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Trapping light to improve solar cells
Scientists have uncovered why thinner solar cells with well organised patterns on their surface are better able to trap and absorb light.

The discovery could mean cheaper and more efficient solar cell technology.

Scientists have known that the efficiency of solar cells is improved when light is allowed to bounce inside the cell, increasing the chances that it will be absorbed.

One way to do this is by 'roughening up' the surface of the solar cell so that photons entering the material tend to be reflected inside it.

Reporting on the pre-publication website ArXiv, Dr Zongfu Yu and fellow researchers from Stanford University in California, have calculated the fundamental limit to the amount of light that can be trapped in a nano photonic solar cell.
Trapping light

The researchers have shown that by using nanotechnology, it is possible to trap more light than previously thought possible.

Yu says this is because light trapping works in a different way on the nano scale.

"Instead of total internal reflection, light becomes trapped on the surface of nano layers, which act like waveguides. This increases the amount of time the photons spend in the material and so also improves the chances of absorption.

"Because of the geometry of the layers, some wavelengths are trapped better than others and this gives rise to resonances at certain frequencies."

The researchers found that by designing the layers in a way that traps light effectively on the surface through the use of a patterned cell, it's possible to capture 12 times the amount of light.

The research also found that thinner layers are also more effective at light trapping.

Thinner solar cells use less material, are cheaper to make, and according to previous research the electrons they produce are also easier to collect, making them potentially more efficient.

"It's now possible to make layers much thinner than the wavelength of the light they are expected to absorb and to carve intricate patterns in these layers," says Yu.
Plastic cells

Professor Martin Green of the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales says the work is important for research into areas like organic semi-conductors, or plastic solar cells.

"There's a lot of interest in trying to make plastic solar cells," he says.

"Plastic is not really an ideal material for solar cells because it's not particularly conductive or durable. The lack of conductivity in the plastic means it has to be very thin."

But Green says, "the thicknesses they're talking about are similar in size to those needed for plastic solar cells [to work]."

He adds, the research highlights another advantage for plastic solar cells.

"Plastics have a low optical density and it just happens that low optical density is ideal for this new effect."

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