Customized Orthogonal Solvent System with Various Hole-Transporting Polymers for Highly Reproducible Solution-Processable Organic Light-Emitting Diodes

Recently, various hosts and emitters for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) have been developed. However, a few studies have been conducted on hole transport materials (HTMs) with differentiated solubility characteristics for manufacturing multilayer OLEDs using a solution process. Here, three new hole transport (HT) styrene polymers, PICz, PPBCz, and PTPCz, were synthesized by radical polymerization. Each of the polymers exhibited increases in their highest occupied molecular orbital (HOMO) levels and better hole-transporting abilities than poly(9-vinylcarbazole) (PVK) as a reference HT polymer. Furthermore, the three HT polymers exhibited different solubilities in toluene. Therefore, it was not possible to use a toluene solution to prepare the emitting layer (EML). To overcome this problem, ethyl acetate (EA), in which the three HT polymers are insoluble, was used as an orthogonal solvent to prepare an EML solution. In EA-solution-processed green-emitting TADF-OLEDs, the three HT-polymer-based devices displayed somewhat low turn-on voltages of 2.8 V and high external quantum efficiencies (EQEs) of >23%. These values are superior to those of a device with a PVK-HT layer. In addition, the devices manufactured with the EA solution showed high-performance reproducibility owing to the stable formation of each layer. In this study, we removed the HTM solubility constraint by dramatically changing the solvent for preparing the EML solution and provided an efficient strategy for the fabrication of OLED devices via solution processes in the future.