Isomeric thermally activated delayed fluorescence emitters based on indolo[2,3- b ]acridine electron-donor: a compromising optimization for efficient orange–red organic light-emitting diodes

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2019, Vol.7 (10), p.2898-2904
• Main Authors: Chen, Jia-Xiong, Tao, Wen-Wen, Xiao, Ya-Fang, Tian, Shuang, Chen, Wen-Cheng, Wang, Kai, Yu, Jia, Geng, Feng-Xia, Zhang, Xiao-Hong, Lee, Chun-Sing
• Format: Article
• Language: English
• Subjects: Conjugation; Diodes; Electrons; Emitters; Emitters (electron); Fluorescence; Optimization; Organic light emitting diodes; Oscillator strengths; Quantum efficiency
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C8TC06081A

Energy differences (Δ E ST s) between the lowest singlet and triplet excited states as well as the oscillator strengths ( f s) are two contradictory factors controlling performance of thermally activated delayed fluorescence (TADF) emitters. To figure out their optimum balance, we developed two orange–red isomeric TADF emitters 12-(13,13-dimethyl-5-phenyl-5,13-dihydro-11 H -indolo[2,3- b ]acridin-11-yl)dibenzo[ a , c ]dipyrido[3,2- h :2′,3′- j ]phenazine ( IDAC–BPPZ ) and 12-(13,13-dimethyl-11-phenyl-11,13-dihydro-5 H -indolo[2,3- b ]acridin-5-yl)dibenzo[ a , c ]dipyrido[3,2- h :2′,3′- j ]phenazine ( ACID–BPPZ ), which are constructed with the same electron-donor (D) and the same electron-acceptor (A) cores via different connecting sites. ACID–BPPZ has a weaker D–A conjugation and realizes an extremely small Δ E ST of 0.02 eV, but its f is also very small (0.001). These factors are better compromised in IDAC–BPPZ which has a more conjugated D–A connections. This successfully realizes a much higher f of 0.068 and a reasonably small Δ E ST of 0.07 eV. As a result, the orange–red OLEDs using IDAC–BPPZ as emitter achieve a maximum external quantum efficiency 18.3% comparing to that of 14.7% in the ACID–BPPZ -based device. The results suggest that a balance between Δ E ST and f is important for developing high performance TADF emitters.