Cryogenic Narrowband Green Emission in Blue-Light Excited Pb-Free Double Perovskite Cs2Ag0.6Na0.4In0.8Bi0.2Cl6@KSCN Via Surface Reconstruction-Induced Energy Level Modification

Pb-free double perovskites, such as Cs2AgInCl6, have received considerable attention in optoelectronic applications as a more stable and less toxic substitute for Pb-based perovskites. However, Cs2AgInCl6 faces challenges in being excited by blue light, and its relatively large spectral line width restricts its development in the display field. This study introduces a novel structure wherein the surface of double perovskite Cs2Ag0.6Na0.4In0.8Bi0.2Cl6 is coated with KSCN, enabling efficient excitation by blue light and producing ultranarrow line width green light emission at low temperatures. Cs2Ag0.6Na0.4In0.8Bi0.2Cl6@KSCN can emit 558 nm green light when excited by blue light (450 nm) at 10-30 K, with a remarkable full width at half-maximum (fwhm) of 36 nm. The emission mechanism of blue light-excited narrow-spectrum green emission has been determined through temperature-dependent photoluminescence (PL) and first-principles calculations. Surface reconstruction with KSCN results in the emergence of new donor energy levels and the potential to be excited by blue light. Below 30 K, the excitons are compressed and concentrated at the bottom of the self-trapped exciton singlet state, directly completing radiative transitions to produce narrowband green emission. The achievement of narrowband green emission provides important guidance for optimizing the photoelectric performance of lead-free double perovskite materials.