Polarization Dependence of Water Adsorption to CH₃NH₃PbI₃ (001) Surfaces

Abstract

The instability of organometal halide perovskites when in contact with water is a serious challenge to their feasibility as solar cell materials. Although studies of moisture exposure have been conducted, an atomistic understanding of the degradation mechanism is required. Toward this goal, we study the interaction of water with the (001) surfaces of CH3NH3PbI3 under low and high water concentrations using density functional theory. We find that water adsorption is heavily influenced by the orientation of the methylammonium cations close to the surface. We demonstrate that, depending on methylammonium orientation, the water molecule can infiltrate into the hollow site of the surface and get trapped. Controlling dipole orientation via poling or interfacial engineering could thus enhance its moisture stability. No direct reaction between the water and methylammonium molecules is seen. Furthermore, calculations with an implicit solvation model indicate that a higher water concentration may facilitate degradation through increased lattice distortion.

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“…Most reports of PeLEDs used a 3D-structured CH 3 NH 3 PbX 3 perovskite. 10,20,[23][24][25][26][27] Lower-dimensional perovskites have drawn researchers' attention recently. 21,28,29 Apart from using organic anions, several more recent reports employed Csbased all-inorganic halide perovskites to make use of their better thermal stability.…”

Mechanical signatures of degradation of the photovoltaic perovskite CH3NH3PbI3 upon water vapor exposure

“…15,16 Recently, specific changes in the chemical composition have been proposed, apt to reduce the tendency to incorporate water. 21 On the theoretical side, computer simulations [22][23][24] have mainly focused on the interaction of water (as a layer or single molecules) with the perovskite surfaces and the mechanism leading to water insertion into the bulk. Clearly, degradation induced by moisture deserves further investigation and several properties of the materials should be monitored so as to gain a comprehensive understanding.…”

Crystalline Clear or Not: Beneficial and Harmful Effects of Water in Perovskite Solar Cells

“…Snaith and coworkers raised the problematic hydrophilic character of Li-TFSI additive commonly used in the Spiro-OMeTAD selective layer. [72] If the CH 3 -end of MA + is pointing towards the contact surface, the water penetration was favoured with very low activation energies (0.023 eV). This strategy added hydrophobicity to the HSL material with low contact angle of water droplets on films, as inferred from Figure 8b-e. [70] Similarly, fluorine-containing hydrophobic Lewis acid dopant have been used in combination with PTAA with excellent results in terms of PCE (~19 %) and stability (> 1600 h in ambient conditions).…”