Novel carbazole-based multifunctional materials with a hybridized local and charge-transfer excited state acting as deep-blue emitters and phosphorescent hosts for highly efficient organic light-emitting diodes

Abstract

In this work, two carbazole- and benzo[d]oxazole-based novel multifunctional materials with a hybridized local and charge-transfer (HLCT) characteristic, namely, OCI and OCT, which could act as deep-blue fluorophors and phosphorescent hosts, were first designed and synthesized. Both OCI and OCT display meritorious thermal stability with a glass transition temperature (T_g) as high as 139 and 137 °C, respectively. The two compounds also reveal excellent bipolar carrier transport ability and emit deep-blue emission with photoluminescence quantum yields (PLQY) of 34.5% and 14.0%, respectively. Used as blue fluorophors, OCI- and OCT-based non-doped devices exhibit a very low turn-on voltage (V_on) of 3.0 V and realize deep-blue emission with peaks located at 438–453 nm, and the optimized OCI-based device achieve the maximum external quantum efficiency (η_ext,max) reaching 3.19%. Remarkably, doped blue devices show even deeper blue emission at 424 and 428 nm and the optimized OCI-based device achieves a significantly improved η_ext,max of 5.06% with CIE_y < 0.04, which could rank among the deepest blue HLCT fluorophors. Additionally, green phosphorescent OLEDs hosted by OCI and OCT reveal a low V_on of 2.7 V with maximum current efficiencies (η_c,max) of 49.43 cd A⁻¹ (η_ext,max of 14.11%) and 52.81 cd A⁻¹ (η_ext,max of 15.57%), respectively, which are superior to that of CBP (V_on of 3.0 V, η_c,max of 51.76 cd A⁻¹ and η_ext,max of 12.88%). The superb electroluminescence (EL) performance of OCI and OCT as deep-blue HLCT emitters and phosphorescent hosts demonstrates that they have great potential for industrial applications.