Solar panels of unprecedented efficiency at accessible prices could be created with very thin wires of gallium arsenide (GaAs) semiconductors combined with existing technology. These semiconductors can be formed into solar cells with exceptional power-to-weight ratios.
GaAs solar cells have excellent efficiency, but are so expensive to manufacture they’re currently only used in places where the price is a minor consideration, such as on satellites. That could be about to change.
In a paper published in ACS Photonics, a team at the Norwegian University of Science and Technology (NTNU) describe the production of wires with p-type GaAs cores surrounded by n-type AlGaAs shells. The wires have thicknesses similar to the wavelengths of the sunlight they capture.
The 7.7 percent efficiency is still well below commercial products, let alone the best produced in the lab, but already the power-to-weight ratio is excellent. Adjusting both the size and the pitch of the nanowires could improve efficiency, and the authors think they’re just getting started.
They foresee a world where GaAs does not replace silicon, but the two work together, with GaAs built on silicon cell substrates to capture higher proportions of the sunlight’s energy than can be done now at a realistic cost.
Silicon requires high-temperature processing to make into cells, but it’s among the Earth’s most abundant materials. Gallium, on the other hand, is scarce, and never concentrates much naturally. The world produces just a few hundred tonnes a year, an obstacle to making millions of solar panels out of it annually. If cells are made out of wires a few atoms thick, rather than thick GaAs substrates, however, the economics could shift.
“Our research group has found a new way to make an ultrahigh power-per-weight ratio solar cell that is more than 10 times more efficient than any other solar cell, by using GaAs in a nanowire structure,” Professor Helge Weman of NUST said in a statement.“Our method uses a vertically standing semiconductor nanowire array structure on a cheap and industry-favorable Si platform to grow the nanowires.”
Better still, if the silicon substrate is actually a silicon cell in its own right, the two can work together to capture up to 40 percent of the energy in sunlight. GaAs harvests blue and ultraviolet light more efficiently than silicon, but lets red and orange photons through, allowing collection by the silicon below. This theoretically enables efficiencies double those of existing commercial solar panels.
Alternatively, if lightweight substrates such as graphene are used, the cells – while less efficient and possibly more expensive than on silicon – would be exceptionally light and flexible, making them suitable for powering drones.
“We grow the nanowires using a method called MBE (molecular beam epitaxy), which is not a tool that can produce materials at a high volume,” said first author and PhD candidate Anjan Mukherjee. However, the team believes metal organic vapor deposition, which is much more suited to scaling up, will also work for nanowire manufacturing.
Silicon cells have made solar power competitive with fossil fuels, and currently dominate the market. However, there is a widespread perception that they have got almost as good as they can become. Indeed, for the first time in many years, the cost of solar panels is rising as shortages of polysilicon create a squeeze that’s undermining many projects’ financial viability.
Gallium arsenide has a long history as a semiconductor, and many attributes that make it well-suited to solar cells, including a wider band gap, allowing it to extract more energy from high frequency photons, and better conductivity.