A transparent spiderweb-like film formed from interconnected carbon nanotubes (CNTs) is highly efficient in solar energy harvesting, say Chinese scientists. 

CNTs are well known for their excellent electrical properties, which offer the potential for use in a broad range of modern technologies from sensors to flexible display panels. However, an appropriate method to produce CNT films that are both conductive and transparent has proved elusive, limiting their applications. 

Anyuan Cao and colleagues at Peking and Tsinghua Universities, Beijing, have developed a direct synthesis technique to achieve highly conductive and transparent CNT spiderwebs. They use chemical vapour deposition to grow ultra-long CNTs, followed by ethanol addition to condense them into bundles. 

We use an extremely slow feeding rate of the chemical precursor, resulting in well controlled formation of thin, uniform CNT films,' explains Cao. The thinner the film, the higher the transparency - a property essential for its effective use as an electrode in solar cells. By tuning the transparency, Cao ensures that most of the incident sunlight can reach the underlying silicon wafer for conversion into electricity, without compromising the film's conductivity. 

'CNTs play multiple roles in the solar cells: capturing the solar energy, forming junctions, collecting the photo-generation carriers and also as the transparent electrodes,' according to Yanqui Zhu at the University of Nottingham in the UK, who has expertise in nanomaterials fabrication and CNTs. He believes the successful fabrication of the 100 cm2 films brings CNTs a step closer to practical applications and paves the way for even larger scale production. 

The spiderwebs are sticky yet robust and sufficiently flexible to be transferred easily to various substrates including metal, paper and micro carbon fibres. Cao foresees numerous potential applications for his CNT spiderwebs, and is pursuing research into their use in flexible devices and window coatings with self-cleaning, sensing, UV-blocking and heating functions.

Erica Wise 

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