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A team from the University of Southern California has created flexible transparent carbon atom films. According to the researchers who have produced them, these films have much potential for a novel kind of solar cells. Professor of electrical engineering, Chongwu Zhou, of the USC Viterbi School of Engineering, wrote in a paper that has lately been published in the ACS Nano journal, that Organic photovoltaic (OPV) cells have been suggested as a way to get low cost energy as a result of their light weight, ease of manufacture as well as compatibility with flexible substrates.

The method that has been described in the articles demonstrates advancement toward a new OPV cell design which has considerable benefits, especially in the area regarding physical flexibility.

A decisive feature of any OPV photo-electronic device consists of a transparent conductive electrode which allows light to couple with active materials in order to create electricity. The novel work suggests that grapheme, which is a highly transparent and highly conductive type of carbon made up of atoms-thick sheets of carbon atoms, has great ability to fulfil this role.

Despite the fact that the existence of grapheme has been a known fact for several decades, it is only since the year 2004 that it has been studied so extensively. This is mainly because of its difficulty to be manufactured in large quantity or high quality.

Three years back, the Zhou lab gave an account of the large scale production of graphene films through vapor deposition. The USC engineering team produces ultra thin graphene sheets in this process. This is done by initially depositing carbon atoms like graphene films on a nickel plate from methane gas. The team subsequently lays down a protective layer of thermo plastic over the graphene layer. They then dissolve the nickel beneath an acid bath. The final step consists of attaching the plastic-protected graphene to an extremely flexible polymer sheet that can be later incorporated into an OPV cell.

The USC team has created polymer/ graphene sheets that range in sizes up to 150 square centimetres. These can in turn be used to produce dense arrangement of flexible OPV cells.

While these OPV devices are able to transform solar radiation into electricity, they do not do so as efficiently as silicon cells. On a sunny day, the energy that is supplied by sunlight is approximately 1000 watts per meter square. Lewis Gomez De Arco who is a doctoral student as well as a member of the team responsible for building the graphene OPVs, explains that for each 1000 watts of sunlight that strikes a one square meter area of the regular silicon solar cell, 14 watts of electricity will be produced. He adds that organic solar cells are less efficient as their conversion rate for that same 1000 watts of sunlight, but in the graphene-based solar cell would be a mere 1.3 watts.

However, what graphene OPVs are deficient in efficiency, they have the potential to make for it in terms of lower price and greater physical flexibility. In accordance to Gomez De Arco it may ultimately be possible to run printing presses laying extensive areas that are covered with low cost solar cells, similar to newspapers presses printing newspapers. He moreover said that they could even be hung like curtains in homes or made into fabric as well and be worn as clothing that generates power. He is even able to project people powering their mobile phones or video/music device while they are jogging in the sun.

According to the USC researchers graphene OPVs would be a significant progress in at least one key area over a competitor OPV design, one that is based on Indium–Tin–Oxide (ITO). During the tests carried out by the USC team, it was observed that ITO cells failed at an incredibly small angle of bending. On the other hand, the graphene-based cells stayed operational even after repetitive bending at much larger stress angles. This result would certainly provide the graphene solar cells with a decided benefit in some uses, taking into account the printed-on-fabric applications suggested by the USC team.

The USC team comprised of Zhou, Yi Zhang, Koungmin Ryu, Gomez de Arco, Mark E. Thompson, Cody W. Schlenker. They are all very excited with the potential that this technology has.

The paper that the USC team wrote concludes that their methodology represents an important progress in relation to the production of transparent conductive electrodes in solar cells. The CVD graphene is able to meet the most essential criteria of low cost, electrode/organic film compatibility, flexibility, abundance, conductivity in addition to stability that are vital to replace ITO in organic photovoltaics, that may entail significant implications for future organic optoelectronic devices.

Source: E-science News and University of Southern California