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Molecular structures known as perovskites have been hailed as a wonder material which could revolutionize the solar cell industry because they are cheap and easy to produce. In fact, after just a few years of development they have become almost as efficient as silicon at converting sunlight in to electricity.
However, they are not without their problems, tiny defects in the crystalline structure of perovskites – called traps – can cause electrons to get stuck before their energy has been harnessed. The easier an electron can move around in a solar cell material, the more efficient that material will be at converting photons, or particles of light, into electricity. These so-called traps hamper the efficiency of the perovskite material.
A team of Researchers from the Universities of Cambridge, Oxford and Bath, Massachusetts Institute of Technology and Delft University of Technology report that these defects can be permanently healed by exposing it to light and just the right amount of humidity.
In perovskite solar cells and LEDs, you tend to lose a lot of efficiency through defects. We want to know the origins of the defects so that we can eliminate them and make perovskites more efficient.
Dr Sam Stranks, Leader of the research and Marie Curie Fellow, MIT and Cambridge
Stranks was part of a team who, in 2016, found that defects in perovskite could be healed by light. When perovskites were exposed to illumination, iodide ions – iodine atoms stripped of an electron so that they carry an electric charge – traveled away from the illuminated region, and in the process cleared away most of the defects in that region along with them. However, the effects were only temporary because the ions migrated back to similar positions when the light was removed.
In their latest paper – published in the inaugural edition of Joule – the team report that the defects can be permanently healed, which could further accelerate the development of cheap, high-performance perovskite-based solar cells that rival the efficiency of their silicon counterparts.
The team used techniques compatible with scalable roll-to-roll processes to print a perovskite-based device, but before the device was completed, they exposed it to light, oxygen and humidity. Perovskites frequently start to degrade when exposed to humidity, but the team found that when humidity levels were between 40 and 50%, and the exposure was limited to 30 minutes, the perovskite did not degrade. Once the exposure was complete, the remaining layers were deposited to finish the device.
When light was applied to the material, electrons bound with oxygen, forming a superoxide that could very effectively bind to electron traps and prevent them from hindering electrons. In the accompanying presence of water, the perovskite surface also gets converted to a protective shell. The shell coating removes traps from the surfaces but also locks in the superoxide, meaning that the performance improvements in the perovskites are now long-lived.
It’s counter-intuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells. We’ve seen an increase in luminescence efficiency from 1% to 89%, and we think we could get it all the way to 100%, which means we could have no voltage loss – but there’s still a lot of work to be done.
Dr Sam Stranks, Leader of the research and Marie Curie Fellow, MIT and Cambridge
Dr Stranks, is now based at Cambridge’s Cavendish Laboratory.
Most solar cells on the market today are silicon-based, but since they are expensive and energy-intensive to produce, Researchers have been searching for alternative materials for solar cells and other photovoltaics. Perovskites are perhaps the most promising of these alternatives as they are cheap and easy to produce.
Image Credit:
Dr Matthew T Klug, University of Oxford
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