## 3 Results
### Properties of the NiO and PTAA/NiO Films  
To further investigate the mechanism behind this output improvement, the detailed optical and electrical properties of the NiO and PTAA/NiO films, surface energy, UV-Vis, energy band level diagram, and PL studies were examined. The energy levels of the individual components and the alignment of the energy levels of device structures were studied using UPS. As shown in Figure 5a, the work functions (WFs) for NiO and PTAA through the UPS were determined to be 4.6 eV and 4.8 eV, respectively [41]. The optical properties of NiO, PTAA/NiO, and PTAA HTL films were investigated by UV-Vis spectroscopy. The UV-Vis spectra data are shown in Figure 5b, and the bandgaps calculated from the Tauc plot method are shown in Figure 5c,d [42]. The obtained band gaps of NiO and PTAA of HTL were 3.7 eV and 4.0 eV, respectively. The energy level alignments of ITO as the bottom electrode, NiO and PTAA as HTL, perovskite as the photoactive layer, and C\({}_{60}\) as the electron transport layer are shown in Figure 5e, and the perovskite was based on the triple  
Figure 4: Statistical comparison of photovoltaic parameters with optimization condition of NiO and PTAA/NiO; (**a**) V\({}_{\text{OC}}\), (**b**) J\({}_{\text{SC}}\), (**c**) FF, and (**d**) PCE. Figure 3: (**a**) J–V curves of the NiO and PTAA/NiO PSCs with active area of 1.12 cm\({}^{2}\). (**b**) EQE spectra of NiO and PTAA/NiO based PSCs. cation composition Cs\({}_{0.175}\)FA\({}_{0.750}\)MA\({}_{0.075}\)Pb(I\({}_{0.880}\)Br\({}_{0.120}\))\({}_{3}\). The obtained valence bands of NiO and PTAA film were 0.6 eV and 0.6 eV, respectively, which means that that of PTAA was closer to the perovskite film than that of NiO. From the UPS results, it was concluded that HTL with bilayers of PTAA/NiO has a more suitable energy level alignment and can better perform the charge extraction role in perovskite active layer. To further investigate the effect of PTAA/NiO film on the performance of the PSCs, the steady-state PL spectra of perovskite and HTL with perovskite film were measured and are shown in Figure 5f. We prepared PL spectra of perovskite films based on HTL films such as perovskite/glass, perovskite/NiO/glass, and perovskite/PTAA/NiO/glass. In the PL peak emission, the perovskite film showed a strong emission peak at about 765 nm. When the NiO as HTL was introduced under the perovskite film, the PL peak intensity decreased dramatically. Further, when PTAA was introduced on NiO to form a PTAA/NiO bilayer, the PL peak intensity was further reduced. Through the inserted cross-section SEM image, it was confirmed that the thickness of perovskite/PTAA/NiO was approximately 30 nm, and that of perovskite was 500 nm. Unfortunately, PTAA was too thin to be distinguished. However, PTAA/NiO bilayer HTL showed more efficient PL quenching than the NiO film obtained from steady-state PL. The finding of a lower PL intensity in the PL results means that carrier  
Figure 5: (**a**) UPS spectra for NiO and PTAA films. (**b**) UV–Vis transmittance spectra of various HTL films; NiO, PTAA, PTAA/NiO (bilayer) on ITO substrate. Tauc plot of (**c**) NiO film and (**d**) PTAA film. (**e**) Energy level diagram of the whole layer PSC device. (**f**) Steady-state PL spectra of perovskite, NiO/perovskite, and perovskite /PTAA/NiO films. Inset: the cross-view SEM image of perovskite/PTAA/NiO film. extraction and transport is most efficient when the PTAA/NiO bilayer is used as HTL, and that the energy level alignment of HTL between electrode and perovskite is substantially improved and the carrier recombination is reduced [43; 44]. These results are consistent with the higher Jsc and FF values of PTAA/NiO bilayer PSCs in their J-V characteristics. Differences in the wettability of the substrates can modulate the nucleation and growth processes of perovskite crystals, thus affecting the quality of the perovskite films.