Creating the most efficient perovskite solar cells to date using ammonium

Scientists at the Arc Centre of Excellence in Exciton Science have created perovskite solar cells with 21% efficiency, the best results ever recorded for a device made from a non-halide lead source.

This new pathway to creating durable and efficient perovskite photovoltaics at industrial scale has been demonstrated through the first effective use of lead acetate as a precursor in making formamidinium-caesium solar cells.

Details of the work, titled ‘Efficient and stable formamidinium–caesium perovskite solar cells and modules from lead acetate-based precursors,’ was published in the journal Energy and Environmental Science.

How was the new device created?

A mini prototype panel featuring ammonium was able to achieve 18.8% efficiency. The large-area perovskite solar cell was fabricated in an ambient atmosphere and made via a single step. This included a blade coating that demonstrated its potential viability for industrial-scale manufacturing.

The test devices also showed strong thermal stability, as they continued to function with no efficiency loss after running at 65°C for 3,300 hours.

Jie Zhao, a PhD student at Monash University and first author of the study, 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. 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.”

Dr Wexin Mao, a corresponding author, added: “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 synthesise cells using this.

“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.”

Problems that have arisen with perovskites

Perovskite solar cells have the potential to disrupt the solar energy sector, thanks to their relatively low manufacturing cost, flexibility, and tuneable band gap relative to silicon.

However, researchers are struggling to solve reliability issues, and they also need to find a way to create devices at a viable commercial scale.

Perovskites are solution processed (made in liquids) using a variety of different ingredients. Most approaches use lead halides, which require the inclusion of strong polar solvents with high boiling points and antisolvent quenching agents to control the perovskite crystallisation process.

This complicated mechanism can lead to defects in the thin films, which causes the perovskite solar cell to rapidly lose efficiency, while also being difficult to control.

A potential solution to these problems is the chemical compound lead acetate. It has emerged as a promising alternative precursor, due to the fact it can create ultrasmooth thin films with fewer defects.

Until now, lead acetate has only been used to make methylammonium or caesium-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. However, previous attempts to synthesise them, using lead acetate as the precursor, have failed.

How are the researchers attempting to solve these problems?

To investigate and solve issues with perovskite solar cells, the researchers examined the material’s 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.

Dr Sebastian Fürer, a contributing author of the study, concluded: “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 behaviour can encourage a greater focus on scalable synthesis and fabrication methods of metal halide perovskite devices.

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