A Little Light Disrupts Advanced Solar Cells, But More Fixes It

Microscope image of a single mixed-halide perovskite crystal. The central region was exposed to intense light, which caused the halide-ions in this region to mix, generating green (540-570 nm) fluorescence. The red emission (>660 nm) is from phase-segregated perovskite showing the stark difference in bandgap produced under more normal light intensity. ARC Centre of Excellence in Exciton Science

Many things in life are good in moderation but hazardous in excess. More unusually, the reverse is true for a promising class of solar cells. Ordinary sunlight exposure reduces their effectiveness, a fairly fundamental problem given their purpose, but more intense light provides a cure.

Perovskite solar cells have the potential to transform energy production, with efficiencies in capturing sunlight already matching those of silicon cells and rising fast, as well as a vanishingly low cost. Nevertheless, a few obstacles remain, including light-induced phase segregation.

All solar cells have a bandgap, the energy level of light required to cause them to produce electricity. For silicon cells, for example, the bandgap is at the edge of visible light, wasting infrared light falling on them. One great advantage of perovskite cells is that their bandgap is tunable, depending on the elements with which they are doped.

However, light falling on metal-halide perovskite cells disrupts the regular distribution of elements within their lattice. This means the carefully tuned bandgap varies across the surface of the cell by up to 100 nanometers, Dr Chris Hall of the University of Melbourne told IFLScience, the difference between blue and yellow light, or green and orange. Efficiency plummets, threatening perovskites' status as the future of solar cells. Many ways around the problem have been proposed but none are ideal.

Then, Hall said in a statement, "we were performing a measurement, looking for something else, and then we came across this process that at the time seemed quite strange.” Under very bright light, such as the concentration of sunlight through a powerful lens, the situation reversed. Hall and colleagues sought help from Dr Stefano Bernardi of the University of Sydney to explain their observations.

“We found as you increase the excitation intensity, the local strains in the ionic lattice, which were the original cause of segregation, start to merge together. When this happens, the local deformations that drove segregation disappear,” Bernardi said. As the material returns to a homogenous state, the bandgap becomes consistent.

Unfortunately, the restoration of the lattice does not last if the intensity of light drops off. Consequently, Hall told IFLScience, the discovery is of no use if you're trying to make solar cells to put on the roof of your house.

However, for applications where sunlight is to be focused onto a cell, for example when multiple layers of solar cells with different band-gaps are stacked on top of one another, the finding could be exactly what is needed.

Moreover, the work could open up entirely different applications for perovskites; the authors have already demonstrated they can store and erase information by varying the light intensity.

The work was published in Nature Materials. The same day, sister publication Nature Electronics published two papers on advances in perovskite technology, one on their potential to make better LEDs, the other on removing defects when made into transistors. Although the applications are different, the timing demonstrates how rapidly our understanding of these semi-conductors is advancing, with developments feeding off each other.

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