White organic light emitting devices based on ultrathin emitting layer and bipolar hybrid interlayer

Abstract

In this paper, efficient phosphorescent white organic light-emitting diodes (WOLEDs) with stable spectra are fabricated based on doping-free ultrathin emissive layers and mixed bipolar interlayers. To achieve WOLEDs, at least three kinds of light-emitting layers, i.e. blue, green and red, are needed. The traditional method to fabricate emissive layers is by co-evaporation, which can improve electroluminescent efficiency. However, the co-evaporation rate and dopant concentration are difficult to control, which leads to a bad reproducibility and thus goes against commercialization. In order to simplify the structures of WOLEDs and improve repeatability, several doping-free ultrathin emissive layers are used in this paper with 3 nm mixed bipolar interlayers separating them. The optimal ratio of bipolar hybrid material is determined by hole-only device, electron-only device and blue phosphorescent OLED. In addition, green, orange and red monochromatic OLED have also been fabricated separately, which are used to prove that mixed bipolar material is also suitable for the three phosphorescent emitting material. The WOLED with TCTA interlayers is fabricated to confirm that mixed bipolar material is beneficial to the characteristics of WOLEDs. The energy transfer process between different emitting materials is verified by studying the transient photoluminescence lifetime. The maximum efficiency of three-color and four-color doping-free WOLED are 52 cd/A (53.5 lm/W) and 13.8 cd/A (13.6 lm/W), respectively, and the maximum external quantum efficiency of three-color and four-color doping-free WOLED are 17.1% and 11.2%, respectively. Due to the sequential energy transfer structure between different emitting layers, the Commission Internationale de L'Eclairage coordinates shows a very slight variation of (0.005, 0.001) from 465 cd/m<sup>2</sup> to 15950 cd/m<sup>2</sup> for three-color WOLED. The Commission Internationale de L'Eclairage coordinates shows a variation of (0.023, 0.012) from 5077 cd/m<sup>2</sup> to 14390 cd/m<sup>2</sup> for four-color WOLED. The four-color WOLED shows a maximum color rendering index of 92.7 at 884 cd/m<sup>2</sup>, and it reaches 88.5 at 14390 cd/m<sup>2</sup>. In addition, the lifetime of phosphorescent OLED is usually poor due to the trap formed by triplet-polaron annihilation. The exciton distribution can be broadened and the exciton concentration can be reduced by using ultrathin light emitting layers (< 1 nm) and mixed bipolar interlayers. Therefore, triplet-polaron annihilation will be reduced, and the lifetime of OLEDs will be improved.