# Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance
## 3 Results and Discussion
This brings about improved pathways for the charge transport from the perovskite crystals to the ETL, although a decreased perovskite grain size, as a result of the SVPT, may potentially create additional charge trap sites at the boundaries, which may cause a reduced charge injection rate. To investigate the latter, the corresponding steady state PL spectra of the perovskite films deposited atop the m-TiO  
Figure 4: Thickness of the MAPbI\({}_{3}\) perovskite films deposited atop the m-TiO\({}_{2}\) layers at various time durations of the SVPT. Figure 3: Normalized (**a**) grain size obtained from the XRD patterns, and (**b**) the surface grain size obtained from the SEM images of the MAPbI\({}_{3}\) perovskite films spun at 5000 rpm, for various time durations of the SVPT. The sizes were normalized with respect to the crystallite/grain sizes of the pristine samples, i.e., 65 nm in (**a**) and 130 nm in (**b**). layer, without and with the SVPT, are shown in Figure 5. The emission peak appearing between 700 and 850 nm indicates the well-known 1.6 eV band gap of the MAPbI\({}_{3}\) perovskite [7]. The PL intensity is stronger for the pristine sample. The reduced PL intensity in the SVPT samples is indicative of higher charge injection into the TiO\({}_{2}\) layer [36], which implies that by applying the SVPT, the contact between the TiO\({}_{2}\) ETL and the perovskite light absorber has improved. It is also speculated that a decrease in the film thickness (c.f. Figure 4), as a result of the imposed vibration, while the film intactness is maintained, contributes to the improvement of the charge transport from the perovskite layer to the ETL. The PL peaks of Figure 5 exhibit a significant systematic red shift occurring due to the SVPT. The occurrence of the red shift may be attributed to a change in the perovskite crystal lattice parameters and/or the inclusion of defects or self-absorption in a thinner perovskite film, as caused by the vibrational excitations. It has been proposed that crystallites of the tetragonal phase of perovskite (as usually observed in PSCs) may co-exist with the orthorhombic phase, which form at room temperature [37]. It is also known that various phases form in different temperatures, where the temperature required for a structural phase transition increases with the thickness of the perovskite film [38]. Given that the imposed vibration affects the film thickness, and consequently, the phase transition temperature, one may speculate that a small amount of the orthorhombic phase may co-exist with the main tetragonal phase. To investigate this, the overall XRD patterns of the pristine and the SVPT MAPbI\({}_{3}\) films were obtained (Figure 6a). The peaks of the tetragonal phase are clearly dominant. Therefore, the hypothesis for the formation of the orthorhombic phase may be ruled out. The detailed XRD of the pristine and the film subjected to the SVPT for 180 s are shown in Fig. 6b. It is observed that a minor peak exists at the (004) plane associated with 28.3\({}^{\circ}\) angle, which disappears when the film is subjected to the SVPT for 180 s, while the peak associated with the (211) plane still exists, which indicates that the crystal structure is still tetragonal. Merger of the peaks at 28.3\({}^{\circ}\) and 28.6\({}^{\circ}\) has been attributed to crystal distortion, and therefore, a change in the lattice parameters [39]. 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].