Balance the Carrier Mobility To Achieve High Performance Exciplex OLED Using a Triazine Based Acceptor.
A star shaped 1,3,5 triazine/cyano hybrid molecule CN T2T was designed and synthesized as a new electron acceptor for efficient exciplex based OLED emitter by mixing with a suitable electron donor (Tris PCz) The CN T2T/Tris PCz exciplex emission shows a high PhPL of 0.53 and a small DET S 0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up conversion. The large energy level offsets between Tris PCz and CN T2T and the balanced hole and electron mobility of Tris PCz and CN T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/4% ReO3:Tris PCz (60 nm)/Tris PCz (15 nm)/Tris PCz:CN T2T(1:1) (25 nm)/CN T2T (50 nm)/Liq (0.5 nm)/Al (100 nm) in which the electron hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9% The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable PhPL (0.50) in which TCTA shows similar energy levels but higher hole mobility as compared with Tris PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex based OLED. Exploiting the Tris PCz:CN T2T exciplex as the host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7% respectively.
ACS applied materials interfaces