Indirect X-ray detection relies on scintillators that convert absorbed X-rays into visible light, and the scintillating layer of an imaging detector is required to be a mechanically robust solid. Metal halide perovskite nanocrystals (NCs) have attracted considerable attention as scintillating materials owing to their strong X-ray absorption, spectrally tunable radioluminescence (RL), and fast emission decay. Nevertheless, the low formation energy of the perovskite lattice and the dynamic binding of surface ligands render the NCs highly susceptible to moisture, and their luminescence deteriorates within hours upon exposure to water. Polymer encapsulation has been shown to extend the aqueous stability of perovskite NCs, and NCs have been incorporated into plastic scintillators. However, these two functions have been established separately, and a film in which a single matrix both protects the NCs from water and takes part in the scintillation has not been reported.

In this study, we report a free-standing CsPbBr₃ NC/PPO/polystyrene (PS) composite scintillator film in which the NCs are combined with 2,5-diphenyloxazole (PPO), an organic fluor that enhances their RL through X-ray-induced charge transfer, and embedded in a PS matrix. PS was selected as the matrix because it protects perovskite NCs from water and, as the base polymer of plastic scintillators, transfers absorbed radiation energy to the dissolved fluor. The films were fabricated by dissolving PS in the NC/PPO solution, followed by freeze-drying and hot-pressing. The NCs remained crystalline and individually dispersed within the film, with PPO distributed around them, as confirmed by X-ray diffraction, Raman spectroscopy, and cross-sectional transmission electron microscopy. Under X-ray irradiation with a clinical linear accelerator, the emission of the film was generated by the NCs and markedly enhanced by PPO, indicating that the sensitization established in solution remains effective in the film. The film retained its emission through more than 1000 h of storage in water as well as in air, with approximately 88% and 76% of the initial emission remaining, respectively, and immersion in water caused no additional degradation. A large-area film approximately 5 cm in diameter prepared by the same procedure visualized the internal structure of small objects under X-ray irradiation. These results demonstrate that the NC/PPO/PS composite film provides a solid, water-stable, and scalable form in which perovskite NC scintillators can be applied to X-ray imaging without the moisture-tight packaging required for hygroscopic scintillators.