Researchers from Monash University, Wuhan University of Technology, CSIRO Manufacturing, The Melbourne Centre for Nanofabrication and Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory have demonstrated "the first effective use of lead acetate as a precursor in making formamidinium-caesium perovskite solar cells". This could lead to a new way of creating durable, efficient perovskite photovoltaics at industrial scale.
Members of Exciton Science, based at Monash University, were able to create perovskite solar cells with 21% efficiency, which they say are the best results ever recorded for a device made from a non-halide lead source.
A mini prototype solar panel featuring these cells achieved 18.8% efficiency. The large-area perovskite layer was fabricated in ambient atmosphere and was made via a single-step blade coating, demonstrating its potential viability for industrial-scale manufacturing.
The test devices also showed strong thermal stability, continuing to function with no efficiency loss after 3,300 hours running at 65 °C.
First author Jie Zhao, a PhD student at Monash University, said: “We’ve been able to use lead acetate in a one-step, spin-coating process to get the perfect, high-quality formamidinium-caesium perovskite thin film. And because we don’t need an anti-solvent agent, we can do this via large-scale techniques, such as blade coating, which means it’s viable at industrial scale.”
Corresponding author and Monash University colleague Dr Wenxin Mao said: “The vast majority of perovskite solar cell research uses lead halides, particularly lead iodide. The lead iodide needs to be 99.99% pure and it’s very expensive to synthesize cells using lead iodide. We’re the first group to make highly stable formamidinium-cesium perovskite solar cells using lead acetate rather than lead iodide. We have provided the entire research community a second way to make high-quality perovskite solar cells.”
Perovskites are solution processed (made in liquid) using a variety of different ingredients.
Most approaches use lead halides and require the inclusion of strong polar solvents with high boiling points and antisolvent quenching agents to control the perovskite crystallization process.
This complicated mechanism can lead to defects in the thin films, which causes the resulting device to rapidly lose efficiency. It’s also hard to control.
The chemical compound lead acetate has emerged as a promising alternative precursor, because it can create ultrasmooth thin films with fewer defects.
Until now, however, lead acetate had only been used to make methylammonium or cesium-based perovskites, which are relatively unstable and not suitable for real-world applications.
A better candidate for commercial use can be found in perovskites made using formamidinium and caesium, thanks to their superior stability. Previous attempts to synthesize them using lead acetate as the precursor failed.
To investigate and solve this issue, the team examined the underlying molecular mechanisms. Through X-ray diffraction and nuclear magnetic resonance spectroscopy, they identified the need to use ammonium as a volatile cation (positively charged ion) at a critical stage.
Contributing author Dr Sebastian Fürer said: “The presence of ammonium served to drive away the residual acetate during annealing, without forming unwanted side products.”
The researchers hope their work on the fundamental chemistry governing precursor behavior can encourage a greater focus on scalable synthesis and fabrication methods of metal halide perovskite devices.