Fig. 1. Schematic diagram of experimental procedure

Fig. 2. （a）X-ray photoelectron spectroscopy of CsPbBr_1.2I_1.8；（b）PL spectra and（c）UV-Vis spectra of CsPbBr_1.2I_1.8，CsPbBr_1.2I_1.8/PMMA，CsPbBr_1.2I_1.8/PIBMA and CsPbBr_1.2I_1.8/PS films.

Fig. 3. FTIR spectra of CsPbBr_1.2I_1.8/PMMA and pure PMMA（a），CsPbBr_1.2I_1.8/PIBMA and pure PIBMA（b），CsPbBr_1.2I_1.8/PS and pure PS（c）. Partial enlargement of FTIR spectra of CsPbBr_1.2I_1.8/PMMA and pure PMMA（d），CsPbBr_1.2I_1.8/PIBMA and pure PIBMA（e），CsPbBr_1.2I_1.8/PS and pure PS（f）.

Fig. 4. XRD patterns of CsPbBr_1.2I_1.8，CsPbBr_1.2I_1.8/PMMA，CsPbBr_1.2I_1.8/PIBMA and CsPbBr_1.2I_1.8/PS films on glass substrates.

Fig. 5. （a）Relative PL intensity variation of films after soaking in water for 1~14 min；（b）Images of film samples soaked in water for 0，8，14 min（Samples from top to bottom：CsPbBr_1.2I_1.8，CsPbBr_1.2I_1.8/PMMA，CsPbBr_1.2I_1.8/PIBMA and CsPbBr_1.2I_1.8/PS films）.

Fig. 6. AFM height images of CsPbBr_1.2I_1.8（a），CsPbBr_1.2I_1.8/PMMA（b），CsPbBr_1.2I_1.8/PIBMA（c）and CsPbBr_1.2I_1.8/PS films（d）.

Fig. 7. Emission spectra from（a）CsPbBr_1.2I_1.8，（b）CsPbBr_1.2I_1.8/PMMA，（c）CsPbBr_1.2I_1.8/PIBMA，（d）CsPbBr_1.2I_1.8/PS films under a 532 nm nanosecond laser excitation with different pump energy densities. Emission intensity（red）and linewidth（blue）as functions of pump energy for（e）CsPbBr_1.2I_1.8，（f）CsPbBr_1.2I_1.8/PMMA，（g）CsPbBr_1.2I_1.8/PIBMA and（h）CsPbBr_1.2I_1.8/PS films.

Fig. 8. Variation in ASE thresholds after 30 days of storage in air environment.（a）CsPbBr_1.2I_1.8；（b）‍ CsPbBr_1.2I_1.8/PMMA；（c）CsPbBr_1.2I_1.8/PIBMA；（d）CsPbBr_1.2I_1.8/PS films.