FULL PAPER 1700012 (1 of 6) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advopticalmat.de Solution-Processed Thermally Activated Delayed Fluorescence Exciplex Hosts for Highly Efficient Blue Organic Light-Emitting Diodes Ye-Xin Zhang, Bo Wang, Yi Yuan, Yun Hu, Zuo-Quan Jiang,* and Liang-Sheng Liao* DOI: 10.1002/adom.201700012 cost-effective, and large-area devices in a manufacturable manner via spin-coating, ink-jet printing, or roll-to-roll produc- tion. [8–11] Although polymeric hosts doped with phosphors can be easily prepared by a solution process, the variations in mole- cular weight, solubility, and purity across different batches may hamper their prac- tical applications. [12,13] In addition, in most polymers it is difficult to tune the highest occupied molecular orbital (HOMO)/ lowest unoccupied molecular orbital (LUMO) levels and the triplet energy for the large singlet–triplet splitting in poly- mers. [14,15] Another strategy for solution processing based on small molecules can also be used to fabricate low-cost, high- performance OLEDs. This method result in higher molecular precision in structure, entails a simpler synthesis, and easier purification, which allows them to occupy an important position in this field. [16–20] In the solution processing of PhOLEDs, two prerequisites should be fully considered: 1) the forma- tion of uniform amorphous films; 2) the charge balance in the emitting layer (EML). The former factor is strongly related to the solubility and solution processability of the applied mate- rials. Regarding the latter, to achieve a charge balance in the EML, many strategies have been employed, such as using a bipolar host [21–24] or a co-host (p-type and n-type together). [25,26] Recently, many extremely high-efficiency PhOLEDs have been reported; these devices use thermally activated delayed fluo- rescence (TADF) materials and exciplexes as hosts. [27–31] TADF hosts with a small singlet–triplet energy difference (ΔE ST ), achieved by separating the intramolecular electron-donor and -acceptor units, can fully utilize both triplet (E T ) and singlet (E S ) energies through triplet up-conversion. [32] In addition to TADF hosts, exciplexes are another type of small ΔE ST hosts; these exciplexes are formed by the intermolecular interaction between the electron-donor (D) and -acceptor (A) molecules in the excited state. [33] In other words, the term exciplex is used to describe such excited charge-transfer (CT) states when D/A pairs do not form a complex in the ground state, but the charge-transfer occurs in the excited state upon the bimolec- ular encounter of one excited molecule and one quencher. The formed exciplex is able to exhibit a balanced charge transfer in the EML according to the D/A composition. Using an exci- plex as a host in a PhOLED, Kim and co-workers reported a Two new wide-bandgap and high-triplet-energy acceptor materials, 1,4- phenylenebis(diphenylphosphine oxide) (2PO) and ((phenylphosphoryl) bis(4,1-phenylene))bis(diphenylphosphine oxide) (3PO), are designed to con- struct thermally activated delayed fluorescence (TADF) exciplexes. By mixing them with tris(4-carbazoyl-9-ylphenyl) amine (TCTA), emission from these exciplexes can be easily observed. The forming exciplexes are also studied by delayed transient fluorescence emission to prove their TADF process. Considering the fact that 2PO and 3PO are constructed in a simple interrup- tion process of the phosphine-oxide group, which has a high triplet energy, the resulting exciplexes can be used to host blue phosphors. Consequently, solution-processed blue phosphorescent organic light-emitting diodes (PhOLEDs) based on these compounds demonstrate a current efficiency of 29.2 cd A -1 and 26.8 cd A -1 , respectively, which is the first reporting of current efficiencies approaching 30 cd A -1 in blue PhOLEDs with solution-processed exciplex hosts. Furthermore, relatively low current-efficiency roll-off values of 3.5% and 0.4% were observed for 2PO and 3PO-based devices, respectively, from 100 cd m -2 to 1000 cd m -2 . Y.-X. Zhang, B. Wang, Y. Yuan, Y. Hu, Dr. Z.-Q. Jiang, Prof. L.-S. Liao Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices Institute of Functional Nano & Soft Materials (FUNSOM) Soochow University Suzhou 215123, P. R. China E-mail: zqjiang@suda.edu.cn; lsliao@suda.edu.cn Organic Light-Emitting Diodes 1. Introduction Since the first practical organic lighth-emitting diode (OLED) was reported, [1] the development of OLEDs has rapidly devel- oped on account of their great potential for use as solid-state lighting sources and in high-quality flat-panel displays. [2,3] Gen- erally, OLEDs can be classified into two types according to their fabrication method: thermal-vacuum evaporation and solution- processing. Producing a device via thermal-vacuum evaporation has some obvious merits, such as the formation of uniform amorphous films and an ideal interface between functional layers; such merits are beneficial for producing high efficiency devices. [4–7] However, the thermal-vacuum-evaporation process entails high fabrication costs, with unavoidable material waste and complicated processes, particularly in doped phospho- rescent OLEDs (PhOLEDs). In contrast, solution processing is preferable because of the benefits of producing flexible, Adv. Optical Mater. 2017, 1700012