Electro-optical Investigation of the Green Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Based on Mixing Hosts by Impedance Spectroscopy

Abstract

In this work, we fabricated green mixed host TADF devices and investigated the luminance and efficiency roll-off characteristics compared to the green single host TADF devices. We also investigated an electrical characteristics of the devices with mixed host and single host, respectively, using the impedance spectroscopy. CBP and TPBi were used as a single host and the mixture of CBP and TPBi (1:1 ratio) was used as a mixed host, 4CzIPN was used as an emitter (Inset of Fig.1 shows device structure of mixed host OLED). As a result, when the mixed host device shows lower efficiency roll-off compared to the single host devices. As shown in Fig. 1, the efficiency roll-off was 49.5% in CBP host device and 49.8% in TPBi host device at 5,000 cd/m2. However, mixed host device had an efficiency roll-off about 19.1% at same luminance. This indicates that a better charge balance is achieved in EML and the recombination zone has expanded compared to the single host devices. Fig. 2 shows the cole-cole plot of those devices, where at the same voltage, a mixed host device having bipolar properties exhibits smaller radius of semi-circle than those of single host devices. It can be explained that the charge carriers from the both transporting layers overcome the interface energy barrier having nearly zero ΔE and migrate to the EML easily. Consequently, the current density of devices were 2.5 mA/cm2, 3.59 mA/cm2 and 5.58 mA/cm2 in CBP, TPBi and CBP:TPBi host devices at the voltage of 8V, respectively. Further details on impedance analysis results for these devices will be discussed in the conference.