Solvent switch adds stability to perovskite solar cells

By removing the solvent dimethyl-sulfoxide and introducing dimethylammonium chloride as a crystallisation agent, the researchers at Oxford University and Exciton Science were able to better control the intermediate phases of the perovskite crystallisation process, leading to thin films of greater quality, with reduced defects and enhanced stability.

Large groups of up to 138 sample devices were subjected to an accelerated ageing and testing process at high temperatures and in real-world conditions.

Formamidinium-caesium perovskite solar cells created using the new synthesis process are said to have significantly outperformed the control group and demonstrated resistance to thermal, humidity and light degradation.

According to the team, this is a strong step forward to matching commercial silicon’s stability and makes perovskite-silicon tandem devices a much more realistic candidate for becoming the dominant next-generation solar cell.

Led by Oxford’s Professor Henry Snaith and Professor Udo Bach, Monash University, Australia, the work has been published in Nature Materials.

In a statement, Oxford University PhD student Philippe Holzhey, a Marie Curie Early Stage Researcher and joint first author on the work, said: “It's really important that people start shifting to realise there is no value in performance if it's not a stable performance.

“If the device lasts for a day or a week or something, there's not so much value in it. It has to last for years.”

During testing, the best device operated above the T80 threshold (the time it takes for a solar cell to reduce to 80 per cent of its initial efficiency) for over 1,400 hours under simulated sunlight at 65°C.

Beyond 1,600 hours, the control device fabricated using the conventional dimethyl-sulfoxide approach stopped functioning, while devices fabricated with the new design retained 70 per cent of their original efficiency, under accelerated aging conditions.

The same degradation study was performed on a group of devices at 85°C, with the new cells again outperforming the control group.

The researchers calculated that the new cells age by a factor of 1.7 for each 10°C increase in the temperature they are exposed to, which is close to the two-fold increase expected of commercial silicon devices.

Dr David McMeekin, the corresponding and joint first author on the paper, was an Australian Centre for Advanced Photovoltaics (ACAP) Postdoctoral Fellow at Monash University and is now a Marie Skłodowska-Curie Postdoctoral Fellow at Oxford University.

He said: “I think what separates us from other studies is that we've done a lot of accelerated aging. We've aged the cells at 65°C and 85°C under the whole light spectrum.”

The researchers hope their work will encourage a greater focus on the intermediate phase of perovskite crystallisation as an important factor in achieving greater stability and commercial viability.

This work was supported by the US Department of Energy’s Stanford Linear Accelerator Center (SLAC) and the National Renewable Energy Laboratory (NREL).