The move could boost silicon solar cell efficiency from its theoretical maximum of 29.1 percent to over 35 per cent using excitons created in a thin film on top of the silicon.
The idea of excitons has been around for many years, and six years ago researchers at the Center for Excitonics at the Massachusetts Institute of Technology (MIT) in the US showed that they could generate two packets of energy, or excitons, from a single photon, rather than a single electron, by using a thin layer of tetracene and an organic solar cell.
The challenge since then has been to build a working silicon cell and transfer the two excitons efficiently into the silicon. This has been shown in a paper in Nature by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University.
As an intermediate step, the team tried coupling the energy from the excitonic layer into quantum dots. “They’re still excitonic, but they’re inorganic,” said Baldo. “That worked; it worked like a charm.”
This showed that the key to the energy transfer lies in the surface of the material, not in its bulk, so the key was in a thin intermediate layer of hafnium oxynitride just 8 angstroms, or a few atoms, thick. This doubles the energy produced by sunlight in the blue and green part of the spectrum, giving the boost in the solar cell efficiency from 29.1 percent, up to a maximum of about 35 percent. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” said Einzinger. So far, no other material they’ve tested can match its properties.
“We still need to optimize the silicon cells for this process,” said Baldo, as these cells can be signficantly thinner and lighter than current versions. Work also needs to be done on stabilizing the materials for a longer lifetime. Overall, commercial applications are probably still a few years off, the team says.
This compares to tandem cells which typically place a perovskite cellon top of the silicon. “They’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells,” said Baldo.