Electron ‘spin’ key to solar cell breakthrough

Organic solar cells, a new class of solar cell that mimics the natural process of plant photosynthesis, could revolutionise renewable energy - but currently lack the efficiency to compete with the more costly commercial silicon cells.

At the moment, organic solar cells can achieve as much as 12 per cent efficiency in turning light into electricity, compared with 20 to 25 per cent for silicon-based cells.

Now, researchers have discovered that manipulating the 'spin' of electrons in these solar cells dramatically improves their performance, providing a vital breakthrough in the pursuit of cheap, high performing solar power technologies.

The study, by researchers from the Universities of Cambridge and Washington, is published today in the journal Nature, and comes just days after scientists called on governments around the world to focus on solar energy with the same drive that put a man on the moon, calling for a "new Apollo mission to harness the sun's power".

Organic solar cells replicate photosynthesis using large, carbon-based molecules to harvest sunlight instead of the inorganic semiconductors used in commercial, silicon-based solar cells. These organic cells can be very thin, light and highly flexible, as well as printed from inks similar to newspapers - allowing for much faster and cheaper production processes than current solar cells.

But consistency has been a major issue. Scientists have, until now, struggled to understand why some of the molecules worked unexpectedly well, while others perform indifferently.

Researchers from Cambridge's Cavendish Laboratory developed sensitive laser-based techniques to track the motion and interaction of electrons in these cells. To their surprise, the team found that the performance differences between materials could be attributed to the quantum property of 'spin'.

'Spin' is a property of particles related to their angular momentum, with electrons coming in two flavours, 'spin-up' or 'spin-down'. Electrons in solar cells can be lost through a process called 'recombination', where electrons lose their energy - or "excitation" state - and fall back into an empty state known as the "hole".

Researchers found that by arranging the electrons 'spin' in a specific way, they can block the energy collapse from 'recombination' and increase current from the cell.

"This discovery is very exciting, as we can now harness spin physics to improve solar cells, something we had previously not thought possible. We should see new materials and solar cells that make use of this very soon" said Dr. Akshay Rao, a Research Fellow at the Cavendish Laboratory and Corpus Christi College, Cambridge, who lead the study with colleagues Philip Chow and Dr. Simon Gélinas.

The Cambridge team believe that design concepts coming out of this work could help to close the gap between organic and silicon solar cells, bringing the large-scale deployment of solar cells closer to reality. In addition, some of these design concepts could also be applied to Organic Light Emitting diodes, a new and rapidly growing display technology, allowing for more efficient displays in cell phones and TVs.

The short film clip above shows the researchers at work in the lab with the web of lasers they used in the research. Simon Gélinas explained that these laser techniques "make ultrafast movies at approximately one million billion frames per second", allowing the team to monitor what happens "from the moment the light is absorbed in the device until electrical current comes out".

"Through this, we've identified what mechanisms prevent the loss of electrical current in good organic solar cells. Now we can design materials knowing specifically how to harvest the most out of this process," he added.

The work on solar cells at Cambridge forms part of a broader initiative to harness high tech knowledge in the physics sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by both the UK Engineering and Physical Sciences Research Council and, in Cambridge, the Winton Programme for the Physics of Sustainability. The work at the University of Washington was supported by the US National Science Foundation and the U.S. Office of Naval Research.