Multijunction solar cell breaks efficiency barrier

A collaboration of scientists from the US Naval Research Laboratory Electronics Technology and Science Division, Imperial College London and MicroLink Devices has proposed a novel triple-junction solar cell with the potential to break the 50% conversion efficiency barrier, which is the current goal in multijunction photovoltaic development.

“This research has produced a novel, realistically achievable, lattice-matched, multijunction solar cell design with the potential to break the 50% power conversion efficiency mark under concentrated illumination,” said NRL research physicist Robert Walters. “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.”

Schematic diagram of a multijunction (MJ) solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency. Image: US Naval Research Laboratory.

In multijunction (MJ) solar cells, each junction is ‘tuned’ to different wavelength bands in the solar spectrum to increase efficiency. High bandgap 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%. The challenge is to develop a semiconductor material system that can attain a wide range of bandgaps 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 has produced a design for an MJ solar cell that can achieve direct bandgaps from 0.7 to 1.8 eV with materials that are all lattice-matched to an indium phosphide substrate.

“Having all lattice-matched materials with this wide range of bandgaps is the key to breaking the current world record,” adds Walters. “It is well known that materials lattice-matched to InP can achieve bandgaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct bandgap.”

The primary innovation enabling this new path to high efficiency is the identification of InAlAsSb quaternary alloys as a high bandgap material layer that can be grown lattice-matched to InP. Drawing from their experience with Sb-based compounds for detector and laser applications, scientists modelled the band structure of InAlAsSb and showed that this material could potentially achieve a direct bandgap as high as 1.8 eV. With this result, and using 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% 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 MicroLink and Rochester Institute of Technology, will execute a three-year materials and device development program to realise this new solar cell technology.