Researchers from Israel's Bar-Ilan University and Weizmann Inst. of Science have measured the effect of humidity on hardness and elastic modulus (E) for two series of lead halide perovskite single crystals. The results indicated the influence of hydrogen (H)-bonding, bond length, and polarization of the ions in lead halide perovskite single crystals.
The team detected an inverse relation between hardness and modulus, which was strengthened with increased humidity. Their findings shed light on the material's distinct structure and properties at the atomic scale. The conclusion of this work was based on the evaluation of outcomes of various nano-indentation techniques that differentiated between surface and bulk E and explored different manifestations of hardness.
Exposing methylammonium lead halides (MAPbCl3, MAPbI3, MAPbBr3) and formamidinium lead bromide (FAPbBr3) to medium relative humidity did not degrade the materials, but their E value increased by 10%, compared to dry conditions.
The unusual increase in E value in the materials exposed to relative humidity was attributed to the underlying structural characteristics that differ for the bulk and the surface of the exposed material. The effect of humidity on E at the material's surface was due to the interaction of anions and cations with H2O, expressed as polarizability/polarity. Additionally, the effect of humidity on E in the bulk of the material was based on H-bonding and bond length.
A substantial increase in E was observed at the surface of MAPbI3 and in the bulk of FAPbBr3. The increased E in the latter was attributed to the diffusion of H2O when exposed to humidity. Moreover, the mechanical properties of cesium lead bromide (CsPbBr3) were independent of humidity, suggesting the critical role of H-bonding.
On the other hand, the effect of humidity on hardness contrasted with that of E, wherein the value decreased by 30% at higher humidity. This decrease in hardness was due to the accumulation of H2O within the perovskite framework and the formation of H-bonds with suitable lattice atoms when exposed to high humidity.
In summary, the team found that while exposing MAPbX3 and FAPbBr3 perovskites to a relative humidity of 55-60% did not degrade the lead halide perovskites, their elastic moduli were increased by 10% compared to the dry condition, and the effect was reversible.
The primary response observed in lead halide perovskites upon exposure to humidity was a decrease in the E, attributed to the inherent structural properties, which were distinct in the bulk and surface.
Furthermore, the modulus was related to fundamental material properties (hardness and bond stiffness). The hardness-humidity response was ascribed to H2O accumulation within the free space of the lead halide perovskite framework and H-bond formation between the lattice atoms. Thus, the present work evaluated and differentiated the surface and bulk E and analyzed various manifestations of hardness.
