Obtaining highly efficient single-emissive-layer orange and two-element white organic light-emitting diodes by the solution process

Abstract

By attaching two electron-withdrawing trifluoromethyl (CF₃) groups to the 2-phenylbenzothiazole cyclo-metalated ligand, a bis-trifluoromethyl-functionalized orange-emitting phosphorescent iridium(III) complex bis-(6-(trifluoromethyl)-2-(4-(trifluoromethyl)phenylbenzothiozolato))iridium(acetylacetonate) [(F₃BT-CF₃P)₂Ir(acac)] was successfully synthesized. The optical, electrochemical and electroluminescence (EL) properties of this new complex were studied. The experimental results support the theoretical expectation that incorporating electron-withdrawing trifluoromethyl groups at the 4-site of the phenyl ring directly bonded to the metal center, and at the 6-site of 2-phenylbenzothiazole, cause a bathochromic shift in the emission peak and bring the emission color much closer to long-wavelength orange light. Moreover, such trifluoromethyl substituents can hinder the π–π stacking or self-polarization effect occurring from the aggregation of the molecules. The new iridium complex gives an unchanged luminescence spectrum, regardless of whether it is in solution, in untreated film or in film doped at different concentrations. Using this iridium complex as a dopant emitter, solution-processed single emissive layer orange and two-element white OLEDs with good performance can be obtained. Highly efficient orange electroluminescence was obtained with a maximum efficiency of 10.5 cd A⁻¹ and CIE coordinates (0.48, 0.51). When combined with a commercial sky-blue phosphorescent emitter, (CF₃BT–CF₃P)₂Ir(acac) can be utilized to achieve two-element white OLEDs that exhibited a high efficiency of 28.3 cd A⁻¹. Such OLEDs retain high efficiency at a luminance suitable for lighting (e.g. 5000 cd m⁻²).