Low turn-on voltage and low roll-off rare earth europium complex-based organic light-emitting diodes with exciplex as the host

Abstract

Low turn-on voltage and low roll-off rare earth europium complex (Eu(DBM)₃Phen)-based organic light-emitting diodes (OLEDs) have been achieved by utilizing an exciplex of TCTA:Bphen as the host. At the optimal doping concentration of 10 wt%, the device shows pure and sharp emission of Eu³⁺ and the turn-on voltage, maximum current efficiency and external quantum efficiency (EQE) are 2.9 V, 3.85 cd A⁻¹ and 2.12%, respectively. More importantly, the roll-off ratio is just 12% from the maximum EQE to the EQE at 100 cd m⁻². Furthermore, by changing the exciplex host of TCTA:Bphen to the thermally activated delayed fluorescence (TADF) exciplex host of mCP:PO-T2T, the turn-on voltage was reduced further to 2.5 V and the maximum current efficiency, power efficiency and EQE reached 5.51 cd A⁻¹, 4.94 lm W⁻¹ and 3.11%, respectively. Barrier-free carrier injection, balanced carrier transport and recombination play key roles in this system. Moreover, the TADF mechanism also helps to improve the efficiency of the system, and more detailed discussions are given.