Process for metallizing a component

The present invention relates to a process for producing one or more electrical contacts on a component, comprising (a) applying one or more coatings on the component, where at least one of the coatings is a coating of an electrically conductive material, (b) applying a self-passivating metal or semiconductor and/or a dielectric material on the coated component, (c) structuring the passivating coating by laser treatment or etching, (d) contacting the structured coating with an electroplating bath, (e) etching the regions not covered with the galvanically deposited metal.

The present invention relates to a process for producing electrical contacts (for example in the form of electrical conductor tracks) on a component, especially an electrical component, for example a solar cell or a light-emitting diode, or a precursor of a printed circuit board. The present invention further relates to devices obtainable via this process.

For the use of many components, it is necessary for electrical contacts, especially in the form of electrical conductor tracks, to be mounted thereon. The electrical contacts serve, for example, to remove current from the component or to tap voltage from the component or to establish an electrical connection between electrical components present on the component. If the component is a solar cell, for example, the photocurrent generated via the photovoltaic effect in this semiconductor component can be removed by the electrical contacts. Alternatively, the component may, for example, be a precursor of a printed circuit board which is ultimately converted to a printed circuit board (PCB) by the application of conductor tracks.

In a known and customary process, a paste comprising silver particles is applied to the component and then treated at a sufficiently high temperature to bring about sintering of the silver particles. For this purpose, temperatures of at least 800° C. may be required. However, such high temperatures are unacceptable for many components.

A heterojunction solar cell, for example a silicon heterojunction solar cell (SHJ solar cell), is an example of an electrical component unsuitable for application of electrical contacts at relatively high temperatures. The SHJ solar cell is a wafer-based crystalline silicon solar cell with an emitter and a back or front surface field of amorphous silicon. The starting material used for this purpose is crystalline, especially monocrystalline, silicon that has been n- or p-doped (base doping). A very thin (about 1 to 10 nm) intrinsic (undoped) amorphous silicon layer is first applied to each side thereof. This is followed on one side by the application of a likewise very thin (about 10 to 50 nm) doped amorphous silicon layer having the opposite doping type (n- or p-type) to the base doping (amorphous emitter layer). On the other side is applied a thin (10 to 50 nm) amorphous silicon layer having the doping type corresponding to the base doping (back or front surface field). Finally, a transparent conductive oxide (TCO), for example indium tin oxide (ITO), of thickness 50-100 nm is applied. Such a TCO layer, at 25° C., typically has a sheet resistance of not more than 300Ω. The construction and mode of function of heterojunction solar cells are described, for example, by S. De Wolf et al., Green, Vol. 2 (2012), p. 7-24.

In order to avoid unwanted crystallization in the amorphous silicon layers of the SHJ solar cell, temperatures of more than 250° C. should be avoided.

For other solar cell types or other electrical components such as light-emitting diodes as well, the application of electrical contacts with minimum thermal stress is desirable.

The use of sufficiently small silver nanoparticles can lower the sintering temperature of silver pastes to below 200° C. A disadvantage here, however, is that the pastes cannot be stored since the sintering process proceeds gradually even at room temperature and that silver nanoparticles constitute a considerable safety risk. Moreover, the costs for nanoparticles are much higher than for large particles or electrolytically deposited metals.

Also known is the use of pastes containing organic binders, for example thermally crosslinking resins and silver particles in flake form. The resin forms a matrix that holds the flakes together and establishes bond strength to the outer layer of the electrical component (for example a layer of a transparent electrically conductive oxide (TCO) such as ITO). But this achieves much lower conductivity than with thermally sintered pastes. As a result, more silver is required and the shadowing of the front side of the solar cell by the conductor tracks is increased.

Alternatively, the conductor tracks can be applied electrolytically. This achieves very good electrical conductivity of the conductor tracks. But the surface has to be printed with a mask of electroplating lacquer as a negative of the conductor track pattern. After the electrolytic deposition, the lacquer has to be removed in a chemical bath. But the necessity of this lacquer mask makes this process very costly owing to the material consumption and the necessary wastewater cleaning. Moreover, the bond strength of the electrolytically applied metal layer on a TCO layer (i.e. a layer of a transparent, electrically conductive oxide such as ITO) is unsatisfactory in some cases.

In the case of particularly high-value components, a thin metal layer or a metal layer stack is first applied over the whole area of the workpiece. Atop that is applied a photoresist, for example, which is structured by photolithography in the form of a negative mask of the conductor tracks to be created. Alternatively, the negative mask is applied in already structured form (for example by means of an inkjet). The non-lacquer-coated surface is thickened with copper by electrolysis and the copper is optionally protected from oxidation by an additional silver layer. Subsequently, the lacquer is removed in a chemical bath and the metal is etched in the previously lacquer-coated regions. A corresponding metallization process is described, for example, in U.S. Pat. No. 8,399,287 B1.

US 2014/0295614 A1 describes a process for metallization of backside contact solar cells. The vapor-deposited aluminum seed layer can be activated over the whole area by a zincate step. Subsequently, a local barrier layer can be applied. After the electrolytic deposition, the barrier layer has to be removed and the activated aluminum seed layer has to be etched.

WO 2015/148572 A1 describes a process for metallization of a solar cell in which an aluminum layer is locally anodized. The anodized regions bring about the electrical separation of the metal contacts of a backside contact solar cell.

R. Rohit et al., Energy Procedia, 124 (2017), p. 901-906, describe a process for metallization of a solar cell. In this process, a self-passivating coating of titanium is first deposited on an SHJ solar cell. No structuring of the titanium layer takes place. Nickel is deposited in defined regions of this self-passivating coating. This is followed by the electrolytic deposition of copper on the nickel.

In the case of printed circuit boards (PCBs) made of plastic, for lack of thermal stability of the board material, it is not possible to print conductor tracks of sinterable metal particles. Conductor tracks made of silver flakes in a resin matrix are an option only in exceptional cases owing to the high costs, lack of conductivity and lack of suitability for soldering processes for coupling of the electrical components.

It is an object of the present invention to apply electrical contacts, for example electrical conductor tracks, on a component via a process that keeps thermal stress on the component low, avoids the use of masks (e.g. lacquer masks) and is performable with maximum efficiency.

As component1on which one or more electrical contacts are to be mounted, a silicon heterojunction solar cell (SHJ) is provided. This has, on its front side and on its back side, a coating of a transparent, electrically conductive oxide (“TCO”), for example indium tin oxide (“ITO”) (not shown inFIG. 1a).

In step (a), a coating2of an electrically conductive material is applied by sputtering on the front side (or alternatively on the back side or both on the front side and on the back side) of the SHJ solar cell1. The structure obtained is shown inFIG. 1a. This electrically conductive coating may have one or more layers. For example, nickel is first applied by sputtering (layer thickness e.g. 10-30 nm), followed by the application of copper (layer thickness e.g. 50-70 nm) and finally the application of nickel (layer thickness e.g. 10-30 nm).

In step (b), by sputtering, a layer of a self-passivating metal (e.g. aluminum) with a layer thickness of about 60-80 nm is applied. Since a natural oxide layer forms at the surface of aluminum, a self-passivating coating3is obtained. Alternatively, a dielectric material, for example aluminum oxide, silicon oxide or silicon nitride, can be applied (preferably likewise by sputtering), which affords a dielectric coating3. An illustrative structure which is obtained in step (b) is shown byFIG. 1b.

In step (c), an HCl-containing etching paste is applied via a printing method atop the self-passivating aluminum layer3in defined regions, heated to 80° C. and rinsed with water. In these etched regions, the aluminum layer3is removed and the underlying nickel layer2is exposed. A structured coating4is obtained, in which the aluminum layer3deposited in step (b) is interrupted by one or more openings. An illustrative structure which is obtained in step (c) is shown byFIG. 1c.

In step (d), the structured aluminum layer4is contacted with a copper salt-containing electroplating bath. For the galvanic deposition, a pulsed current (i.e. alternating cathodic and anodic pulses) is used. The alternation of deposition pulses (i.e. cathodic pulses) and dissolution pulses (i.e. anodic pulses) of the electrical current results in selective galvanic deposition in the openings (i.e. the regions in which the etching exposed the nickel layer2), while galvanic metal deposition on the surface of the self-passivating aluminum of the structured coating4takes place only to a very minor degree, if at all. The galvanic copper deposition is effected until the openings are completely filled with copper5and an excess of copper has even formed (i.e. the copper projects out of the openings). An illustrative structure which is obtained in step (d) is shown byFIG. 1d.

In step (e), the exposed regions (i.e. regions not covered with galvanically deposited copper) of the self-passivating aluminum of the structured coating4and the underlying regions of the coating2deposited in step (a) are etched away down to the SHJ solar cell1. In these regions that have been etched away, the ITO surface of the SHJ solar cell1is thus exposed again. Adjacent to these regions that have been etched away, there remain layer stacks in which the coating2deposited by sputtering in step (a) and the copper5deposited galvanically in step (d) are each present. Optionally, at the edge of the galvanically deposited copper regions5, residues of the self-passivating metal of the structured coating4may still be present (for example when excess copper5, i.e. copper projecting out of the openings, covers part of the surface of the self-passivating aluminum and has therefore provided protection from the etchant in step (e)). An illustrative structure which is obtained in step (e) is shown byFIG. 1e.