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

Abstract

Recently, various hosts and emitters for solution-processable thermally activated delayed fluorescence organic lightemitting 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(9vinylcarbazole) (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 EAsolution-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.

Rational Molecular Design Strategy for Host Materials in Thermally Activated Delayed Fluorescence-OLEDs Suitable for Solution Processing

“…The fabrication process of OLEDs is elaborated in the Supporting Information. The EML solution was prepared with EA, an eco-friendly solvent proposed previously, as an orthogonal solvent . Poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonic acid) (PEDOT:PSS) was used as the hole-injection layer, PVK as the hole-transporting layer, and 1,3-bis­(3,5-dipyrid-3-ylphenyl)­benzene (BmPyPB) as the electron-transporting layer.…”

Opportunities for cost-effective manufacturing of fully printed high performance displays enabled by vertical light-emitting transistor pixels

“…Key research areas in this field include the careful solvent selection and development of solvent resistant film. Je et al recently publish the study on a customized orthogonal solvent system with three HTLs with different solubilities in toluene, while being insoluble in ethyl acetate, which is the orthogonal solvent in EML solution [174]. The approach showed high reproducibility in the fabrication of OLED devices as evidenced by the achievement of devices using all three HTL materials, exhibiting EQE exceeding 23% and a favorable turn on voltage of 2.8 V.…”

Universal Polymeric Hole Transporting Material for Solution-Processable Green and Blue Thermally Activated Delayed Fluorescence OLEDs

“…In a previous study, ethyl acetate (EA), which is recognized as an eco-friendly solvent, was selected as the solvent for the EML instead of an aromatic solvent. 33 An OLED device fabricated using this system possessed a stable lower layer and exhibited a high reproducible performance. Additionally, polymeric HTMs used in OLEDs are suitable for green-emitting OLEDs, owing to their low T 1 values.…”

“…A new solvent system for EML fabrication has been proposed for polymeric HTM that can replace PVK. In a previous study, ethyl acetate (EA), which is recognized as an eco-friendly solvent, was selected as the solvent for the EML instead of an aromatic solvent . An OLED device fabricated using this system possessed a stable lower layer and exhibited a high reproducible performance.…”