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New Process for producing solar cell contacts

As the prices for solar modules have constantly been decreasing in the international market for many years, cost reduction is crucial for module manufacturers to remain competitive. One possibility to cut costs is to reduce the content of silver in a solar module, which is contained in its contact fingers. Contact fingers, i.e. its conductive lines made of metal, are applied to the solar cell surface to transport the produced energy out of the cell. The standard process for producing contact fingers is screen printing, in which silver paste is applied through a sieve onto the front side of the solar cell. This process is called electrochemical plating. As a rule, solar cells are encased in an electrically insulating coating that needs to be opened to make the contact fingers which are electrochemically grown on top of these openings using first nickel, then copper and lastly silver.

Now (2020), scientists at the Fraunhofer Institute for Solar Energy Systems have developed an alternative process for producing solar cell contacts – the laser transfer and firing (LTF) technique. This process consists of two steps: firstly, a direct laser printing process is employed to apply metal from a metal-coated foil to the desired contact finger layout. This process is also referred to as LIFT (Laser Induced Forward Transfer). The transferred metal structures have to be very narrow so that the cell area is not excessively shadowed. For the LIFT process in the new system, the solar cell is put on a vacuum chuck and moved under a metal-coated foil from which the metal is transferred to the solar cell. After the metal transfer, the metal foil is transported through a roll-to-roll system for the metal transfer of the next solar cell. Secondly, the metal structures from the first step are transformed into contacts using a laser selective heating (LSH) process. The wavelength of the laser beam is absorbed by the metal but not by the underlying silicon. As a result, the silicon material does not get damaged in this process, which is useful for high solar cell efficiencies. Both process steps can be fully automated.

Scientists have long sought to improve the design of contact fingers and reduce the costs of solar panels. In 2017, an interconnection concept for solar cells was designed that enabled the soldering of solder coated copper wires directly onto the contact fingers of the front side metallization without the need of busbars or contact pads. By reshaping the copper wires the scientists constructed a wave-shaped stress relief structure. This decreased the yield force up to 90 % compared to straight wires, and therefore minimized mechanical stresses in the joint after the soldering process. Experimental analyses showed the mechanical long-term stability of the interconnection. The method facilitated a significant silver reduction through omitting solder pads or busbars on the front side and was especially suitable for the interconnection of very thin, stress sensitive solar cells or back contact solar cells with minimized cell bows.

In 2018, scientists used an interconnect electrode called conductive belt in modules instead of interconnection ribbons. The conductive belt had multiple wires and achieved a multibusbar structure by forming ohmic contacts with the cell electrodes. Furthermore, the wire number and diameter were optimized according to full, half, and one-third cell sizes and finger wet weights of 80 mg, 40 mg, and 20 mg. The result showed that multibusbar and half-cell structures with a wire number of 16 and a wire diameter of 200 μm could achieve maximum power output. Finally, the reliability of the modules made with conductive belts was tested and was qualified according to International Electrotechnical Commission standards.

The advantages of this new technique to create contacts are that using the laser transfer and firing (LTF) process to open the insulating layer and create the metal contact fingers allows for a greater variety of contact metals to be used, such as, for example, aluminum, titanium, or bismuth. In contrast to screen printing, the LTF process also enables more flexibility in the layout of the contact fingers. The metallization process is also suitable for solar cells with temperature-sensitive layers, as it does not heat the solar cell above room temperature.

The functionality of the LTF process had been proven previously in the laboratory. Now it has been applied to an industry-scale system for the first time. In addition to the production of minuscule 3D structures and the localized coating of sensitive components, the LTF technology could possibly be applied in a variety of fields, as the pilot system has successfully proved.