# Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance
## 3 Results and Discussion
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.  
Figure 5: Effect of the SVPT on the photoluminescence (PL) spectra of the MAPbI\({}_{3}\) perovskite layers deposited atop the m-TiO\({}_{2}\) layers. The spin speed in the second stage of the spinning is 5000 rpm. The effect of the crystal size on the device performance is controversial. Some studies are in favor of smaller crystal sizes, whereas some other recommend otherwise [41]. [PERSON] et al. have reported that larger grain sizes are beneficial [42], whereas here we observed that while the application of the SVPT results in a decrease in the grain size, the charge transport from the perovskite layer to the ETL layer is improved (PL spectra). Therefore, it may be deduced that the improved coverage and the contact surface between the perovskite layer and the TiO\({}_{2}\) nanoparticles, as caused by the SVPT, is responsible for the improved charge transport. Figure 7 illustrates the SEM images of the pristine and SVPT perovskite layers spun atop the m-TiO\({}_{2}\) layer at 5000 rpm. It is observed that the morphology of the perovskite film is improved with an increase in the duration of the SVPT, and the best morphology (Figure 7) and PL spectra (Figure 5) are obtained at the SVPT time duration of 180 s. The SVPT imparts energy to the wet film, and therefore creates micromixing [25] and in-situ heating, which tends to homogenize the perovskite solution, leading to the formation of a uniform and high coverage film. Figure 6: (**a**) Effect of the SVPT time on XRD patterns of the MAPbI\({}_{3}\) perovskite films. (**b**) Effect of the SVPT on crystal distortion. The “**”and “**” denote the perovskite and FTO peaks, respectively. Figure 7: SEM images of the MAPbI\({}_{3}\) perovskite surface morphology, spun at 5000 rpm for (**a**) pristine sample, and the samples subjected to the SVPT for (**b**) 60 s, (**c**) 120 s, and (**d**) 180 s.  
Since the perovskite film thickness and crystal size vary with the spinning speed, in another series of experiments, the spinning speed was decreased from 5000 to 4000 rpm, to further investigate how the SVPT affects thicker films. The PL spectra of a perovskite film spun at 4000 rpm is compared with that of the best sample of the previous cases, i.e., the film spun at 5000 rpm, using 180 s of the SVPT (Figure 8a). Figure 8a shows that the lower spin speed of 4000 rpm results in a better charge extraction. Hence, the effect of the SVPT was further explored on the perovskite films that were spun at 4000 rpm (Figure 8b). A much weaker PL intensity is observed in the sample subjected to the SVPT for 240 s, which is an indicative of a better charge transport. To study the surface morphology, Figure 9 shows the SEM images of pristine and the SVPT MAPbI3 samples spun at 4000 rpm. It is observed that the long-duration vibration of 240 s results in a decrease in the grain size and emergence of pinholes. Despite the appearance of the pinholes, this sample shows a superior charge transfer as compared with the pristine sample (Figure 8b). While vibration has a similar positive effect on both cases, comparison of the SEM images of the perovskite films spun at 4000 rpm (Figure 9) and 5000 rpm (Figure 7) reveals that the film spun at 5000 rpm and subjected to 180 s of the SVPT shows a more desirable uniformity and coverage, and therefore 5000 rpm is selected as the optimum case for device fabrication. The absorbance spectra of the pristine and the SVPT MAPbI3 films spun at 5000 rpm are shown in Figure 10. Absorbance is a measure of the capability of the film to absorb the photon energy to  
Figure 8: Photoluminescence (PL) spectra of the MAPbI3 perovskite films deposited atop the m-TiO2 layers.