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Huanhuan Zhou1 Mingchao Wang4 Jingfan Wang3 Shangchao Lin2

1, Materials Science and Engineering, Florida State University, Tallahassee, Florida, United States
4, Materials Science and Engineering, Monash University, Melbourne, Victoria, Australia
3, Mechanical Engineering, Florida State University, Tallahassee, Florida, United States
2, Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, Shanghai, China

Organic-inorganic hybrid perovskites, such as the prototypical methylammonium lead iodide (CH3NH3PbI3 or MAPbI3), have emerged as promising light absorbers in photovoltaic (PV) cells or as emitters in light-emitting diodes (LEDs). The strikingly high energy conversion efficiency up to ~ 20% ensures hybrid perovskites as the key component of highly efficient solar cells. However, they generally suffer from moisture instability, which limits the long-term use of perovskite-based devices in ambient environment. In this work, in order to improve the surface moisture stability of MAPbI3 in common humidity atmosphere, more specifically, to discover better ligands under a specific coverage on the [MAI]0 surface to greatly improve the moisture stability and decrease the ion dissociation rate, we have applied molecular dynamics (MD) simulations and reaction kinetics theory to model the ion dissociation process and estimate the associated free energy barrier with and without ligand passivation. We have developed a new MD force field for MAPbI3, which can match the density functional theory (DFT)-predicted elastic properties, experimental water contact angle on MAPbI3, and DFT-predicted water infiltration and adhesion energies. We design MAPbI3 with ligand-passivated surfaces by replacing MA+ with ligands composed of long-chain alkyl-ammoniums. Ligands with different chain lengths, such as CH3(CH2)nNH3+ (n = 3, 5, 7), and under different surface coverages (σ = 25%, 50%, 75%, 100%), are considered here. We discover that ligand passivation can greatly help protect MA+ on the surface due to the much higher dissociation free energy barriers of these ligands compared to that of MA+. For iodine ions, ligand passivation can also shield them from water contacts, except for long-chain ligands, such as CH3(CH2)nNH3+ (n = 5, 7) under full surface coverage (σ = 100%), due to the reduced dissociation free energy barriers of long-chain ligands. As an interesting finding, the reduced dissociation free energy barriers for long-chain ligands under high surface coverages could be explained by their larger tendencies to micellize, which serves as additional driving force for their dissociation. This work significantly motivates future experimental efforts in designing new surface ligands to improve the moisture stability of hybrid perovskites.

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