Impact of Donor and Acceptor Modification on TADF and Roll-Off Behaviors in Solution Processed OLED

Up-conversion of triplet into singlet exciton in the emitting layer is believed to be one of the ways that thermally activated delayed fluorescent (TADF) materials may employ to reduce triplet exciton density hence preventing device quenching by triplet excitons. Yet, two donor-acceptor type molecules; 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12-(3-(triphenylsilyl) phenyl)-5,12-dihydroindolo[3,2-a]carbazole (SiPhCzTrz) and 5-phenyl-12-(4-(4-phenyl-6-(3-(triphenylsilyl) phenyl)-1,3,5-triazin-2-yl)pheny-l)-5,12-dihydroindolo [3,2-a]carbazole (SiTrzPhCz), which exhibited different TADF properties depending on the relative positions of their electron donor unit (PhCz) and electron acceptor unit (Trz), show opposite behaviors. These materials are used as sensitizer in phosphorescent solution-processed organic light emitting diodes (s-OLEDs) showing moderately high current efficiencies of 19.3 and 20 cd/A, respectively. SiTrzPhCz exhibits stronger TADF properties compared to SiPhCzTrz; however, these stronger TADF characteristics lead to a more pronounced efficiency roll-off, mainly due to the longer residence time of excitons in SiTrzPhCz, leading to exciton quenching. In contrast, due to their twisted structures, the efficiency roll-off is efficiently suppressed, particularly for SiTrzPhCz, when both materials are used as hosts. Their twisted structures promote aggregate-induced emission and prevent aggregation-caused quenching. Further analysis of exciton dynamics reveals faster decay rate for both singlet and triplet densities in SiPhCzTrz compared to SiTrzPhCz, indicated by its higher fast prompt emission, k_FRET and k_nr^T.