Taming a promising, but flaky, solar perovskite

Author: EIS Release Date: Sep 7, 2023

A cubic perovskite α-formamidinium lead iodide known as ‘α-FAPbI3’ would be a promising solar cell material if it did not gradually turn into useless δ-FAPbI3 at room temperature under the influence of, of all things, light. Water has the same effect.

 

TokyoTech stablised perovskite

“Solar cells made of α-FAPbI3 exhibit a 25.8% conversion efficiency and an energy gap of 1.48eV, specifications that are highly desirable for real-life applications. Unfortunately, α-FAPbI3 is metastable at room temperature and undergoes a phase transition to δ-FAPbI3. The energy gap of δ-FAPbI3 is much larger than the ideal value for solar cell applications,” according to Tokyo Tech, which has worked out the mechanism behind one particular proposed stabilisation technique – swapping some of the iodine ions with thiocyanate ions (SCN-).

 

“Previous studies have shown that partial replacement of surface anions stabilises the α-phase. However, it is unclear how SCN- incorporate themselves within the perovskite lattice and increase the interfacial stability,” said Takafumi Yamamoto, who lead the research.

 

 

For the first time, according to Tokyo Tech, single crystal and powder samples of the thiocyanate-stabilised pseudo-cubic perovskite were prepared by the team, and detailed study revealed the relationship between the internal crystal structures and defects as the material moved between its phases.

 

 

And this substitution did indeed tip the balance, so that undesirable δ phase regions now converted to the desirable α phase at room temperature – effectively stabilising the material – something that only happened 100°C hotter in unmodified FAPbI3 perovskite.

 

The new material exhibited a 1.91eV bandgap at room temperature in a dry atmosphere.

 

This may not be ideal for solar cells, but the research did revealed the underlying stabilisation mechanism “which might prove beneficial to scientists trying to develop new thermodynamically stable solar cell materials with ideal band gaps,” said Yamamoto.

 

Tokyo Tech worked with Colorado State University, the University of Oxford, the Kanagawa Institute of Industrial Science and Technology. Results are published in the Journal of American Chemical Society as ‘Thiocyanate-stabilized pseudo-cubic perovskite CH(NH2)2PbI3 from coincident columnar defects lattices‘.

 

A note for the chemically-interested: ‘FA+, the formamidinium part, is CH(NH2)2+.