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Here comes the Sun

Marc Baldo shines light on phycobilisomes

By Hamish Johnston

It’s a dull morning here in Pittsburgh — and the view out the press room window is of a rusting railway viaduct, grey pavement, hills covered in leafless brown trees and a leaden sky.

However in a couple of months it will be hot and hazy here as the city basks under a Mediterranean-strength Sun. You might think this would make Pittsburgh a perfect place to deploy solar panels (at least in the summer) — but there is a problem, all that haze makes it difficult to focus sunlight onto high-performance photovoltaics.

This focussing is necessary because it is very difficult to make large-area photovoltaics from semiconductors. The materials and processing are expensive and it is tricky to make large devices without defects, which reduce their efficiency.

One solution is to simply use an optical system of lenses and/or reflectors to concentrate the light at a photovoltaic. The problem is that such systems must track the Sun precisely — which is tough to do when it is lurking in the haze.

A better way would be to take a hint from nature and capture diffuse light and then concentrate it on an efficient photovoltaic. However like most biomimicry, this is easier said than done.

This morning MIT’s Marc Baldo talked us through a number of approaches that he was taking in his lab. The most successful one, it seems, is using glass plates containing fluorescent dye. Sunlight enters the plate via the broadside and causes the dye to emit light. This light then travels along the plate to one edge, where it can enter a photovoltaic.

The advantage is that much of the light captured by the large broadside of the plate is re-emitted as light that leaves the plate via the much smaller edges — concentrating it where it can be converted to electricity by a relatively small photovoltaic.

However, Baldo and team had to cleverly engineer the energy levels in the dye to ensure that the light destined for the phovoltaic is not reabsorbed and ultimately scattered out of the plate.

This is done using a molecules called phycobilisomes — proteins that are involved in photosynthesis.

Hopefully this will allow Baldo’s devices to be a bit more like plants — not solid chunks of semiconductor — by separating the functions of light absorbtion ad charge production.

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