Colour stability of Blue–Green and white phosphorescent organic light-emitting diode employing a 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole host

An ambipolar compound, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole, is employed as a universal single host for efficient sky-blue (B), green (G), orange–red (R), BG, and white (W) phosphorescent organic light-emitting diodes (PhOLEDs) with an external quantum efficiency (ηEQE) of more than 20%. By integrating B and G emitters into the emitting layer (EML), an efficient BG PhOLED exhibits a maximum current efficiency (ηC) of 81.7 cd/A, power efficiency (ηP) of 73.2 lm/W, and ηEQE of 23.1%. Another BG PhOLED exhibits excellent colour stability with a slight shift (Δx of 0.0006, Δy of 0.0014) in CIE1931 colour coordinates while its luminance varies from 3.5 to 38360 cd/m2. Furthermore, a 1 nm R EML is employed as a probe inserted at three distinct positions to investigate the main recombination zone; the main recombination zone is detected near the interface between B/G EML. At the same time, a W spectrum combines individual R, G, B emission peaks. Besides, a W PhOLED simply comprises of a B and R EML, exhibiting good colour stability (Δx of 0.0004, Δy of 0.0023 between 4 and 12 V) and high efficiency (a maximum ηC, ηP, ηEQE of 58.4 cd/A, 50.7 lm/W, 21.9%). In addition, transient electroluminance signals described green emitters have less trapped charges and a longer exciton lifetime which benefit the device lifetime.