Healing the defects in perovskites. Photo Credit: Matthew T. KlugA new study by an international team of researchers has shown that defects in perovskites, the material touted to provide the next generation of cheap solar cells and LEDs, can be corrected using a combination of light and humidity. They demonstrated that defects in the molecular structure of perovskites could be resolved by exposing them to light and specific amounts of humidity to improve their performance in converting sunlight into electricity.
The research, which was published in the journal Joule [Brenes et al. Joule (2017) DOI: 10.1016/j.joule.2017.08.006], was based on earlier work by the team that found defects in perovskites could be temporarily healed by exposing them to light. They had found that when perovskites were illuminated, iodide ions moved away from the light, removing most of the defects in the area along with them. However, it was a temporary solution as the ions then migrated back to similar positions when the light was taken away.
This time around, an expanded research team were able to show the defects in the crystalline structure – which can result in electrons being trapped before their energy can be harnessed – can be healed for much longer, making the material potentially useful in inexpensive, high-performance solar cells and other light-emission applications to replace the more expensive and energy-intensive silicon currently used.
“It's counterintuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells”Sam Stranks
The team produced a perovskite-based device that was printed based on techniques compatible with scalable roll-to-roll processes. However, before completion, they exposed the device to light, oxygen and humidity. On testing, it was shown that when humidity levels were 40–50% and exposure was limited to just half an hour, there was no degradation at all. Once the exposure was complete, the remaining layers were deposited to finish the device.
When light is applied, electrons bind with oxygen, forming a superoxide able to effectively bind to electron traps and prevent them from hindering electrons. With water present, the surface of the perovskite converts into a protective shell, with the shell coating removing traps from the surfaces but also enclosing the superoxide so that the effect is more permanent. The combination of light and humid air produces significant enhancements in the films – as research leader Sam Stranks said “It's counterintuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells”.
They are now looking to optimise the treatments through accelerated stability tests so that the performance improvements will last for years rather than the weeks demonstrated here, as well as to test whether they are applicable to other types of perovskites, including the colored perovskite films used in LEDs and photovoltaic devices.