Structure Design and Luminescence Control of Eu⁺²-doped Orthorhombic Phosphors for Solid-State Lighting Applications

Next-generation solid-state lighting devices urge the innovation of bright and robust phosphors. Most of the current phosphors either have low quantum efficiencies (internal and external, IQE/EQE) or have poor luminescence thermal stability, significantly affecting the performance of phosphor-converted white light emitting diodes (pc-WLEDs). This work focuses on the exploration of novel phosphors used for high-performance WLED, using orthorhombic host materials as the structural prototype. The major findings from this thesis can be summarized as follows: (1) An orthorhombic-type K2SrBa(PO4)2:Eu2+ with a narrow emission band was successfully designed and synthesized through a high-temperature solid-state reaction. This phosphor can be excited by an ultraviolet spectrum (250-400nm), showing an ultra-narrow blue emission peaked at 425 nm with a full width at half maximum (FWHM) of 37 nm (2088 cm-1 ). The obtained phosphor exhibits an excellent color purity of 0.93 with superior thermal stability and quantum efficiency. Its external quantum efficiency exceeds that of the blue phosphors reported thus far. It also has excellent thermal stability showing near-zero thermal quenching up to 300 °C, retaining 102%, 100%, and 96% of its initial integrated emission intensity at 75, 200, and 300 °C, respectively. Rietveld refinements and density functional theory (DFT) calculations reveal that the Eu2+ ions preferentially occupy the Sr/Ba sites, rather than the K sites, of the host structure. (2) A new solid-solution phosphor with a formula of K2(SrxBa2-x)(PO4)2: Eu2+ (x = 0-2) was synthesized. The optimized phosphor K2Sr1.25Ba0.75(PO4)2: Eu2+ demonstrates an extra-narrow blue emission (425 nm, FWHM =37.3 nm), enhanced quantum efficiencies (IQE = 96.4%, EQE = 76.4%) and superior thermal stability (90% retention at 300 ℃) enabled by defect-induced trap centers. To the best of our knowledge, this phosphor is one of the bestperforming phosphors, with much better overall performance than commercial blue phosphors. We unraveled the controlling factors for enhanced thermal stability by first-principles calculation and experimental investigation. The introduced size mismatch defect (Sr/Ba) can add extra crystal trap density and trap levels, leading to improved thermal stability luminescence. In addition, the improved P-O symmetric stretching by blending Sr/Ba with an appropriate ratio reduces the energy loss from the activators to the kill centers. (3) A new structure of K1-xBa(1+x)/2Sr(1+x)/2(Six-P1-x)O4: 0.01Eu2+ (0 ≤ x ≤ 1, KBSSxP) phosphors with a high-purity phase have been designed by chemical units co-substitution strategy. It shows photoluminescence (PL) emission redshift from 423 nm to 525 nm due to the designed centroid shift and crystal field splitting effects of the 5d levels. Compared with the original orthosilicate BaSrSiO4:Eu2+ phosphor, the KBSSxP exhibits better thermal stability and higher quantum efficiency. Our experiments indicate the symmetric stretching vibrations have contributed to the variation of quantum efficiency. This work shed light on understanding the relationship between quantum efficiency and lattice structure. In addition, by controlling the lattice structure, the bandgap size and defect levels were changed accordingly, leading to improved thermal stability.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Chemical and Materials Engineering

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2022

Author dc:creator

• Yang, Tingxuan

Advisor dc:contributor.advisor

• Cao, Peng

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/60977

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/60977