Effect of the Ultrasonic Substrate Vibration on Nucleation and Crystallization of PbI2 Crystals and Thin Films

Abstract

Preparation of defect-free and well-controlled solution-processed crystalline thin films is highly desirable for emerging technologies, such as perovskite solar cells. In this work, using PbI 2 as a model solution with a vast variety of applications, we demonstrate that the excitation of a liquid thin film by imposed ultrasonic vibration on the film substrate significantly affects the nucleation and crystallization kinetics of PbI 2 and the morphology of the resulting solid thin film. It is found that by applying ultrasonic vibration to PbI 2 solution spun onto an ITO substrate with a moderate power and excitation duration (5 W and 1 min for the 40 kHz transducer used in this study), the nucleation rate increases and the crystals transform from 2D or planar to epitaxial 3D columnar structures, resulting in the suppression of crystallization dewetting. The effects of various induced physical phenomena as a result of the excitation by ultrasonic vibration are discussed, including microstreaming and micromixing, increased heat transfer and local temperature, a change in the thermodynamic state of the solution, and a decrease in the supersaturation point. It is shown that the ultrasonic-assisted solution deposition of the PbI 2 thin films is controllable and reproducible, a process which is low-cost and in line with the large-scale fabrication of such solution-processed thin films.

Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance

“…Thus, excitation of the perovskite by the vibration results in a slight lattice distortion and a change in the lattice parameters. A change in the peak position, intensity, and width has been observed in PbI 2 crystals subjected to the SVPT, as well [35]. Additionally, the red shift may be due to an increase in the extrinsic defects, such as the fraction of the grain boundaries in the crystal lattice [40], which indeed increases with the application of vibration, through a decrease in the crystallite size, a conclusion that is backed by the findings of this work, e.g., Figure 3.…”

“…Nevertheless, the results based on the XRD and SEM affirm that the size of the perovskite grains almost linearly decrease with the duration of the SVPT. Figure 3a shows about 40% decrease in the grain size, if the SVPT is applied for 120 s. Imposed vibration results in an increase in the evaporation rate [23], an increase in the nucleation rate, and a decrease in the activation energy of crystallization [35], which seem to result in a decrease in the grain size. However, the main reason for a decrease in the perovskite grain size in the perovskite/m-TiO 2 system is due to an enhanced infiltration of the pores of the mesoporous layer by the perovskite precursors.…”

Investigation on Microstructures and Mechanical Properties of Isotactic Polypropylene Parts Fabricated by Different Process Conditions with Different Aging Periods

“…Thus, the shearing effect during mold-filling is strong, which benefits the orientation of molecular chains to generate crystals with regular shapes. However, the mechanical shocking effect caused by the high-frequency ultrasonic vibration makes polymer molecules difficult to quickly discharge into crystal lattices or even leads to crystal damage [27]. As a result, the crystallinity of each sample increases slowly due to the competition between these two opposite effects.…”

Inducing Crystallinity of Metal Thin Films with Weak Magnetic Fields without Thermal Annealing

“…The authors declare no conflict of interest. Magnetic forces can induce crystallization and is a valuable strategy for chemical free reactions, together with electric field [41], pressure [42], ultrasound [43] and sonication treatment [44].…”