Copper Metallization – Long-Term Stable and Industrially Feasible (KuLi)

Duration: June 2012 - May 2015
Contracting Authority/ Sponsors: German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)
Project Partners: RENA, Heraeus, Q-Cells
© Fraunhofer ISE

Solar module featuring solar cells with fine-line, screen-printed seed layer plated with nickel, copper and silver, in front of copperplating facility at Fraunhofer ISE.

© Fraunhofer ISE

Fig. 2 Cross-section through fully plated contact consisting of nickel (1), copper (2) and tin (3).

Silver and lead are the two most critical materials in PV solar module construction, for cost and availability constraints or health concerns, respectively. At Fraunhofer ISE, a multi-step contacting process was developed to avoid using these materials. A first approach is based on currently used contacting technology and complemented by plating, already leading to drastically reduced silver consumption. In a second stage, silver and lead are eliminated completely from the front contacts, which are instead formed by nickel, copper and tin plating. We have now demonstrated both approaches successfully at the module level.

 

Currently, screen printing of silver pastes is the standard technology to form the front contacts of silicon solar cells. Despite recently achieved reductions in consumption, silver used in solar cell front contacts is still an important cost driver in PV production. Replacing silver by more readily available and about 100x cheaper copper has the potential to reduce costs and susceptibility to price fluctuations. The efficiency potential is similar or even higher, as copper plating allows the formation of very compact and highly conductive contacts.

At Fraunhofer ISE, plating of nickel and copper on fineline, screen-printed seed layers and directly on silicon are approaches to replace silver by copper. Contact adhesion has been one of the most challenging issues to tackle. In that respect, breakthroughs have been achieved very recently in cooperation with our partners. These developments allowed the construction of two prototype modules with the above mentioned techniques using standard soldering for interconnection. All adhesion criteria have been exceeded.

The first module features cells with easy-to-implement plating of nickel, copper and silver on fine-line, screen-printed contacts. Silver consumption was reduced from approx. 120–150 mg to 13 mg per cell for metallization by an improved screen-printing process (only 15 mg laydown of wet paste with low Ag content).

For the second module, direct plating of nickel enables cells with completely silver-free and lead-free front contact architecture. A detailed understanding of the Ni-Si solid state reactions enabled us to develop well-adhering contacts. These are comparable to high-efficiency contacts for laboratory solar cells or integrated circuits, at low cost and industrial feasibility. This makes them promising candidates for next-generation solar cell concepts, such as n-type cells.

With these results, such cost-saving metallization processes can be adapted by a multitude of manufacturers with relatively little effort. Based on our experience, Fraunhofer ISE can offer support with these processes, together with our partners.