Hydrophilicity, dissolution and water contact angle on methylammonium lead iodide
Claudia Caddeo a, Simone Meloni b, Alessandro Mattoni a
a Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali, CNR-IOM, Cagliari, Cittadella Universitaria, Monserrato 09042-I (CA), Italy
b Dipartimento di Scienze Chimiche e Farmaceutiche (DipSCF), Università degli Studi di Ferrara (Unife), Via Luigi Borsari 46, I-44121, Ferrara, Italy
Online Conference
Proceedings of Internet Conference on Theory and Computation of Halide Perovskites (ComPer)
Online, Spain, 2020 September 8th - 9th
Organizers: Giacomo Giorgi and Linn Leppert
Oral, Claudia Caddeo, presentation 016
Publication date: 4th September 2020

Surface properties are often assessed with measurements of the contact angle of a water drop. The process is however flawed for the very important class of hybrid perovskite materials, extensively employed in solar cells and optoelectronics research, because they are water soluble and their surface degrades during contact angle measurements. While hybrid perovskites are considered to be highly hydrophilic, a contact angle with water of 83◦ can be measured, as if they were almost hydrophobic. By combining experiments and simulations, we explain the actual value as the result of the interaction of water with degraded superficial layers that form over sub millisecond time scale at the water/perovskite interface. The molecular details of dissolution of MAPI crystals in liquid water are studied by finite-temperature molecular dynamics by developing a MYP-based force field for water/MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI2 films and solvation of MA and I ions. Our models are validated against contact angle measurements for water on a variety of substrates, and are referenced to with the Young-Dupré relation between liquid-solid adhesion and contact angle. Present work not only clarifies the mechanisms of MAPI dissolution, but also reconciles the hydrophilic nature of methylammonium lead iodide with the apparent hydrophobic behavior in contact angle measurements, proposing a methodology for the study of contact angle on evolving substrates.

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