Lotus leaf solar cells soak up more power...

New Scientist, Paul Marks

Solar cells have an unfortunate habit of reflecting back much of the light that hits them, rather than converting it into electricity. A technique that peppers the cells' surface with nanoscale domes could curb this tendency and improve efficiency by as much as 25 per cent.

Peppering the cell's surface with nanodomes increases efficiency by as much as 25 per cent

To ramp up the proportion of light solar cells converted into electricity, Yi Cui of Stanford University in California has focused on ensuring that more light gets through the layers of silicon.

Low-power solar cells in devices such as wristwatches and calculators are made of amorphous silicon, which converts light in the range of 400 to 800 nanometres into electricity. However, around 35 per cent of light is reflected back into the sky.

Cui's team cut this to a mere 6 per cent by depositing the solar cell's usual sandwich of layers onto a quartz base, studded with an array of 100 nm-wide cones set at 450-nm intervals (Nano Letters, DOI: 10.1021/nl9034237).

The first layer deposited is a silver reflector that ensures photons that would otherwise be wasted are bounced back up towards the active layer (see diagram). Then comes a transparent electrode, one made of an active semiconductor, and a final layer of transparent electrode.

Viewed under an electron microscope the surface of the solar cells is covered with dome shapes that look a little like ranks of eggs.

The lack of reflectivity is in fact pretty clear at a glance, says Cui: "The nanodome device looks black while the flat device looks reflective and red, as the red light does not get absorbed." In optical simulations, Cui's team found that the domes acted as waveguides, channelling light towards the active area, a bit like an optical fibre.

In lab tests the extra light made the nanodome solar cells 5.9 per cent efficient, compared with 4.7 per cent for traditional flat film amorphous cells. Cui expects that the efficiency of commercial thin film nanodome cells will be much higher.

The roughness of the new cells at nanoscales also mimics the fibrous bumps on the leaves of the lotus plant, which help it repel water. Water droplets landing on the leaf cannot achieve a contact angle that breaks their surface tension, so they form beads on the leaves rather than wetting them. In the same way water drops will roll off the surface of the nanodome solar panel taking any light-blocking dust with them.

Cui is confident the nanodome technique will also work for solar panels made from the widely used and more efficient polycrystalline silicon. The cells in these panels gather light at wavelengths up to 1500 nanometres.

Darren Bagnall, a solar cell engineer at the University of Southampton, UK, is impressed. "It's a beautiful device," he says. But he cautions that changing the geometry of the cells to achieve the higher efficiencies required of commercial versions could undermine their anti-reflective and self-cleaning properties.

Cui is undaunted. "The geometry of the nanodomes will need to be tuned slightly for different materials but it will work."

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