In the project “C3PO” researchers at the Fraunhofer Institute for Solar Energy Systems ISE, together with industry partners, have come up with a process for mass-producing solar cell contacts. The novel laser transfer and firing (LTF) technique, which is applied in a fully-automated roll-to-roll set up, is far more effective than conventional processes.
So-called contact fingers (conductive lines made of metal) are applied to the solar cell surface to transport the electric current outside of the cell. To date, the production method has consisted of applying strips of silver paste to the front of a solar cell via a screen printing technique. There are clear economic reasons for seeking alternatives to silver contacts or even electrochemically-plated contacts (where nickel, copper and then silver paste are applied in layers).
In the new process, the LTF technique is used to open minute contact cavities beneath the insulating layer of the cell, and it allows for greater flexibility regarding the metals injected: aluminium, titanium, or bismuth can be used in place of nickel, for example.
In the first step, a laser printing process is used to transfer the metal in a precise layout from a foil to the cell. These metal lines are so narrow (5 micrometres width) they aren’t visible to the naked eye. In a second step, the metal structures are formed into contacts using a laser selective heating (LSH) process. Here the laser beam is absorbed by the metal but not by the underlying silicon, which can be damaged in the process.
The new LTF process has now been tested on an industrial scale system as part of a collaborative effort between the partners which include PULSAR, Soliton GmbH, RENA GmbH, ROWO GmbH, as well as Fraunhofer ISE. C3PO has been funded by the Federal Ministry for Economic Affairs and Energy.