U.S. Naval Research Laboratory (NRL) scientists in the Electronics Technology and Science Division – in collaboration with the Imperial College London and MicroLink Devices Inc. – have proposed a triple-junction solar cell that they say has the potential to break the 50% conversion efficiency barrier. This is the current goal in multi-junction photovoltaic development.
‘This research has produced a novel, realistically achievable, lattice-matched, multi-junction solar cell design with the potential to break the 50 percent power conversion efficiency mark under concentrated illumination,’ says Robert Walters, NRL research physicist. ‘At present, the world-record triple-junction solar cell efficiency is 44 percent 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 challenge is to develop a semiconductor material system that can attain a wide range of bandgaps and be grown with high crystalline quality, according to the scientists.
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 multi-junction solar cell that can achieve direct band gaps from 0.7 to 1.8 electron volts (eV) with materials that are 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,’ says Walters. ‘It is well known that materials lattice-matched to InP can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band gap.’