Multi-junction solar cell tipped to break efficiency barrier

Scientists at the US Naval Research Laboratory, in collaboration with Imperial College London and MicroLink Devices, have proposed a novel triple junction solar cell with the potential to break the 50% conversion efficiency barrier.

"At present, the world record triple junction solar cell efficiency is 44% under concentration, and it is generally accepted that a major technology breakthrough will be required for the efficiency of these cells to increase much further," said research physicist Robert Walters.

In multi-junction solar cells, each junction is 'tuned' to different wavelength bands in the solar spectrum to increase efficiency. A high band gap semiconductor material is used to absorb the short wavelength radiation with longer wavelength parts transmitted to subsequent semiconductors.

"In theory, an infinite junction cell could obtain a maximum power conversion percentage of nearly 87%," Walters noted. "The challenge is to develop a semiconductor material system that can attain a wide range of band gaps and be grown with high crystalline quality."

By exploring novel semiconductor materials and applying band structure engineering, via strain balanced quantum wells, the research team was able to produce a design for a multi junction solar cell with direct band gaps from 0.7 to 1.8 electron volts (eV), with materials that were all lattice matched to an indium phosphide (InP) substrate.

"Having all lattice matched materials with this wide range of band gaps is the key to breaking the current world record" Walters added. "It is well known that materials lattice matched to InP can achieve band gaps of about 1.4eV and below, but no ternary alloy semiconductors exist with a higher direct band gap."

According to Walters, the key innovation was the discovery of InAlAsSb quaternary alloys, a high band gap material layer that can be grown lattice matched to InP

The scientists modelled the band structure of InAlAsSb and showed that this material could potentially achieve a direct band gap as high as 1.8eV.

With this result, and using a model that includes both radiative and non radiative recombination, the team created a solar cell design which they believe has the potential to break the 50% conversion efficiency barrier under concentrated solar illumination.

Laura Hopperton

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