Isotope Effect of Host Material on Device Stability of Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

Organic light‐emitting diodes (OLEDs) exhibiting thermally activated delayed fluorescence (TADF) have attracted great interest because of their excellent exciton harvesting ability in electroluminescence (EL). While TADF‐OLEDs show a high EL efficiency, the device operational stability is not necessarily satisfactory for commercial applications. Herein, the isotope effect of the host material on the device operational stability in TADF‐OLEDs is investigated. It is unveiled that the deuterated host, i.e., PYD2Cz‐d16, forms a denser film than that of the nondeuterated host, PYD2Cz, demonstrating enhanced stable amorphous nature and balanced carrier transport properties. In green TADF‐OLEDs, the PYD2Cz‐d16‐based OLED considerably lengthens the device operational stability of LT95 (95% of the initial luminance) to ≈140 h at an initial luminance of 1000 cd m−2, which is 1.7 times longer than that of PYD2Cz. Device stabilities of blue TADF‐OLEDs with PYD2Cz‐d16 as a host are also demonstrated with an enhancement of 2 times in LT50 at an initial luminance of 1000 cd m−2. It is suggested that the deuterated materials have a positive effect on the device stability in not only TADF‐OLEDs, but also all other OLEDs having fluorescence and phosphorescence emitters.