Solution-processable ambipolar host oligomers with high triplet energies for phosphorescent green emitters

Abstract

Four carbazole–diaryloxadiazole oligomers 14–17 based on 9,9′-dioctyl-[3,3′]bicarbazolyl and 2,5-diaryl-[1,3,4]-oxadiazole monomer units have been synthesised by Suzuki cross-coupling reactions. The molecular weights of the oligomers were estimated using gel permeation chromatography: M_w 3130–4266 Da; polydispersity indices M_w/M_n 1.41–1.76 (i.e. 3–4 repeat units). These oligomers are, therefore, more like low molar mass polymers than monodisperse molecular materials. Thermal gravimetric analysis (TGA) indicated their good thermal stability with decomposition temperatures T_d5% 381–440 °C. The photophysical properties have been investigated in detail in solution and as thin films. The data are consistent with the existence of an intramolecular charge transfer state (ICT) in all of these oligomers, which is enhanced by the more electron withdrawing pyridyl-oxadiazole substituent in 15. The triplet energy is sufficiently high (E_T = 2.57 eV for λ_max of the phosphorescence) for oligomers 14–17 to host a soluble green phosphorescent iridium guest emitter, as demonstrated in electroluminescence studies which showed emission exclusively from the guest complex. Single-emitting-layer organic light-emitting diodes (OLEDs) were constructed with the architecture glass/ITO/PEDOT:PSS/oligomer host + Ir guest (10% w/w)/Ba/Al. Devices with 16 as host demonstrated the best results reaching maximum current efficiency, η_c 3.93 cd A⁻¹, external quantum efficiency (EQE) η_ext 1.1% and good stability. The solution processability of the oligomers, their ambipolar structure, and the simplicity of the device architecture are attractive for further development.