Efficient Thermally Activated Delayed Fluorescence Conjugated Polymeric Emitters with Tunable Nature of Excited States Regulated via Carbazole Derivatives for Solution-Processed OLEDs

For main-chain-type thermally activated delayed fluorescence (TADF) polymeric emitters, conjugated or nonconjugated monomers are generally copolymerized as the spacers to suppress the inter- and intramolecular exciton concentration quenching. In this work, carbazole derivatives are introduced into the main chains of the conjugated polymers based on the TADF unit, 2-(10<i>H</i>-phenothiazin-10-yl)­dibenzothiophene-<i>S</i>,<i>S</i>-dioxide (DBTO2-PTZ). It is found that the content of carbazole derivatives units could not only effectively suppress exciton quenching and nonradiative transition but also manipulate the distribution of molecular orbits and Δ<i>E</i>_ST values and even regulate the nature of excited states. Therefore, the upconversion of triplet exciton from triplet to singlet excited state could be regulated by introducing the different contents of carbazole derivatives. Among three synthesized polymers, COP-10 displays a relatively higher <i>k</i>_RISC and PLQY in the film state. The optimized OLED device without any TADF assistant dopant could reach to an EQE_max of 15.7% with a lower turn-on voltage of 3.2 V

Thermally Activated Delayed Fluorescence Pendant Copolymers with Electron- and Hole-Transporting Spacers

To study the effect of hole- and electron-transporting spacers in copolymers on the thermally activated delayed fluorescence (TADF) properties and device efficiency of copolymers, two series of copolymers PCzPT-<i>x</i> and POPT-<i>x</i> have been designed and synthesized successfully. In these copolymers, 2-(10<i>H</i>-phenothiazin-10-yl)­dibenzothiophene-<i>S</i>,<i>S</i>-dioxide units give green-yellow TADF, while hole-transporting 9-(4-vinylphenyl)-9<i>H</i>-carbazole units or electron-transporting diphenyl­(4-vinylphenyl)­phosphine oxide act as spacers or hosts. Their thermal, electrochemical, photophysical, and electroluminescent properties and theoretical calculations are systematically investigated to illustrate the relationships between molecular structures and photophysical properties. By optimizing the upconversion and radiative decay rate and managing the energy transfer, a green-yellow device based on POPT-25 achieves a maximum external quantum efficiency of 5.2%, a current efficiency of 16.8 cd/A, and a power efficiency of 7.8 lm/W with CIE coordinates of (0.36, 0.50). Moreover, an external quantum efficiency of 3.5% at the practical luminescence of 100 cd/m² is obtained