Carbon nanotubes could up organic solar cells’ efficiency

Researchers have found a way to improve the performance of organic solar cells while maintaining the flexibility of the solution-based process, using carbon nanotubes.

Organic photovoltaics (PV) potentially promise cheap, lightweight devices, albeit at lower conversion efficiencies than crystalline materials such as silicon.

‘Everything is about efficiency versus cost — which method gives a little bit of headroom over the other,’ said Prof Ravi Silva, who heads Surrey University’s Advanced Technology Institute, where the latest work was performed.

Nevertheless, this balance has yet to properly favour the large-scale commercial uptake of organic PV, as Silva explained to The Engineer.

‘For the last 10 or 15 years lots of people have been working on organic materials, but the key breakthrough hasn’t come in yet — that is to achieve efficiencies as high as eight or 10 per cent.’

Some research groups have therefore looked at developing hybrid PVs that include semiconductor nanoparticles such as zinc oxide and tin oxide to overcome the low charge mobility in traditional organic materials.

‘We have so much knowledge in the area of inorganic materials, why not use the best properties of the inorganic materials to improve on the organics without compromising the ability to apply this over large areas in solution-processable routes?’ said Silva.

In their current work, Silva and his team have used multi-walled carbon nanotubes mixed with organic polymers. Essentially the nanotubes facilitate the creation of so-called triple junctions, ultimately improving the photo-generated charge transfer from the solar cell to the electrical circuit. 

‘In terms of efficiency the work we are doing will never compete on the same scale as the crystalline material, but in terms of cost we will be able to drop the cost of the materials by 100- or 1,000-fold,’ Silva said.

Crucially, the team’s fabrication method allows even mixing of the nanotubes with the organic materials, retaining the flexible handling characteristics.

‘In the future you might be able to just print out these materials, because what you might have is a metallic inkpot and a semiconducting inkpot, all based on a solution-processable system,’ Silva said.