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

Abstract

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- <i><b>d</b></i> _{<i><b>16</b></i>} , 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- <i><b>d</b></i> _{<i><b>16</b></i>} -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- <i><b>d</b></i> _{<i><b>16</b></i>} 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.