More power from plastic solar cells

This week, scientists from the universities of Groningen (the Netherlands), Cambridge (UK) and Mons (Belgium) published an article in an online Science publication on the physics behind functionality of plastic solar cells.  They have discovered a crucial step in the charge transfer process that explains why this transfer is only partially efficient. This new knowledge paves the way to higher electricity yields of this sustainable source of energy.

The physicists hope for a breakthrough: ‘Our new understanding of the mechanism behind electricity generation in plastic solar cells will contribute to choosing the right materials boosting the efficiency of the next generation of solar cells.’

Mass Production

Conventional solar cells are made of silica. They have been available for some time with the highest efficiency to date of 28 per cent. Plastic solar cells have only just appeared on the market. They are less efficient but they hold important promise for the future: cheap mass production. Rows upon rows of plastic solar cells -- as far as the eye can see -- could convert the deserts of today into the power stations of tomorrow.

Charge Transfer

Plastic (also known as ‘organic’) solar cells convert sunlight into electricity by transferring electrons between two different types of molecules: the donor and the acceptor. Before now, it was not exactly understood how the electron transfer occurs, and which material characteristics determine the efficiency of the process, critical factors in how solar cells function.

It has been generally assumed that the energy difference between the quantum states of the two types of molecules determined the efficiency of the charge transfer and thus the electricity production. However, the research published in Science demonstrates that despite the importance of this difference, it is not decisive at the end of the day. The decisive mechanism is an extremely short-lived charge-transfer state. Within this state, the positive and negative charges of the various molecules are only very weakly linked to each other. This allows them to be split apart relatively easily and thereby release electricity.

Partially effective

When an organic solar cell catches sunlight, a part of this light is converted directly into charge carriers via the short-lived state. However, the majority of the captured light is converted into a different electronically excited state, out of which only an inefficient pathway leads to further charge carrier production.

Using advanced laser technology, the researchers have demonstrated how this inefficient electronically excited state can be converted into an efficient short-lived charge-transfer state through application of short bursts of infrared light. This results in a substantial increase in the number of charge carriers and thus more electricity.

The University of Groningen researchers are affiliated to the Zernike Institute for Advanced Materials. The research was partially funded with a Rubicon grant from the Netherlands Organisation for Scientific Research (NWO) and by the Zernike Institute for Advanced Materials.

For more information:
- Prof. Paul van Loosdrecht

- Dr Maxim Pchenitchnikov

Referentie
‘The Role of Driving Energy and Delocalised States for Charge Separation in Organic Semiconductors’, Artem A. Bakulin, Akshay Rao, Vlad G. Pavelyev, Paul H.M. van Loosdrecht, Maxim S. Pshenichnikov, Dorota Niedzialek, Jérôme Cornil, David Beljonne, and  Richard H. Friend. Science (2012). DOI: 0.1126/science.1217745

The article appeared online on 23 February 2012 as a Science Express paper