X-ray diffraction (XRD) was employed to perform the crystalline structure of the WBG films with Pb(SCN)
2 additive and subsequent treatment with MACl [
41]. Two distinctive diffraction peaks are seen at 14.06° and 28.19°, which correspond to the (100) and (200) crystal planes of the α-phase, as presented in Fig.
3A [
36,
42]. Additionally, a new peak appears at 12.38° corresponding to PbI
2, indicating that Pb(SCN)
2 additive formed excessive PbI
2 on the surface [
43]. The intensity of (100) and (200) planes is enhanced in the samples with the Pb(SCN)
2 additive and MACl posttreatment [
44]. Meantime, the diffraction peaks shifted to a higher degree (Fig. S5), resulting from the excessive PbI
2 and smaller chloride doped in the crystal. As shown in Fig. S6, the diffraction intensity of PbI
2 and (100) plane is enhanced with the concentration of Pb(SCN)
2 additive increasing. Comprehensively, the optimal concentration of Pb(SCN)
2 additive is 2%. As presented in Fig. S7A, the optimal concentration of MACl solution is 3 mg/ml, as demonstrated by the highest diffraction intensity and the disappearance of the PbI
2 peak. Moreover, Fig. S7B illustrates that the (100) peak shifts to a higher degree as the concentration of MACl increases. This demonstrates that the compressive strain is formed on the surface owing to the smaller crystal lattice of MAPbI
3–xCl
x than WBG perovskite [
45]. Based on these results, we speculate that chloride ions are incorporated into the crystal lattice to form GHJ. To further investigate this effect, we prepared samples containing 2% Pb(SCN)
2 additive and 3 mg/ml MACl. As presented in Fig.
3B, a distinct absorption onset is at around 750 nm in ultraviolet (UV)-visible (vis) absorption spectra. The absorption edge blue-shifted in the perovskite films with Pb(SCN)
2 additive and MACl posttreatment, indicating the presence of excessive PbI
2 and GHJ [
39,
46]. In addition, the absorbance is also enhanced owing to the improved crystallization after adding Pb(SCN)
2. Tauc curves were calculated using absorption spectra in Fig. S8. In comparison with the control sample, the optical bandgap of WBG films with excessive PbI
2 and GHJ enhanced from 1.686 to 1.696 and 1.698 eV, respectively. The steady-state photoluminescence (PL) spectra were measured using the samples prepared on an insulated substrate and covered by poly (methyl methacrylate) (PMMA). The control sample exhibits an emission peak located at around 749 nm, whereas the perovskite films with excessive PbI
2 and GHJ exhibit blue shift and enhanced intensity resulting from the suppression of nonradiative recombination [
19,
47]. Moreover, the PL spectra were measured from both the glass and PMMA sides to perform the difference between bulk and surface in Fig. S9. The perovskite films with GHJ exhibit a blue shift measured from the PMMA side, indicating that the GHJ is mainly located on the surface. The PL spectra were recorded after light soaking for 2 h in Fig. S10, where the sample with GHJ did not show halide segregation under continuous illumination in comparison with the control one. This demonstrates that the compressive strain resulted in higher barrier of ion migration, suggesting that the GHJ can improve light stability and enhance operational stability [
45,
48]. Moreover, time-resolved photoluminescence (TRPL) is utilized to determine the carrier lifetime [
26]. As presented in Fig.
3D and Table S1, the average lifetime was increased from 92.31 ns to 173.91 and 231.12 ns in the WBG films with excessive PbI
2 and GHJ, indicating enhanced carrier lifetime resulting from improved crystallization and passivated defects. Next, we further investigate the effect of GHJ on WBG films through electronic characterizations.