Bismuth could replace lead in solar cells

BISMUTH could offer a non-toxic alternative to lead in next-generation perovskite solar cells, according to researchers at the University of Cambridge, UK.

Conventional silicon solar cells require extremely pure, defect-free silicon, which makes them energy-intensive to produce. Alternative materials like perovskite do not require such high purity levels, making them cheaper to produce. However, one of the sticking points has been the need to use lead, which is integral to their functional structure. The team at Cambridge, led by materials scientist Robert Hoye, sought to determine why perovskite cells are so defect tolerant, and subsequently find ways to replace lead with non-toxic alternatives. The researchers worked with colleagues at MIT, the National Renewable Energy Laboratory and Colorado School of Mines in the US, and identified bismuth, which sits next to lead in the periodic table, as a possibility.

Bismuth oxyiodide has previously been investigated for solar cells and water splitting, but has low efficiencies and degrades in liquid electrolytes. Hoye and the multi-institution team instead tested it for use in solid-state solar cells. They generated bismuth oxyiodide thin films for testing by a chemical vapour transport technique commonly used in industry, meaning that devices based on them could be produced cheaply at scale.

They found that bismuth oxyiodide is as tolerant to defects as lead halide perovskites. It is stable in air for at least 197 days, which is much better than some other lead halide perovskites. The researchers also tested bismuth oxyiodide between two oxide electrodes and found that it could convert 80% of light into electrical charge.

In separate work at University College London, a team led by David Scanlon, a reader in computational materials and inorganic chemistry, looked at Hoye and his team's work and estimate that the bismuth oxyiodide-based solar cells will have an efficiency of 22%, similar to conventional silicon solar cells and the best perovskite solar cells.

“Bismuth oxyiodide has all the right physical property attributes for new, highly efficient light absorbers. I first thought of this compound around five years ago, but it took the highly-specialised experimental and theoretical skills of a large team for us to prove that this material has real practical potential,” said co-author and materials science professor Judith Driscoll.

Hoye added: “This work shows that earlier theories about bismuth oxyiodide were not wrong, and these compounds do have the potential to be successful solar cells. We’re just scratching the surface of what these compounds can do.”

Advanced Materials doi.org/b9xb