Highly Efficient, Solution-Processed Organic Light-Emitting Diodes Based on Thermally Activated Delayed-Fluorescence Emitter with a Mixed Polymer Interlayer

The efficiency of solution-processed organic light-emitting diodes based on pure organic thermally activated delayed-fluorescence (TADF) emitters is limited by the hole-injection/severe exciton quenching of poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) and the exction quenching of TADF emitters due to the self-aggregation. In order to effectively enhance the hole-injection from PEDOT:PSS and to alleviate the exciton quenching of the PEDOT:PSS, the mixed interlayer of poly(9-vinyl-carbazole) (PVK) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-s-butylphenyl)diphenylamine)) (TFB) was inserted between emitting layer (EML) and the PEDOT:PSS layer. Additionally, the cohost of 3',5'-di(carbazol-9-yl)-[1,1'-biphenyl]-3,5-dicarbonitrile (DCzDCN) and bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2) was used to suppress the self-aggregation of the TADF emitter of 2-(9-phenyl-9H-carbazol-3-yl)-10,10-dioxide-9 thioxanthone (TXO-PhCz) and tune the charge carrier transport. By optimizing the mix ratio, highly efficient, solution-processed, green-emitting TADF-OLEDs with the hole-transport layer (HTL) of TFB:PVK (1:1) and the EML host of DCzDCN:SimCP2 (1:2) can be turned on at 4 V and can show a maximum current efficiency (CE) of 55.6 cd/A, a maximum power efficiency (PE) of 47.2 lm/W, and an external quantum efficiency (EQE) of 18.86%. The devices are among the most efficient OLEDs fabricated by solution-processed methods. These results indicate that the employment of a mixed HTL and EML cohost is a promising way toward the high-efficiency solution-processed OLEDs.