Ultra-broadband emitter is critical to advancing the applications of light sensing, spectrum analysis, and life sciences imaging, et al. With the development of high-capacity optical data communications and ultra-precision metrology1,2, efficient ultra-bandgap emission becomes particularly important. Traditional ultra-broadband light sources generally include halogen tungsten lamps (HTLs)3, super-luminescent diodes (SLDs)4, ultra-broadband 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 properties8,9,10, making them as the promising monochromatic bright emitters. However, toxicity and poor material stabilities of traditional lead perovskites impede their further commercialization11. Accordingly, lead-free halide double perovskites (DPs) have drawn increasing attention recently owing to their fascinating optical properties and excellent stabilities. In particular, lanthanide (Ln3+) 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 detecting12. The progresses leverage the opportunity to realize ultra-broadband emission using Ln3+-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 Cs2AgInCl6 DPs, by combining the self-trapped exciton (STE) and extra luminescence channel induced by Ln3+ doping13 (Fig. 1a). In particular, the Bi/Ln co-doped Cs2AgInCl6 (Bi/Ln (Ln = Nd, Yb, Er, Tm): Cs2AgInCl6) exhibit both visible STE and multiple NIR Ln3+ 4f-4f emissions under excitation14, which enables ultra-broadband emission (Fig. 1b). Energy transfer mechanism was proposed to explain the origin of the Ln3+ emission in Bi/Ln: Cs2AgInCl6 DPs. Notably, Bi3+ doping is critical to enabling Ln3+ emission, since Bi3+ 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 emission15. Moreover, two intense absorptions transitions of Bi3+ were observed, which were ascribed to the 1S0 → 1P1 and 1S0 → 3P1 transition. The process effectively transfers energy to Ln3+ dopants, to enable multiple emission of 4f–4f transitions that resulted in NIR emissions16.

Fig. 1: u-LED relying on the synergy of (STE) recombination and Ln3+ dopants’ 4f-4f transitions of the multi-Ln3+-DiG.
figure 1

a Structure diagram of Bi/Ln:Cs2AgInCl6 DPs. b The ultra-broadband emission mechanisms in Bi/Ln:Cs2AgInCl6 DPs. c PL spectrum of the u-LED device. The inset is the photographs of the multi-Ln3+-DiG u-LED by visible camera (yellow) and NIR camera (white)

The synergy of STE broadband emission (400–800 nm) and narrowband NIR emissions from Ln3+ (Yb3+, Tm3+, Er3+, and Nd3+) thus induce ultra-broadband continuous luminescence. As shown in Fig. 1, multiple Ln3+ 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 Ln3+ emissions in the multi-doped DPs17. To solve the problem, they constructed a unique DPs-in-glass (DiG) monolithic composite to confine different Ln3+ dopants and avoid their interaction. Specifically, Nd:Cs2AgInCl6, Yb/Er: Cs2AgInCl6 and Yb/Tm:Cs2AgInCl6 DPs were dispersed into an inorganic glass matrix by low temperature co-sintering. The above bottom-up strategy endows the prepared Ln3+-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 lighting13.

The brand-new strategy conceived by Chen et al. thus provides a powerful toolbox to tailoring multi-Ln3+-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.