A team of Australian researchers from the University of New South Wales is racing another tam from MIT in the US to boost the efficiency limits of silicon solar cells using quantum mechanics.Â
Silicon solar cell efficiency – the fraction of energy supplied by the sun that can be converted into electricity – is theoretically limited to an “absolute limit” of 29.4 per cent, according to Ned Ekins-Daukes from UNSW’s School of Photovoltaics & Renewable Energy Engineering.
In the real world, the highest efficiency was set earlier this year by China solar giant LONGi, which demonstrated a 27.3 per cent efficient silicon solar cell.
Attempts to crack the silicon ceiling have spanned from trying to stablise perovskites to just adding another side with bifacial panels. Another potential solution – and one being pursued by a multi-disciplinary team from the UNSW – is singlet fission.
The idea is that ordinary silicon panels can get more bang for their buck by splitting light energy into two, so there’s more for a silicon panel to absorb and less energy is wasted as heat.
“The basic effects of the process is that light can be absorbed in this material and then be split into two separate energy packets. If you do that, you can use high energy light more efficiently in a solar cell,” says lead researcher Professor Timothy Schmidt from UNSW Sydney’s School of Chemistry.
“We’re confident we can get silicon solar cells to an efficiency above 30 per cent.”
ARENA wants cheap quantum-led solar
This is good news for the Australian Renewable Energy Agency (ARENA), which is funding the UNSW work to the tune of a $4.8 million grant in the Ultra Low Cost Solar program, which supports technologies with the potential to deliver more than 30 per cent efficiency below a 30 cents per watt cost by 2030.
Schmidt says they’re barely a year into the five-year project but they’re well ahead of their targets, having already hit the 18 month milestone and are now six months away from the three year milestone.
The breathtaking pace is because the project is the result of 15 years of previous research.
Schmidt and his longtime collaborator Professor Ned Ekins-Daukes have been pitching for funding to get the project off the ground since 2015, but it was only when MIT published their first demonstration model that Australian funders were comfortable enough with the technical readiness of the concept.
“This is our big chance,” Schmidt says.
“We’re at the stage where we’ve got a cell that generates current in silicon, generated from singlet fission. It does not yet give us a net improvement because whenever you put a coating on the front of a solar cell you’re absorbing some light.
“We’re not quite there but we’re confident we’ll get there within the next six months because MIT have already achieved that.”
Spin doctors
The quantum element is in the “spin”.
“[Singlet fission is] the journey that the energy takes between being created by a photon and being two separate energy packets, which are confusingly called triplets,” Schmidt says.
“A spin is a quantum mechanical property. Electrons have a spin of a half. If the spins cancel out it’s a singlet, if they reinforce each other to add to one it’s called a triplet, because it splits into three states if you apply a magnetic field.”
But applying this to silicon solar cells is difficult, because scientists don’t know the details of how the two triplets move away from each other. Without that knowledge, it’s tricky to design new singlet fission materials that will do what researchers want them to.
Currently the UNSW research team is working with a material called tetracene, it’s well-studied but “a bit fragile and it won’t survive in full sunshine”.
Schmidt’s hope is for automotive paints, which are designed to survive for decades in the sun and for an as-yet unknown reason have really good singlet fission properties.
The latest research by Schmidt, Ekins-Daukes, and their team of 15 researchers addresses that very problem: what do triplets do after they separate from a singlet?
They created an experimental tool that uses a single wavelength laser to “excite” singlet fission material. An electromagnet reduces the speed of the singlet fission process which makes it easier to see and measure what is happening.
If the team can measure the singlet fission process and simultaneously measure the current in a silicon cell, they can tell whether the material is working, Schmidt says.
