Solar cell material can recycle light to boost efficiency

Scientists have discovered that a group of perovskite-based materials can recycle light, a finding that could lead to large gains in the efficiency of solar cells.
The group of synthetic materials known as hybrid lead halide perovskites appear to promise a revolution in the field of solar energy, researchers said.
Perovskite solar cells are cheap and easy to produce, and have become almost as energy-efficient as silicon - the material currently used in most household solar panels.
Solar cells work by absorbing photons from the Sun to create electrical charges, but the process also works in reverse, because when the electrical charges recombine, they can create a photon.

The research shows that perovskite cells have the extra ability to re-absorb these regenerated photons - a process known as "photon recycling".
This creates a concentration effect inside the cell, as if a lens has been used to focus lots of light in a single spot.

"It's a massive demonstration of the quality of this material and opens the door to maximising the efficiency of solar cells," said Felix Deschler from the University of Cambridge.
"The fabrication methods that would be required to exploit this phenomenon are not complicated, and that should boost the efficiency of this technology significantly beyond what we have been able to achieve until now," said Deschler.

The study involved shining a laser on to one part of a 500 nanometre-thick sample of lead-iodide perovskite.
Perovskites emit light when they come into contact with it, so the team was able to measure photon activity inside the sample based on the light it emitted.
Close to where the laser light had shone on to the film, the researchers detected a near-infrared light emission.
However, this emission was also detected further away from the point where the laser hit the sample, together with a second emission composed of lower-energy photons.
Researchers also manufactured the first demonstration of a perovskite-based back-contact solar cell.

This single cell proved capable of transporting an electrical current more than 50 micrometres away from the contact point with the laser; a distance far greater than the researchers had predicted, and a direct result of multiple photon recycling events taking place within the sample.
The researchers now believe that perovskite solar cells may be able to reach considerably higher efficiencies than they have to date.
"The fact that we were able to show photon recycling happening in our own cell, which had not been optimised to produce energy, is extremely promising," said Richard Friend, from the University of Cambridge.

"If we can harness this it would lead to huge gains in terms of energy efficiency," Friend said.
The study was published in the journal Science.