Improved luminance and external quantum efficiency of red and white organic light-emitting diodes with iridium(iii) complexes with phenyl-substituted 2-phenylpyridine as a second cyclometalated ligand

Abstract

Two novel red Ir(III) complexes, (PQ)₂Ir(Pppy) and (DMPQ)₂Ir(Pppy), based on 2-phenylquinoline (PQ) and 2-(3,5-dimethylphenyl)quinoline (DMPQ) as cyclometalating ligands and 2-[4-(phenyl)phenyl]pyridine (Pppy) as a second cyclometalated ligand were synthesized for use in phosphorescent organic light-emitting diodes (OLEDs). The photoluminescence (PL) of (PQ)₂Ir(Pppy) and (DMPQ)₂Ir(Pppy) produces red emissions with maximum emission peaks at 602 and 615 nm, respectively. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of (PQ)₂Ir(Pppy) and (DMPQ)₂Ir(Pppy) were −5.15/−2.97 and −5.13/−2.96 eV, respectively. Red phosphorescent organic light-emitting devices were fabricated based on (PQ)₂Ir(Pppy) or (DMPQ)₂Ir(Pppy) as red dopants: ITO (50 nm)/PEDOT:PSS (40 nm)/NPB (20 nm)/TCTA (10 nm)/TCTA:TPBi:red dopant (25 nm, 10%)/TPBI (35 nm)/LiF (1 nm)/Al (200 nm). (PQ)₂Ir(Pppy) and (DMPQ)₂Ir(Pppy) exhibited a very broad full width at half maximum (FWHM) at around 100 nm in electroluminescent (EL) devices, which makes it suitable for fabricating white OLEDs with high colour quality. Among the these devices, the device fabricated by (DMPQ)₂Ir(Pppy) of higher PL quantum yields (Φ_pl) than that of (PQ)₂Ir(Pppy) showed an EL emission peak at 617 nm and a maximum external quantum efficiency (EQE_max) of 19.2%. Moreover, a white OLED prepared from (DMPQ)₂Ir(Pppy) with FIrpic and Ir(ppy)₃ gave the best performance, with an EQE_max of 21.9%, maximum power efficiency of 24.8 lm W⁻¹, luminous efficiency of 40.2 cd A⁻¹, and Commission Internationale de L'Eclairage coordinates of (0.39,0.41) at a luminance of 1000 cd m⁻².