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Flexible optoelectronics yields cheaper solar cells, LEDs

Posted: 19 Mar 2015     Print Version  Bookmark and Share

Keywords:flexible electronics  roll-to-roll  solar cell  LED  perovskite 

Scientists from the European Union-funded TREASORES project developed prototype flexible solar cell modules and silver-based transparent electrodes. TREASORES stands for Transparent Electrodes for Large Area Large Scale Production of Organic Optoelectronic Devices.

In order to make solar energy widely affordable scientists and engineers all over the world are looking for low-cost production technologies. Flexible organic solar cells have a huge potential in this regard because they require only a minimum amount of (rather cheap) materials and can be manufactured in large quantities by roll-to-roll (R2R) processing. This requires, however, that the transparent electrodes, the barrier layers and even the entire devices be flexible.

The EU-funded project "TREASORES," which started in November 2012 with an overall budget of more than 14 Mio Euro and is led by Empa researcher Frank Nüesch, aims at developing and demonstrating technologies to facilitate R2R production of organic optoelectronic devices such as solar cells and LED lighting panels.

Transparent electrodes with superior performance

The TREASORES project recently completed its mid-term review and has already achieved some major milestones. The international team that comprises researchers from 19 labs and companies from five European countries has, for instance, developed an ultra-thin transparent silver electrode that is cheaper than, and outperforms, currently used indium tin oxide (ITO) electrodes.

The researchers could also demonstrate a record efficiency of 7 per cent for a perovskite-based solar cell using such novel transparent electrodes. What's more, their first fully R2R-produced solar cells already achieved commercially acceptable lifetimes when tested in the field. The next step, said Nüesch, is to scale up and improve the most promising technologies identified so far, say, to produce barrier materials and transparent electrodes in larger quantities, i.e. in rolls of more than 100m in length.

Flexible solar cell

A flexible organic solar cell from TREASORES project undergoing mechanical testing: the cell is repeatedly flexed to a 25mm radius whilst monitoring its performance. Such cells have shown lifetimes in excess of 4000 hours. (Source: National Physical Laboratory (NPL), England)

In its second half, the TREASORES project will also continue to develop other promising technologies such as transparent and flexible electrodes based on woven fabrics, nanowires and carbon nanotubes (CNTs).

"We are working on the most crucial issues in large-scale organic optoelectronics. Our new low-cost electrode substrates already outperform existing conductive oxide electrodes in many ways", said Nüesch. "But we must further improve the resulting device yields from large-scale production by reducing the defect density of the substrates."

The new materials have been thoroughly tested using special instruments for mechanical, electrical and optical testing, and their performance in practical devices has been characterised, e.g. for lifetime and quality of illumination. Silver nanowires were used to produce flexible electrodes with a sheet resistance of below 20Ω/square—a measure for the electrical conductivity of thin films—and an optical transmission of 80 per cent.

Copper nanowires were even better, yielding a sheet resistance of below 10Ω/square and an optical transmission of 90 per cent on glass. They clearly outperformed current ITO electrodes, which typically have sheet resistance values of 100Ω/square and above for such high transparency.

Solar cell devices with an energy conversion efficiency of over 3 per cent have been made on these substrates with copper electrodes. CNT electrode performance likewise made significant progress during the first half of the project, reaching a sheet resistance of 74Ω/square with an optical transmission of 90 per cent. The organic solar cells that were produced with these electrodes reached an energy conversion efficiency of 4.5 per cent.


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