Direct Observation of Radical States and the Correlation with Performance Degradation in Organic Light-Emitting Diodes During Device Operation

摘要

Microscopic characterization of radical states in organic light-emitting diodesrn(OLEDs) during device operation is useful for elucidating the degradationrnmechanism because the radical formation has been considered as non-radiativernrecombination centers. Electron spin resonance (ESR) spectroscopy is suitablernfor such characterization because it can directly observe radicals in OLEDs. Inrnthis work, the detailed ESR investigation into the radical states in OLEDs duringrndevice operation is firstly reported using a typical light-emitting Alq_3-basedrnOLEDs. The simultaneous measurements of the ESR signal and the luminancernof the same OLED are performed to study the direct correlation between thernradical states and the performance degradation. These characteristics show thatrnthe luminance monotonically decreases and an ESR signal concomitantlyrnincreases as the duration of the device operation increases after operating thernOLED. Using the analysis of density functional theory (DFT) calculation, thernorigin of the newly emerged ESR signal is ascribed to the cationic species due torndecomposed Alq3 molecules. The elucidation of the radical species formed inrnOLEDs during device operation has been demonstrated at a molecular level forrnthe first time. This ESR analysis would provide useful knowledge forrnunderstanding the degradation mechanism in the OLEDs at the molecular level.