Efficiency Breakthrough of Fluorescence OLEDs by the Strategic Management of "Hot Excitons" at Highly Lying Excitation Triplet Energy Levels

Authors：Lin, Chengwei; Han, Pengbo; Xiao, Shu; Qu, Fenlan; Yao, Jingwen; Qiao, Xianfeng; Yang, Dezhi; Dai, Yanfeng; Sun, Qian; Hu, Dehua; Qin, Anjun^*; Ma, Yuguang^*; Tang, Ben Zhong^*; Ma, Dongge^*

Source：Advanced Functional Materials, 2021, 31(48): 2106912.

DOI：10.1002/adfm.202106912

• aggregation-induced emission
• Forster resonance energy transfer
• hot exciton
• hybridized local and charge-transfer
• organic light-emitting diodes

Aggregation-induced emission (AIE) and hybridized local and charge-transfer (HLCT) materials are two kinds of promising electroluminescence systems for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) by harnessing "hot excitons" at the high-lying triplet exciton states (T-n, n >= 2). Nonetheless, the efficiency of the resulting OLEDs did not meet expectations due to the possible loss of T-n -> Tn-1. Herein, experimental results and theoretical calculations demonstrate the "hot exciton" process between the high-lying triplet state T-3 and the lowest excited singlet state S-1 in an AIE material 4 ''''-(diphenylamino)-2 '',5 ''-diphenyl-[1,1 '':4 ',1 '':4 '',1 ''':4 ''',1 ''''-quinquephenyl]-4-carbonitrile (TPB-PAPC) and it is found that the Forster resonance energy transfer (FRET) between two molecules can facilitate the "hot exciton" process and inhibit the T-3 -> T-2 loss by doping a blue fluorescent emitter in TPB-PAPC. Finally, the doped TPB-PAPC blue OLEDs achieve a maximum external quantum efficiency (EQE(max)) of 9.0% with a small efficiency roll-off. Furthermore, doping the blue fluorescent emitter in a HLCT material 2-(4-(10-(3-(9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d] imidazole (PAC) is used as the emission layer, and the resulting blue OLEDs exhibit an EQE(max) of 17.4%, realizing the efficiency breakthrough of blue fluorescence OLEDs. This work establishes a physical insight in the design of high-performance "hot exciton" molecules and the fabrication of high-performance blue fluorescence OLEDs.