Organic Solar Cells: Spinning efficiency

Researchers at the University of Cambridge (Cambridge, U.K.) and the University of Washington (Seattle, Washington USA) have tackled the problem of relative inefficiency of organic solar cells by orchestrating their electronic ‘spin.’ It could be a breakthrough….

Organic solar cells have so much promise. Instead of silicon, these solar cells are constructed with polymers – long molecular chains, often combinations of carbon, oxygen, nitrogen and hydrogen – essentially a form of what we call plastic. Like most plastics, these solar cells can be thin, flexible, easily molded and even ‘printed’ with the new 3-D printing techniques. They’re also inexpensive to manufacture in quantity and absorb energy quite well. Yet organic solar cells have not stormed the market. Three reasons are usually given: They not very strong (they bend but are easily pulled apart). They are not very stable (a persistent tendency to molecular decomposition when bombarded with sunlight. If you’ve even owned a nylon tent you know what this means.), and they’re not very efficient relative to silicon cells (the killer, about 10-12% efficiency versus 20-25%).

Publishing the journal Nature [07 August 2013, paywalled, The role of spin in the kinetic control of recombination in organic photovoltaics ], the Washington and Cambridge researchers discovered that manipulating the ‘spin’ of electrons in organic solar cells significantly improves their efficiency. How much better? They don’t say and in all likelihood, they can’t say because the work is at the stage just beyond theoretical, the first demonstrations of concept. Without much larger samples of quantitative testing, accurate measurements of efficiency are not possible. Therefore, they’re going with their preliminary measurements and intuition – all they can say is this new way of production will make a significant improvement in organic solar cells.

Which gets to the issue of why so? It starts with a long-standing problem for the organic polymers used in solar cells – the molecules involved can be very efficient at capturing sunlight, but they have a tendency to ‘recombine’ (the physical configuration of the molecules shifts, unpredictably) and lose the stored energy. In fact, they go from an ‘excited’ state to an empty state known as the ‘hole,’ which greatly decreases their ability to produce electricity. In short, organic solar cells are not very efficient.

What started the researchers along a new path was a chance meeting at a conference in Italy, where they began discussing a new polymer developed at the University of Washington that included sulfur in the typically Carbon-Hydrogen-Nitrogen molecules. In essence, this new material exhibited fewer ‘dead ends’ (the holes) than other such materials. Intrigued by this property, the scientists at Cambridge put the material under sophisticated new electron spectrography (a type of microscope), which showed that a quantum mechanical property known as ‘spin’ was involved.

“Spin” when applied to electrons is a commonly used term, but not easy to explain. It doesn’t mean that the electrons spin like balls (rotation), for one thing an electron is too small to have a surface that could spin, only that they have a tendency for motion (called angular momentum) in a certain direction, labeled ‘spin up’ or ‘spin down.’ When materials containing electrons with different spin are mixed together, as they are in organic polymers, there is a strong tendency for spins to ‘cancel out’ (a metaphor) and produce configurations that lose energy.

The question was, how to control spin in the polymers so that (less) energy would be lost? The researchers solved this by discovering that, in their own words, in the polymer material, “…the relaxation of electron states…can be strongly suppressed by wavefunction delocalization…thereby reducing recombination and enhancing device performance.” Translated: by electromagnetically manipulating the electrons of the polymer’s molecules so that they remained connected with other molecules (delocalization), they wouldn’t recombine and lose their energy.

This approach worked, and in fact, worked so well that it the researchers believe this is the route to closing the efficiency gap between organic and silicon solar cells – or, to put it another way, levels the energy conversion playing field so that all the other strengths of organic solar cells can be in play.

This may be true, but a note of caution needs to be applied in that this is early research. It may be robust – translatable to manufacturing processes; but there is no guarantee that there won’t be technical problems that reduce the competitiveness of organic solar cells. Also, keep in mind that silicon cell technology is not standing still.

Research Spectrum

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