Tunable Optical Properties and Enhanced Stability of Organic‐Inorganic Hybrid Cu‐Based Halide Thin Films for Advanced Anti‐Counterfeiting Applications

Abstract The development of highly stable and optically tunable materials is crucial for next‐generation patterning and anti‐counterfeiting applications. This study presents a comprehensive investigation into the synthesis, structural dynamics, and optoelectronic properties of MA 4 Cu 2 X 6 (X = Cl, Br, Br/Cl) films fabricated via a three‐step spin‐coating technique. By leveraging halide anion engineering, the precise control over the coordination environment of Cu centers are achieved, as evidenced by synchrotron‐based extended X‐ray absorption fine structure analysis. Both their strong electron–phonon coupling and high exciton binding energies facilitate efficient self‐trapped exciton emission. The photoluminescence emission is tailored from yellow to green and eventually to blue‐green under ultraviolet excitation (254 nm). STE localization mitigates thermal quenching, achieving PLQYs >64%. Stability assessments indicate that mixed‐halide compositions effectively suppress Cu+ oxidation, prolonging luminescence performance. Mixed halides enhance environmental stability, retaining >25% initial PLQY after 720 h in ambient conditions. Furthermore, the films demonstrate tunable diffraction properties in patterned photonic architectures, highlighting their potential for advanced optoelectronic devices. The films demonstrate high‐resolution patterning capabilities via photolithography and screen printing, underscoring their potential for advanced optical security applications.