## 3 Results and Discussion
### Study of Photophysical Properties of Multicomponent Hybrid Ferroelectric Thin Films  
Figure 4 shows the ultraviolet/visible (UV/Vis) absorption spectra of the multicomponent ferroelectric hybrid films. It can be seen from the figure that the absorption edges of these films were all in the range of 630 nm to 680 nm, which is the same trend as that of the two ferroelectric films of HDA-BiI\({}_{5}\) and [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]SbI\({}_{5}\) (Figure S2). In the range from 450 nm to 600 nm, the light absorption intensity of the ferroelectric films with a mixing ratio of 0.8:0.2 between Bi and Sb elements was slightly higher, and the absorption spectra of the other films in this range almost overlapped. This trend reflects the transition from [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]BiI\({}_{5}\) to [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]SbI\({}_{5}\) films, which can be well integrated with Figure S2. Because the absorbance values of pure HDA-BiI\({}_{5}\) and HDA-SbI\({}_{5}\) differed very little in the range of 400-550 nm, the absorbance values of the mixed materials almost overlapped after changing the concentration of the combination. It is noteworthy that, when the mixing ratio of Bi and Sb elements was 0.8:0.2, the position of the absorption edge of the film was significantly different from the other mixing ratios. With the increase in the atomic ratio of Sb elements, the absorption band edge of the film showed a tendency of red-shifting, even toward the long wavelength compared with the [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]BiI\({}_{5}\) film of Figure S2. In order to determine the bandgap of the multicomponent mixed ferroelectric films, the (ahv)\({}^{1/2}\)-hv relationship diagram is given in Figure 5, the calculation of which is described in the supporting literature and is not repeated here. From the figure, it can be observed that the bandgap of the ferroelectric film was about 2.02 eV when the mixing ratio of Bi and Sb elements was 0.8:0.2. The bandgap of the ferroelectric films gradually decreased with the increase in the atomic percentage of Sb elements, and the bandgaps of the films were 1.94 eV, 1.93 eV, and 1.91 eV when the mixing ratios of Bi and Sb elements were 0.6:0.4, 0.5:0.5, and 0.4:0.6, respectively. When the mixing ratio of Bi and Sb elements was 0.2:0.8, the bandgap of the ferroelectric film increased again to 1.95 eV. By comparison, it can be found that the mixing of two elements of group VA narrowed the bandgap of ferroelectric films (in most cases). A narrow bandgap means that the material can absorb a larger range of visible light, as shown in Figure 5, with an absorption band edge close to 700 nm, which has important implications for the study of optical absorption layers in ferroelectric photovoltaic devices. Optical absorption is closely related to the bandgap of the material. Solar power materials are generated by absorbing photons in electron-hole pairs so that electrons leap to the conduction band to become free electrons, generating a flow of electricity; hence, the incident light energy hv must be greater than or equal to  
Figure 4: Ultraviolet/visible (UV/Vis) absorption spectra of multicomponent ferroelectric films. the bandgap Eg of the material used. An appropriate reduction in the bandgap value can result in a higher utilization of light energy, which is conducive to light energy conversion. We believe that the reason for bandgap narrowing is mainly due to the mixture of two ferroelectric materials, HDA-Bil\({}_{5}\) and [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]Sbl\({}_{5}\), where Bi\({}^{2+}\) is replaced by Sb\({}^{2+}\), forming the [C\({}_{6}\)N\({}_{2}\)H\({}_{18}\)]Bi\({}_{1-x}\)Sb\({}_{x}\)I\({}_{5}\) compound. Since the B-site divalent metal ions in the chalcogenide type material (ABX\({}_{3}\)) occupy the main electronic orbitals at the bottom of the conduction band, it is obvious that B-site ion doping can tune the bandgap of chalcogenide materials.