Wang Sanhai ¹, Xu Yanqiao ², Hu Qing ¹, Xu Zhifang ³, Wang Lianjun ⁴, Jiang Wan ⁴

(1. School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China;2. National Engineering Research Center for Daily & Building Ceramics, 

Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 3. China National Light Industry Ceramic Research Institute, Jingdezhen 333001, Jiangxi, China;

4. School of Materials Science and Engineering, Donghua University, Shanghai 201620, China)

Extended abstract:

[Significance] White light-emitting diodes (WLEDs) have emerged as the dominant next-generation solid-state lighting technology, valued for their energy efficiency, long lifespan and environmental friendliness. A critical factor determining WLED performance, particularly color rendering index (CRI) and correlated color temperature (CCT), is optical properties of the down-conversion phosphors used in conjunction with LED chips. While blue-pumped YAG:Ce³⁺-based WLEDs are commercially prevalent, they suffer from a deficiency in red spectral components, resulting in high CCT and low CRI, which limits their suitability for high-quality illumination. An alternative and superior approach employs near-ultraviolet (NUV, 380–420 nm) LED chips combined with red, green and blue (RGB) tri-phosphors to achieve full-spectrum high-CRI (>90) white light. However, the development of efficient NUV-excitable red phosphors remains a major bottleneck. Eu³⁺-activated red phosphors are highly attractive, due to their excellent color purity (dominant ⁵D₀→⁷F₂ emission at ~615 nm), chemical/thermal stability and low cost. Nevertheless, their practical application is severely hindered by intrinsically weak and narrow-line f-f excitation peaks at about 395 nm, which exhibit poor spectral overlap with commercial NUV LED chips, leading to low absorption efficiency. Overcoming this fundamental limitation is therefore of paramount significance for advancing high-performance full-spectrum WLED technologies.

[Progress] To address the weak NUV absorption of Eu³⁺, researchers have developed four primary strategies, each with distinct mechanisms and varying degrees of success. Firstly, high-concentration doping is used to increase the number of Eu³⁺ absorption sites. By engineering crystal structures with large cationic distances or insulating polyhedral networks, non-radiative concentration quenching can be mitigated, enabling doping concentrations up to 100% and significantly enhanced absorption. However, the improvement in absorption efficiency remains modest (typically <40%) and the intrinsic line-like nature of the absorption is unchanged. Secondly, sensitization via co-doping introduces ions like Sm³⁺, Tb³⁺ or Bi³⁺, which possess stronger or broader absorption bands. Sm³⁺ and Tb³⁺ can absorb light at specific NUV wavelengths and transfer energy to Eu³⁺, but their own narrow absorption limits the overall gain. In contrast, Bi³⁺, with its intense, spin-allowed ¹S₀→³P₁ broadband transition in the NUV region, has shown greater promise. Notably, systems like LiKBi₂(MoO₄)₄:Eu³⁺ have achieved an ultrabroad excitation band (200–400 nm) and a high external quantum efficiency (EQE) of 84.7%, primarily through Bi³⁺→Mo⁶⁺ metal-metal charge transfer (MMCT). Thirdly, macroscopic structural engineering focuses on enhancing light-matter interaction by fabricating phosphors as dense ceramics, transparent ceramics or phosphor-in-glass (PiG) composites. These architectures extend the effective optical path length of the incident NUV light, thereby increasing the probability of absorption. Transparent ceramics, in particular, offer excellent thermal management and high optical quality, but require complex high-cost fabrication processes. Finally, host matrix engineering seeks to create intrinsic broadband NUV absorption by rationally designing the host to position its charge transfer band (CTB) or MMCT band within the NUV window. Molybdate and tungstate hosts containing [MoO₆] octahedra have been extensively explored for this purpose, as their CTB can be tuned into the NUV range, providing a direct and efficient excitation channel for Eu³⁺.

[Conclusions and prospects] In summary, while significant advances have been made in enhancing the NUV excitation of Eu³⁺-activated red phosphors, no single strategy yet delivers an ideal combination of strong broadband absorption, high quantum efficiency, stability and cost-effective scalability. Future efforts should prioritize synergistic approaches: refining host engineering through predictive crystal-chemical design to tune charge transfer bands into the 380–410 nm range, optimizing Bi³⁺ co-doping by tailoring its local coordination to maximize energy transfer and developing scalable fabrication methods for transparent or glass-integrated phosphor architectures. Achieving these goals will enable the practical deployment of Eu³⁺-based red phosphors in high-CRI full-spectrum WLEDs for human-centric lighting applications.

Key words: white LED; Eu³⁺; red phosphor; excitation peak; charge transfer band; energy transfer