Phase Engineering of Perovskites Unlocks Brilliant UVB Luminescence and Hour‐Long Afterglow for Solar‐Blind Optoelectronics

Achieving both high‐efficiency luminescence and long‐lasting afterglow in the solar‐blind ultraviolet‑B (UVB) region remains a fundamental challenge for metal halide perovskites, whose intrinsically soft lattices and strong electron–phonon coupling typically suppress one property at the expense of the other. Here we overcome this trade‑off by deliberately engineering electron–phonon coupling through structural transformation in the Cs‐Cd‐Cl perovskite system. Leveraging phase diversity and the site‑sensitivity of Pb 2+ doping, we enhance lattice rigidity and tailor the defect landscape, enabling spectral emission to be shifted from the green to the UVB region. The resulting Ruddlesden‐Popper phase, Cs 2 CdCl 4 :Pb 2+ , achieves a photoluminescence quantum yield of 34% in the UVB band together with an afterglow exceeding 10 h, establishing a new dual‑mode benchmark for halide perovskite UVB emitters. This dual‑mode operation unlocks three advanced solar‑blind applications of dynamic information encryption via time‑gated UVB readout, sensitive ion detection using the narrowband 314 nm emission as a high‑purity absorption probe, and effective surface sterilization within a biosafety‑enhanced spectral window. By establishing a direct causal link between lattice dynamics and macroscopic photonic behavior, this work provides a transformative design paradigm for next‑generation UV photonic technologies.