## 3 Results and Discussion  
We prepared HDA-BiI5 powder and HDA-BiI5 ferroelectric films and characterized them. By comparing the XRD plots of Figure S1, the characteristic peak positions are consistent with a orthorhombic crystal structure of space group Pna21, which is consistent with the literature Ref. [8]. It has been reported that its crystal structure consists of one-dimensional zigzag chains of the corner-shared distorted BiI6 octahedra and hexane-1,6-diammonium cations. The ferroelectric film absorbs light in the wavelength range of 450 nm to 600 nm, i.e., most of the sunlight, as shown in Figure S2. The ultraviolet photoelectron spectroscopy of the ferroelectric films were also measured, and the Fermienergy level, valence band maximum and optical bandgap of this ferroelectric material were calculated from the processing of Figures S2 and S3 to be about 4.99 eV, 6.57 eV and 1.99 eV, respectively, which are similar to the energy band values reported by [PERSON] et al. This implies that this organic-inorganic hybrid material is an N-type semiconductor, so a P-N junction can be formed in the heterojunction interface between it as an optical absorber layer material and a P-type hole transport layer, thus accelerating the carrier separation and transport. Therefore, we use conventional inverted structure of solar cell as shown in Figure 1.  
In this study, we used chlorobenzene as the anti-solvent to optimize the ferroelectric film, and after several experiments, we found that when chlorobenzene was added drop-wise at the 10 th second of the second stage of film spin coating, the film was more uniform and crack-free, but the overall film denseness was poor, as shown in Figure 2. Therefore, we adjusted the amount of counter-solvent and observed in Figure 3 that the surface of the film was the flattest and densest when 40 uL of chlorobenzene was added in the 10 th second, and there were almost no cracks and holes. The detailed performance parameters of the device are provided in Table 1. The forward scan test yields a PCE of 0.053% and the reverse scan of 0.040%, which are 152% and 74% higher, respectively, compared with the device without the anti-solvent process; the open-circuit voltage (V\({}_{\text{oc}}\)) of the device is 0.69 V and 0.74 V under the forward scan and reverse-scan test conditions, respectively, which are 30% and 16% higher, respectively, compared with the device without the anti-solvent process. The short-circuit current densities of 0.41 mA/cm\({}^{2}\) and 0.20 mA/cm\({}^{2}\) under forward and reverse scan test conditions are shown in Table 1, which are improved by 105% and 43%, respectively. The flat and dense film structure facilitates the transport of photogenerated carriers and reduces carrier complexation at the interface, thus improving the performance of ferroelectric solar cells. For ferroelectric photovoltaic devices, polarization can improve its photovoltaic performance, so we conducted a study by polarizing one sample with structure ITO/NiO\(\times\)/HDA-BiI\({}_{\text{S}}\)/PCBM/Ag at different voltages. The measured J-V curves of the ITO/NiO\(\times\)/HDA-BiI\({}_{\text{S}}\)/PCBM/Ag structured device after polarization at different constant voltages are shown in Figure S4, where the maximum values of the forward and reverse scan ranges of each experiment are consistent with the polarization voltage values. The trends of each \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Optimized Conditions** & **Scanning Direction** & **PCE (\%)** & **\(\boldsymbol{V_{\text{oc}}\) (V)** & **\(\boldsymbol{J_{\text{sc}}}\) (mA/cm\({}^{2}\))** & **FF (\%)** \\ \cline{2-6}  & Forward scan & 0.021 & 0.53 & 0.20 & 19.56 \\ \cline{2-6}  & Reverse scan & 0.023 & 0.64 & 0.14 & 25.94 \\ \hline \multirow{2}{*}{Chlorobenzene adding} & Forward scan & 0.053 & 0.69 & 0.41 & 18.26 \\ \cline{2-6}  & Reverse scan & 0.040 & 0.74 & 0.20 & 27.26 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance parameters of unpolarized devices before and after optimization.