(Invited, Digital Presentation) Chemical Insights into Perovskite Ink Stability

Abstract

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 – methylammonium and formamidinium – in the precursors solutions, that leads to ink variations over time, which in turn means variation in the final polycrystalline material properties. In fact, the characteristics of precursor solution eventually dictate the optoelectronic quality of resulting perovskite films that proceed with the solution aging and determine undesirable decrease of solar cells performances. We have studied different parameters that affects such reactions kinetics and proposed solutions to overcome these issues.4 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.5