Perovskite solar cells degrade rapidly under natural conditions; as such they are yet to become technically viable despite their great potential. Now, UK researchers think they have uncovered the precise mechanism by which degradation occurs at the atomic scale, which might lead to a way to prevent it from happening.
Solar cells are already widely used for harnessing energy from the Sun for "clean" electricity generation. However there is always room for improvement with any technology, an efficiency boost, an opportunity to make devices more cheaply and to make them longer lasting. Saif Haque at Imperial College London and his team have turned to light-absorbing materials based on organic lead halide perovskites for their huge promise in improving the outlook in solar energy conversion.
Earlier work at Imperial College London had revealed that the breakdown is due to the formation of superoxides, which attack methylammonium lead halide these form through photochemical activation of atmospheric oxygen. Now, Haque's team, working with Christopher Eames and Saiful Islam at the University of Bath, have found that defects within the perovskite crystal structure that should normally be filled with iodide ions help promote superoxide formation. When they dosed the material with extra iodide subsequent to manufacturing, they could improve the stability somewhat. However, they suggest that a more permanent answer would be to engineer out the iodide defects altogether. [Aristidou et al. Nature Commun (2017) 8: 15218 DOI: 10.1038/ncomms15218]
Currently, perovskite cells must be protected from the air by encasing them in glass, but given that one of the big advantages mooted for perovskite solar cells is their flexibility this essentially defeats their purpose as well as adding a lot of weight and cost to what would otherwise be a lightweight and less costly device.
The team reports how they have shown that, "Thin-film passivation with iodide salts enhances film and device stability." Adding that, "The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells." With the role of iodide defects in generating superoxide having now been identified, an obvious way to improve the material's stability is to fill the vacancies with additional iodide ions. This could open up a new way of optimizing the material for enhanced stability by controlling the type and density of defects present.
"In terms of future work, we next plan test the stability of the solar cells in real-world settings where the devices would be exposed to a combination of both oxygen and moisture," Haque told Materials Today.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".