Studies Of The Degradation Mechanism Of Organic Light-emitting Diodes Based On Tris(8-quinolinolate)aluminum Alq And 2-tert-butyl-9,10-di(2-naphthyl)anthracene Tbadn

摘要

Previously, radical cation of tris(8-quinolinolate)aluminum (Alq~(·+)) has been associated with the instability of Alq films subjected to holes-only electrical current. Yet, the questions remain (ⅰ) whether Alq~(·+) is the primary source of the intrinsic degradation of bipolar organic light-emitting diodes (OLEDs) based on Alq, (ⅱ) whether Alq~(·+) reactions result in deep charge traps in holes-only devices as found in bipolar counterparts, and (ⅲ) whether radical cations can be a common source of degradation of OLEDs irrespective of materials. With regards to generality of hole-current-related degradation, it is interesting to examine the behavior of 9,10-diarylanthracenes (DAAs)-the practically important class of blue-fluorescing light-emitting-layer hosts. These questions prompted our comparative study of the effects of unipolar currents in Alq and 2-t-buty1-9,10-di (2-naphthyl)anthracene (TBADN), which was chosen as a representative material of the DAA class. First, we identified device structures allowing for rigorous and stable unipolar conduction. Interestingly, even in pristine holes-only devices, our voltammetric measurements indicated that Alq contains a substantial density of deep hole traps (far deeper than what can be explained by energetic disorder), which can be charged by passing holes-only current and seemingly discharged by exposure to white light. As for aged holes-only Alq devices, they exhibited symptoms qualitatively matching those of aged bipolar Alq devices, viz., photoluminescence (PL) loss, transition voltage (V_0) rise, and drive voltage (V_d) rise. Notably, PL and V_0 are linearly correlated in both holes-only and bipolar devices, which reinforces the supposed link between Alq~(·+) and the degradation in both types of devices. Yet, there are indications the Alq~(·+) instability may not be the only degradation pathway in bipolar devices. Even though our observations for holes-only Alq devices agree qualitatively with previously reported ones, we observe far slower degradation rates [Alq PL fades up to ～500 times slower in holes-only devices, while Alq electroluminescence (EL) fades ～50 times slower in bipolar control devices]. It is possible that impurities play a significant, perhaps crucial role in the degradation mechanism of both bipolar and holes-only devices, especially the relatively shorter-lived ones. In sharp contrast to Alq, all three observables (PL, V_0, and V_d) indicate that holes-only current in TBADN (neat or doped with a perylene-based blue dopant) does not result in degradation in the time that is sufficient for the corresponding bipolar control devices to lose 60%-80% of EL and 20%-30% of PL. We find that the electrons-only current in Alq or TBADN does not result in degradation either. Thus, the degradation of Alq and DAA bipolar devices may be caused by fundamentally dissimilar mechanisms: while hole current may damage the former, it does not appear to affect the latter, suggesting that the initiation step is different.