Recent progress in trivalent europium (Eu3+)-based inorganic phosphors for solid-state lighting: an overview.

Narrow band red-emitting phosphors are significant constituents but still a bottleneck for next-generation smart displays and high-performance lighting (solid-state lighting based white light-emitting diodes (WLEDs)) technology. This review emphasizes the fundamental understanding and comprehensive overview of the recent progress and challenges associated with inorganic phosphors or down (wavelength) convertors, providing special attention to narrowband red-emitting oxide phosphors for phosphor-converted WLEDs (pc-WLEDs). In this context, the comprehensive progress on trivalent europium (Eu3+, in scheelite and double perovskite structures) based oxide phosphors with special emphasis on structure-composition-property-correlations is briefly reviewed. Furthermore, the challenges faced in the design of new oxide red phosphors and strategies to improve their absorption, emission efficiency, and future research direction are highlighted.

Stable and efficient narrow-band red emitters with high colour purity for white LEDs and plant growth applications.

The search for extremely narrow-band new red emitters is essential for smart displays and lighting applications. Herein, we report the synthesis of a series of red-emitting Eu3+-substituted Li3BaSrY3(WO4)8 and solid-solution (Li3BaSrY0.3Eu2.7(WO4)8-y(MoO4)y) phosphors with a stratified scheelite structure and the systematic investigation of their optical properties. All the compositions show broad absorption (charge transfer (O2- → M6+)) extended up to the blue region along with strong characteristic Eu3+ excitation lines. The phosphor compositions exhibit proficient narrow-band red emission (quantum yield up to 85%; full width at half maximum (FWHM) = 6 nm; CIE colour coordinates x = 0.65, y = 0.35, i.e. approaching the NTSC standard for red colour) of exceptional colour purity (94% to 98%), and this host supports heavy trivalent Eu activation (showing zero concentration quenching of the emission (Eu-rich lattice)). The dominating electric dipole (5D0 → 7F2, red emission) transition shows Eu3+-ion occupancy in the non-centrosymmetric site in the host lattice. The selected composition shows better absolute quantum efficiencies compared to commercial phosphors. The presently synthesized phosphors have proven to be thermally stable against the temperature quenching effect. The fabricated red LED showed excellent performance, colour purity, LER and CIE values. The excellent optical properties, quantum yield and thermal stability make the phosphor applicable as a red component in solid-state lighting devices and as a light source for plant growth applications.

Narrow-Band Red-Emitting Phosphors with High Color Purity, Trifling Thermal and Concentration Quenching for Hybrid White LEDs and Li3Y3BaSr(MoO4)8:Sm3+, Eu3+-Based Deep-Red LEDs for Plant Growth Applications.

Trivalent europium-based monochromatic red light-emitting phosphors are an essential component to realize high-performance smart lighting devices; however, the concentration and thermal quenching restrict their usage. Here, we report a series of efficient Eu3+-substituted Li3Y3BaSr(MoO4)8 red-emitting phosphors based on a stratified scheelite structure with negligible concentration and thermal quenching. All of the host and phosphor compositions crystallize in monoclinic crystal structure (space group C2/c). All of the phosphor compositions produce narrow-band red emission (FWHM ∼6 nm), which is highly apparent to the human eyes, and lead to exceptional chromatic saturation of the red spectral window. Concurrently, detailed investigations were carried out to comprehend the concentration and thermal quenching mechanism. Absolute quantum yields as high as 88.5% were obtained for Li3Y0.3Eu2.7BaSr(MoO4)8 phosphor with virtuous thermal stability (at 400 K, retaining 87% of its emission intensity). The light-emitting diodes were constructed by coupling Li3BaSrY0.3Eu2.7(MoO4)8 red phosphor with a near-UV LED chip (395 nm) operated at 20 mA forward bias, and the hybrid white LED (an organic yellow dye + red Li3Y3BaSr(MoO4)8:Eu3+ phosphor integrated with an NUV LED chip) showed a low CCT (6645 K), high CRI (83) values, and CIE values of x = 0.303; y = 0.368, which indicated that the synthesized phosphors can be a suitable red component for white LEDs. In addition, we have systematically investigated the Sm3+ and Sm3+, Eu3+ activation in Li3Y3BaSr(MoO4)8 to display the latent use of the system in plant growth applications and establish that the phosphor exhibits orange red emission with an intense deep-red emission (645 nm (4G5/2 → 6H9/2)). The phytochrome (Pr) absorption spectrum well matched the fabricated deep-red LED (by integrating a NUV LED + Li3Y3BaSr(MoO4)8:Sm3+ and Eu3+ phosphor) spectral lines.

Efficient and ultra-thermally stable Eu3+ and Sm3+-activated narrow-band red/deep red-emitting phosphors and their versatile applications.

A succession of Eu3+-activated Na2Y4(WO4)7 (NYW) red phosphors were synthesised and their optical properties were studied in detail for white LED, latent fingerprint and plant growth applications. The phosphors crystallised in a tetragonal system with space group I41/a. The NYW:Eu3+ red phosphors demonstrated a line-like emission at 616 nm owing to electric dipole transition, and a systematic concentration-dependent PL study revealed that concentration quenching occurs at x = 1.8 with a color purity of 96.06%. The thermal stability and internal quantum efficiency of the phosphor were found to be ∼75.54% (at 423 K) and 88%, respectively. Furthermore, solid solution phosphors were synthesized to increase QE, which was found to be 91.27%. Specifically, the hybrid white LED exhibits warm white light with high CRI (80) and low CCT (5730 K) values, and these values are further improved (CRI-81, CCT-4274 K) when the WLED is fabricated using the most efficient solid solution phosphor Na2Y2.2Eu1.8(WO4)3(MoO4)4. The currently synthesized phosphors can be potential candidates for security applications. The selected phosphor compositions can be used for the detection of latent fingerprints. Besides, a succession of Eu3+ and Sm3+ co-doped phosphors were synthesized and their photophysical properties were studied systematically. The deep red LED was fabricated using the same and this could be a possible light source for plant growth usage.

Narrow-band red-emitting phosphor with negligible concentration quenching for hybrid white LEDs and plant growth applications.

Narrow-band red-emitters are the key to solving problems encountered by the current white LED technology. In this context, a series of new red-emitting Li3BaSrLa3(MoO4)8:Eu3+ phosphors were synthesized and characterized through various spectroscopic methods. All phosphor compositions were crystallized in the monoclinic phase, with space group C2/c. A broad charge transfer (O2-→ Mo6+) extended up to the blue region along with strong 7F0→5L6, 5D3 absorption, making them looked-for materials for warm white LED applications. The concentration quenching study reveals that there was no concentration-quenching occuring and the quantum yield of this non-concentration-quenching Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor reaches 92.6%. The Li3BaSrLa0.3Eu2.7(MoO4)8 retain >80% of its emission intensity at 150 °C. The best red-emitting composition was integrated with near UV LED and obtained bright red emission with CIE x = 0.6647, y = 0.3357. White LED was fabricated by integrating the blue LED with yellow dye + red phosphor and white LED showed bright white light with CCT (5546 K), CIE (0.331, 0.385), and CRI (81%). In addition, the red LED spectrum is well-matched with the phytochrome (Pr) absorption spectrum and is useful for plant growth applications.

Novel narrow band red emitters based on mixed metal oxides and their application in hybrid white light-emitting diodes.

A series of high-efficiency narrow band red-emitting La2 M2 O9 :Eu3+ (M = Mo/W) phosphors for white LEDs was synthesized using a conventional solid-state reaction method. All the compositions show absorption in the near ultraviolet (UV) light region due to charge transfer from O to M (M = W and Mo). In order to investigate the luminescence quenching effect, the Eu3+ concentration was varied in the La2 M2 O9 lattice. The tungstate analogue had a quantum yield of 46.5%, whereas the molybdate equivalent had a comparatively subordinate value (15.4%). The phosphor could be competently excited by ~395 or 465 nm photons (could be integrated well with a near-UV or blue LED chip) and showed dominant red emission electric-dipole transition (5 D0 →7 F2 ) with sharp spectral lines due to 4f-4f electronic transition of the Eu3+ ion and potential red-emitting colour converters for white LEDs. The red LED was fabricated by integrating the best phosphor composition with a near-UV LED and a white hybrid LED was fabricated by conjugating with a yellow organic dye and a red phosphor with near-UV LEDs. The white hybrid LED showed an excellent colour rendering index (83%), with CIE colour coordinates (0.313, 0.365) and CCT (6280 K).

Controlled Energy Transfer from a Ligand to an EuIII Ion: A Unique Strategy To Obtain Bright-White-Light Emission and Its Versatile Applications.

A new diphenylamine-functionalized ancillary-ligand-coordinated europium(III) ß-diketonate complex showed incomplete photoexcitation energy transfer from a ligand to a EuIII ion. A solvatochromism study led to a balancing of the primary colors to obtain single-molecule white-light emission. Thermal-sensing analysis of the europium complex was executed. The europium complex, conjugated with a near-UV-light-emitting diode (395 nm), showed appropriate white-light-emission CIE color coordinates (x = 0.34 and y = 0.33) with a 5152 K correlated color temperature.

Versatile Luminescent Europium(III)-ß-Diketonate-imidazo-bipyridyl Complexes Intended for White LEDs: A Detailed Photophysical and Theoretical Study.

Three ancillary ligands based on imidazo-bipyridyl with phenyl (Ph), naphthyl (Np), and triphenylamine (TPA) substitution were synthesized and secondhand to formulate the consistent europium(III) ternary complexes using thenoyltrifluoroacetone as an anionic ligand. The complete investigation of spectroscopic, photophysical, and electrochemical properties was carried out. The attained results for all the ancillary ligands and their corresponding Eu complexes were compared with one another. All the Eu complexes reveal a broad excitation band ranging from the near-UV to blue region, along with high intense emission and apposite color purity. To further understand the ligand-to-metal energy transfer (ET) process, the geometry of the ligand was optimized and the energy level location (singlet and triplet) was calculated by using DFT and TD-DFT calculations. On the basis of the theoretical calculation, the ET mechanism was proposed. From PL emission spectra in the solid state, complete ET occurs from Ph, Np based ancillary ligands to the Eu3+ ion, which yields a pure red emission, whereas the TPA functionalized based Eu complex shows incomplete ET. Fortunately, white emission was observed in the TPA based Eu complex in the solid state. The white LED was fabricated by using a white emitting complex integrated with 395 nm emitted LED (InGaN) chips under 20 mA forward-bias current. The excitation source from LED was fully observed by the complex shown for 3Eu and showed yellowish emission in different concentrations (the similar observation also reflected in solid). However, in the case of 1Eu and 2Eu complexes, they showed close to white emission. The Commission International de I'Eclairage (CIE) chromaticity coordinates are close to the National Television Standard Committee standard value for white emission, and in addition, the complex 3Eu coated with the blue LED chip (460 nm) by PMMA (1:10) showed bright white emission with CIE x, y values of 0.30, 0.33, respectively.