Insights into Hybrid Perovskites Ink Reactivity and Evolution to Polytypes

Metal halide perovskite (MHP) semiconductors are excellent candidates for contemporary optoelectronics innovation, particularly for photovoltaics.[1] The advantages of this class of materials derive from their hybrid nature, allowing for straightforward fabrication processes, and from their unique optoelectronic properties. A typical 3D organic-inorganic perovskite has a chemical formula of ABX3, where A is an organic cation (such as MA [methylammonium] or FA [formamidinium]), B is a metal cation (such as Pb2+), and X is a halogen anion (such as I or Br).[2] However, recent advances have also explored more complex compositions embedding diverse cations/anions.[3] These materials are prepared by simple and straightforward solution processing, the material precursors dissolved in a solvent undergoes self assembly into a perovskite structure during spin-coating onto a substrate under mild thermal annealing. As the technology continues to mature, this still is a key advantage, allowing for affordable and scalable processing. Understanding perovskite ink properties is therefore a fundamental requirement towards industrialization, with special regards to their evolution over time. It has been demonstrated that even for the simplest system, the precursor solution is a complex – and dynamic – dispersion which contains not only solvated ions but also lead halide complexes, colloids and aggregates of different natures and dimensions. In these complex dispersions, multiple chemical species are present and can interact – or react – between each other or with the solvent. We have proved the existence of a reactivity between two of the perovskite components – MA and FA – in the precursors solutions, that leads to the formation of a novel condensation product, methylformamidinium (MFA). We have studied different parameters that affects such reactions kinetics, therefore modifying the ink composition over time, and proposed solutions to overcome these issues.[4] With the aim of correlating the solution chemistry with the film structural properties, through the synergic use of solution Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray Diffraction and Density Functional Theory (DFT) calculations, we have recognized and explained for the first time a correlation between the aging of perovskite precursor solutions, the presence of MFA species in solution and the emergence of photoinactive hexagonal polytypes (6H and 4H) [5] Starting from the known reactivity of the chemical species present in ink solutions, we outline the directions towards which future research efforts should be directed.[6] References: [1] Jeong, J., Kim, M., Seo, J., Lu, H., Ahlawat, P., Mishra, A., Yang, Y., Hope, M.A., Eickemeyer, F.T., Kim, M., et al. (2021). Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells. Nature 592, 381–385. [2] Snaith, H.J. (2013). Perovskites: The emergence of a new era for low-cost, high-efficiency solar cells. Journal of Physical Chemistry Letters 4, 3623–3630. [3] McMeekin, D.P., Sadoughi, G., Rehman, W., Eperon, G.E., Saliba, M., Hörantner, M.T., Haghighirad, A., Sakai, N., Korte, L., Rech, B., et al. (2016). A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science 351, 151–155. [4] Valenzano, V., Cesari, A., Balzano, F., Milella, A., Fracassi, F., Listorti, A., Gigli, G., Rizzo, A., Uccello-Barretta, G., and Colella, S. (2021). Methylammonium-formamidinium reactivity in aged organometal halide perovskite inks. Cell Reports Physical Science 2, 100432. [5] Gianluca Bravetti, Nicola Taurisano, Anna Moliterni, Jose Manuel Vicent-Luna, Davide Altamura, Federica Aiello, Nadir Vanni, Agostina Lina Capodilupo, Sonia Carallo, Giuseppe Gigli, Gloria Uccello-Barretta, Federica Balzano, Cinzia Giannini, Shuxia Tao, Silvia Colella, Aurora Rizzo (2023). Solution Aging Promotes Hexagonal Polytype Formation in Mixed Cation/Halide Perovskites, accepted. [6] Rizzo A., Listorti A., Colella S. (2022). Chemical insights into perovskite ink stability. Chem, 8, 1, 31-45