Boffins boost solar cell efficiency

The "breakthrough" at Northwestern University promises cheaper solar cells which are more easy to manufacture and implement.

Tobin J. Marks, a research professor in the Weinberg College of Arts and Sciences, and Robert Chang, professor of materials science and engineering in the McCormick School of Engineering and Applied Science, led the research team.

The scientists explained that solar cells fabricated from plastic-like organic materials are attractive because they can be printed cheaply and quickly by a process similar to printing a newspaper, i.e. roll-to-roll processing.

To date, the most successful type of plastic photovoltaic cell is called a 'bulk-heterojunction cell'.

This uses a layer of a mixture of a semi-conducting polymer (an electron donor) and a fullerene (an electron acceptor) sandwiched between two electrodes, one a transparent electrically conducting electrode (the anode, which is usually a tin-doped indium oxide) and a metal (the cathode) such as aluminium.

When light enters through the transparent conducting electrode and strikes the light-absorbing polymer layer, electricity flows due to formation of pairs of electrons and holes that separate and move to the cathode and anode respectively.

The Northwestern researchers employed a laser deposition technique that coats the anode with a very thin (5 to 10 nanometre) and layer of nickel oxide.

This material is an excellent conductor for extracting holes from the irradiated cell but, equally important, is an efficient 'blocker' which prevents misdirected electrons from straying to the 'wrong' electrode (the anode), which would compromise the cell energy conversion efficiency.

"In contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive," the team stated.

"In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40 per cent and the power conversion efficiency from approximately three to four per cent to 5.2 to 5.6 per cent."

The researchers are currently working on further tuning the anode coating technique for increased hole extraction and electron blocking efficiency and moving to production-scaling experiments on flexible substrates.

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