Ion exchange-assisted surface passivation toward highly stable red-emitting fluoride phosphors for light-emitting diodes

A facile and environmentally friendly ion exchange-assisted surface passivation (IASP) strategy is presented for synthesizing red emitting Mn4+-activated fluoride phosphors. A substantial, pristine Mn4+-free shell layer, applied as a coating to Mn4+ doped potassium fluorosilicate K2SiF6:Mn4+ (KSFM) phosphors, enhances both water resistance and luminescence efficiency. The stability test of fluoride in water at ambient temperature and boiling water demonstrates that IASP-treated KSFM phosphors are highly water resistant. Furthermore, both the negative thermal temperature (NTQ) fitting results and the photoluminescence (PL) decay confirm that the IASP process effectively passivates surface defects, leading to enhanced luminescence performance. The maximum internal quantum yield (QYi) of the IASP-KSFM phosphor is 94.24%. A white LED realized a high color rendering index (CRI) of 93.09 and luminous efficiency (LE) of 149.48 lm/W. This work presented a novel technique for the development of stable fluoride phosphors and has the potential to increase the use of KSFM phosphors in plant supplementary lighting systems and white light-emitting diodes.

Harnessing solid-state ion exchange for the environmentally benign synthesis of high-efficiency Mn4+-doped phosphors

In this study, a solid-state ion exchange (SSIE) method is proposed to synthesize a series of Mn4+-doped fluoride phosphors, which avoids the use of HF solution during the Mn4+-doping process. The obtained Mn4+-doped fluoride phosphors exhibit strong red emission with a quantum yield of 46%.

Limited energy migration and circumscribed multiphonon relaxation produced non-concentration quenching in a novel dazzling red-emitting phosphor

White LED applications are still constrained by extremely efficient narrow band red emitting phosphors. Meanwhile, the concentration quenching induced by energy migration is the main reason that limits the emission intensity of a red emitting phosphor. Therefore, developing a novel red emitting material with energy migration limitations seems necessary. Here, we proposed and realized the non-concentration quenching doping of Eu3+ ions in a Sr9Y2-2xW4O24:xEu3+ (0 ≤ x ≤ 1.0) phosphor for the first time by means of host preferential selection. By clearly investigating the crystal structure and luminescence kinetics, the long-distance between the nearby Eu3+ ions and the low phonon energy are the main reasons that suppress the energy migration and the cross-relaxation among Eu3+ ions. These advantages result in a high internal (90.47%) and external quantum efficiency (42.1%) of Sr9Eu2W4O24. With the help of the Judd-Ofelt theory and the large value of oscillator strength Ω2, Eu3+ ions are verified to occupy the non-symmetric lattice site with high color purity (94.4%). In addition, only 5.2% emission intensity loss at 140 °C can guarantee its application in LED devices. Moreover, the SYWO:Eu3+ phosphor has high thermal tolerance, high color stability, excellent moisture resistance and superior physical/chemical stability, and thus has broad practical spectral application prospects. The prepared WLED shows superior performance, and the calculated NTSC values are as high as 101.8% and 104.7%, respectively. For comparison, the optical performances of the Sr9Y2W4O24:Eu3+ phosphor outperform those of the standard commercial red phosphors, Y2O3:Eu3+ and Y2O2S:Eu3+, and almost match that of K2MnF6. These results may pave the way for fresh approaches to the study of high-performance Eu3+-activated phosphors.

Research progress on surface modifications for phosphors used in light-emitting diodes (LEDs)

Stable and efficient phosphors are highly important for light-emitting diodes (LEDs) with respect to their application in solid-state lighting, instead of conventional lamps for general lighting. However, some problems, like low stability, low photoluminescence (PL) efficiency, and serious thermal degradation, are commonly encountered in phosphors, limiting their applications in LEDs. Surface modifications for some phosphors commonly used in LED lighting, including fluoride, sulphide, silicate, oxide, nitride, and oxynitride phosphors, are presented in this review. By forming a protective surface layer, the stabilities against moisture and high temperature of fluoride- and sulphide-based phosphors were strengthened; by coating inorganic and organic materials around the particle surface, the PL efficiencies of silicate- and oxide-based phosphors were improved; by passivation treatment upon the phosphor surface, the thermal degradation of nitride- and oxynitride-based phosphors was reduced. Various technologies for surface modification are described in detail; moreover, the mechanisms of stability strengthening, PL efficiency improvement, and thermal degradation reduction are explained. In addition, embedding of phosphors in inorganic glass matrix, especially for quantum dots, is also introduced as an effective method to improve phosphor stability for LED applications. Finally, future developments of surface modification of phosphors are proposed.

Advancing Red-Emitting Fluoride Phosphors for Highly Stable White Light-Emitting Diodes: Crystal Reconstruction and Covalence Enhancement Strategy

Mn⁴⁺-activated fluoride red phosphors exhibit excellent luminescence properties. However, a persistent technical challenge lies in their poor moisture resistance. Current strategies primarily focus on surface modifications to effectively shield the [MnF₆]^2- species from water molecules while neglecting the underlying structure of the fluoride matrix. In this study, we introduce Si⁴⁺ and Ge⁴⁺ ions into the K₂TiF₆:Mn⁴⁺ crystal to create covalent fluoride solid solutions, namely, K₂Ti_1-xSi_xF₆:Mn⁴⁺ and K₂Ti_1-yGe_yF₆:Mn⁴⁺, through crystal reconstruction. The findings reveal that the incorporation of Si⁴⁺ leads to increased particle size, enhanced luminescence intensity (by 40%), and improved moisture resistance. Furthermore, after undergoing 1000 h of aging at high temperature and high humidity conditions, the white LED featuring the K₂Ti_0.97Si_0.03F₆:Mn⁴⁺ phosphor demonstrates remarkable durability by retaining 90% of its initial luminous efficacy. This performance surpasses that of the device utilizing the K₂TiF₆:Mn⁴⁺ phosphor, which only retains 74% of its original efficacy. The crystal reconstruction method and covalent enhancement strategy proposed in this work contribute to enhancing the luminescence efficiency and moisture resistance of fluoride phosphors, thereby offering new insights for advancing the development of high-efficiency and highly stable white light LED devices.

Recent Research Progress of Mn4+-Doped A2MF6 (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge, or Sn) Red Phosphors Based on a Core-Shell Structure

White light emitting diodes (WLEDs) are widely used due to their advantages of high efficiency, low electricity consumption, long service life, quick response time, environmental protection, and so on. The addition of red phosphor is beneficial to further improve the quality of WLEDs. The search for novel red phosphors has focused mainly on Eu²⁺ ion- and Mn⁴⁺ ion-doped compounds. Both of them have emissions in the red region, absorption in blue region, and similar quantum yields. Eu²⁺-doped phosphors possess a rather broad-band emission with a tail in the deep red spectral range, where the sensitivity of the human eye is significantly reduced, resulting in a decrease in luminous efficacy of WLEDs. Mn⁴⁺ ions provide a narrow emission band ~670 nm in oxide hosts, which is still almost unrecognizable to the human eye. Mn⁴⁺-doped fluoride phosphors have become one of the research hotspots in recent years due to their excellent fluorescent properties, thermal stability, and low cost. They possess broad absorption in the blue region, and a series of narrow red emission bands at around 630 nm, which are suitable to serve as red emitting components of WLEDs. However, the problem of easy hydrolysis in humid environments limits their application. Recent studies have shown that constructing a core-shell structure can effectively improve the water resistance of Mn⁴⁺-doped fluorides. This paper outlines the research progress of Mn⁴⁺-doped fluoride A₂MF₆ (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge or Sn), which has been based on the core-shell structure in recent years. From the viewpoint of the core-shell structure, this paper mainly emphasizes the shell layer classification, synthesis methods, luminescent mechanism, the effect on luminescent properties, and water resistance, and it also gives some applications in terms of WLEDs. Moreover, it proposes challenges and developments in the future.

Structural evolution of organic-inorganic hybrid crystals for high colour-rendering white LEDs

Luminescent crystals with high efficiency have huge potential in applications for white light-emitting diodes (LEDs). Herein, organic-inorganic hybrid crystals doped with Mn⁴⁺, [N(CH₃)₄]₂XF₆:Mn⁴⁺ (X = Si, Ge, and Ti), were grown under mild conditions. Their crystal structural evolution was confirmed by single-crystal X-ray diffraction at different temperatures. These crystals exhibit intense red emission with a high external quantum efficiency (73.0% for [N(CH₃)₄]₂TiF₆:Mn⁴⁺) and good thermal stability. The warm white LEDs were fabricated by combining these red-emitting phosphors with a YAG:Ce³⁺ ceramic chip. As-grown crystals can significantly optimize the performances of white LEDs (colour rendering index up to 95). Hence, this work provides a new strategy to explore Mn⁴⁺-activated organic-inorganic hybrid materials for white LEDs.

Enhancement of emission and luminescent thermal stability of K2SiF6 : Mn4+ by synergy of co-doping with Na+ and coating with GQDs

The thermal stability and luminescent intensity of the sample are obviously enhanced by co-doping of Na+ and coating of GQDs. Mechanism of the strong NTQ is attributed to conversion of thermal energy into light energy via phonon-induced transition.

Enhancing Structural Rigidity via a Strategy Involving Protons for Creating Water-Resistant Mn4+-Doped Fluoride Phosphors

The poor water resistance property of a commercial Mn⁴⁺-activated narrow-band red-emitting fluoride phosphor restricts its promising applications in high-performance white LEDs and wide-gamut displays. Herein, we develop a structural rigidity-enhancing strategy using a novel KHF₂:Mn⁴⁺ precursor as a Mn source to construct a proton-containing water-resistant phosphor K₂(H)TiF₆:Mn⁴⁺ (KHTFM). The parasitic [HMnF₆]^- complexes in the interstitial site from the fall off the KHF₂:Mn⁴⁺ are also transferred to the K₂TiF₆ host by ion exchange to form KHTFM with rigid bonding networks, improving the water resistance and thermostability of the sample. The KHTFM sample retains at least 92% of the original emission value after 180 min of water immersion, while the non-water-resistant K₂TiF₆:Mn⁴⁺(KTFM) phosphor maintains only 23%. Therefore, these findings not only illustrate the effect of protons on fluoride but also provide a novel insight into commercial water-resistant fluoride phosphors.

Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion

Red phosphor materials play a key role in improving the lighting and backlit display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). However, the development of a red phosphor with simultaneous high efficiency, excellent thermal stability and high colour purity is still a challenge. In this work, unique non-concentration quenching in solid-solution Cs₃Gd_1 - xGe₃O₉:xEu³⁺ (CGGO:xEu³⁺) (x = 0.1-1.0) phosphors is successfully developed to achieve a highly efficient red-emitting Cs₃EuGe₃O₉ (CEGO) phosphor. Under the optimal 464 nm blue light excitation, CEGO shows a strong red emission at 611 nm with a high colour purity of 95.07% and a high internal quantum efficiency of 94%. Impressively, this red-emitting CEGO phosphor exhibits a better thermal stability at higher temperatures (175-250 °C, >90%) than typical red K₂SiF₆:Mn⁴⁺ and Y₂O₃:Eu³⁺ phosphors, and has a remarkable volumetric negative thermal expansion (coefficient of thermal expansion, α = -5.06 × 10^-5/°C, 25-250 °C). By employing this red CEGO phosphor, a fabricated pc-WLED emits warm white light with colour coordinates (0.364, 0.383), a high colour rendering index (CRI = 89.7), and a low colour coordinate temperature (CCT = 4508 K). These results indicate that this highly efficient red-emitting phosphor has great potential as a red component for pc-WLEDs, opening a new perspective for developing new phosphor materials.

Improved Moisture-Resistant and Luminescence Properties of a Red Phosphor Based on Dodec-fluoride K3RbGe2F12:Mn4+ through Surface Modification

Mn⁴⁺-activated red-emitting fluoride phosphors are essential for white light-emitting diodes (WLEDs) with desirable color rendition index (CRI) because of their unique and efficient luminescence characteristics. Herein, we synthesized a novel Mn⁴⁺-activated dodec-fluoride phosphor K₃RbGe₂F₁₂:Mn⁴⁺ (KRGF:Mn) through a facile ionic exchange method at room temperature. A surface-modified strategy using weak reducing agents such as oxalic acid and citric acid is proposed to improve the moisture-resistance ability of KRGF:Mn phosphor dramatically, and the possible mechanism of surface modification has been investigated. A shell formed on the surface of the KRGF:Mn phosphor reduces the concentration of Mn⁴⁺ on the surface, which can prevent the internal KRGF:Mn group hydrolysis by the external moisture and effectively decreased the probability of energy migration to surface defects, thereby increasing both the emission efficiency and the moisture-resistance ability of KRGF:Mn. More interestingly, the KRGF:Mn phosphor is quenched after soaking in water for 72 h but recovered to the initial brightness after soaking in the modifier solutions for 2 min. This work fabricates a new efficient red phosphor KRGF:Mn for application in warm WLEDs and provides insight into the mechanism of the strategy to improve the moisture resistance of the stability of Mn⁴⁺ through surface modification.

Phase Transformation of a K2GeF6 Polymorph for Phosphors Driven by a Simple Precipitation-Dissolution Equilibrium and Ion Exchange

Tuning crystal phase transformations is very important for obtaining polymorphs for phosphors with the ideal optical properties and stability. Mn⁴⁺-doped K₂GeF₆ (KGF) is a typical polymorphic phosphor, but the thermodynamic and kinetic mechanism of its phase transformation is still unclear. Herein, the phase transformation of polymorphs varying from P6₃mc KGF and trigonal KGF to P6₃mc Si⁴⁺-doped KGF is realized by introducing the synergistic action of an HF solution and Si⁴⁺ ions. The full structural refinements of KGF polymorphs at room temperature and the electronic band structure calculations were performed. The results show that the Si⁴⁺-doped hexagonal KGF polymorph with good photoluminescence properties is the most stable phase according to the calculated total energy landscape and relative formation energy. The morphologic changes were monitored in situ to clearly understand the rapid phase transformation mechanism, which proves that the phase transformation is driven by a simple precipitation-dissolution equilibrium and ionic exchange.

Dual-Shelled RbLi(Li3 SiO4 )2 :Eu2+ @Al2 O3 @ODTMS Phosphor as a Stable Green Emitter for High-Power LED Backlights

The stability of luminescent materials is a key factor for the practical application in white light-emitting diodes (LEDs). Poor chemical stability of narrow-band green-emitting RbLi(Li₃ SiO₄ )₂ :Eu²⁺ (RLSO:Eu²⁺ ) phosphor hinders their further commercialization even if they have excellent stability against thermal quenching. Herein, we propose an efficient protection scheme by combining the surface coating of amorphous Al₂ O₃ and hydrophobic modification by octadecyltrimethoxysilane (ODTMS) to construct the moisture-resistant dual-shelled RLSO:Eu²⁺ @Al₂ O₃ @ODTMS composite. The growth mechanisms of both the Al₂ O₃ inorganic layer and the silane organic layer on the phosphor surface are investigated. The results remarkably improve the water-stability of this narrow-band green emitter. The evaluation of the white LED by employing this composite as the green component demonstrates that RLSO:Eu²⁺ @Al₂ O₃ @ODTMS is a promising candidate for the high-performance display backlights, and this dual-shelled strategy provides an alternative method to improve the moisture-resistant property of humidity-sensitive phosphors.

Cs2MnF6 Red Phosphor with Ultrahigh Absorption Efficiency

To improve absorption efficiency (AE) and subsequently improve external quantum efficiency (EQE) remains one of the significant challenges for Mn⁴⁺-doped red-emitting fluoride phosphors. In this study, we propose to use Mn⁴⁺ as a part of matrix to enhance the AE of fluoride phosphors. Red-emission phosphors Cs₂MnF₆, Cs₂MnF₆:Sc³⁺, and Cs₂MnF₆:Si⁴⁺ were synthesized successfully by a coprecipitation method. The Rietveld refinement of X-ray diffraction reveals that this red phosphor exhibits a cubic structure in Fm3̅m space group. Owing to Mn⁴⁺ being a part of matrix, this kind of red phosphor possesses an extremely high AE, which can be promoted to 88%. The doping of Sc³⁺ and Si⁴⁺ ions into Cs₂MnF₆ can effectively increase the luminescence intensity to 253 and 232%, respectively, relative to that of Cs₂MnF₆. The relative emission intensity of Cs₂MnF₆:5%Si⁴⁺ red phosphor preserves about 115% when temperature rises to 175 °C. By employing Cs₂MnF₆:5%Si⁴⁺ as a red-emitting component, high-performance LED-1 with Ra = 86.2, R9 = 82.1 and CCT = 3297 K, and LED-2 with an ultrawide color gamut (NTSC value of 122.3% and rec. 2020 value of 91.3%) are obtained. This work may provide a new idea to explore a new type of fluoride phosphor with high EQE for high-performance white-light-emitting diodes.

Synthesis, Crystal Structure, and Photoluminescence Characteristics of High-Efficiency Deep-Red Emitting Ba2GdTaO6:Mn4+ Phosphors

In this article, a series of novel Mn4+-doped Ba2GdTaO6 (BGT) red-emitting phosphors were successfully synthesized via a high-temperature solid-state method. The crystal structure, morphology, and luminescent performance of the samples were investigated in detail with X-ray diffraction, field emission scanning electron microscopy, photoluminescence (PL) spectra, decay curves, and internal quantum efficiency (IQE). Excited at 358 nm, these samples showed an intense deep-red emission band peaking at 688 nm in the wavelength region of 620-800 nm. The excitation spectra of these samples monitored at 688 nm exhibited two broad excitation bands from 250 to 600 nm with peaks at 358 and 469 nm. The systematic investigation of the concentration-dependent PL properties of BGT:Mn4+ phosphors revealed that the deep-red emission intensity reached the maximum when the Mn4+ doping concentration was 0.6 mol %. The critical distance (R c) between Mn4+ ions for concentration quenching was 36.57 Å, and the major mechanism of energy transfer among Mn4+ activators in BGT:Mn4+ was dipole-dipole interaction. The decay lifetimes decreased from 0.285 to 0.248 ms with the increasing Mn4+ doping concentration from 0.2 to 1.2 mol %. The Commission Internationale de l'Éclairage coordinates of the optimal BGT:0.6%Mn4+ sample were (0.7294, 0.2706). The values of the IQE for all BGT:Mn4+ samples were measured, and the highest value could reach up to 62%. The above results revealed that these high-efficiency BGT:Mn4+ deep-red-emitting phosphors had promising potential for application in indoor plant growth lighting.

Alcohol-Guided Growth of Two-Dimensional Narrow-Band Red-Emitting K2TiF6:Mn4+ for White-Light-Emitting Diodes

The use of red phosphors with low light-scattering loss could improve the luminous efficacy and color rendering of white-light-emitting diodes (LEDs). Thus, the discovery of such phosphors is highly desired. In this work, high-efficiency two-dimensional red-emitting K₂TiF₆:Mn⁴⁺ (KTFM) were synthesized via an alcohol-assisted coprecipitation route. The synergistic effects of 1-propanol and hydrofluoric acid on the growth of KTFM microsheets (MSs) were studied through the first-principles calculations, which revealed that 1-propanol promoted the growth of KTFM MSs by preferentially adsorbing on the H-terminated K₂TiF₆ (001) surface. The photoluminescence quantum efficiency (QE) of Mn⁴⁺-activated K₂TiF₆ MSs was highly related to their size and thickness. The morphology-optimal KTFM MSs presented high internal QE (>90%), external QE (>71%), and thermal quenching temperature (102% at 150 °C relative to that at 25 °C). A prototype phosphor-converted LED with KTFM as the red-emitting component showed excellent color rendition ( R_a = 91, R9 = 79) and high luminous efficacy (LE = 156 lm/w).

A highly luminescent Mn4+ activated LaAlO3 far-red-emitting phosphor for plant growth LEDs: charge compensation induced Mn4+ incorporation

A highly luminescent far-red-emitting phosphor LaAlO₃:Mn⁴⁺,Mg²⁺ with a quantum yield of 78.6% synthesized with the assistance of MgF₂.