Highly Efficient and Thermally Stable K₃AlF₆:Mn⁴⁺ as a Red Phosphor for Ultra-High-Performance Warm White Light-Emitting Diodes

Abstract

Following pioneering work, solution-processable Mn-activated fluoride pigments, such as ABF (A = Na, K, Rb, Cs; A = Ba, Zn; B = Si, Ge, Ti, Zr, Sn), have attracted considerable attention as highly promising red phosphors for warm white light-emitting diodes (W-LEDs). To date, these fluoride pigments have been synthesized via traditional chemical routes with HF solution. However, in addition to the possible dangers of hypertoxic HF, the uncontrolled precipitation of fluorides and the extensive processing steps produce large morphological variations, resulting in a wide variation in the LED performance of the resulting devices, which hampers their prospects for practical applications. Here, we demonstrate a prototype W-LED with KAlF:Mn as the red light component via an efficient and water-processable cation-exchange green route. The prototype already shows an efficient luminous efficacy (LE) beyond 190 lm/W, along with an excellent color rendering index (Ra = 84) and a lower correlated color temperature (CCT = 3665 K). We find that the Mn ions at the distorted octahedral sites in KAlF:Mn can produce a high photoluminescence thermal and color stability, and higher quantum efficiency (QE) (internal QE (IQE) of 88% and external QE (EQE) of 50.6%.) that are in turn responsible for the realization of a high LE by the warm W-LEDs. Our findings indicate that the water-processed KAlF may be a highly suitable candidate for fabricating high-performance warm W-LEDs.

Co-Precipitation Synthesis and Optical Properties of Mn4+-Doped Hexafluoroaluminate w-LED Phosphors

“…For K 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ the measured Mn concentration is very close to the 1 mol % Mn added during the synthesis, which demonstrates that our synthesis method provides good control over the Mn 4+ doping concentration. Also a substantially higher fraction of Mn 4+ is incorporated into K 3 AlF 6 compared to previously reported cation exchange methods for preparing K 3 AlF 6 :Mn 4+ [18]. …”

“…Recently, the synthesis and luminescence properties of Mn 4+ -doped hexafluoraluminates with the compositions M 3 AlF 6 :Mn 4+ (M = Li, Na, K) were reported [17,18,19,20]. The ionic radius of Mn 4+ is similar to the ionic radius of Al 3+ (0.53 versus 0.54 Å), and as a result, Mn 4+ can easily substitute for Al 3+ in fluoride hosts [17,21].…”

“…Song et al prepared Na 3 AlF 6 :Mn 4+ phosphors via a co-precipitation method [17], while others synthesized K 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ by cation exchange [18,19]. Furthermore, K 2 LiAlF 6 :Mn 4+ was synthesized via a hydrothermal method [20].…”

“…A single convenient synthesis method for preparing M 3 AlF 6 :Mn 4+ phosphors is thus so far lacking. The previous works on M 3 AlF 6 :Mn 4+ have reported luminescence spectra and decay curves for different Mn 4+ doping concentrations in the temperature range of 300 to 500 K [17,18,19,20]. However, they provided no insight into the influence of (charge compensating) defects on the luminescence spectra and quantum yield of M 3 AlF 6 :Mn 4+ .…”

Photoluminescence properties of a novel red fluoride K2LiGaF6:Mn4+nanophosphor

“…It can be seen that the emission intensity is almost the same in the temperature range from 300 K to 375 K and then drops sharply with the temperature above 400 K. At 425 K (150 C), it remains 88.8% of the initial intensity at 300 K. Similar thermal quenching behavior of Mn 4+ ion in uoride phosphors had been reported for several times. 49,56,57 We determine the activation energy (E a ) of K 2 LiGaF 6 :Mn 4+ for thermal quenching using eqn (1).…”