Lanthanide doped lead-free double perovskites as the promising next generation ultra-broadband light sources

Efficient ultra-broadband emitter is realized by using lanthanide ion doping coupled with “DPs-in-glass composite” (DiG) structure. The synergy of self-trapped exciton together with the energy transition induce this ultra-broadband emission emerge. Efficient ultra-broadband emitter is realized by using multi-Ln-doped Cs2Ag(In/Bi)Cl6 lead-free double perovskite (DP) coupled with “DPs-in-glass composite” (DiG) structure. The synergy of self-trapped exciton together with the energy transition induces this ultra-broadband emission emerge.

Ultra-broadband emitter is critical to advancing the applications of light sensing, spectrum analysis, and life sciences imaging, et al. With the development of highcapacity optical data communications and ultra-precision metrology 1,2 , efficient ultra-bandgap emission becomes particularly important. Traditional ultra-broadband light sources generally include halogen tungsten lamps (HTLs) 3 , super-luminescent diodes (SLDs) 4 , ultrabroadband semiconductor lasers (UBSLs) 5 , laser-driven light sources (LDLSs) 6 , super-continuum light sources (SCLSs) 7 , etc. However, many shortcomings still exist, such as spectral instability, high electrical consumption, short lifetime, substantial heat generation, and noncompactness. Hence, alternative ultra-broadband light sources with outstanding optical and structural properties are highly demanded.
Metal halide perovskites have attracted widespread attention due to their outstanding optoelectronic properties [8][9][10] , making them as the promising monochromatic bright emitters. However, toxicity and poor material stabilities of traditional lead perovskites impede their further commercialization 11 . Accordingly, leadfree halide double perovskites (DPs) have drawn increasing attention recently owing to their fascinating optical properties and excellent stabilities. In particular, lanthanide (Ln 3+ ) ion doping to tailor the optical or electrical properties of DPs has been well documented, aiming for their applications in white LED, NIR-LED, scintillator, anti-counterfeiting, and X-ray detecting 12 . The progresses leverage the opportunity to realize ultrabroadband emission using Ln 3+ -doped DPs, which has never been explored.
Chen's group here reported the pioneer work to realize ultra-broadband continuous emission from visible to near-infrared spectral region (400-2000 nm) in Cs 2 A-gInCl 6 DPs, by combining the self-trapped exciton (STE) and extra luminescence channel induced by Ln 3+ doping 13 (Fig. 1a). In particular, the Bi/Ln co-doped Cs 2 A-gInCl 6 (Bi/Ln (Ln = Nd, Yb, Er, Tm): Cs 2 AgInCl 6 ) exhibit both visible STE and multiple NIR Ln 3+ 4f-4f emissions under excitation 14 , which enables ultra-broadband emission (Fig. 1b). Energy transfer mechanism was proposed to explain the origin of the Ln 3+ emission in Bi/Ln: Cs 2 AgInCl 6 DPs. Notably, Bi 3+ doping is critical to enabling Ln 3+ emission, since Bi 3+ doping can modulate the density of states at the band edge, break parity forbidden transition of STE states and promote exciton localization, giving rise to new optical channels at a lower energy level and promoting efficiency of STE emission 15 . Moreover, two intense absorptions transitions of Bi 3+ were observed, which were ascribed to the 1 S 0 → 1 P 1 and 1 S 0 → 3 P 1 transition. The process effectively transfers energy to Ln 3+ dopants, to enable multiple emission of 4f-4f transitions that resulted in NIR emissions 16 .
The synergy of STE broadband emission (400-800 nm) and narrowband NIR emissions from Ln 3+ (Yb 3+ , Tm 3+ , Er 3+ , and Nd 3+ ) thus induce ultra-broadband continuous luminescence. As shown in Fig. 1, multiple Ln 3+ activators need to be doped into DPs host, but the energy transfer and cross-relaxation processes among them typically led to the energy loss via non-radiative relaxation, resulting in quenched Ln 3+ emissions in the multi-doped DPs 17 . To solve the problem, they constructed a unique DPs-in-glass (DiG) monolithic composite to confine different Ln 3+ dopants and avoid their interaction. Specifically, Nd: Cs 2 AgInCl 6 , Yb/Er: Cs 2 AgInCl 6 and Yb/Tm:Cs 2 AgInCl 6 DPs were dispersed into an inorganic glass matrix by low temperature co-sintering. The above bottom-up strategy endows the prepared Ln 3+ -doped DiG with an improved PLQY of 40% and superior long-term stability.
The DiG was then coupled with commercial 350 nm UV chip to fabricate lighting devices, representing the record ultra-broadband light source covering spectral region from 400 to 2000 nm with full width at half maxima (FWHM) of~365 nm (Fig. 1c). Furthermore, Chen et al. showcase the compact ultra-broadband LED's (u-LED's) applications in nondestructive spectroscopic analysis and multifunctional lighting 13 .
The brand-new strategy conceived by Chen et al. thus provides a powerful toolbox to tailoring multi-Ln 3+doped DPs to realize efficient ultra-broadband emitters. The strategy certainly will attract widespread attention from the whole community, and facilitate their application in various fields such as multi-functional lighting, optical communication, and nondestructive spectral analysis. The lanthanide-doped lead-free DPs thus represent a promising candidate for next generation ultra-broadband light sources. /Ln 3-e N lR 4 0 0 -8 0 0 n m V is 1 S 0 3 P 0 3 P 1 3 P 2 1 P 1 Fig. 1 u-LED relying on the synergy of (STE) recombination and Ln 3+ dopants' 4f-4f transitions of the multi-Ln 3+ -DiG. a Structure diagram of Bi/Ln:Cs 2 AgInCl 6 DPs. b The ultra-broadband emission mechanisms in Bi/Ln:Cs 2 AgInCl 6 DPs. c PL spectrum of the u-LED device. The inset is the photographs of the multi-Ln 3+ -DiG u-LED by visible camera (yellow) and NIR camera (white)