Bright prospects for tin perovskite solar cells

Perovskite solar cells are widely regarded as the next generation photovoltaic technology. However, they are not yet stable enough in the long term for widespread commercial use. One reason for this is migrating ions, which cause degradation of the semiconducting material over time. A team from HZB and the University of Potsdam has now investigated the ion density in four different, widely used perovskite compounds and discovered significant differences. Tin perovskite semiconductors produced with an alternative solvent had a particular low ion density — only one tenth that of lead perovskite semiconductors. This suggests that tin-based perovskites could be used to make solar cells that are not only really environmentally friendly but also very stable.

For over ten years, researchers worldwide have been investigating organometallic perovskite semiconductors for use in photovoltaics. Their research has focused mainly on perovskite solar cells that contain lead as the B cation in the ABX₃ structure of the perovskite. Thanks to their efforts, the efficiency of lead-based perovskite solar cells has increased from 4% to over 27% today.  However, these materials still have some issues. It's not just the problematic heavy metal lead which is toxic; the semiconductor material also lacks long-term stability.

Tin-perovskites - a neglected topic

The lead in the structure can be replaced by tin, a non-toxic element. Although tin-based perovskite solar cells currently have a significantly lower efficiency than lead-based solar cells, this may be because research on the tin-perovskites is still in its infancy, says Dr Artem Musiienko, head of a research group at HZB and points out: ‘In purely theoretical terms, tin-based perovskite solar cells could even exceed the efficiency of lead-based perovskites.’

A new study provides a compelling argument for paying more attention to tin-based perovskites. One of the main reasons for the insufficient stability of perovskite solar cells to date is the presence of mobile halide ions in the material. As halide ions migrate, the material degrades, and the solar cell’s efficiency decreases over time.

Mobile ions in perovskites

Musiienko's team has now investigated this phenomenon together with the Antonio Abate group, HZB, and Felix Lang group, University of Potsdam. They examined four of the most relevant perovskite compositions and quantitatively measured the ion density and migration of ions in the material.

'We found not only that tin based perovskites have a lower concentration of mobile ions but also that they intrinsically exhibit a degradation time five times slower than that of lead based perovskites,’ says Musiienko. The tin perovskite materials were produced in the HZB's Hysprint laboratory. One of the tin perovskites was synthesized using a dimethyl sulfoxide (DSMO) solvent and the other using an alternative DMF-DMI  solvent. The solvent variation approach was used to demonstrate a route for avoiding tin oxidation due to strong DMSO coordination, as previously shown in several investigations [Chemistry of Materials (2022)].

Huge differences in ion density

In fact, they discovered that the lead-based perovskite exhibited the greatest ion density. This density was slightly lower in the lead-tin mixture and the tin perovskite. A big surprise was by the tin perovskite sample that had been produced using the alternative solvent: ‘This was really unexpected: these FASnI₃ solar cells have ten times fewer mobile ions than the Pb-based solar cells. We also found that they exhibited excellent stability during operation for over 600 hours,’ says Shengnan Zuo, a PhD student in the team of Antonio Abate.

A compelling argument for tin-perovskites

‘We are convinced that tin-based perovskites have enormous potential and that investigating these materials is a very good idea. There are chances to significantly increase their efficiency and stability. This study paves the way for the development of innovative, stable thin-film solar cells with suppressed ion migration,’ says Musiienko.