Novel solar cell could break conversion efficiency barrier

Scientists from the US Naval Research Laboratory (NRL) have proposed a novel triple-junction solar cell with the potential to break the 50 per cent conversion efficiency barrier.

‘This research has produced a novel, realistically achievable, lattice-matched, multi-junction solar cell design with the potential to break the 50 per cent power conversion efficiency mark under concentrated illumination,’ said Robert Walters, PhD, NRL research physicist. ‘At present, the world record triple-junction solar cell efficiency is 44 per cent 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.’

The work was carried out in collaboration with Imperial College London and MicroLink Devices.

According to NRL, each junction in multi-junction (MJ) solar cells is tuned to different wavelength bands in the solar spectrum to increase efficiency.

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 per cent. 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 NRL research team has produced a design for a MJ solar cell that can achieve direct band gaps from 0.7eV to 1.8eV with materials that are 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’ said Walters in a statement. ‘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.’

The primary innovation enabling this new path to high efficiency is claimed to be the identification of InAlAsSb quaternary alloys as a high-band-gap material layer that can be grown lattice-matched to InP.

Drawing from their experience with Sb-based compounds for detector and laser applications, NRL 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 radiative and non-radiative recombination, the NRL scientists created a solar cell design that is a potential route to more than 50 per cent power conversion efficiency under concentrated solar illumination.

Recently awarded a US Department of Energy, Advanced Research Projects Agency-Energy (ARPA-E) project, NRL scientists, working with Niles, Illinois-based MicroLink and Rochester Institute of Technology, will execute a three-year materials and device development programme to realise this new solar cell technology.