Insights into A-Site Ion Engineering for Boosting Luminescence Efficiency in Lead-Free Perovskites

Zero dimensional (0D) lead-free perovskites, particularly Cs₄SnBr₆, have emerged as promising alternatives to toxic lead-based perovskites due to their distinctive broadband emission originating from self-trapped excitons (STEs). However, their practical implementation has been hindered by low photoluminescence quantum yields (PLQYs) and an insufficient understanding of the STE enhancement mechanism. In this work, we systematically investigate the role of A-site ion engineering (NH₄⁺, MA⁺, and FA⁺) in enhancing STE emission by inducing lattice distortion in the [SnBr₆]^4- octahedron. By combining advanced characterization techniques (XRD, Raman, solid-state NMR, and XPS) and theoretical calculations, we established an atomic-level correlation between A-site ion radii and the degree of octahedral distortion and clarified the PLQY enhancement mechanism by symmetry-breaking distortions localizing excitons, accelerating STE radiative recombination. Notably, larger A-site ions (FA⁺) induce pronounced lattice expansion and asymmetrically elongate Sn-Br bonds, boosting PLQY from 6.79% to 71.88%. A proof-of-concept white LED fabricated with FA-doped Cs₄SnBr₆ achieves a high color rendering index (CRI > 90) and long-term operating stability. This work provides a mechanistic understanding of STE enhancement via A-site doping, advancing the design of efficient lead-free optoelectronic materials.