A multifunctional hole-transporter for high-performance TADF OLEDs and clarification of factors governing the transport property by multiscale simulation

Abstract

To date, a limited number of reports have been published on thermally activated delayed fluorescent (TADF) organic light-emitting devices (OLEDs) that simultaneously achieved high efficiencies and long operational lifetimes. The development of tailored-hole transporters is an effective solution because extensively used conventional hole-transport materials (HTMs), such as NPD or TAPC, are unsuitable for simultaneous realizations of high-efficiency and long-lifetime in TADF OLEDs. In this study, we developed a new four-dibenzofuran (DBF) end-capped hexaphenylbenzene (HPB)-based HTM, referred to as T4DBFHPB. Using this as the HTM, we simultaneously achieved high external quantum efficiency (η_ext = 22.0%), long operational lifetime (LT₅₀ = 28 000 h), and low-drive voltage (3.83 V) at 1000 cd m⁻² in green TADF OLEDs. Our research reveals the importance of a multifunctional HTM with (i) high triplet energy (E_T), (ii) high glass transition temperature (T_g), and (iii) high bond dissociation energy (BDE) of the C–N bonds in the anion state. Moreover, we conducted multiscale simulations to improve the hole-mobility (μ_h). Consequently, the simulation suggested that permanent-dipole-induced site energy and reorganization energy are critical factors for improving μ_h among HPB derivatives.