High-triplet-energy Bipolar Host Materials Based on Phosphine Oxide Derivatives for Efficient Sky-blue Thermally Activated Delayed Fluorescence Organic Light-emitting Diodes with Reduced Roll-off

Abstract

We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO, which contain electron-donating carbazole and electron-accepting triphenylphosphine oxide moieties. Thermally activated delayed fluorescence (TADF)-based OLEDs employing CzPO and Cz3PO as host materials and the 9-(4-(4,6diphenyl-1,3,5-triazin-2-yl)phenyl)-1,3,6,8-tetramethyl-9H-carbazole (CzTRZ2) as the emitter resulted in improved maximum external quantum efficiencies, EQEmax, of 13.1% and 13.2%, respectively, together with small efficiency roll-offs, while the device based on bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) showed a much more pronounced efficiency roll-off. The reduced efficiency roll-off in the devices based on CzPO and Cz3PO can be ascribed to their improved ambipolar charge transport capacity compared to that of DPEPO, which results in a broader recombination zone in the emitting layer using these ambipolar hosts.

Aggregation‐Induced Emission in a Flexible Phosphine Oxide and its Zn(II) Complexes—A Simple Approach to Blue Luminescent Materials

“…[ 11,12 ] One approach that has been extensively pursued for organic light‐emitting diode (OLED) applications is the use of phosphines or phosphine oxides as hosts, emitters, or ligands in luminescent metal complexes. [ 13–25 ] Used as hosts, particularly in blue phosphorescent (2nd generation) and thermally activated delayed fluorescence (TADF, 3rd generation) OLEDs, phosphine oxides stand out due to their high triplet‐state energy (T 1 ) and charge mobility, and can improve device properties and performances. [ 15,26 ] The drawback of phosphines and phosphine oxides is their rather labile PC bond.…”

A meta-linkage strategy towards high-performance hosts for efficient blue thermally activated delayed fluorescence OLEDs

“…Owing to the high T 1 energies and appropriate electron-transporting ability, phosphine oxide derivatives were applied successfully to construct n-type and bipolar host materials for blue OLEDs. [32][33][34][35][36][37][38][39][40] Furthermore, the linkage mode of the functional groups, which influence the molecular configuration and molecular packing, is equally important in determining the properties of the host materials. [41][42][43] The influence of positional isomerism on electronic decoupling and the steric hindrance has been widely investigated.…”

Asymmetric Host Molecule Bearing Pyridine Core for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs

“…[16,19,25] In contrast, althought here has not been much research performed on chemical stability in the excited state of electron-accepting units, pyridine, cyano, and phos-phine oxide groups are generally used as electron-accepting moieties in various bipolar host materials. [4,9,[26][27][28] In this study,t wo host materials (i.e., pCzBzbCz and pCzPybCz)w ere designed and synthesized to achieve ah igh T 1 value for blue TADF-OLEDs.T he molecular designs trategy was to introduce different core structuresw ith phenyla nd pyridine groupsi nt he pCzBzbCz and pCzPybCz structures, respectively,a nd also to substitute carbazole and bicarbazole in the 2,5-position. These host materials demonstrate excellent thermals tabilitye ven in doped films, and ah igh T 1 ,w hich are both suitable qualitiesf or blue TADF-OLEDs.…”

“…Thus, the chemical stability in the excited state of the carbazole unit has been researched more than other moieties [16, 19, 25] . In contrast, although there has not been much research performed on chemical stability in the excited state of electron‐accepting units, pyridine, cyano, and phosphine oxide groups are generally used as electron‐accepting moieties in various bipolar host materials [4, 9, 26–28] …”