New world record set for solar cell efficiency

Scientists have developed a new solar cell with efficiency of 34.5 per cent - establishing a new world record for unfocused sunlight and nudging closer to the theoretical limits for such a device.

The record was set by Mark Keevers and Martin Green of University of New South Wales (UNSW) in Australia using a 28 square centimetres four-junction mini-module - embedded in a prism that extracts the maximum energy from sunlight.

It does this by splitting the incoming rays into four bands, using a hybrid four-junction receiver to squeeze even more electricity from each beam of sunlight.

The result, confirmed by the US National Renewable Energy Laboratory, is almost 44 per cent better than the previous record made by Alta Devices of the US, which reached 24 per cent efficiency, but over a larger surface area of 800 square centimetres.

“This encouraging result shows that there are still advances to come in photovoltaics research to make solar cells even more efficient,” said Keevers.

“Extracting more energy from every beam of sunlight is critical to reducing the cost of electricity generated by solar cells as it lowers the investment needed, and delivering payback faster,” he said.

The result was obtained by the same UNSW team that set a world record in 2014, achieving an electricity conversion rate of over 40 per cent by using mirrors to concentrate the light a technique known as CPV (concentrator photovoltaics)  and then similarly splitting out various wavelengths.

The new result, however, was achieved using normal sunlight with no concentrators. “What’s remarkable is that this level of efficiency had not been expected for many years,” said Green.

The record-setting mini-module combines a silicon cell on one face of a glass prism, with a triple-junction solar cell on the other.

The triple-junction cell targets discrete bands of the incoming sunlight, using a combination of three layers: indium-gallium-phosphide; indium-gallium-arsenide; and germanium.

As sunlight passes through each layer, energy is extracted by each junction at its most efficient wavelength, while the unused part of the light passes through to the next layer, and so on.

Some of the infrared band of incoming sunlight, unused by the triple-junction cell, is filtered out and bounced onto the silicon cell, thereby extracting just about all of the energy from each beam of sunlight hitting the mini-module.

The 34.5 per cent result with the 28 square centimetre mini-module is already a world record, but scaling it up to a larger 800 square centimetre - thereby leaping beyond Alta Devices’ 24 per cent - is well within reach.