Washington, Nov 12 : Engineers have said that to really increase the efficiency of solar cells, they should be made to look dull.
According to Darren Bagnall, an engineer at the University of Southampton in the UK, engineers are not interested in producing gleaming new solar panels and the familiar lustrous deep blue of silicon solar cells has to change.
"Look at a solar cell at the wrong angle and you seem to get the whole sun reflecting back at you," he said.
A bare silicon solar cell absorbs only two-thirds of the light shone upon it, with the protective glass layer on top making things worse.
To address that, anti-reflective coatings are added to the silicon of commercial solar cells, but they still allow much light to be reflected.
Most cells use a coating that is designed to force reflecting photons to get tangled up with new photons striking the cell.
The interference causes both outgoing and incoming photons to be absorbed, but typically only cuts reflectance over a fraction of the visible spectrum.
The record-setting efficiency of the world's most effective solar cells, unveiled this month, was in part due to their dullness.
An anti-reflective surface cuts reflection to 3 percent across most of both the visible and infrared spectrums, helping them to convert 25 percent of light energy into electricity.
Martin Green at the University of New South Wales, Australia, led the team that set the new world standard.
Their complex coating also uses interference. But it is built to contain a reflecting layer of its own, increasing the chances of interference occurring by delaying photons' exit from the cell.
A new approach to the reflection problem has just been suggested by Shawn-Yu Lin's team at the Rensselaer Polytechnic Institute in Troy, New York.
Lin's anti-reflective coat contains seven layers, made from just two materials - silica and titanium dioxide.
Instead of creating abrupt reflecting surfaces to generate interference, it presents photons with a kind of ramp that smoothes their entry into a cell.
The top layer of their coating is silica-rich, with successive layers progressively richer in titanium dioxide towards the bottom. That produces a gradient in refractive index between air and the underlying solar cell, so photons are much less likely to bounce.
According to Lin, his coating, made from cheap materials, should add 4 percent at most to the cost of producing a cell, and cut reflection to just 4 percent across the visible and near-infrared spectrums.