This image shows how the presence of a transparent phosphate salt in a perovskite solar cell stops the lead from leaching out when the cell is exposed to water. Image: Endre Horváth (EPFL)."The solar-energy-to-electricity conversion of perovskite solar cells is unbelievably high, around 25%, which is now approaching the performance of the best silicon solar cells," says László Forró in the School of Basic Sciences at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. "But their central element is lead, which is a poison; if the solar panel fails, it can wash out into the soil, get into the food chain and cause serious diseases."
The problem is that most of the lead in halide perovskites can dissolve in water. This solubility in water and other solvents is actually a great advantage, as it makes building perovskite solar panels simple and inexpensive – another perk along with their performance. But the water solubility of lead can become a real environmental and health hazard when the panel breaks or gets wet, such as when it rains.
To prevent the lead becoming a hazard, it must be captured before it gets to the soil, and ideally recycled. This issue has spurred intensive research effort because it is the main obstacle preventing regulatory authorities approving the production of perovskite solar cells on a large, commercial scale. However, attempts to synthesize non-water-soluble and lead-free perovskites have not been very successful, yielding solar cells with poor performance.
Now, Forró's group has come up with an elegant and efficient solution, based on using a transparent phosphate salt, which does not block solar light and so doesn't affect performance. If a solar panel fails, the phosphate salt immediately reacts with the lead to produce a water-insoluble compound that cannot leach out to the soil, and which can be recycled. Forró and his team report their work in a paper in ACS Applied Materials & Interfaces.
"A few years ago, we discovered that cheap and transparent phosphate salt crystals, like those in soil fertilizers, can be incorporated into various parts of the sandwich-like lead halide perovskite devices, like photodetectors, LEDs or solar cells," explains Endre Horváth, the paper's first author. "These salts instantaneously react with lead ions in the presence of water, and precipitate them into extremely non-water-soluble lead phosphates."
"The 'fail-safe' chemistry keeps lead ions from leaching out and can render perovskite devices safer to use in the environment or close to humans," says Márton Kollár, the chemist behind the growth of the perovskite crystals.
"We show that this approach can be used to build functional photodetectors, and we suggest that the broad community of researchers and R&D centers working on various devices like solar cells and light-emitting diodes implements it in their respective prototypes," adds Pavao Andricevic, who characterized the sensitive photodetectors.
"This is an extremely important study – I would say, a central one – for large-scale commercialization of perovskite-based solar cells," Forró concludes.
This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.