Nature still has plenty to teach us about harnessing the sun's energy. The latest lesson comes from the ancient stromatolites living in Australia's Shark Bay. Their cyanobacteria contain a newly discovered form of chlorophyll – the fifth known – that absorbs sunlight in the red and infrared part of the spectrum. It could be harnessed to help solar cells convert more light into electricity.
Stromatolites are among the most primitive of life forms, with a fossil record stretching back over 3.4 billion years. Their layered structures are built up by sediment-trapping cyanobacteria. Stromatolites suffered markedly with the evolution of animals that munched on the defenceless algal mats, and are now found only in inhospitable environments where such animals are rare – including the very salty Shark Bay.
Min Chen of the University of Sydney in Australia, and her colleagues, went looking for interesting chlorophyll in the stromatolites there because the water in which they live – and the trapped sediment that bulks them out – filter out much of the visible light reaching the stromatolitic cyanobacteria. The team suspected that the cyanobacteria might therefore be better-than-average at absorbing the infrared radiation that makes it through.
Their hunch proved correct, but rather than familiar chlorophyll in new formations, they found a completely new type of chlorophyll – chlorophyll f – made by an as-yet unnamed filamentous bacterium.
Look and learn
Because over half of the light from the sun comes in at infrared wavelengths, the makers of photovoltaic panels have been working on ways to extend the section of the spectrum that solar cells can absorb to beyond red.
"Nature can use such simple modification of chlorophyll to acquire more sunlight," says Chen. "Why can we not learn from it?"
Shuguang Zhang of the Massachusetts Institute of Technology thinks that he and his colleagues can do more than learn from the chlorophyll – they can use it directly.
Zhang's previous work includes making solar cells using proteins from spinach leaves. These proteins, known as photosystem I, contain arrays of some 200 light-gathering chlorophyll molecules that use photons to free up electrons for fixing carbon dioxide into sugars.
If the electron is not immediately harvested from the photosystem for use in a solar cell, it will recombine with its hole – the region of positive charge it left behind – and re-emit a new photon. But in Zhang's set-up, the photosystem is anchored to a semiconductor nanowire capable of transferring that electron to a metal, putting photosystem I to work generating a current.
Widen the net
Adding different types of chlorophyll to these sorts of solar cell makes sense, says James Barber of Imperial College London. "Having a range of different chlorophylls with different absorption properties allows more of the viable solar spectrum to be captured," he says. "This is important for the design of photovoltaic cells and artificial photosynthesis technology."
Zhang agrees: "It's like a wider net to catch more fish." He is currently working with Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne, developer of low-cost dye-sensitised solar cells that use inorganic molecular dyes to absorb light in the same way that chlorophyll does. The pair is exploring whether new models of the cell could use cyanobacteria photosystems instead.
The discovery of the new chlorophyll is also important from an evolutionary standpoint: the anaerobic bacteria which dominated early earth absorbed infrared wavelengths, but the oxygen-loving bacteria of today tend to live on the visible spectrum. "There are very few examples of 'bridging organisms' which absorb in the 700 to 800 nanometres region," says Barber.
Journal reference: Science, DOI: 10.1126/science.1191127
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