In the race towards lower costs photovoltaics, silicon wafers are increasingly getting larger and thinner. However, any attempt to handle this fragile and costly solar-cell material with a thickness of a mere 0.15 millimetres, is enough to break it. In particular the process in which several solar cells are interconnected and mounted in a solar panel, is critical. As part of the European project CrystalClear, ECN is responsible for solving this problem.
The market for solar energy is growing for some years now, with an annual rate of more than 40 percent. The biggest contribution in recent years has come from Germany, where sustainable energy has been high on the national agenda. In the mean time this successful policy has now been taken up by a number of southern European nations as well, amongst them Spain, Greece, Italy and France.
The production of solar cells requires high-grade silicon. Up to 2004 this silicon was obtained from overcapacity in the semiconductor industry. Last year, however, the demand for high-grade silicon – so called feedstock – from the solar energy industry began to outstrip production capacity. Paul de Jong, research manager Module Technology at the Energy research Centre of the Netherlands (ECN), explains: "This tight market has resulted in a significant price increase of feedstock. Companies lacking a contract for forward delivery could not obtain any silicon at all.
Processing station where the copper strips are attached to the solar cell.
Meanwhile, companies are investing considerably in the construction of new factories for the production of high-grade silicon, but it will take some years before production capacity again exceeds demand. As a reaction to high prices and tight supply there is enormous pressure to further reduce the costs of solar power. Moreover, production of solar cells is planned as efficient as possible."
To achieve a cost reduction, one has to be far more economical with the silicon. As a result, solar cells are becoming thinner all the time. "A few years ago," De Jong recalls, "cells had a thickness of 0.3 millimetres. Today they are just 0.2 millimetres thick. Within the next year they will probably be no thicker than 0.1 millimetres. Furthermore, cells have become larger. Until recently cells were 10 × 10 square centimetres, nowadays they are 15 × 15 square centimetres. That, of course, is a challenge towards manufacturing of the cells and the assembly of the solar panel. Present, technologies are not capable to prevent breakage of the cells".
To transport the generated electrical power the cells are interconnected using copper strips. De Jong explains, "The problem lies within the difference in expansion rate of copper and silicon. When heated, copper expands much more as compared to silicon. The soldering process takes place at a temperature of about 200° Celsius, thus wedging the silicon between copper strips. Once cooling starts the whole unit literally stiffens under tension. Indeed there is not much needed to break the silicon."
Pilot line on which copper strips are attached to the solar cell.
"This problem is intensified as solar cells become thinner. Moreover, the amount of electrical power being produced by solar cells is increasing all the time, and the copper strips have to be designed thicker and thicker. It would literally be a waste of energy if increased yields were negated by resistance losses. To cap it all, we have to deal with new environmental legislation requiring that all solder material must be lead-free. This pushes up the temperature even more. In short, there's no end to it."
Applying adhesive to the contact surfaces on the solar cell.
To tackle these problems, ECN has developed a pilot line for connecting solar cells in a unique way. "At the moment," De Jong says, "we are working a lot with electrically conductive adhesive. This is because the required processing temperature is much lower. And since this adhesive layer is rather spongy, it absorbs mechanical tensions between the solar cell and the copper strip. Only last week we produced the first working solar panel with solar cells just 0.15 millimetres thick involving conductive adhesive technology. A real breakthrough!"
One great advantage of this pilot line set-up is its flexibility. It can be used to experiment with different materials and with a range of process parameters. As a result, ECN is able to respond to the whole spectrum of market demand.
Information
For more information about research into solar cells and the pilot line for interconnection, visit www.ipcrystalclear.info and www.ecn.nl
Contact
Paul de Jong
ECN Solar Energy
Tel. +31 (0)224 - 56 47 31
p.dejong@ecn.nl
Rome, 7-8 October 2013
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