Improving operating lifetime of organic light-emitting diodes with polycyclic aromatic hydrocarbons as aggregating light-emitting-layer additives

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 (t(50)) 4-200 times, depending on the additive, up to 100 000 h upon operation at 40 mA/cm(2). 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. (c) 2006 American Institute of Physics.