Plastic solar cells have long been a distant promise. Flexible, thin, light and cheap, polymer or ‘organic’ solar cells could be wrapped around surfaces, rolled up or even painted onto structures. You could even glue them onto your windowpanes or on the roof of your car and get solar power practically for free.

The main problem until now has been the low efficiency of polymer solar cells. Two years ago, they converted only 3% of the solar radiation into electric power. For silicon solar PV cells, the current efficiency is typically 15 – 17%. But last month, researchers from the Wake Forest nanotechnology centre and the Korea Institute of Science and Technology published their results hitting over 6% efficiency.

The centre’s director David Caroll said: “Within only two years we have more than doubled the 3% mark. I fully expect to see higher numbers within the next two years, which may make plastic devices the photovoltaic of choice.” Carroll said the researchers hope to reach 10% efficiency in the next year. Solar cells are considered to be commercially viable if they can convert 8% of solar radiation into electricity.

However the US National Renewable Energy Laboratory (NREL) corrected the result of the Wakefield University to 3.3% in Scientific American.

The explanation of these contradictory claims lies in the measurement of the efficiency. Basically, the solar cell is put under an artificial sun and the output is measured. But to have comparable results, a standard has been set for the intensity and the spectral composition (‘the colour’) of the light source. Whenever another light source is used for testing, a correction factor should be used.

Apparently, the authors have not applied the correction, which resulted in the unusually high result of 6% efficiency.

The triumphant claim has turned into a shameful lesson for the Wakefield team, but also for solar cell science at large. Colleagues now suggest that anyone with a claim to a record high efficiency should go to a certified benchmarking institute: the NREL in the United States, or the Fraunhofer Institute in Germany. This way, when researchers really get to 4 or 5%, they will be appreciated for it.

More information:

NREL on the false claim in Scientific American

‘Plastic Solar Cell Efficiency hits 6% in US Lab’, Renewable Energy Access, 4 May 2007

 ‘Roles of donor and acceptor nanodomains in 6% efficient thermally annealed polymer photovoltaics’, Applied Physics Letters, 16 April 2007