Making ‘perovskite’ solar cells lead free

A team led by researchers at the University of Oxford has demonstrated that the lead (Pb) in ‘perovskite’ solar cells can be replaced with tin, writes our technology editor Steve Bush.

Oxford University

Pb perovskite solar cells have been causing a storm – reaching 17% efficiency from a standing start only three years ago. They are made from cheap raw materials, are atmospherically stable, and easy to make over large areas.

Although the amount of lead in each cell is tiny, the presence of the toxic metal could be a barrier to commercialisation.

“To our knowledge, this is the first time tin perovskites have been used as the active layer in a solar cell,” Oxford physicist Nakita Noel told Electronics Weekly.

“We wanted to try and replace the Pb with something similar but non-toxic. Tin is safe, cheap and abundant, and has been reported in perovskites before, but not in a solar cell, so we decided to see if it would work,” she said.

The cells work like other solar cells – absorbed photons create electron-hole pairs which have to be separated or they uselessly re-combine.

The absorber is the tin perovskite which, unfortunately, has poor electron diffusion length.

To counter this, the crystals are grown in a sensitising structure – a layer of highly porous (‘mesoporous’ with 10-20nm pores) TiO2 which acts as an electron-selective transporter.

When pairs are created, electrons get injected into the intimately-mingled TiO2, while the more frisky holes can diffuse all the way out to an adjacent hole transport layer, made from an organic compound called ‘spiro-OMeTAD’.

This structure is delivering 6% efficiency, but Noel sees no reason this should not be brought up to the 17% already demonstrated with Pb perovskites, and on to around 20%.

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Image: Oxford University

Steve Bush

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