Multifunctional carbazolocarbazoles as hole transporting and emitting host materials in red phosphorescent OLEDs

Abstract

A new series of carbazolo[4,3-c]carbazoles (1–3) have been examined as hole-transporting and emitting host materials in the fabrication of red phosphorescent OLEDs (PhOLEDs). The presence of different N-substituents including hydrogen, octyl and 4-butylphenyl attached to the carbazolo[4,3-c]carbazole skeleton was aimed to condition the charge transporting properties. Due to their resemblance in the electronic structure, these carbazolocarbazoles have been compared to a commonly used hole-transporting material such as N,N′-bis(naphthalen-1-yl)-N,N′-bisphenylbenzidine (NPB). Interestingly, the introduced structural differences endow compounds 1–3 with a wide range of hole-mobilities, which provide room for adjusting the carrier balance of OLEDs. Different approaches, including bi-layer and tri-layer architectures, have been employed for the successful fabrication of Ir(piq)₂(acac) doped OLEDs using NPB and these novel carbazolocarbazoles as hole transport and host materials. The simplified bi-layer device demonstrated high performance with maximum efficiencies of 8.7%, 5.6 cd A⁻¹ and 3.4 lm W⁻¹ when using the N-alkylated derivative 2. Furthermore, the red PhOLEDs with tri-layer architecture using 3 as the HTL showed peak efficiencies of 12.2%, 8.7 cd A⁻¹, and 9.3 lm W⁻¹. In addition, both compounds 2 and 3 used in OLEDs exhibited superior performance to those of devices using NPB, demonstrating their high potential for employment in phosphorescent OLEDs.