Enhanced performance of solution processed OLED devices based on PFO induced TADF emission layers

Abstract

The internal quantum efficiency (IQE) of the OLED devices based on thermally activated delayed fluorescence (TADF) materials was thought to nearly 100 % by assuming the light outcoupling efficiency of 20–25 %. However, the external quantum efficiency (EQE) is still limited by the ηout. One crucial factor that affects the light output coupling efficiency is the molecular transition dipole moment of the emission molecule. In this study, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was chosen as an orientation inducer to enhance the horizontal orientation ratio of a TADF emitter, 2-Phenyl-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl] pyrimidine (DMAc-PPM), prepared using a solution method. With doping PFO in weight ratios ranging from 1 wt% to 9 wt%, the orientation factor of TADF emitter increased, which testified the inducing function of PFO. The TADF-OLED devices using PFO as an inducer exhibited an EQE increasing with the PFO doping weight ratio. The maximum EQE of 7.3 % was achieved at the doping weight ratio of 7 wt%, which increase about 1.7 times compared to the device without the PFO inducer. This observed variation of EQE differs from the trend observed in maximum luminance intensity. The increase of EQE can be attributed to the improvement of light output efficiency caused by the variation of orientation factor. The lower carrier mobility lead to the lower maximum luminance intensity. The synergistic effect of light output efficiency and carrier mobility enhanced the performance of OLED devices.