Researchers at European research project CrystalClear have succeeded in developing solar modules with technology that can be used for very thin solar cells. The solar cells will not be interconnected through soldering, but by using conductive adhesive. This technology will pave the way to cheaper solar modules and thus helps to reduce the cost of solar electricity.

An important objective for research into solar modules is to supply solar electricity at the same price as the power that arrives at your outlet (consumer price). To achieve this, manufacturing costs of solar modules must be reduced substantially, down to €1 per watt module power or less. At this time, costs are still approximately €2 per watt module power. More than 85 percent of the current global market for solar energy systems relies on wafer silicon modules. Since the very pure silicon used to make cells represents a substantial part of the total manufacturing cost of a module, it is important to reduce the amount of silicon required. This results in the use of very thin cells with a thickness of only 0.1 to 0.2 millimetres. However, solar cells made from such extremely thin wafers are very delicate and break easily during the module manufacturing process. The soldering process that has been in use for decades to connect the cells is a source of stress and may cause breakage in the case of very thin cells. This results in loss during manufacturing, or reduced performance of the modules. Modules might also stop working at a later time, when already in use.

Conductive Adhesive
To enable the manufacture of modules using very thin cells, the CrystalClear consortium and its partners developed and tested a low-stress method of interconnecting the cells using a conductive adhesive. This research is coordinated by ECN as partner of the consortium. Conductive adhesives need curing in order to become active, but this process can be performed at significantly lower temperatures than conventional soldering, whereas the process time is comparable. Curing has been done using halogen lamp heating prior to encapsulation, but a “single shot” process for combined curing of cell interconnections and encapsulation is under development. Such a process would reduce the number of process steps and the process time, which is important for low-cost, very large-scale manufacturing.

Test Results
Since the quality of the interconnections and the encapsulation is decisive for the lifetime and reliability of solar modules (which often come with a 25 years performance warranty!), innovations in this area require very thorough testing and are extremely rare. CrystalClear researchers have therefore built dedicated equipment, to enable highly reproducible application and curing of adhesives. With this, they have made strings of cells that have subsequently been encapsulated by the industrial partners. The cells used varied in silicon type, thickness and technology. The strings were encapsulated in modules adjacent to strings made with similar cells interconnected by soldering. This allowed a direct comparison of the conductive adhesive with soldering during climate chamber, outdoor and mechanical tests. The tests showed that the conductive adhesives strings performed as well as the soldered cells.

To summarise, the CrystalClear consortium has developed a new technique for the application of very thin solar cells. This paves the way towards cheap, efficient solar modules, and as a result, to solar energy that can compete with consumer electricity prices.

However, before this new technique can be enjoyed, manufactures must start to use it in large-scale production. This is not a change that will take place overnight, and it may therefore take a few years before the new solar modules will be available on the market. However, as ECN scientist Wim Sinke stresses: “Do not wait for that; already today it is attractive to invest in solar energy systems. Because of the feed-in tariff that now also applies in the Netherlands (through the “SDE”, see website SenterNovem), the investment can be recouped in 15 years, which is much shorter than the expected lifetime of the systems”.

Information:
Wim Sinke
Coordinator CrystalClear
Tel.: (+31) 224 564539
sinke@remove-this-part-ipcrystalclear.info
www.ipcrystalclear.info