Quantifying energy transfer to organic dyes through self-trapped excitonic and dopant-mediated emission in Cs ₂ NaBiCl ₆ lead-free double perovskite nanocrystals

As a promising alternative, lead-free halide double perovskites offer enhanced stability and reduced toxicity by combining monovalent (B⁺) and trivalent (B³⁺) cations in place of two Pb²⁺ ions. However, they exhibit weak photoluminescence due to indirect bandgaps or forbidden transitions. In this work, we demonstrate that controlled doping of Ag⁺ and Mn²⁺ ions into Cs₂NaBiCl₆ (CNBC) nanocrystals (NCs) activates strong self-trapped excitonic (STE) and dopant emission, respectively, in an otherwise non-emissive host. Ag⁺ incorporation induces lattice distortions that promote STE formation, yielding broadband emission (500 nm-1200 nm) with a large Stokes shift. Mn²⁺ doping results in dopant-induced emissions, arising from spin- and parity-forbidden transitions. We further investigate co-doping to harness both these effects simultaneously. Their energy transfer studies reveal that the doped NCs exhibit efficient energy transfer to organic dyes (rhodamine 6G and cyanine-7 amine). Overall, these findings highlight the role of dopants in creating broadband photoluminescence (350 nm-1200 nm) in an otherwise non-emissive host, essentially providing a robust way to produce intrinsic white light from a single material. It opens new pathways for advanced photonics, solar concentrators, bioimaging, and improved light harvesting applications.