Molecular Insights into Water Vapor Adsorption and Interfacial Moisture Stability of Hybrid Perovskites for Robust Optoelectronics

Abstract

• Grand canonical Monte Carlo predicts BET vapor adsorption isotherms on perovskites. • Vapor-induced degradation cannot be simply correlated with surface hydrophilicity. • Humidity affects ion dissociation rates rather than degradation energy barriers. • Coupled water adsorption and diffusion mechanism leads to perovskite ion solvation. • Material loss rates of 5.8 ~ 13.1 μm/s are estimated at 30 ~ 80% relative humidity. Understanding interfacial mass transfer processes, such as the water vapor adsorption-induced degradation of hybrid perovskites, is vital for improving the durability and performance of their optoelectronic devices in the ambient atmosphere with humidity. In this paper, vapor adsorption on prototypical MAPbI 3 , terminated by [MAI] 0 and [PbI 2 ] 0 surface, at different relative humidity (RH) levels is studied using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The resulting vapor adsorption isotherms match the Brunauer-Emmett-Teller (BET) adsorption model with heats of adsorption of 0.510 and 0.609 eV, respectively, for water monolayers on [MAI] 0 and [PbI 2 ] 0 . The formation of water monolayer on [MAI] 0 (for RH ≥ 30%) is consistent with its lower hydrophilicity compared to [PbI 2 ] 0 (for RH ≥ 10%), reflected from the larger water contact angle predicted. Based on predicted water surface coverages at various RHs, the moisture-induced surface degradation kinetics is studied using MD simulations and transition-state theory. Humidity has a minor impact on the degradation energy barriers due to the similar ion removal pathway by water solvation, but strongly affects the ion dissociation rates through the frequency of attempts to attack surface ions. [MAI] 0 is more vulnerable against water than [PbI 2 ] 0 , despite its lower hydrophilicity, implying that long-term exposed MAPbI 3 are mostly terminated by [PbI 2 ] 0 . Furtherly, averaged degradation material loss rates of 5.8 ~ 13.1 μm/s are estimated at 30 ~ 80% RH levels and 300 K, which is consistent with experiment observations. Finally, we offer a picture for the water-diffusion-based degradation mechanism and elucidate interesting hydrogen-bonding features at the vapor-MAPbI 3 interface. This research provides quantitative insights into the inherent vapor-perovskite interactions and addresses the moisture instability mechanisms in metal halide perovskites towards the rational design of water-resistant, long term stable and efficient optoelectronic devices.