Closer to Printed Organic Solar Cells

Friday, March 20, 2015 @ 05:03 PM gHale

Flexible organic solar cells have a huge potential to make solar energy widely affordable using low-cost production technology.

That is because organic solar cells require only a minimum amount of (rather cheap) materials and can end up manufactured in large quantities by roll-to-roll (R2R) processing.

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To be able to do that, however, requires the transparent electrodes, the barrier layers and even the entire devices be flexible.

The EU-funded project “TREASORES” (Transparent Electrodes for Large Area Large Scale Production of Organic Optoelectronic Devices) aims to develop and demonstrate technologies to facilitate R2R production of organic optoelectronic devices such as solar cells and LED lighting panels. The project, led by Empa researcher Frank Nüesch, started in November 2012.

The TREASORES project just 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 developed an ultra-thin transparent silver electrode that is cheaper than, and outperforms, currently used indium tin oxide (ITO) electrodes.

The researchers also demonstrated a record efficiency of 7 percent for a perovskite-based solar cell using these 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 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, Nüesch said.

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,” Nüesch said. “Our new low-cost electrode substrates already outperform existing conductive oxide electrodes in many ways. 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 undergone testing using special instruments for mechanical, electrical, and optical and their performance in practical devices.

Silver nanowires ended up used to produce flexible electrodes with a sheet resistance of below 20 Ohms/square – a measure for the electrical conductivity of thin films – and an optical transmission of 80 percent. Copper nanowires were even better, yielding a sheet resistance of below 10 Ohms/square and an optical transmission of 90 percent on glass. They clearly outperformed current ITO electrodes, which typically have sheet resistance values of 100 Ohms/square and above for such high transparency. Solar cell devices with an energy conversion efficiency of over 3 percent ended up 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 74 Ohms/square with an optical transmission of 90 percent. The organic solar cells produced with these electrodes reached an energy conversion efficiency of 4.5 percent.

All these electrode technologies suffer, however, to some extent from waviness or roughness and require a flattening layer to allow defect-free deposition of optoelectronic device stacks. That’s why the researchers set out to develop yet another electrode technology, which uses thin silver (Ag) films sandwiched between two metal oxide (MO) layers.

These films turned out to be much flatter. MO/Ag/MO electrode stacks provide a sheet resistance of 6 Ohms/square with an optical transmission of 85 percent and allowed the construction of more efficient optoelectronic devices compared to the other electrode technologies, which is due, at least in part, to the low peak-to-valley roughness of about 20 nm. With these ultra-flat electrodes, record efficiencies of up to 7 percent were possible for organic solar cells using commercially available materials for light harvesting.

Using the very same electrode materials, the team achieved 17 lm/W for the production of white light organic LEDs (OLEDs) and more than 20 lm/W for organic light-emitting electrochemical cells (OLECs). Although not quite record values for flexible OLED and OLEC devices, Nüesch said “all electrodes were produced by an R2R process in an industrial environment or with industrially relevant processes on large areas of the polymer substrate. We can thus say that the processes we used are robust and reproducible.”

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