Degradation Mechanisms of Organic Light-Emitting Diodes Based on

Article pubs.acs.org/JPCC

Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules Atula S. D. Sandanayaka,* Toshinori Matsushima, and Chihaya Adachi* Center for Organic Photonics and Electronics Research (OPERA) and Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADFOLEDs.

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OLEDs.19−25 Therefore, we chose to investigate the degradation mechanisms of 4CzIPN. To investigate the degradation mechanisms, we used steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements.26−29 From the steady-state PL measurement results, we found that the excited-state instability of host molecules and the electrochemical instability of radical cations and anions are the possible sources of OLED degradation. Furthermore, TRPL measurements revealed that OLED degradation is attributable to the formation of exciton quenchers,27,30 as well as nonradiative carrier recombination centers,31,32 which are generated by excited-state and electrochemical instabilities.

INTRODUCTION Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) molecules have attracted great attention because of their high electron-tophoton conversion efficiencies.1−6 Because TADF molecules have very low energy gaps