Their results, reported in the journal Advanced Materials, suggest that solar cells incorporating bismuth can replicate the properties that enable the exceptional properties of lead-based solar cells, but without the same toxicity concerns.
Most solar cells in use today are made from silicon which is very efficient at converting light into energy but is very energy-intensive to produce.
Researchers have been searching for materials which can perform at similar or better levels to silicon, but that don’t need such high purity levels, making them cheaper to produce. The most promising of these new materials are called hybrid lead halide perovskites.
Perovskite solar cells are cheap and easy to produce however lead is integral to their chemical structure. Whether the lead contained within perovskite solar cells represents a tangible risk to humans, animals and the environment is being debated, however, some scientists are now searching for non-toxic materials which could replace the lead in perovskite solar cells without negatively affecting performance.
“We wanted to find out why defects don’t appear to affect the performance of lead-halide perovskite solar cells as much as they would in other materials,” said Dr Robert Hoye of Cambridge’s Cavendish Laboratory and Department of Materials Science & Metallurgy, and the paper’s lead author. “If we can figure out what’s special about them, then perhaps we can replicate their properties using non-toxic materials.”
For this study, Hoye and his colleagues looked at bismuth oxyiodide, a material which was previously investigated for use in solar cells and water splitting, but was not thought to be suitable because of low efficiencies and because it degraded in liquid electrolytes. The researchers used theoretical and experimental methods to revisit this material for possible use in solid-state solar cells.
They found that bismuth oxyiodide is as tolerant to defects as lead halide perovskites. Bismuth oxyiodide is also stable in air for at least 197 day. By sandwiching the bismuth oxyiodide light absorber between two oxide electrodes, they were able to demonstrate a record performance, with the device converting 80% of light to electrical charge.
The bismuth-based devices can be made using common industrial techniques, suggesting that they can be produced at scale and at low cost.
“Bismuth oxyiodide has all the right physical property attributes for new, highly efficient light absorbers,” said co-author Professor Judith Driscoll, of the Department of Materials Science and Metallurgy. “I first thought of this compound around five years ago, but it took the highly specialized experimental and theoretical skills of a large team for us to prove that this material has real practical potential.”
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