A bipolar triphenylamine-dibenzothiophene S,S-dioxide hybrid compound for solution-processable single-layer green OLEDs and as a host for red emitters

Abstract

A novel bipolar compound triphenylamine-dibenzothiophene S,S-dioxide (TPAFSO) based on a dibenzothiophene-S,S-dioxide (DBTO) moiety and a triphenylamine (TPA) unit, in which the TPA unit is linked at its 4-position to the DBTO group at its 3,7-position, is designed and synthesized. The experimental results clearly show that TPAFSO exhibits a high photoluminescence efficiency of 0.52 in a film form and good/balanced carrier injecting properties, as well as an adequate triplet energy (E_T) of 2.1 eV to sensitize red phosphor. Hence, solution-processable single-layer green OLEDs using TPAFSO as emitters exhibit a maximum luminous efficiency (LE) of 4.7 cd A⁻¹ and a maximum brightness of 5952 cd m⁻² with a low turn-on voltage of 2.5 V, which is superior to the performance of double-layer devices. Furthermore, considering the TPAFSO as a solution-processed host for red emitters, 5% bis(2-(2′-benzothienyl)-pyridinato-N,C³′)iridium(acetylacetonate) (Ir(btp)₂(acac))-doped single-layer devices give a maximum quantum yield of 3.9% and a saturated red emission with CIE coordinates of (0.68, 0.32), which are much better than that of Ir(btp)₂(acac)-doped poly(N-vinylcarbazole)-2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK-PBD) single-layer devices. Additionally, the red OLEDs using 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB)-doped TPAFSO as the emitting layer exhibit a maximum LE of 1.8 cd A⁻¹, and can be compared to the performances of DCJTB/Alq₃-based multilayer devices via an evaporation process.