Recent electroluminescence (EL) detected magnetic resonance and transient EL studies reveal the presence and role of holes that drift beyond the recombination zone and approach the cathode in small molecular organic light-emitting diodes (OLEDs) with specific materials and structures. In particular, these studies suggest that these holes are responsible for trion (i.e., a bipolaron stabilized by a counterpolaron on an adjacent molecule) formation in the electron transport layer, and may contribute to EL spikes observed at the end of a bias pulse. The significance of these holes to overall OLED performance is discussed.
Typical guest-host small molecular OLEDs (SMOLEDs) exhibit an emission spike at 100 - 200 ns and a tail that
extends over several μs following a bias pulse. The spike and tail are attributed to recombination of correlated
charge pairs and detrapped charges (mostly from the host shallow states), respectively. They may also be associated
with other OLED layers and other phenomena, e.g., triplet-triplet annihilation. The implications of the spike and tail
for OLED-based, photoluminescent oxygen sensors operated in the time domain are evaluated and compared to the
behavior observed when using undoped OLEDs or inorganic LEDs as the excitation sources.
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)-1,1'-
biphenyl (DPVBi), and some guest-host small molecular OLEDs, fabricated on 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, there is no observable
threshold current or bias associated with this spectral narrowing. 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, but it is not
clear if such a misalignment can account for the for the observed evolution of the edge emission spectra with l.
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