Roll-to-roll printing of organic solar cells and LEDs gets closer

March 18, 2015 // By Paul Buckley
An EU-funded research project is claiming a number of successes at its mid-term review of technologies being developed to facilitate roll-to-roll (R2R) production of organic optoelectronic devices such as solar cells and LED lighting panels.

The 'TREASORES' (Transparent Electrodes for Large Area Large Scale Production of Organic Optoelectronic Devices) project, which started in November 2012 with an overall budget of more than 14 million Euros, and is led by Empa researcher Frank Nüesch, has already 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% for a perovskite-based solar cell using such novel transparent electrodes. The first fully R2R-produced solar cells already achieved commercially acceptable lifetimes when tested 'in the field'.

The TREASORES international team, which comprises researchers from 19 labs and companies from five European countries, is aiming to develop flexible organic solar cells that can be manufactured in large quantities by roll-to-roll (R2R) processing. To achieve that objective requires that transparent electrodes, the barrier layers and even the entire devices also need to be flexible.

"The next step," explained 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 100 meters in length".

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