A General Ammonium Salt Assisted Synthesis Strategy for Cr³⁺‐Doped Hexafluorides with Highly Efficient Near Infrared Emissions

Abstract

The discovery of highly efficient broadband near infrared (NIR) emission material is urgent and crucial for constructing NIR lighting sources and emerging applications. Herein, a series of NIR emission hexafluorides A2BMF6:Cr3+ (A = Na, K, Rb, Cs; B = Li, Na, K, Cs; M = Al, Ga, Sc, In) peaking at ≈733–801 nm with a full width at half maximum (FWHM) of ≈98–115 nm are synthesized by a general ammonium salt assisted synthesis strategy. Benefiting from the pre‐ammoniation of the trivalent metal sources, the Cr3+ can be more efficiently doped into the A2BMF6 and simultaneously prevent the generation of the competitive phase. Particularly, Na3ScF6:Cr3+ (λem = 774 nm, FWHM ≈ 108 nm) with optimal Cr3+‐doping concentration of 35.96% shows a high internal quantum efficiency of 91.5% and an external quantum efficiency of ≈40.82%. A lighting emitting diode (LED) device with a NIR output power of ≈291.05 mW at 100 mA driven current and high photoelectric conversion efficiency of 20.94% is fabricated. The general synthesis strategy opens up new avenues for the exploration of Cr3+‐doped high efficiency phosphors, and the as‐obtained record NIR output power demonstrates for NIR LED lighting sources applications.

Glass‐Crystallized Luminescence Translucent Ceramics toward High‐Performance Broadband NIR LEDs

“…[ 23b ] More specifically, the magnitude of tetrahedral shrinkage due to the replacement of larger Al 3+ by smaller Si 4+ is larger than that of dodecahedral expansion due to the simultaneous substitution of Ca 2+ for Y 3+ (Table S3 , Supporting Information), leading to overall shrinkage of the garnet structure (Table S1 , Supporting Information); meanwhile, the large shortening of the Si/Al—O bonds pushes the oxygen atoms of the common apex of AlO 6 octahedra away from the central Al 3+ and thus leads to the octahedral enlargement. Only when Cr 3+ occupies a large octahedral site with weak crystal field can it generate broadband NIR emission; [ 23a ] accordingly, nearly all of the reported phosphors are gallate/germanate compounds or those containing rare metal ions like Sc 3+ and In 3+ , [ 8 , 9 , 11 , 12 , 14 ] which make them high in cost. By contrast, the case of YCAS:Cr 3+ implies that it is possible to obtain weak‐field octahedral Al 3+ sites for generating broadband NIR emission with low cost.…”

“…Trivalent chromium ion (Cr 3+ ) [ 9 ] has shown its superiority to other activator ions [ 10 ] in emitting far‐red/NIR light of 650–1100 nm upon blue light excitation. For example, high IQE (>90%) was achieved through cation substitution [ 9c,d ] and optimizing synthesis strategy; [ 11 ] single‐phase extra‐broadband emission with FWHM > 200 nm was also realized by exploring host crystals allowing multiple‐site occupation of Cr 3+ . [ 12 ] Nevertheless, the fundamental problem for Cr 3+ activated phosphors is the small absorption cross‐section (10 −19 –10 −20 cm 2 ) of Cr 3+ as a result of the parity‐forbidden nature of intraconfigurational d–d transitions ( 4 A 2 → 4 T 1 / 4 T 2 ).…”

“…[ 12 ] Nevertheless, the fundamental problem for Cr 3+ activated phosphors is the small absorption cross‐section (10 −19 –10 −20 cm 2 ) of Cr 3+ as a result of the parity‐forbidden nature of intraconfigurational d–d transitions ( 4 A 2 → 4 T 1 / 4 T 2 ). [ 13 ] Increasing the doping concentration, [ 11 , 14 ] and introducing the odd‐parity crystal field by lattice distortion, [ 15 ] can expectedly improve the light absorption efficiency (AE), but inevitably impose serious adverse effects on IQE and thermal stability because of the enhanced (thermally activated) concentration quenching and/or the reduced structural rigidity. [ 16 ] Another knotty problem for traditional NIR pc‐LEDs with phosphor powders embedded in organic resin is the massive heat accumulation resulting from large quantum defect (>40%) in down‐converting blue light into NIR one and the very low thermal conductivity (≈0.2 W m −1 K −1 ) of organic binders, which fundamentally limit the performance of the pc‐LED devices.…”

A broadband near‐infrared Sc1−x(PO3)3:XCr3+ phosphor with enhanced thermal stability and quantum yield by Yb3+ codoping

“…Zhang et al. found Na 3 ScF 6 :Cr 3+ exhibiting an ultrahigh EQE of 40.82% and the photoelectric conversion efficiency reaching 20.94% 35 . For the Cr 3+ ‐doped broadband NIR phosphors with longer wavelength emission, however, they are usually having an inferior IQE with lower thermal quenching temperature, because the decrease of the energy gap from the excited state to the ground state may yield an increasing possibility of the non‐radiative transition 36–41 .…”

“…34 Zhang et al found Na 3 ScF 6 :Cr 3+ exhibiting an ultrahigh EQE of 40.82% and the photoelectric conversion efficiency reaching 20.94%. 35 For the Cr 3+ -doped broadband NIR phosphors with longer wavelength emission, however, they are usually having an inferior IQE with lower thermal quenching temperature, because the decrease of the energy gap from the excited state to the ground state may yield an increasing possibility of the non-radiative transition. [36][37][38][39][40][41] A novel NIR phosphor LiScP 2 O 7 :Cr 3+ has been reported by Shao et al, showing a pure broadband NIR emission peaking at 880 nm with a full width at half-maximum (FWHM) of ∼170 nm, and further improved by codoped with Yb 3+ .…”