Structure Design and Mechanism Investigation of a Yellow Phosphorescent Organic Light-Emitting Diode with Simple Structure, High Efficiency, and Low Roll-Off Efficiency

With its innate reverse intersystem crossing (RISC) process, the exciplex system has great potential for improving the efficiency of organic light-emitting diodes (OLEDs). However, the traditional emitting layer involves codoping with the host to form the exciplex, complicating the device manufacturing process. In this work, we reported the structural design simplification and optimization of yellow OLEDs based on the interfacial exciplex cohost, in which the exciplex is formed with tris(4-carbazoyl-9-ylphenyl)amine(TCTA) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBI), and bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) (acetylacetonate) iridium(III)(PO-01) is doped into TPBI as a yellow phosphorescent guest. Experimental results showed that this emitting layer (EML) design was helpful to efficient host–guest energy transfer due to the matching of the host’s and guest’s excited state energy levels. The carrier recombination mechanism of the device was analyzed by the ideality factor to prove the advantages of the EML designed in this study when compared with the other EML structures. Then, through UPS, capacitance–voltage, contact angle, and AFM measurements, it was suggested that the appropriate guest doping concentration could help increase carrier accumulation and formation of the exciplex at the interface by improving the balance of carrier transport as well as reducing the efficiency roll-off. Finally, it was stated that the best yellow OLED exhibited excellent EQE_max, CE_max, and PE_max of 27.4%, 77.8 cd/A, and 50.9 lm/W, respectively, with an EQE of 26.2% at 1000 cd/m² and a low roll-off efficiency of only 4.4%.