A novel form of perovskite materials shown to improve the stability of perovskite solar cells

Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences and the Faculty of Chemistry of Warsaw University of Technology have designed what they consider to be an improved version of a perovskite, containing in the crystal structure a relatively large organic ion, a guanidinium cation. Lab tests at the EPFL have reportedly shown that photovoltaic cells made of the new perovskite work more efficiently than the cells prepared using its original form.

The guanidinium cation was incorporated into the crystal structure of the classic perovskite using a 'solvent-less' mechanochemical approach. The experiments proved that from many aspects the new, modified perovskites are clearly better than the parent (CH3NH3)PbI3.

"One of the main areas of perovskite applications today is photovoltaics. The organic-inorganic halide perovskites used here generally contain organic methylammonium cations (component A) located in the centres of the crystal lattice cubes. These cations easily undergo gradual degradation under the influence of external factors, e.g. water, but also as a result of photochemical reactions or the action of an elevated temperature. We attempted to mechanochemically replace some of the methylammonium cations by guanidinium ions. The choice of the latter was not accidental", says Marcin Saski, PhD student at the IPC PAS, one of the co-authors of the publication describing the achievement.

In the perovskite structure, the methylammonium ion CH3NH3 + is often replaced with a slightly larger formamidinium ion CH(NH2)2 + , which is more thermally stable. In addition, it has a delocalized bond between the carbon atom and two nitrogen atoms, improving the semiconductor properties. Scientists from the IPC PAS ambitiously attempted to incorporate an even larger guanidinium ion, C(NH2)3 + . Its both an internal high symmetry and an extended conjugated unsaturated bond system stabilize the perovskite material and make it possible to increase the charge carrier lifetime. A mechanochemical method was used to introduce guanidinium into the perovskite crystal lattice by grinding the precursors in the proper stoichimetry.

"In contrast to standard chemical reactions taking place in solutions, the mechanochemical reaction takes place entirely in the solid phase, and the chemical transformations are induced by mechanical force. The mechanochemical reactions offer a significant advance by avoid the use of harmful solvents and high temperatures. This is truly green chemistry, and due to the fact that the laboratory equipment required is not demanding, it is also easy on the wallet", clarifies Dr. Daniel Prochowicz, beneficiary of the individual Marie Sklodowska-Curie grant of the European Union framework programme Horizon 2020, carried out in the team of Prof. Grätzel from EPFL.

During many tests conducted in Prof. Lewinski`s group, it was found that the crystal structure of the perovskite remains stable when even 25% of the methylammonium ions are replaced by guanidinium cations. Advanced diffraction studies and nuclear magnetic resonance (NMR) spectroscopy show that guanidinium homogeneously incorporates into the perovskite crystal structure.

"The improvement of the photovoltaic parameters in perovskites containing guanidinium ions is perhaps not particularly spectacular, but it is very clear. What's more, increases can be seen in not one, but several important parameters", emphasizes Prof. Lewinski and adds: "It is worth noting that our perovskites with guanidinium contain less volatile amine and are more resistant to external factors than the parent perovskite. The higher durability of the photovoltaic cells is a considerable argument in favor of their commercialization".