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UK project discovers why tin-based perovskite solar cells are poor

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he finding, could open the door to low-Pb perovskite cells, a silicon-beating dual-junction perovskite solar cell, and possibly efficient Pb-free perovskite solar cells.

Excitement around perovskite solar cells comes from them, despite only being invented a few years ago, already challenging conventional mono-crystal silicon cells for efficiency, and almost certainly being cheap to make on roll-to-roll processes.

However, the most efficient perovskite solar cells contain Pb, which no one really wants in products due to its toxicity – and they are also very vulnerable to moisture damage, but that is another story.

Except for a few compounds, tin (Sn) is considered far less toxic than Pb, so tin is an attractive substitute – as it has been in solder – and many research teams are trying to displace Pb with tin in perovskite solar cells.

However, one of the things that happens when tin is introduced into the perovskite crystal structure is that efficiency drops.

Luckily, the Surrey team has uncovered the mechanisms responsible for a good chunk of this drop.

Tin-based perovskite

“In the past, adding Sn to Pb perovskites has degraded the performance of the solar cell significantly,”Professor Ravi Silva told Electronics Weekly. “We believe we now know a major reason for this degradation, and it has to do with Sn4+ [ions].”

The perovskite lattice needs Sn2+ ions in its structure, but these are unstable in the presence of moisture and oxidises into Sn4+. Also, some Sn4+ ions arrive alongside one of the precursors, SnI2.

“Sn4+ ions are smaller in size than Sn2+ and do not fit in the perovskite crystal lattice,” said Silva. “They can act as unwanted dopants in the perovskite and increase recombination by capturing the mobile electrons, which hinder the performance of solar cells. By removing this harmful material, we can increase the efficiency of Sn-based solar cells.”

Surrey’s perovskite is made by spin-coating precursor materials onto a surface, then adding a solvent.

…chip away everything that doesn’t look like an elephant*

This solvent works in an unusual way – it is chosen to dissolve and wash away those ions amongst the precursors not wanted in the final crystals, and what remains on the surface then crystallises into the required perovskite – known as an ‘anti-solvent’ method.

The researchers were trying a range of solvents to see which produced the best Sn-Pb perovskite solar cells. The winner was toluene.

It was also discovered that efficiency of cells produced by the different solvents was in reverse proportion to the number of Sn4+ ions remaining in the cell, and that toluene was best at washing away Sn4+ ions.

The perovskite concerned is Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3, where FA and MA are formamidinium and methylammonium, and the best cells produced by toluene washing during the solvent selection process had charge carrier mobilities of ~32cm2/Vs together with ~28mA/cm2 short circuit current and 11.6% efficiency.

Once Sn4+ ions were suspected, the team came up with an improved toluene wash, followed by heat treatment, that removed all of the Sn4+ ions – efficiency rose to 12.04%.

Short circuit current did suffer somewhat in this last cell, although, according to the team, this looks to be due to roughness in the thin-film caused by the harsh proof-of-concept techniques needed for total Sn4+ removal.

Tandem cells

Replacing some Pb with tin also increases the wavelength at which the solar cell operates.

“It extends the absorption into the near-infra-red regime, up to 950nm, allowing the bandgap to be low [1.26eV for Pb-Sn, 1.62eV for Pb-only],” said Silva, suiting such a cell to be stacked with a Pb-only perovskite cell making a dual junction device.

“There is a significant drive for tandem perovskites to break the theoretical limits of single junction solar cells of 33% using a very cheap material system that is competitive in delivering affordable energy,” said Silva.

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