Red/Orange-Red Emitting Phosphors for Solid State Lightings: Structure-Compositions-Property-Correlations

Abstract

Highly efficient narrowband red-emitting phosphors based on oxides remain a significant challenge for white LED applications. The current thesis addresses the design, synthesis, and photophysical analyses of Eu3+-activated oxide-based narrowband red-emitting, Sm3+/Eu3+ red/deep-red emitting phosphors, and trivalent Eu molecular complexes. These materials are investigated for their potential applications in solid-state lighting (including white LEDs and light sources for plant growth). The study methodically examines the relationships between the structure, composition, and properties of these phosphors. Chapter 1 introduces solid-state lighting, phosphor materials, the luminescence process involved, and white light-emitting diodes. Literature surveys of recently developed Eu3+ and Sm3+-based red/deep-red emitters for solid-state lighting were also discussed. Additionally, a brief introduction to latent fingerprint detection and security ink is provided. The importance of trivalent Europium molecular complexes for white light emission, along with a review of recent literature, is also covered. This chapter concludes by summarizing the primary goals and significance of the thesis. Chapter 2 describes the Eu3+ luminescence in Na2La4(WO4)7 and its application in solid-state lighting. All NLW:xEu3+ phosphors exhibited a sharp red emission at ~616 nm due to the ED transition (5D0 → 7F2) under 394 nm excitation. In addition, the Na2La3.2(WO4)7:0.8Eu3+ phosphor demonstrated a high color purity of 96.79% and IQE of 83.8%. A temperature-dependent PL study revealed the thermal stability of the phosphor as 69.75% at 423 K. To assess their practical applicability, red and white LEDs were fabricated using the synthesized phosphor. The EL spectrum of the red LED displayed intense red emission, while the white LED exhibited remarkable performance with a high CRI of 80 and a low CCT of 5730 K. These Eu3+ doped red phosphors can also be utilized for latent fingerprint applications. Moreover, a series of Sm3+ doped and Sm3+/Eu3+ co- doped NLW phosphors were synthesized and investigated for their optical properties. Red/deep-red LEDs were fabricated using Sm3+ co-doped Eu3+-activated phosphors for potential applications in plant growth. Chapter 3 describes the optical characteristics of Eu3+ doped Na2Y4(WO4)7 red emitters. The synthesized phosphors exhibited intense red-light emission (5D0 →7F2, ED transition) due to the non-centrosymmetric site occupation of Eu3+ ions within the crystal lattice. The Abstract solid solution between tungstate and molybdate groups enhanced the emission intensity. The thermal stability and internal quantum efficiency of the phosphor were found to be ~75.54% (at 423K) and 88%, respectively, under excitation at 395 nm. Furthermore, solid solution phosphors were developed to enhance the QE, which increased to 91.3%. The hybrid white LED exhibited superior white light emission with a high CRI of 80 and a low CCT of 5730 K. These values were further improved (CRI-81, CCT-4274 K) when the WLED was fabricated using the most efficient solid solution phosphor, Na2Y2.2Eu1.8(WO4)3(MoO4)4. Additionally, Sm3+ and Eu3+ co-doped deep-red phosphors were synthesized and studied for their optical properties for plant growth. The emission from the fabricated LED (Sm3+ and Eu3+ co-doped) completely covers the phytochrome PR absorption spectrum. Chapter 4 describes zero concentration quenching in Eu3+-activated Na5Ln(WO4)4 [Ln = Y, Gd] red-emitting phosphors with a scheelite structure. The emission spectra were dominated by the ED transition (5D0 → 7F2) under UV/NUV excitation, indicating the non- centrosymmetric site occupancy of the activator ions in the lattice. This was further confirmed by AR analysis and Judd-Ofelt parameters. For fully Eu3+ substituted phosphors, the color purity and IQE were found to be approximately 97.05% and 85.6%, respectively. In a high-temperature environment (150 ℃), Na5Eu(WO4)4 retained 69.03% of its initial emission intensity under 395 nm excitation, while the solid solution phosphor Na5Eu(WO4)1.5(MoO4)2.5 retained 85.56% of its initial emission intensity at 466 nm excitation. The fabricated white LEDs exhibited good CRI (81) and CCT (5734 K) values. In addition to lighting applications, the synthesized red phosphors demonstrated potential applicability in areas, such as LFP detection and anti-counterfeiting. Furthermore, Sm3+/Eu3+ co-doped Na5Ln(WO4)4 phosphors were developed to explore their potential use in plant growth applications as red/deep-red LEDs. Chapter 5 describes the trivalent Eu3+ luminescence in Li2La4(MoO4)7 and its applications in various fields. Either 395 nm near-UV light or 465 nm UV light can efficiently excite these synthesized phosphors, producing red light with a prominent wavelength at 616 nm. The optimal phosphor composition for high concentration quenching was identified as Li2La4(MoO4)7:1.8Eu3+, which achieved a high color purity (CP) of 97.28% and an internal quantum efficiency (IQE) of 89.6%. The Eu3+ emission from this phosphor exhibited excellent thermal stability, retaining 81.75% of its initial intensity at 423 K. To further enhance photophysical properties, solid solution phosphors were synthesized, Abstract increasing the IQE and thermal stability to 92.5% and 86.12%, respectively. When combined with a yellow organic dye and a blue LED chip, the red component enhanced the CRI and CCT of customizable white light emitting diodes (WLEDs). The WLED fabricated using the Li2La4(MoO4)7:1.8Eu3+ red phosphor demonstrated pure white light emission with a high CRI of 83 and a low CCT of 4925 K. These values were further improved to a CRI of 86 and a CCT of 5371 K when using the Li2La2.2Eu1.8(MoO4)4(WO4)3 solid solution phosphor. Prospective uses of the phosphors that are now being synthesized include security applications (to identify latent fingerprints and in the anti-counterfeiting). Additionally, Eu3+/Sm3+ co-doped red/deep red emitting phosphors were synthesized, and their photophysical properties were extensively studied for potential use in fabricating red/deep-red LEDs as a light source to promote plant growth. Chapter 6 describes the computation-aided design, synthesis, and photophysical study of N1-functionalized phenanthrol-imidazole based Eu(III) molecular complexes. Two ancillary ligands were synthesized, and their effects on the photophysical properties of the respective Eu(III) complexes were studied. Both ligands exhibited blue emission, where as their respective Eu(III) complexes displayed pure red emission. Detailed photophysical and electroluminescence analyses were conducted. In contrast to DBM complexes, TTA based complexes demonstrated a longer lifetime. The fabrication of red LED was achieved by integrating red-emissive Eu(III) complexes with near-UV LED chips. Using the synthesized complexes, white LEDs were fabricated by mixing them with a yellow dye and coating the mixture onto the surface of blue LED chips. The Eu-complexes currently under research have also demonstrated outstanding reversible on-off-on luminescence behavior on exposure to acid-base vapors. Chapter 7 briefly summarizes the results obtained from the investigations and the major conclusions drawn from these studies. Furthermore, it outlines the future scope of the present study.