Unraveling the Position Effect of Spiroxanthene-Based n-Type Hosts for High-Performance TADF–OLEDs

Abstract

For developing high-performance organic light-emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) emitters, the diphenyltriazine (TRZ) unit was introduced onto the 2′- and 3′-positions of xanthene moiety of spiro[fluorene-9,9′-xanthene] (SFX) to construct n-type host molecules, namely 2′-TRZSFX and 3′-TRZSFX. The outward extension of the TRZ unit, induced by the meta-linkage, resulted in a higher planarity between the TRZ unit and xanthene moiety in the corresponding 3′-TRZSFX. Additionally, this extension led to a perched T1 level, as well as a lower unoccupied molecular orbital (LUMO) level when compared with 2′-TRZSFX. Meanwhile, the 3′-TRZSFX molecules in the crystalline state presented coherent packing along with the interaction between TRZ units; the similar packing motif was spaced apart from xanthene moieties in the 2′-TRZSFX crystal. These endowed 3′-TRZSFX superior electron transport capacity in single-carrier devices relative to the 2′-TRZSFX-based device. Hence, the 3′-TRZSFX-based TADF–OLED showed remarkable electroluminescent (EL) performance under the operating luminance from turn-on to ca. 1000 cd·m−2 with a maximum external quantum efficiency (EQEmax) of 23.0%, thanks to its matched LUMO level with 4CzIPN emitter and better electron transport capacity. Interestingly, the 2′-TRZSFX-based device, with an EQEmax of 18.8%, possessed relatively low roll-off and higher efficiency when the operating luminance exceeded 1000 cd·m−2, which was attributed to the more balanced carrier transport under high operating voltage. These results were elucidated by the analysis of single-crystal structures and the measurements of single-carrier devices, combined with EL performance. The revealed position effect of the TRZ unit on xanthene moiety provides a more informed strategy to develop SFX-based hosts for highly efficient TADF–OLEDs.

Theoretical Insights into the Photophysical Properties of 4CzIPN Doped in Different Hosts: A Multiscale Study

“…1,2,3,5-Tetrakis­(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) is a typical TADF emitter with green emission, and high photoluminescence quantum yield (PLQY) and external quantum efficiency (EQE) have been achieved for the OLEDs with 4CzIPN as emitters. − Significant efforts have been dedicated to the development of new hosts to enhance the performance of OLEDs with 4CzIPN. − To date, several kinds of host materials have been reported to effectively improve the performance of OLEDs with 4CzIPN as emitters. , However, theoretical studies on the photophysical properties of 4CzIPN are limited to research in gas phases or in solvents. − Since most of the 4CzIPN emitters used in OLEDs are in doped films, the influence of the host on the photophysical properties should be considered. Recently, four kinds of hosts (m-CzPym, m-CzTrz, p-CzPym, and p-CzTrz shown in Figure a), which comprise two peripheral carbazole donors that link to benzonitrile-substituted triazine and pyrimidine acceptors, were employed as the host materials of 4CzIPN with different photophysical properties .…”