Optimization of charge injection in the active emitting layer and balanced transport of carriers are important in realizing
high efficiency and good reliability in organic light emitting devices (OLEDs). Electrical doping of such molecular
materials with a view to enhancing their conductivity is an attractive route for enhancing the performance and versatility
of these optoelectronic devices, in particular by enhancing carrier injection and lowering operating voltages. In the
present study, we demonstrate efficient n-type doping of tris-(8-hydroxyquinoline) aluminum (Alq3) with the inorganic
insulator lithium fluoride (LiF) by co-evaporation. The effect of dopant concentration on charge injection and carrier
transport in this system is studied. We demonstrate that optimal doping not only leads to enhanced device currents and
lower operating voltages, but also changes the charge transport from trap-limited to space-charge-limited transport.
Using this scheme, we achieve efficient electron injection without using low work function cathodes. Finally, we employ
the optimally-doped electron transport layers in OLED architectures to demonstrate devices with enhanced efficiency
and lowered operating voltages.
In this paper, we demonstrate that the light extraction efficiency of an OLED is a strong function of the location of the
recombination zone and the ratio of the extracted mode to the substrate guided mode varies from 22% to 55%. The large
variation of the extraction efficiency in most OLEDs is the direct result of optical cavity effect present in the devices. In
addition, we show that the light intensity profile varies from a Lambertian shape to a non-Lambertian shape depending
of the device geometry.
It has been demonstrated that Air Products(R) HIL (hole injection layer) material based on the
conducting polymer polythienothiophene (PTT) and poly(perfluoroethylene-perfluoroethersulfonic
acid) (PFFSA) dramatically improves the lifetime of polymer light emitting diodes. Compared with
other conductive polymer HILs, PTT based HILs have some unique properties. The resistivity of
PTT:PFFSA films is sensitive to the annealing temperature. The resistivity dependence on annealing
temperature is not favorable for certain applications (e.g., in passive matrix display applications,
where too low a resistivity after annealing can lead to cross-talking), or from the point view of
process control. We have found that raising the pH of PTT:PFFSA dispersions can suppress the
resistivity sensitivity to annealing conditions. At the same time, raising the pH of PTT:PFFSA
dispersions also lowers the work function of PTT:PFFSA films. When LumationTM Green 1304 light
emitting polymer is used as the emitting layer, all PTT:PFFSA based devices showed lifetime that is
several times longer than that of PTT:PSSA based devices. Among the PTT:PFFSA dispersions, pH
adjusted ones show a lower leakage current, lower efficiency and shorter device lifetime compared
with the original dispersion. We have also explored the application of PTT:PFFSA in small molecule
devices. Longer device lifetime has been obtained in devices using PTT:PFFSA as HIL and
aluminum tris(8-hydroxyquinoline) (Alq3) as emitter compared with devices using copper
phthalocyanine (CuPc) as HIL. We have also found that hole injection from PTT:PFFSA into hole
transport materials commonly used in small molecule devices is very efficient.
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