Application of a novel cationic iridium(iii) complex as a red phosphor in warm white light-emitting diodes

Abstract

A novel red-emitting cationic iridium(III) complex, [(L_m)₂Ir(L_a)]PF₆, (L_m: 2-(9-(2-ethylhexyl)-9H-carbazol-3-yl)benzo[d]thiazole, L_a: N,N-diphenyl-4-(5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl)aniline) was synthesized. Its ultraviolet-visible absorption and photoluminescent properties show that the complex can be efficiently excited by a 465 nm-emitting blue GaN chip, its decomposition temperature (T_d) is 340 °C, and its relative emission intensity at 100 °C is 88.3% of 25 °C. Perfect red light with a CIE value of (0.65, 0.34) was obtained when it was used as a phosphor at 6.0 wt% blending concentration in epoxy resin in a blue GaN-based LED. A 465 nm-emitting blue GaN-based LED only using yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) as a phosphor (1.0 wt% in epoxy resin) emitted cold white, and its corresponding color rendering index (CRI) was 74.1, correlated color temperature (CCT) was 6026 K, and luminous efficiency (η_L) was 25.3 lm W⁻¹. It became a neutral white light LED when the iridium(III) complex was added at 0.5 wt%, the corresponding CRI was 79.5, CCT was 4004 K, and η_L was 32.6 lm W⁻¹. It further became a warm white LED when the complex was blended at 1.0 wt% and 1.5 wt%, the corresponding CRI were 80.0 and 79.6, CCT were 3650 K and 3133 K, η_L were 25.5 lm W⁻¹ and 22.8 lm W⁻¹, and CIE values were (0.40, 0.39) and (0.43, 0.40), respectively. This complex is a promising red phosphor candidate for red LEDs and warm white LEDs.