electrode units 310 (e.g. by releasing the support arms 350 or, if the solar cell is held by the pins 315 of the patterned electrode 310 being inserted through a resilient or adhesive encapsulant 540, by lifting the cell off the pins). The cells are then removed for the subsequent process.

In the preferred arrangement, the plating belt 305 then returns to the beginning of the nickel plating bath where further solar cells 100 are placed on the plating belt 305. Preferably the plating belt 305 returns along side of the plating baths, however in environments where there is a limited floor space the belt can return either above or below the plating baths. If the patterned electrode units 310 are employed in an anode configuration, then as the belt returns to the start of the Ni metal plating step they are periodically checked to determine the extent of their corrosion and are replaced, by freshly-plated units 310 where necessary. This corrosion detection and replacement is preferably done by manual inspection, however alterative arrangements could employ automated systems where images of patterned electrode units are captured and analysed in software to determine the extent of corrosion. Units, thus identified for replacement, can then be replaced in an automated manner, reducing the requirement for a further manual step in the automated process.

When the LIP system is operated in the anode configuration, the plating belt 305 can be passed through an electroless nickel plating solution on its return to plate additional fresh metal on the patterned electrode units 310. Although this arrangement has the advantage that the patterned electrode units 310 are always freshly plated, it requires significantly more bath maintenance and can be expensive with regard to energy consumption in that typically nickel electroless plating solutions must be operated at temperatures in the range of 80-90°C. Electrode surfaces can be cleaned prior to re-plating in a further bath on the return section of the plating belt 300.

Returning to the preferred arrangement, which is depicted in Figure 2, the , plated nickel is then sintered in step 230 at 350°C for 2-10 mins, with the duration of the sintering step depending on the depth of the junction from the surface in the solar cell. Cells which employ deeper junctions at the base of their grooves can typically tolerate sintering durations at this temperature of 10 mins, however solar cells having shallower junctions are preferably sintered for durations as, short as 2 mins. Nickel sintering is preferably performed under nitrogen with oxygen levels being maintained at less than 20 ppm and preferably less than 5 ppm to ensure minimal oxidation of the nickel. After cooling for 10-15 mins under nitrogen, in the preferred arrangement the solar cells 100 are then directly loaded onto the plating belt 300 of the copper plating system.

In alternative processes, the nickel that remains on the surface of the grooves after the sintering step can be removed by immersing the solar cells 100 in a solution comprising 35% nitric acid for 2 mins (see step 235 in Figure 2). The nitric acid dissolves any excess nickel but in doing so can leave a residual oxide layer in the base of the grooves. This layer can be removed by then performing additional deglaze and rinse steps as described for steps 205 and 210.

The copper plating system operates substantially as described above for the nickel plating system with the exceptions that the plating bath 615 contains a copper plating solution rather than a nickel plating solution. Also, the length of the plating bath 615 is 2.m rather than 0.6 m to achieve a copper LIP duration of 10 mins rather than the 3 mins used for nickel LIP in the preferred arrangement. Alternatively, if the length of the bath is required to be shorter than 2 m then 10 mins of LIP can be achieved by using a slower belt speed. The anodes 810 of the (copper) plating belt 300, used when the LIP apparatus is operated in the cathode configuration, are plated with copper metal, and more preferably with phosphorous-doped copper to minimise the possibility of anode passivation.

Preferably, a copper pyrophosphate plating solution is used for copper LIP step

250. This plating solution which comprises: 52-84 g/L copper pyrophosphate; 200-350 g/L potassium pyrophosphate; and 3-6 g/L potassium nitrate, is operated at a temperature of 40°C and a pH of 8.0-8.7. Copper pyrophosphate plating solutions demonstrate good throwing power and ductility, and are non-corrosive and non-toxic. A further advantage of this class of plating solution for the described LIP process is the operation at a weakly alkaline pH which further inhibits leaching of aluminium oxide from the rear surface of the solar cells 100. Although the central regions of the rear surface become almost completely protected by the arrangement of pins 315 of the patterned electrode units 310, which are encased in a protective hydrous oxide layer 505 (as shown in Figure 5), there can be some dissolution of aluminium oxide from the edge regions (which are more exposed to the plating solution 620) of the solar cells 100 as they move through the plating bath 615 if the plating solution is either strongly acidic or strongly alkalinei

In alternative arrangements, in which acidic copper sulphate solutions are used for the copper LIP process, some dissolution of aluminium ions into the plating solution 620 can occur due to the acid pH and the increased solubility of aluminium oxide in acid solution. However, if the dissolution only results at the edges of the solar cell 100, levels of the aluminium in the plating bath rise very slowly. Furthermore, due to the limited amounts of corroded aluminium, the final electrical performance of the solar cells 100 are not affected. The plating process 200 depicted in Figure 2 then completes with a final rinse and dry step 255 which can be incorporated into the plating apparatus as described for the nickel LIP step 220. Optionally, and not shown in Figure 2, the final copper surface of the plated metal contacts can be replaced by a thin silver layer by immersion plating in a solution, comprising silver nitrate and nitric acid, and as provided by chemical suppliers such as MacDermid and Enthone. The resulting silver surface layer provides resistance to corrosion, without the unnecessary cost of having to plate the entire font- contact scheme in highly expensive silver. Alternatively, a surface layer of tin can be similarly formed thus eliminating the need to use the more expensive silver entirely.

In alternative arrangements, when the aluminium layers are formed on the rear surface by screen printing, silver tabs can be included to ensure low-resistance contact between the rear of the solar cell 100 and solder used to connect the rear surface of the cell to electrical wiring within the module. In the. event that such tabs are provided on the rear surface, then it is preferable to ensure that these tabs are located as centrally as possible to eliminate the possibility of the plating electrolyte 390 coming in contact with the screen-printed silver. Silver can act as an oxidation catalyst and effectively "transiently stores" electrons, accepted from an electropositive metal such as aluminium. This results in galvanic corrosion and the silver tabs in. contact with the plating electrolyte 390, become cathodic sites and also are plated, thus consuming plating current and reducing the efficiency of the LIP process.

Alternatively, the use of silver solder tabs can be eliminated by the use of conductive epoxies or glues which can form a conductive pathway between the aluminium and the module wiring. Before these epoxies are applied (and after the. plating process 200), the solar cells 100 can be immersed for 15 seconds in a solution comprising anhydrous ammonium fluoride in water-free organic solvents, such as acetic acid or ethylene glycol, essentially as described in US patent 4,087,367 granted to Rioult et. al. The latter solution preferentially etches aluminium oxide without significant etching of aluminium or front-surface dielectric layer.

Also as described earlier in this disclosure, if the rear surface of the solar cell 100 is further protected with an encapsulating layer 540, the silver tabs are also protected from any actions of the plating electrolyte. The locations of the silver tabs can be clearly seen through the transparent encapsulating layer 540 and interconnects can be soldered through the thermoplastic polymer layer because the polymer reflows on application of heat.

An alternative LIP process 700 which can ensure even greater protection of the aluminium rear surface of the solar cell 100 is shown in Figure 7. It differs from the process 200 depicted in Figure 2 in that the solar cells are maintained on a single plating belt for the entire nickel/copper plating process. In this case the sinter step is performed after the copper plating step. .

In this alternative process the solar cells 100 are placed on the patterned electrode units 310 in step 705. Then in step 710, the rear aluminium surface is oxidised preferably by anodising the aluminium by immersing the plating belt 300 with the patterned electrode units 310 in contact with their individual solar cells 100 into an anodising bath under illumination. Preferably the electrolyte used for the anodising bath comprises 2.3 M boric acid, however clearly other anodising electrolytes, such as 1 M sulphuric acid can also be used. The light source used for illuminating the solar cells 100 can be as described previously for the nickel and copper plating steps in process 200. The solar cell substrates are immersed in the anodising electrolyte for a period of 2 to 10 minutes, and more preferably 5 minutes. Step 710 results in the formation of a thick aluminium oxide layer in aluminium regions which are not contacted by the pins 315 of the patterned electrode units 310. Other methods of thickening or strengthening the protective aluminium oxide layer can also be used, for example immersion in boiling deionised water for 2-5 minutes, however the anodising step is preferred because it does not result in extensive oxidation of the exposed n-type silicon surfaces of the solar cells 100. On completion of step 710 the solar cells are rinsed in deionised water for 1-2 minutes whilst remaining on the plating belt 300.

Then in step 715 the plating belt 300 enters a deglazing bath, comprising the same deglazing solution used in step 205 and is rinsed in step 720 .substantially as described for step 210 in process 200. Nickel is then plated in step 725, using the LIP process described for step 220, with the exception that the plating duration for this step is increased slightly to ensure that the resulting nickel layer plated on the n-type surfaces is preferably 1-4 μιη thick, and more preferably 3 μηι thick. This thicker layer is required if the sintering step is performed after the copper plating step. Following nickel plating, the plating belt 300 enters a water rinse station in step 730 for 1 minute before commencing copper plating in step 735. The copper plating is performed substantially as described for step 250 of process 200. Finally the solar cells 100 are rinsed and dried in step 740 (as described for step 225 of process 200) and then sintered in step 745 (as described for step 230 of process 200).

This alternative processing sequence has the advantage that it requires less loading and unloading of the solar cell substrates 100. It also enables the continued usage of silver tabs for the purpose of forming electrical connections to the final cell. These silver tabs, due to the electronegative nature of silver are not affected by the anodising step, and so remain unoxidised in process 700. It is not necessary to further treat the rear surface to eliminate the aluminium oxide formed by the anodisation process. In fact, the aluminium oxide serves to further protect the rear of the solar cell 100 once integrated into a module. The key requirement for process 700 is that low- resistance connections are made to the solar cell 100 by the pins 315 of the patterned electrode units 310 before the aluminium oxide is thickened into a highly protective layer.

The process 700 can also be performed without the anodising step 710. This would result in a process which is substantially as described for process 200 but with the nickel sintering step 750 still being performed after the copper plating step/

This proposed method can also be used to form metal contacts to exposed n-type silicon regions of solar cells in which both polarity of contact are formed on the same surface (typically the rear surface). This solar cell arrangement is advantageous because it eliminates losses due to shading from front-surface metal contacts. When the present method is applied to this solar cell arrangement, patterned electrode units substantially as described above for the preferred arrangement can be used, however it is necessary to shape the patterned electrode units so they do not cover the n-type regions that are to be plated. The solar cells 100 are illuminated from the side which is free of contacts, and the patterned electrode unit 310 is designed such that the n-type regions on the rear surface are not covered by the unit and exposed to the plating electrolyte 390. The patterned electrode unit 310 in this case can be operated in both anode and cathode configurations substantially as described above for the preferred arrangement. When operated in the cathode configuration, the anode can be provided as a sheet at the bottom of the plating bath 615 thus enabling a field-driven flow of electrons in a direction perpendicular to the surface being plated. When operated in the anode configuration, the flow of metal ions from the anodic surface of the patterned electrode unit is largely facilitated by the electric fields that form between the adjacent anode and cathode regions on the rear surface of the solar cell.

Figure 10 A shows how the use of a pin patterned electrode in the anode configuration (see Figure 9A) can both increase the plating rate of copper and decrease the amount of aluminium dissolved (released) into the plating electrolyte from that which occurs if the rear aluminium surface is exposed to the plating solution. In the latter configuration, the rear aluminium surface of the solar cell is acting as the anode during 10 mins LIP of the solar cell. In other words, the oxidation of the aluminium is completing the electrochemical circuit. Due to the acidic nature of the acid copper solution used in the plating the cells from which in the data from Figure 10A was derived, a large fraction of the oxidised aluminium was dissolved in the plating electrolyte. Use of a copper pyrophosphate solution can further increase the molar ratio . of plated metal to released aluminium as shown in Figure 10B, due to the higher pH of the pyrophosphate solution resulting in less oxidised aluminium being released into the plating electrolyte, however the plating rate of the copper is much slower than that which is observed in the acid copper solution.

Figure IOC shows how the use of the patterned electrode in the cathode configuration, with different applied voltage values, can increase the molar ratio of plated metal to released aluminium to a value that is more than 15 times that achieved using a patterned electrode in the anode configuration. The results shown in Figure IOC were obtained by 6 mins LIP using an acid copper plating electrolyte and therefore can be increased even further by the use of a^' copper pyrophosphate electrolyte. Although the plating rate using the latter electrolyte was much slower than that observed using the acid copper electrolyte in the anode configuration of the patterned electrode, the increased plating rate enabled in the cathode configuration would make the use of the higher pH copper pyrophosphate solution more viable.

Claims

CLAIMS:-

1. A method of depositing metal over a region of n-type semiconducting material of a solar cell, the solar cell comprising a p-n junction with an electropositive metal in electrical contact with a p-type semiconductor material of the solar cell, the method comprising:

i. electrically contacting a patterned electrode to the electropositive metal, the patterned electrode comprising a plurality of raised contact areas having a total area less than a total area of the electropositive metal contacted by the patterned electrode; ii. placing the solar cell and patterned electrode in an electrolyte comprising ions of a metal to be deposited; and

2. The method of claim 1 wherein less than 5% of a total area of the electropositive metal is contacted by the patterned electrode.

3. The method of claim 1 or 2 wherein the electropositive material is selected from one of aluminium, alloys of aluminium, zinc and alloys of zinc, tin and alloys of tin.

4. The method of claim 1, 2 or 3 wherein the raised contact areas of the patterned electrode comprise a pattern of pins extending generally perpendicularly from a planar surface of the patterned electrode.

5. The method of claim 4 wherein the raised contact areas are pins which have a tip surface area of 0.1 to 0.5 mm²

6. The method of claim 1, 2 or 3 wherein the raised contact areas of the patterned electrode comprise a pattern of ridges projecting from a planar surface of the patterned electrode

7. The method as claimed in any one of claims 1 to 6 wherein the electropositive metal surface of the solar cell is covered by a layer of water resistant polymer.

8. The method of claim 7 wherein the raised areas of the patterned electrode makes electrical contact to the solar cell through the water resistant polymer layer.

9. The method of claim 8 wherein the metal being deposited is selected from one of nickel, copper, tin and silver.

10. The method as claimed in any one of claims 1 to 9 wherein the patterned electrode is carried on a belt and the belt passes the patterned electrode and solar cell- through a bath containing the electrolyte and simultaneously under a source of the illumination.

11. The method of claim 10 wherein the solar cell is maintained in electrical contact with the raised contact areas of the patterned electrode by a mechanism involving downward pressure.

12. The method of claim 11 wherein the pressure is applied by removal of electrolyte from below the solar cell.

13. The method of claim 11 or 12 wherein the pressure is applied by directing a flow of the electrolyte over an illuminated surface of the solar cell.

14. The method of claim 12 or 13 wherein the patterned electrode contains one or more openings through which electrolyte is removed by a pumping mechanism.

15. The method as claimed in claim 10, 11 12, 13 or 14 wherein the belt carries a plurality of patterned electrodes, each patterned electrode being shaped so that when a section of the belt carrying one of the patterned electrodes passes through the electrolyte, electrolyte is deflected toward an n-type material surface of a solar cell carried on the patterned electrode from a surface of a previous patterned electrode exposed to the electrolyte.

16. The method of claim 15 wherein a leading edge of each patterned electrode is shaped to deflect the electrolyte by movement of the patterned electrodes through the electrolyte. ,

17. The method as claimed in any one of claims 1 to 16 further comprising exposing the one or more regions of n-type semiconductor material of the solar cell to illumination, whereby the illumination results in the depositing of metal onto the region of n-type material.

18. The method as claimed in any one of claims 1 to 17 wherein the one or more n- type regions is exposed to illumination though openings in an ahtireflection coating.

19. The method as claimed in any one of claims 1 to 18- wherein the one or more regions of n-type semiconductor material of the solar cell is plated with a metal barrier layer, whereby the illumination results in the depositing of a metal layer onto the barrier layer.

20. The method as claimed in any one of claims 1 to 19 wherein the patterned' "electrode operates as a metal anode.

21. The method of claim 20 wherein the patterned electrode is composed, at least in part, of the metal being deposited to the exposed region of n-type material.

22. The method of claim 20 or 21 wherein the patterned electrode is formed of a base material which is then plated with the metal to be deposited.

23. The method of claim 22 wherein the base material comprises either a metal or a plastic material

24. The method of claim 22 wherein the base material comprises stainless steel or acrylonitrile butadiene styrene.

25. The patterned electrode of claim 22, 23 or 24 wherein the metal to be deposited is selected from one of nickel, silver and copper.

5 26. The method of claim 22, 23, 24 or 25 wherein metal is deposited onto a plurality of solar cells by repeatedly reusing the patterned electrode and the patterned electrode is periodically replated between uses.

27. The method as claimed in any one of claims 1 to 19 wherein the patterned electrode operates as a cathode.

10 28. The method of claim 27 wherein the patterned electrode is electrically connected to a separate anode via a voltage source, the separate anode being in contact with the electrolyte.

29. The method of claim 28 wherein a potential applied between the patterned electrode and the separate anode by the voltage source is arranged to reverse bias the

15 solar cell/

30. The method as claimed in any one of claims 27 to 29 in which all surfaces of the patterned electrode except for the raised areas are sealed from the electrolyte.

31. The method of claim 30 wherein surfaces are sealed by being covered with an organic sealing material.

20 32. The method of claim 31 wherein the organic sealing material is applied in liquid form and then cured to form a barrier which is impermeable to the electrolyte.

33. A patterned electrode comprising a planar metal member and a plurality of raised contact areas arranged to electrically contact an electropositive metal electrical contact of a solar cell, the plurality of raised contact areas having a total area of less than a total

25 area of the electropositive metal when contacted by the patterned electrode.

34. The patterned electrode as claimed in claim 33 wherein the plurality of raised contact areas have a total area of less than 5% of a total area of the electropositive metal when contacted by the patterned electrode.

35. The patterned electrode as claimed in claim 33 or 34 wherein the raised contact 30 areas of the patterned electrode comprise a pattern of pins extending generally perpendicularly from a planar surface of the patterned electrode.

36. The patterned electrode of claim 35 wherein the pins have a tip surface area of 0.1 to 0.5 mm² . ,

37. The patterned electrode as claimed in claim 33 or 34 wherein the raised contact 35 areas of the patterned electrode comprise a pattern of ridges projecting from a planar surface of the patterned electrode.

38. The patterned electrode as claimed in any one of claims 33 to 37 wherein a leading edge of the patterned electrode is shaped so that when it passes through an electrolyte it deflects the electrolyte towards a n-type surface of a solar cell carried on the patterned metal electrode.

39. The patterned electrode as claimed in any one of claims 33 to 38 in wherein the patterned electrode operates as a metal anode.

40. The patterned electrode of claim 39 wherein the patterned electrode is composed, at least in part, of the metal being deposited to the exposed region of n-type material.

41. The patterned electrode of claim 39 or 40 wherein the patterned electrode is formed of a base material which is then plated with a metal to be deposited.

42. The patterned electrode of claim 41 wherein the base material comprises either a metal or a plastic material

43. The patterned electrode of claim 41 wherein the base material comprises stainless steel or acrylonitrile butadiene styrene.

44. The patterned electrode of claim 41, 42 or 43 wherein the metal to be deposited is selected from one of nickel, silver and copper.

45. The patterned electrode as claimed in any one of claims 33 to 37 wherein the patterned electrode operates as a cathode.

46. The patterned electrode of claim 45 wherein the patterned electrode is electrically connected to a separate anode via a voltage source, the separate anode being in contact with an electrolyte.

47. The patterned electrode of claim 45 or 46 wherein a potential applied between the patterned electrode and the separate anode by the voltage source is arranged to reverse bias the solar cell.

48. The patterned electrode as claimed in any one of claims 45 to 47 in which all surfaces of the patterned electrode except for the raised areas are sealed against contact with the electrolyte.

49. The patterned electrode of claim 48 wherein surfaces are sealed by being covered with an organic sealing material.

50 . The patterned electrode of claim 49 wherein the organic sealing material is applicable in liquid form and then cured to form a barrier which is impermeable to the electrolyte.

51. A plating system comprising:

i. a patterned electrode having a planar metal member and a plurality of raised contact areas arranged to electrically contact an electropositive metal electrical ^■ contact of an anode of a solar cell, the plurality of raised contact areas having a total area of less than a total area of the electropositive metal when contacted by the patterned metal anode;

ii. a continuous belt to which the patterned electrode is attached;

52. The plating system as claimed in claim 51 wherein the plurality of raised contact areas have a total area of less than 5% of a total area of the electropositive metal when contacted by the patterned metal anode.

53. The plating system as claimed in claim 51 or 52 wherein the raised contact areas of the patterned electrode comprise a pattern of pins extending generally perpendicularly from a planar surface of the patterned electrode.

54. The plating system of claim 53 wherein the pins have a tip surface area of 0.1 to 0.5 mm²

55. The plating system as claimed in claim 51 or 52 wherein the raised contact areas of the patterned electrode comprise a pattern of ridges projecting from a planar surface of the patterned electrode.

56. The plating system as claimed in any one of claims 51 to 55 wherein the light source comprises a fluorescent lamp,

57. The plating system as claimed in any one of claims 51 to 56 wherein the patterned electrode is releasably connected to the belt.

58. The plating system as claimed in any one of claims 51 to 57 wherein the belt carries a plurality of patterned metal anodes, each patterned electrode being shaped so that when a section of the belt carrying one of the patterned metal anodes passes through the electrolyte, electrolyte is deflected toward an n-type material surface of a solar cell carried on the patterned electrode from a surface of a previous patterned electrode exposed to the electrolyte.

59. The plating system as claimed in claim 58 wherein a leading edge of the patterned electrode is shaped so that when it passes through an electrolyte, the electrolyte is deflected toward an n-type material surface of a solar cell carried on the patterned electrode from a surface of a previous patterned electrode exposed to the electrolyte.

60. The plating system as claimed in any one of claims 51 to 59 in wherein the patterned electrode operates as a metal anode.

61. The plating system of claim 60 wherein the patterned electrode is composed, at least in part, of the metal being deposited to the exposed region of n-type material.

62. The plating system of claim 60 or 61 wherein the patterned electrode is formed of a base material which is then plated with a metal to be deposited.

63. The plating system of claim 62 wherein the base material comprises either a metal or a plastic material

64. The plating system of claim 62 wherein the base material comprises stainless steel or acrylonitrile butadiene styrene.

65. The plating system of claim 62, 63 or 64 wherein the metal to be deposited is selected from one of nickel, silver and copper.

66. The plating system of claim 62, 63, 64 or 65 wherein metal is deposited onto a plurality of solar cells by repeatedly reusing the patterned electrode and the patterned electrode is periodically replated between uses.

67. The plating system as claimed in any one of claims 51 to 59 wherein the patterned electrode operates as a cathode.

68. The plating system of claim 67 wherein the patterned electrode is electrically connected to a separate anode via a voltage source, the separate anode being in contact with the electrolyte.

69. The plating system of claim 67 or 68 wherein a potential applied between the patterned electrode and the separate anode by the voltage source is arranged to reverse bias the solar cell.

70. The plating system as claimed in any one of claims 67 to 69 in which all surfaces of the patterned electrode except for the raised areas are sealed from the electrolyte.

71. The plating system of claim 70 wherein surfaces are sealed by being covered with ah organic sealing material.

72. The plating system of claim 71 wherein the organic sealing material is applied in liquid form and then cured to form a barrier which is impermeable to the electrolyte.