Spectrally narrowed edge emission from leaky waveguide modes in organic light-emitting diodes

A dramatic spectral line narrowing of the edge emission at room temperature from tris(quinolinolate) Al (Alq₃), N, N′ -diphenyl- N, N′ -bis(1-naphthylphenyl)- 1, 1′ -biphenyl- 4, 4′ -diamine (NPD), 4, 4′ -bis(2, 2′ -diphenyl-vinyl)-, 1′ -biphenyl (DPVBi), and some guest-host small molecular organic light-emitting diodes (OLEDs), fabricated on indium tin oxide (ITO)-coated glass, is described. In all but the DPVBi OLEDs, the narrowed emission band emerges above a threshold thickness of the emitting layer, and narrows down to a full width at half maximum of only 5-10 nm. The results demonstrate that this narrowed emission is due to irregular waveguide modes that leak from the ITO to the glass substrate at a grazing angle. While measurements of variable stripe length l devices exhibit an apparent weak optical gain 0≤g≤1.86 cm^-1, there is no observable threshold current or bias associated with this spectral narrowing. In addition, in the phosphorescent guest-host OLEDs, there is no decrease in the emission decay time of the narrowed edge emission relative to the broad surface emission. It is suspected that the apparent weak optical gain is due to misalignment of the axis of the waveguided mode and the axis of the collection lens of the probe. However, it is not clear if such a misalignment can account for all the effects of the observed evolution of the edge-emission spectra with l.