KONG Yuqiang ^{1, 4}, CAO Wangsheng ^{1, 2}, WANG Guilu ^{1, 4}, ZHANG Zhiyong ^{1, 4}, ZHENG Xigui ^{1, 5},

WANG Yanming ^{1, 4}, MA Yuelong ^{3, 6}, CHEN Lumin ^{1, 4}

(1. School of Mechanical Engineering, Zhengzhou University of Science and Technology, Zhengzhou 450064, Henan, China;

2. School of Mechanical and Electrical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, Henan, China;

3. School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou 450001, Henan, China;

4. Zhengzhou Advanced Manufacturing Engineering Research Center of Key Parts for Dental Implant,

Zhengzhou University of Science and Technology, Zhengzhou 450064, Henan, China;

5. Henan Digital Intelligent Equipment Engineering Research Center, Zhengzhou University of Science and Technology,

Zhengzhou 450064, Henan, China; 6. Songshan Laboratory, Zhengzhou 450046, Henan, China)

Extended abstract:

[Significance] Solid-state lighting and display technologies, particularly laser diode (LD) and light-emitting diode (LED)-based systems, are revolutionizing various applications from general illumination and automotive headlamps to projection displays and visible light communication. Their advantages, including high brightness, superior efficiency, long lifespan and environmental friendliness, are driving their widespread adoption. A key component in these systems is the phosphor conversion material, which transforms high-energy blue or ultraviolet light into white or other desired colors. Traditional phosphor-silicone composites suffer from poor thermal stability and low thermal conductivity, leading to performance degradation and failure under high-power excitation. Phosphor-in-glass films (PiGFs), a novel class of color converters fabricated by low-temperature sintering of phosphor and glass matrix mixtures onto high thermal conductivity substrates, have emerged as a promising solution. They offer excellent optical performance, structural diversity and significantly improved thermal management, as compared with bulk phosphor-in-glass or conventional composites. This review was attempted to comprehensively examine the latest research progress in PiGFs, focusing on their fabrication, the influence of high-conductivity substrates (such as sapphire, AlN, Al₂O₃, YAG ceramics, BN and diamond) on performance and their structure- property relationships, aiming to provide a clear roadmap for their development in high-power lighting and display applications.

[Progress] This paper was aimed to systematically review the fundamental aspects and recent advances in PiGFs. It begins by introducing the basic composition of PiGFs, typically a blend of phosphor particles (e.g., YAG:Ce³⁺, LSN:Ce, CASN:Eu²⁺) and a low-melting glass matrix, and the prevalent fabrication techniques, like screen printing and doctor blade coating, highlighting their impact on film uniformity and thickness control. The core discussion is organized around the critical role of substrate materials in determining the optical and thermal performance of PiGFs. For sapphire substrates, strategies, such as sandwich structures (S@PiGF@S), patterned films and gradient designs, are shown to enhance the luminescence saturation threshold and reduce operating temperature. AlN substrates are emphasized for their excellent thermal conductivity and thermal expansion match with glass, which minimizes interfacial stress and improves thermal stability, especially when combined with reflective or scattering layers, like BN or TiO₂. Al₂O₃ ceramic substrates, valued for their cost-effectiveness and promising thermal properties, are widely used in reflective configurations. Structural innovations, including composite designs with metal heatsinks or surface modifications, further boost their heat dissipation and luminous output. YAG transparent ceramic substrates enable composite structures (e.g., PiGF@YAG ceramic@PiGF) that combine high thermal conductivity with superior color rendering, achieved by compensating for the red spectral deficiency of YAG:Ce. BN, utilized both as a substrate and as a filler, provides a unique combination of high thermal conductivity and optical scattering, enhancing both heat dissipation and light extraction efficiency. Diamond substrates represent the frontier for extreme thermal management, offering unmatched thermal conductivity that dramatically increases the laser power threshold, although challenges related to cost and interfacial compatibility remain significant. Other innovative substrates, including metals for passive cooling and thermoelectric generators for active energy recycling, are also explored, pointing toward multifunctional and energy-efficient designs. Throughout the review, key performance parameters, such as luminescence saturation threshold, correlated color temperature (CCT), color rendering index (CRI), luminous efficacy (LE) and thermal stability under laser excitation, are analyzed and compared across different substrate systems, often summarized in comparative tables. The discussion also delves into how the macro- and microstructure of PiGFs, including phosphor distribution, film thickness, porosity and the presence of secondary phases, like Al₂O₃ or BN particles, directly influence light scattering, thermal conduction paths and ultimately the luminescent performance.

[Conclusions and prospects] PiGFs demonstrate considerable potential as robust, tunable and scalable color converters for next-generation high-power solid-state lighting and displays, offering a favorable balance of performance, cost and manufacturability. Current research has successfully addressed many thermal management challenges through innovative substrate engineering and composite structural design. However, several critical issues require further investigation to enable full-scale commercialization. These include (1) mitigating interfacial reactions between the phosphor and glass matrix during sintering to preserve phosphor quantum efficiency, (2) enhancing the long-term reliability and environmental stability of PiGFs under continuous high-power operation, (3) optimizing optical performance to eliminate artifacts like the "yellow ring" effect in transmission mode, potentially through refined microstructure control and advanced optical design, (4) developing more effective solutions to the thermal expansion mismatch between films and substrates to prevent delamination and (5) reducing the cost of high-performance substrates like diamond and single-crystal sapphire. Future research directions should be focused on developing novel glass compositions with higher intrinsic thermal conductivity, exploring advanced patterning and 3D structural designs for more effective heat and light management and deepening the understanding of the photothermal coupling mechanisms within PiGFs through multiscale modeling. The integration of PiGFs with intelligent thermal management systems and their application in emerging fields like micro-LED displays and LiFi are also promising avenues. With continued interdisciplinary efforts in materials science, optics and thermal engineering, PiGFs are poised to play a pivotal role in advancing high-brightness, high-efficiency and reliable solid-state lighting and display technologies.

Key words: phosphor-in-glass film; solid-state lighting and display; phosphor conversion materials; thermal conductive substrate; macro/microstructure; photothermal properties