Design of phenanthroline and diindolocarbazole based emitter for efficient upconversion of triplet excitons by improving rISC

Abstract

There has been significant advancement in display technology in recent times. However, synthetic efforts toward designing more efficient emitters are still underway. TADF emitters offer the advantage of an all organic based design with IQEmax of 100%. To study the effect of phenanthroline-based moieties, we designed and synthesized two yellow-green thermally activated delayed fluorescent (TADF) emitters, DICz-DBPZ and DICz-DPPN. Due to structural variations associated with steric strain, the two emitters displayed different photophysical and organic light-emitting diode (OLED) performances. For DICz-DPPN, the absorption and emission maxima were recorded at 313-368 nm and 516 nm, respectively, while for DICz-DBPZ, they were observed at 311-393 nm and 559 nm. The DPPN-substituted TADF emitter had an EQE of 5.0% and color coordinates of (0.37, 0.53), whereas the DBPZ-substituted emitter displayed a high EQE of 15% with color coordinates of (0.47, 0.51). Due to the closed structure of DICz-DBPZ, thermal decomposition temperature occurred at 269oC as compared to 213oC for DICz-DPPN, which significantly influenced the difference in EQE between the two emitters. In order to simultaneously obtain high EQE and a long lifetime, this study offers an approach to synthesize yellow TADF emitters by utilizing a combination of diindolocarbazole (DICz) and 1,10-phenanthroline-5,6-diamine as a stable donor-acceptor.