Halide perovskites are the next big thing in solar energy. State-of-the-art perovskite solar cells outperform established technologies with the advantage of processing from solution. Solution-processed perovskite solar cells are made by spraying a precursor ink onto a substrate; the perovskite forms as the chemicals in the ink crystallise. This production process makes halide perovskites a valid low-cost alternative to established materials such as silicon. However, the most effective photovoltaic perovskites contain more than 10% by weight of lead, which oversteps the limits adopted in most countries to regulate the use of heavy metals in electronics.
Within the FREENERGY project, we have developed a strategy for achieving efficient and cost-effective lead-free perovskites using tin (Sn) as a safe alternative to lead (Pb). The main challenge we faced was the relatively low chemical stability of Sn2+, which is prone to be oxidised to Sn4+, degrading the perovskite structure and, thus, the solar cell performances. We have successfully implemented three key strategies:
• Inorganic cations: We knew that organic cations comprising perovskite are more prone than inorganic ones to activate the Sn oxidation. We have screened a series of inorganic cations and identified caesium (Cs) as a valid replacement.
• Alternative solvents: dimethyl sulfoxide was the most used solvent for processing perovskite but contributes to oxidation. We identified alternative solvent compositions to overcome this issue.
• Reducing additives: we have identified processing additives to control the oxidation of Sn during the perovskite processing.
The technical results collected within FREENERGY allow the development of a lead-free perovskite ink that is compatible with most of the industrial solution processing of semiconductor materials for a wide range of applications, including photovoltaic, light-emitting diodes, displays, memories and transistors. An accurate business feasibility study conducted within the framework of FREENERGY has demonstrated considerable market potential for lead-free perovskite ink. The PI is currently working on possibly creating a start-up company to exploit the business potential of lead-free perovskite ink.
