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-butyl-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 (V0) rise, and drive voltage (Vd) rise. Notably, PL and V0 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, V0, and Vd) 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.

We describe a synergistic effect of a lifetime-extending light-emitting-layer (LEL) additive and improved electron injection and transport in organic light-emitting diodes (OLEDs). Previously reported di(2-naphthyl)perylene (DNP) serves as the LEL additive capable of extending the operating lifetime of OLEDs by over two orders of magnitude. Using 2-phenyl-9,10-di(2-naphthyl)anthracene (PADN) as an electron-transport layer (ETL) and a separate layer of 4,7-diphenyl-1,10-phenanthroline (BPhen) as an electron-injection layer (EIL) significantly improves electron delivery into the charge recombination zone relative to traditional ETL made of tris(8-quinolinolate)aluminum (Alq). This ETL∣EIL combination not only results in approximately seven times lower electric field in the ETL and, thus, lower drive voltage and higher efficiency devices, but can also increase device lifetime substantially. In a representative device containing a red-emitting LEL dopant [Commission Internationale de l’Eclairage 1931 2° color chromaticity coordinates (CIEx,y) of 0.65, 0.35], the external quantum efficiency, electroluminescence yield, drive voltage, and operating half-life (t50) can reach 5.8%, 6.5cd∕A, 4.5V, and ∼1000000h, respectively, all at 20mA∕cm2 current density.

It is common in organic light-emitting diode technology to construct a light-emitting-layer (LEL) host with materials that resist luminescence-reducing aggregation, which is one of the common reasons behind a phenomenon widely referred to as concentration quenching. However, if a host material in its aggregated state has a substantial quantum yield of fluorescence (e.g., at least several percents), it may yet be useful. We describe a group of aggregating flat and rigid polycyclic aromatic hydrocarbons (PAHs) as LEL additives. These molecules readily form emissive aggregates when added to the LEL. In the resulting devices, the aggregates show low-to-moderate external quantum efficiencies (EQE) of 0.2%–1.3%. Significantly, the addition of these PAHs increases device half-life (t50) 4–200 times, depending on the additive, up to 100000h upon operation at 40mA∕cm2. The lifetime increase occurs with many diverse classes of PAHs. The EQE can be improved to 3.7% by further adding a proper dopant while maintaining the increased lifetime. A possible link between the ability to aggregate and the lifetime increase is illustrated by comparing aggregation-prone perylene and aggregation-resistant 2,5,8,11-tetra-t-butylperylene (TBP). Despite the similarity between the two additives with respect to their initial device performance, perylene’s stronger ability to aggregate correlates with the eight times longer half-life versus that for TBP.