The most efficient flexible solar cells

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The technique developed at Empa, has managed to create a flexible solar cells that have been record-breaking energy efficiency of 18.7%, was published in the renowned "Nature Materials". The technological breakthrough by the Empa researchers based on the modeling of the electronic band gap in the semiconductor material of the solar cell.

The critical layer, which absorbs sunlight and converts it to electricity is made of copper-indium-gallium-diselenide (CIGS short). The team succeeded, during the various stages of the evaporation process of the semiconductor layer to control the flow rate of the elements to produce a well-defined band gap.

Flexible and lightweight high-performance solar cells - such as on plastic films - represent an important economic potential: the production costs for solar cells made by the "roll to roll" production process significantly reduced. Due to its high efficiency and also the cost would fall for the entire system. This would be another step on the path to cost-effectively produced solar power. Until now, however, have flexible, plastic solar cells deposited films not shown the desired efficiency, they were far behind the values ​​that were achieved with solar cells on glass substrates. The reason: The deposition of semiconductor material that converts sunlight into electricity, keeping in polymer films only far lower process temperatures than glass plates. This resulted in lower efficiency of the produced cell.

Records in team work

The research team from the "Thin Films and Photovoltaics" at Empa, led by Ayodhya N. Tiwari was concerned with the development of high efficiency CIGS solar cells - both on glass and on flexible substrates. The group initially conducted research at the ETH Zurich and worked for three years at Empa is, succeeded several times in recent years, setting new efficiency record for CIGS cells. With the current record level of 18.7% graduated Tiwari and his team now almost the performance gap that existed previously between CIGS cells on flexible materials and CIGS cells on glass or polycrystalline silicon solar wafers. The details of the new low-temperature manufacturing method and the layer structure of the new cell have now been published in "Nature Materials".

New manufacturing method

"To achieve such high efficiency rates had we minimize the recombination of charge carriers generated by light," says Tiwari. The CIGS layers, caused by simultaneous evaporation of the elements at around 450 ° C, have a strong gradient in the composition: the diffusion of elements to form the desired CIGS phase is insufficient, especially diffuse gallium (Ga), preferably to the electric back contact. To circumvent this problem, the students developed Chiril Adrian? Patrick Bloesch and a new vapor-deposition process in which the order of gallium and indium in the various stages of production could be accurately controlled. The result was an optimized composition of the individual components within the CIGS layer. Thus, the charge carriers are efficiently captured, and the recombination losses are smaller. CIGS solar cells with such high efficiency were previously possible only on glass substrates, because only there production reached temperatures of more than 600 ° C were - not plastic sheets to keep out those temperatures.

Even cheap metal films suitable as a carrier

Tiwari's research group succeeded not only in setting a record efficiency of 18.7% based on polymer films, they also reached on a steel foil an efficiency of 17.7% - and to use this without a nitride or oxide barrier layer, which usually for high temperature processes on metal foils is necessary. The record values ​​were independent of the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg (Germany) certified. "We have thus shown that we have developed low-temperature manufacturing process is also applicable to low-cost metal foil, such as aluminum-forged steel or simple slides. With our technology, so it will be possible, the cost of highly efficient solar cells to push down tremendously, "says Tiwari.

Researchers at the start-up company FLISOM and Empa researchers are now working together to scale up the technology of low temperature production of solar cells on industrial scale and to commercialize. The results will be interconnected solar modules in the "roll-to-roll" process can be produced. The projects were funded by the Swiss National Science Foundation (SNF) of the Commission for Technology and Innovation (CTI), the Federal Office for Energy (BFE), EU-funding and under the Swiss Company Ltd. and W. Blösch FLISOM.

Source: EMPA - Swiss Federal Laboratories for Materials Testing and Research Institute - 9/21/2011.

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