At present, most high-power white Light-emitting diode and laser diode (LED&LD) package is usually constructed by a blue LED&LD chip with a Cerium doped Yttrium Aluminum Garnet (YAG:Ce³⁺) yellow phosphor, but its color rendering performance is severely challenged due to the lack of red light emission spectrum. The CaAlSiN₃:Eu²⁺ red phosphor can effectively improve the color quality of traditional yellow phosphor converted white LED&LDs(pc-wLED&LDs), however, it is often susceptible to degradation under high temperature and high humidity environments, which will directly affect the color quality of pc-wLED&LDs. In this study, a series of water immersion tests on CaAlSiN₃:Eu²⁺ red phosphor are used to quantitatively study its hydrolysis reaction kinetics. Then, the degradations of its photoluminescence and photothermal performances are evaluated by characterizing the crystal structure, micromorphology and chemical element composition. Finally, an atomic level hydrolysis reaction mechanism of CaAlSiN₃:Eu²⁺ red phosphor is investigated by using the first-principles density functional theory (DFT) calculation. The results show that: (1) By modelling the in-situ monitored electrical conductivity of CaAlSiN₃:Eu²⁺ red phosphor water solution with a first-order reaction function, the calculated hydrolysis reaction rate satisfies the Arrhenius relationship and the reaction activation energy is estimated as 49.19 kJ/mol; (2) The increased self-heating effect of CaAlSiN₃:Eu²⁺ red phosphor after water immersion test attribute to its drastic drop of light emission efficiency; (3) The hydrolysis reaction mechanism of CaAlSiN₃:Eu²⁺ red phosphor is confirmed, which sequentially results in the dissolution of Ca²⁺ and OH⁻, the crash of host lattice and the accumulation of reaction residues.

This study selects yellow, red and orange phosphors used in high color rendering white packages, analyzes their luminescence and thermal properties, crystal structures and micro morphologies during both the high temperature and high moisture ageing test and the water immersion test, and finally figures out the degradation mechanisms of different phosphors. The results show that: 1) the hydrolysis reaction of phosphors under hygrothermal environment produces OH^- and increases the environment's pH value, which can lower the crystallinity of phosphor hosts, increase the lattice vibration and light-induced heat, and enhance the thermal quenching effect of phosphors. Meanwhile, the surface roughening of phosphor particles can increase the blue light scattering and lower its absorption and conversion efficiency; 2) the degradation trends of both luminescence and thermal properties of phosphors under the water immersion test are similar to those under high temperature and high moisture ageing test. So it is an effective method to assess the reliability of LED phosphors under hygrothermal environment.