An environmental failsafe for lead-based perovskites

Perovskite solar cells (PSCs) represent a big hope for the future of solar cell technology, with the potential for low-cost production, high efficiency and incorporation into all sorts of flexible or multilayered devices. As the technology overcomes more of the challenges related to performance and stability that have held it back from commercial development, questions are increasingly being asked about the use of lead, a toxic material that’s at the heart of all of the best performing perovskite solar cell materials developed to date.

Opinion is divided on the level of risk posed by lead in perovskites – some studies have shown that the amounts of lead present in most of the cell materials being researched are too small to pose an environmental threat even in a ‘worst case' scenario, and were production to be scaled up, options could be explored for safe disposal and recycling – as has long been established for the lead used in today’s car batteries.

However, there is still the possibility that if a perovskite module in the field suffered damage or moisture ingress, lead from the device could dissolve into water and make its way into the soil or even the food chain – and this will likely be a factor for regulators in allowing large commercial solar installations to use the material. Plenty of research is underway into alternatives to lead in perovskites, with both tin and bismuth showing some promise. But abandoning lead would be a setback in terms of cell performance; and finding ways to prevent it from entering the environment could be a more effective solution.

Chemical failsafe

This was the approach of scientists led by Switzerland’s Ecole Polytechnique Fédérale de Lausanne (EPFL), who discovered a material that reacts with lead only in the presence of water, forming a new substance that cannot be dissolved in water and thus reducing the risk of lead from a PSC ending up in the environment in almost any possible scenario. The group incorporated this material, a phosphate salt, into the structure of a PSC; showing that it did not affect the properties or performance of the cell, and only reacts with the lead when water is present (and water coming into contact with the active cell material would likely mean the panel has already failed).

Work with the phosphate salt is described in the paper Fighting Health Hazards in Lead Halide Perovskite Optoelectronic Devices with Transparent Phosphate Salts, published in ACS Applied Materials & Interfaces. “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 EPFL scientist Márton Kollár.

The group expects that its discovery will see perovskites solar cells downgraded to a similar risk category as existing thin-film PV technologies like CIGS or cadmium telluride and hopes to see its approach adopted in other prototype perovskite devices. “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,” added Pavao Andričevic, another EPFL researcher who worked on characterizing the cells’ performance.