Efficient phenanthroimidazole-styryl-triphenylamine derivatives for blue OLEDs: a combined experimental and theoretical study

Abstract

Blue emitting devices based on N-(4-(4′-(1-(4-benzylpiperidine)-1H-diphenylimidazol-2-yl)-styryl-1)phenyl)-N-phenylbenzenamine (TPA-BDIS), N-(4-(4′-(1-(4-benzylpiperidine)-1H-phenanthro[9,10-d]imidazol-2-yl)-styryl-1)phenylbenzenamine (TPA-BPIS) and 2-(4′-9H-carbazol-9-yl)-[1,1′-styryl]-4-yl-1-benzylpiperidine-1H-phenanthro[9,10-d]imidazole (Cz-BPIS) have been synthesized and characterized. The fully twisted TPA-BDIS exhibits a narrow electroluminescence emission at 441 nm with a full-width at half maximum of 40 nm. Density functional theory (DFT) calculations show that the decreased energy barrier between the transitions of TPA-BDIS causes a strong orbital coupling resulting in a deep blue emission. The electroluminescence performance of the non-doped device based on TPA-BPIS shows a high brightness of 2852 cd m⁻², a current efficiency of 1.88 cd A⁻¹, a power efficiency of 1.51 lm W⁻¹ and an external quantum efficiency of 2.53%. Compared with TPA-BPIS, the weak donor carbazole substituted Cz-BPIS exhibits an excellent performance: a blue emission with CIE coordinates of (0.16, 0.09), a current density of 1.91 cd A⁻¹, a power efficiency of 1.63 lm W⁻¹ and an external quantum efficiency of 2.61%. A fluorescent molecule with a combination of the HLCT state and hot excitons may be an ideal candidate in the design of high efficiency, low cost, fluorescent OLED materials.