US team closes in on naturePlants are still well ahead of the best semiconductor photocells in the photon-collecting game, and researchers are getting closer to harnessing the technology that has taken millions of years to evolve – but they have not quite cracked it yet, writes Steve Bush. Anyone who studied biology at school knows that water and carbon dioxide are converted to sugar and oxygen inside plants, and that this reaction is driven by photosynthesis. What is less well known is that electric current is the intermediary between photon absorption and chemical reaction. The parts of a plant that convert light into current are called reaction centres and it has long been possible to extract these undamaged and measure their properties using micro-probes. They have even been attached to surfaces, but not in any structured way that might offer a route to making photocells. Honda’s Dream… Raced across Australia in 1996, its monocrystaline silicon cells produce 1.9kW with efficiencies above 22%. Now a team of three workers at Oakridge National Laboratory in Tennessee has found a way to construct rectangular planar arrays of reaction centres on an artificial substrate, all orientated in the same direction. “We have got the reaction centres to self-align so that the electron-emitting ends face the same way,”said Eli Greenbaum of the ORNL team. Greenbaum is quick to point out that this is all he has achieved so far: “We have measured the voltage-current characteristics of reaction centres in the array, but have not tried to detect photocurrent yet.” Then continued: “Although without crazy science fiction you can see some interesting applications.” One is a micro-imager. “If you could wire up the reaction centres, and this is not outside sound scientific principles, you could make an optical sensor with a resolution between 10 and 100nm,” said Greenbaum. Another suggestion is to lay down reaction centres onto nano-machines as a way of powering them. How efficient a solar cell could be made using plant reaction centres is still unknown. Reaction centres are known to capture around 99 per cent of photons that impinge on them. Potential cell efficiencies of 50 per cent have been mentioned in the past. Beans in a row… ORNL scientists have successfully aligned photosynthetic reaction centres on an artificial substrate for the first time. Cleaved mica is chosen as a base because it is molecularly flat. A thin layer of gold adheres to the mica and provides electrical contact. The key to the ORNL breakthrough is the use of sulphur-headed complex molecules which stick to the gold and attract a particular end of photosynthetic reaction centres. The result is a flat layer of photoreceptor which could, in future, convert light usefully into electricity.Greenbaum’s research partners are: Dr James Weifu Lee and Dr Ida Lee.