As a photoisomeric molecule, BTTM can undergo isomerization between closed- and open-ring forms under UV light and visible light (Fig.
1A) [
46,
47]. First, we dissolved BTTM in an isopropyl alcohol solution and observed that its color deepened under 365-nm UV light irradiation. As shown in Fig.
1B, the absorption spectrum of the BTTM solution in the UV–visible range (300 to 600 nm) indicates a structural change. Meanwhile, it can be seen that the color of the BTTM solution that has changed after UV aging will return to the state before irradiation after the UV light is removed (inset in Fig.
1B), indicating the reversibility of its tautomeric reaction. Herein, we introduced the BTTM as an additive into the perovskite solution, allowing it to interact with the perovskite. Prior investigations have demonstrated that iodide ions in halide perovskite materials will migrate and form iodine vacancies under UV light irradiation, ultimately leading to the photodegradation of perovskite [
48,
49]. The C=O functional group in the BTTM molecule engages in a strong bond with Pb in the perovskite. Meanwhile, the N–H group coordinates with I (in the [PbI
6]
2− octahedron) through hydrogen bonding. These interactions will anchor the Pb–I octahedral structure of the perovskite, thereby effectively suppressing the migration of iodide ions and effectively passivating perovskite defects, as shown in Fig.
1C. To verify the interaction of BTTM molecules with iodide ions, we dissolved formamidinium iodide (FAI), BTTM, and FAI+BTTM in isopropyl alcohol, respectively, and irradiated under UV light (365 nm) for different periods of time. As shown in Fig. S1, it can be seen that the color of the FAI+BTTM solution is different from that of the BTTM solution after UV light irradiation. It is speculated that an interaction occurs between FAI and BTTM (Fig. S1B and C). Meanwhile, it can be seen that the FAI solution turned yellow after 4 h of UV light irradiation, indicating that iodide ions escape under UV light irradiation (Fig. S1C). After the FAI+BTTM solution irradiated with UV light for 4 h is placed in an environment without UV light for a period of time, a change in the solution color is also observed, indicating that there is still a certain tautomerism after the interaction between the BTTM and FAI (Fig. S1D). To further verify the interaction between BTTM and the perovskite, we carried out Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) tests. The FTIR results show that the BTTM with –C=O and –N–H groups interacts with FAI and with PbI
2, as shown in Fig.
1D and Fig. S2. After the interaction between the –C=O in the BTTM and PbI
2, the observed downward shift of the –C=O symmetric stretching band is from 1,643 to 1,637 cm
−1, indicating a strong interaction between PbI
2 and –C=O. Meanwhile, the stretching vibration of N–H also shifts notably, indicating an obvious interaction between the N–H group and I
−, which further indicates that the BTTM is beneficial for passivating defects in the perovskite film. Figure
1E and F shows the XPS Pb 4f spectra and I 3d spectra of the control and BTTM perovskite films (denoted as control and BTTM, respectively). The XPS spectrum of Pb 4f in the BTTM perovskite film shows that the peak positions of the 2 main peaks, Pb 4f 7/2 and Pb 4f 5/2, are 138.2 and 143.1 eV, respectively, while those of the control perovskite film are located at 138.5 and 143.3 eV. This observation signifies a robust interfacial interaction between the perovskite and BTTM, driven by electron cloud redistribution-induced shifts in binding energy. Meanwhile, the shifts of the I 3d and N 1s peak positions (Fig. S3) further indicate that the N–H group in the BTTM perovskite film interacts with I
− to form hydrogen bonds. The results of FTIR and XPS indicate that BTTM would passivate defects in the perovskite film and achieve an anchoring effect on Pb–I, reducing the migration of halide ions, which is expected to improve the photoelectric properties and UV stability of the perovskite film.