# Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance
## 3 Results and Discussion  
Given that the mechanism of the crystal formation is comprised of two steps of nucleation and crystal growth [13; 30], the deposition of perovskite precursors was performed in two consecutive spinning speeds. Evaporation rate is lower at lower speeds, and therefore nucleation starts without substantial crystal growth. At a higher spinning speed, the evaporation rate increases, and the resulting supersaturation boosts the crystallization process. To further expedite the solvent extraction, toluene anti-solvent was dripped at the second spinning stage. Therefore, unless stated otherwise, the perovskite films in this work were prepared by one-step deposition of a mixture of precursor solutions, employing two consecutive spinning speeds. In the first stage of all cases, the film was spun at 1000 rpm for 15 s, whereas the speed of the second stage was either 4000 or 5000 rpm, for 40 s. Based on several optimization steps, we found that 60 ul of toluene is the suitable amount of the anti-solvent to prepare a pure and high coverage MA-based perovskite film with an area of 10 \(\times\) 10 mm, spun at 5000 rpm (Figure 2). The film coverage and the perovskite conversion mainly determine the rate of the charge generation and transport within the cell [10; 31]. Figure 2, however, shows some manufacturing defects in the film, which affects the device performance. In the next step, to further control the mechanism of the crystal growth, we performed the SVPT on the as-spun wet perovskite films deposited atop the m-TiO\({}_{2}\) layer for certain time durations. The XRD patterns and SEM images were employed to assess the perovskite inner crystallite and surface grain sizes, respectively. The x-ray incident angle was set at 14.2\({}^{\circ}\) and the constant in the Scherrer equation was set to 0.9. The top view SEM images show the size and shape of the surface grains, whereas the values obtained using the XRD-based Scherrer equation [32; 33] are more representative of the perovskite grains in the three-dimensional (3D) lattice. It is also noted that the Scherrer equation is accurate for crystals smaller than 100 nm [34], and large surface grains cannot be estimated using the XRD patterns. The XRD and SEM grain sizes are shown in Figure 3a,b, respectively. The sample prepared without the SVPT has XRD and SEM grain sizes of 65 and 130 nm, respectively. As the measurements show, the surface grains observed under the SEM are larger than those within the film detected and measured by the XRD. Nevertheless, the results based on the XRD and SEM affirm that  
Figure 2: XRD pattern and SEM image of the MAPbI\({}_{3}\) perovskite films fabricated atop the m-TiO\({}_{2}\) layer using the one-step deposition method at two consecutive spin speeds (spin speed in the second stage is 5000 rpm). The “**” denotes the perovskite peaks and “#” denotes the fluorine-doped tin oxide (FTO) peaks. 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. This improved perovskite penetration hypothesis is supported by the thickness data of the MAPbI\({}_{3}\) perovskite layers deposited atop the m-TiO\({}_{2}\) layer, as illustrated in Figure 4, which shows a decrease in the perovskite thickness with the vibration time. The enhanced infiltration of the m-TiO\({}_{2}\) pores by the perovskite precursors leads to an increase in the contact surface between the perovskite and the TiO\({}_{2}\) nanoparticles, within the m-TiO\({}_{2}\) layer.