Highly Efficient Solution Processed Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Devices with a Mixed Hole Injection Layer

In the study of the solution-processed organic light-emitting devices (OLEDs) PEDOT: PSS is often used as a hole injection layer (HIL) in OLEDs due to its good film-forming property and high light transmittance. However, related studies have shown that PEDOT: PSS itself has defects, such as poor stability and low work function, which may result in poor and unstable device performance. At the same time, blue light is one of the three primary colors of the display, and the preparation of high efficient blue light OLEDs is indispensable for realizing high-quality white light and solid-state lighting devices. At present, most solution-processed blue OLEDs suffer from poor device performance, hence the research on the highly efficient solution-processedblue OLEDs will be of great significance. In this paper, we applied an efficient blue thermally activated delayed fluorescent (TADF) emitter DMAC-DPS to fabricate solution-processed blue TADF OLEDs, and the mixed hole injection layers (mix-HILs) were prepared by doping PEDOT: PSS with PSS-Na and its effect on device performance of blue-light TADF OLEDs was investigated. First, we mixed different volumes of PSS-Na solution in PEDOT: PSS aqueous solution and the mix-HILs were spin-casted under the same conditions to fabricate blue OLEDs. The electroluminescence (EL) spectrum is blue-shifted after the incorporation of PSS-Na, which can be attributed to the decrease in the thickness of the mix-HIL layer. The reduction in the thickness of the mix-HIL layer results in a blue shift in the EL spectrum under the effect of the microcavity. By comparing the current density-voltage-luminance (J-V-L) curves and its calculated current efficiency of each device, the results show that with the incorporation of PSS-Na, the brightness and current of the device increase, and the current efficiency of the device is also improved. The increase is the highest when the doping ratio is 0. 5: 0. 5 (PEDOT: PSS/PSS-Na) of which the device brightness is increased by 86. 7% and the current efficiency is increased by 34. 3%. Finally, the behavior of internal carriers of blue OLEDs based on mix HILs with different doping ratios was observed by means of transient electroluminescence (EL) test system. Forward bias of 10 V was applied to the blue OLEDs. When the electroluminescence of the device reached a steady-state, the bias was removed, and the intensity of the delayed EL peak was observed. After a time delay of 50 mu s, a reverse bias of 7 V was applied to observe the intensity of transient EL peak. It shows that after the removal of the forward bias, the EL spike of the device decreases as the PSS-Na incorporation ratio increases, indicating that the accumulated charge at the internal interface of the device is reduced. And the increase in the EL spike of the device after application of the reverse bias indicates that the injection of holes is more efficient after the incorporation of PSS-Na, and the injection barrier at the mix-HIL/EML interface is decreased. With the help of transient EL test system, the change of EL intensity of the device was observed by applying or removing the driving voltage, and the charge accumulation at the mix-HIL/light-emitting layer (mix-HIL/EML) interface was analyzed. The incorporation of PSS-Na increases the work function of the hole injection layer, and the accumulation of charge at the interface is reduced. By preparing the mix-HIL with doping PSS-Na into PEOT PSS, the device performance of blue TADF OLEDs is improved, which is a feasible method for obtaining high efficient solution-processed OLEDs.