Triple Junction Solar Cell Proposal May Break 50% Efficiency
The world record for solar cell efficiency we wrote up last week may have competition soon. Scientists at the U.S. Naval Research Laboratory in association with Imperial College of London and MicroLink Devices, Inc, have proposed a triple-junction solar cell design. The cell has the potential to bust through the 50 percent conversion efficiency barrier, a present goal in multi-junction photovoltaics.
“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,” said NRL research physicist Robert Walters, Ph.D. “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.”
Multi-junction (MJ) solar cells are cells in which each junction is ‘tuned’ to different solar spectrum wavelength bands in order to enhance efficiency. High bandgap semiconductor material is used to absorb the short wavelength radiation with longer wavelength parts transmitted to subsequent semiconductors.
Theoretically, an infinite-junction cell would be able to achieve maximum power conversion percentage of almost 87 percent. The test for scientists is to develop a semiconductor material scheme that can arrive at a wide range of bandgaps and also be grown with high crystalline quality.
Through the investigation of novel semiconductor materials and engineering the band structure via strain-balanced quantum wells, the Naval Research Laboratory researchers have come up with a design for a MJ 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” explains Walters. “It is well known that materials lattice-matched to indium phosphide can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band-gap.”
The main innovation allowing this new corridor to high efficiency is the identification of InAlAsSb quaternary alloys as a high band gap material layer that can be grown lattice-matched to indium phosphide.
Drawing from their experience with Sb-based compounds for detector and laser applications, Naval Research Laboratory scientists modeled the band structure of InAlAsSb and showed that this material could potentially achieve a direct band-gap as high as 1.8eV.
Using that result, with a model that includes both radiative and non-radiative recombination, the NRL scientists created a solar cell design that is a potential route to over 50 percent power conversion efficiency under concentrated solar illumination.
Illustration Credit: U.S. Naval Research Laboratory