Host sensitized photoluminescence in Sr2.9-3x/2LnxAlO4F: 0.1Eu3+ (Ln = Gd,Y) for innovative flexible lighting applications

Tetragonal structured Sr₃AlO₄F is highly strained as reported from its global instability index estimation. Moreover, our results of X-ray photoelectron spectroscopy (XPS) also ascertained that the structure of Sr₃AlO₄F is highly strained with oxygen vacancies. Herein, aliovalent substitutions of divalent Sr ions with trivalent Ln (Ln = Gd/Y) ions were carried out to improve the stability of Sr₃AlO₄F lattice, which subsequently enhanced the photoluminescence in a series of Sr_2.9-3x/2Ln_xAlO₄F: 0.1Eu³⁺ phosphors. All the phosphors showed intense red-orange emission (⁵D₀ → ⁷F_1,2) at excitation with UV and near-UV light. The critical concentrations of Gd³⁺ and Y³⁺ up to which the Eu³⁺ emission intensities increased linearly were observed to be x = 0.09 and x = 0.07, respectively. Nevertheless, further enhancement in the Eu³⁺ luminescence of the optimized phosphors was realized by subsequently annealing in low oxygen atmospheres. The enhancement in oxygen deficiency during the post-annealing in Ar or vacuum led the energy transfer (O^2--Eu³⁺) to a greater extent which afterward increased the Eu³⁺ luminescence. The optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ and Sr_2.795Y_0.07AlO₄F: 0.1Eu³⁺ phosphors showed high red color purity (~99%), as well as CIE coordinates of (0.62, 0.38), indicated that these phosphors could be appropriate red-emitting components for making flexible optical films for many lighting devices. Therefore, flexible polydimethylsiloxane based films were also fabricated using optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ phosphor. The electroluminescence of a flexible PDMS-phosphor composite film showed an intense and pure red color with good thermal stability suggesting its suitability in flexible lighting and display devices.