Pseudohalogen Engineering at X-Site for Highly Efficient and Stable Self-Trapped Excitons in Double Perovskites

The manipulation of self-trapped excitons (STE) in lead-free double perovskites is crucial for achieving efficient broadband emission but remains challenging due to their highly symmetric lattice. The highly rigid lattice of double perovskites typically restricts strong exciton–phonon coupling-induced lattice distortion, which consequently leads to a low photoluminescence quantum yield (PLQY). Here, we demonstrate a novel and effective X-site engineering strategy using the pseudohalogen thiocyanate (SCN–) anion to dramatically enhance the blue STE emission in Sb3+-doped Cs2NaInCl6. Combined experimental and theoretical analyses reveal that SCN– preferentially substitutes for Cl– anions coordinated with Sb3+, inducing pronounced lattice distortion and strong electron–phonon coupling. Moreover, significant lattice distortion accompanied by a highly distorted Jahn–Teller deformation accelerates the singlet-to-triplet transition, suppresses nonradiative pathways, and consequently enhances the photoluminescence quantum yield (PLQY) from 36.26% to 73.27%. The incorporation of SCN– also modifies the electronic structure of the excited state, resulting in enhanced resistance to thermal quenching from 303 to 383 K. Finally, leveraging these superior properties, we fabricated a stable white light-emitting diode (WLED) with broad-spectrum emission. This work provides fundamental insights into tailoring STE emission through pseudohalogen doping and establishes a new paradigm for X-site engineering in perovskite optoelectronics.