Ultrabroadband light sources that emit over an extremely wide spectral range are of great interest in many fields, such as photonics, medical treatment, high-capacity optical data communications, ultra-precision metrology and spectroscopy.Conventionally, halogen tungsten lamps (HTLs) are used in most studies and applications, but they generate large amounts of heat and have limited operational lifetimes. Recently, super-luminescent diodes (SLDs), ultrabroadband semiconductor lasers (UBSLs), laser-driven light sources (LDLSs), supercontinuum light sources (SCLSs), and other sources of light have been developed to satisfy the needs of various applications. However, each of these approaches has intrinsic drawbacks, such as complex structure design for coupling multiple LEDs, difficulty in tailoring IS transitions, low efficiency, and expensive laser triggers.
Lanthanide (Ln3+) or rare-earth ion incorporation or doping is considered a promising approach to imparting and tailoring the optical and optoelectronic performances of inorganic materials spanning the ultraviolet (UV), visible (Vis), near-infrared (NIR) and middle-infrared (MIR) regions. For example, Er-doped fibre amplification (EDFA), praseodymium-doped fibre amplifier (PDFA) and thulium-doped fibre amplifier (TDFA) with broadband NIR emissions have been successfully applied in optical telecommunication. However, the parity-forbidden 4f-4f transition of Ln3+ leads to weak light absorption, which limits the practical application of these materials. Sensitizing Ln3+ emission via semiconductor quantum dots (QDs) can effectively address this difficult challenge because QDs undergo efficient allowed band-to-band absorption and have tuneable bandgaps. Recently, various lanthanide ions have been successfully doped into the hottest lead-halide perovskite QD (PeQD) lattices to endow them with optical functionality. Unfortunately, their use of toxic lead and their poor stability to heat, water, electric fields and light hamper their commercialization and industrialization on a large scale. As an alternative, lead-free halide double perovskites (DPS) have drawn extensive attention for their fascinating optical performance and excellent stability. Among the DP family, Cs2AgInCl6 has been in the spotlight for its direct band gap characteristics and various alloy compositions with mono- and trivalent cations other than Ag+ and In3+.
In a new paper published in Light Science & Application, Prof. Chen Daqin’s team at Fujian Normal University and Prof. Chen Xueyuan’s team at Fujian Institute of Material Structure, Chinese Academy of Sciences, have explored the possibility of combining the self-trapped exciton (STE) recombination of Cs2AgInCl6 with Ln3+ dopant-induced extra light-emitting channels to produce lead-free Vis-NIR (400~2000 nm) ultrabroadband emissive DPs for the first time. To our knowledge, there has been no report on Ln3+-doped DPs aimed at the application of ultrabroadband light sources. Herein, a family of Ln3+ dopants (Ln=La-Lu) with Bi3+ ions are successfully incorporated into Cs2AgInCl6 DPs (Fig.1a). The Bi/Ln (Ln=Nd, Yb, Er, Tm):Cs2AgInCl6 samples can yield both visible STE radiation and multiple NIR Ln3+ 4f-4f emissions under UV light excitation, and the related energy transfer mechanisms are systematically discussed (Fig.1b). We further provide a strategy of dispersing Nd:Cs2AgInCl6, Yb/Er:Cs2AgInCl6 and Yb/Tm:Cs2AgInCl6 DPs into an inorganic glass matrix to prevent the depletion of nonradiative migration from the co-doping of multiple Ln3+ ions in a sole DP host, leading to detrimental luminescent quenching (Fig.1c, d). To the best of our knowledge, this is the first report on a DP-in-glass (DiP) composite. The DiP composite with superior stability and a high PLQY of ~40% can be coupled with a commercial UV LED chip to construct an ultrabroadband LED (u-LED) device (Fig.1e). Finally, we demonstrate its promising applications in spectroscopic analysis and multifunctional lighting (Fig.2). Notably, compared to previously reported ultrabroadband light sources, this u-LED device shows the advantages of cost-effective fabrication, compactness, excellent optical performance, and extremely long-term stability.