ECO SPECIAL...Solar cells made from plant waste

Within a few years,people in remote villages could make their own solar panels,at low cost,using otherwise worthless agricultural waste as the raw material In a project that began eight years ago,researcher Shuguang Zhang was able to use complex of molecules known as photosystem-I (PS-I ),the tiny structures within plant cells that carry out photosynthesis. Zhang and colleagues derived the PS-I from plants,stabilised it chemically and formed a layer on a glass substrate that could like a conventional photovoltaic cell produce an electric current when exposed to light. But that early system had some drawbacks.Assembling it required expensive chemicals and equipment.Whats more,the resulting solar cell was weak: Its efficiency was low,it had to be blasted with a strong laser to produce current. ANY LAB ANY TIME Now Massachusetts Institute of Technology researcher Andreas Mershin,whose work appears in the journal Scientific Reports claims the process has been simplified to the point that virtually any lab could replicate it,allowing researchers around the world to start exploring the process and making further improvements.The new systems efficiency is 10,000 times greater than in the previous version although in converting just 0.1 per cent of sunlights energy to electricity,it still needs to improve another tenfold or so to become useful,he says. EFFICIENT LIKE PINE TREES The key to achieving this huge improvement in efficiency,Mershin explains,was finding a way to expose much more of the PS-I complex per surface area of the device to the sun.Zhangs earlier work simply produced a thin flat layer of the material;Mershins inspiration for the new advance was pine trees in a forest. Mershin noticed that while most of the pines had bare trunks and a canopy of branches only at the very top,a few had small branches all the way down the length of the trunk,capturing any sunlight that trickled down from above.He decided to create a microscopic forest on a chip,with PS-I coating his trees. Mershin was able to create a tiny forest of zinc oxide (ZnO) nanowires as well as a spongelike titanium dioxide (TiO2) nanostructure coated with the light-collecting material derived from bacteria.The nanowires not only served as a supporting structure,but also as wires to carry the flow of electrons,from which it could be connected to a circuit.Its like an electric nanoforest, he says. Because the ingredients are so cheap and the processing so simple progress,toward improving their efficiency should be rapid.Ultimately,once the efficiency reaches 1 or 2 per cent. You can use anything green,even grass clippings as the raw material,he says.While centrifuges were used to concentrate the PS-I molecules,the team has proposed a way to achieve this concentration by using inexpensive filters.No special lab conditions are needed,Mershin says: It can be very dirty and it still works,because of the way nature has designed it. Mershin says,within a few years a villager in a remote,off-grid location could take that bag,mix it with anything green and paint it on the roof to start producing power,which could then charge cellphones or lanterns.

(MM7FEB12)