# Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance
## 3 Results and Discussion
(**a**) The best SVPT sample spun at 5000 rpm, and the pristine sample spun at 4000 rpm, and (**b**) samples spun at 4000 rpm with and without the SVPT. Figure 4:0 SEM images of the MAPbI3 perovskite films spun at 4000 rpm. (**a**) Pristine sample, and the samples subjected to the SVPT for (**b**) 60 s, (**c**) 120 s, (**d**) 180 s, and (**e**) 240 s. In (**e**), the voids are circled. overcome the band gap. Some studies indicate that a larger perovskite crystal size (\(\sim\)1 \(\upmu\)m) favors the light absorbance [43,44]. Also, a thicker and highly covered and defect-free film favor the absorbance. Since in this study, the imposed vibration affects the crystal size, film thickness, and coverage, the sole effect of the vibration or crystal size on the absorbance cannot be deduced. The perovskite layer vibrated for 180 s has the lowest absorbance. Figure 3 shows that this sample has one of the smallest crystal sizes, and Figure 4 shows that the thickness of this film is almost half of that of the pristine film. Therefore, the reduced absorbance may be due to both a decreased crystal size and also a lower film thickness. It is noted that although the film that was subjected to the SVPT for 180 s has the lowest absorbance, it is the most intact, defect-free, and impurity-free film, with the best charge transfer capability. It was previously observed that the PL spectra undergo a red shift as a result of the vibration (c.f. Figure 5), due to some changes in the lattice structure of the film. On the other hand, Figure 10 shows that there is no shift in the position of the sudden drop in the absorbance intensity (\(\sim\)780 nm), as a result of the vibration, which may be an indicative of insensitivity of the band gap to the vibration. Table 1 summarizes the effect of the vibration time on major characteristics of the perovskite films spun at two speeds. It is corroborated that as the vibration time increases, the PL intensity decreases (charge transfer between the perovskite and the ETL is improved), the film thickness decreases (perovskite fills the gaps of the m-TiO\({}_{2}\) layer), and the full width at half maximum (FWHM) in the XRD peaks increases (grain size decreases). Thus, one may conclude that the SVPT improves all of the characteristics of the MAPbI\({}_{3}\) films, except for the absorbance. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Spin Speed (rpm)** & **Vibration Time (s)** & **PL Intensity (a.u)** & **Film Thickness (nm)** & **FWHM at 14\({}^{\circ}\) (degree)** \\ \hline
4000 & 0 & 0.44 & 450 & 0.12 \\
4000 & 60 & 0.36 & 400 & 0.19 \\
4000 & 120 & 0.34 & 400 & 0.20 \\
4000 & 180 & 0.33 & 390 & 0.22 \\
4000 & 240 & 0.27 & 388 & 0.26 \\
5000 & 0 & 1.6 & 410 & 0.13 \\
5000 & 60 & 0.73 & 310 & 0.14 \\
5000 & 120 & 0.48 & 280 & 0.18 \\
5000 & 180 & 0.46 & 245 & 0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Effect of the vibration time for two spinning speeds on the characteristics of MAPbI\({}_{3}\) layers. Figure 10: Absorbance of the MAPbI\({}_{3}\) perovskite films spun atop the m-TiO\({}_{2}\) layers at 5000 rpm for several time durations of the SVPT. The previous experiments were performed on MAPbI\({}_{3-x}\)pcovskite. In an attempt to generalize the findings, selected experiments were performed on mixed halide perovskite (MAPbI\({}_{3-x}\)Cl\({}_{x}\)) films, spun at 5000 rpm. The XRD patterns of the MAPbI\({}_{3-x}\)Cl\({}_{x}\) films are shown in Figure 11, substantiating a complete conversion of the precursors to perovskite. Comparison of the peaks of the MAPbI\({}_{3}\) with those of the MAPbI\({}_{3-x}\)Cl\({}_{x}\) reveals that the position of the peak of the (110) plane for MAPbI\({}_{3}\) is at 14.12\({}^{\circ}\), while it changes to 14.25\({}^{\circ}\) for the sample with 30 mg of PbCl\({}_{2}\). The other peaks remain at the same positons. Therefore, the addition of Cl slightly changes the perovskite lattice parameters.