Inverted organic light-emitting diodes (IOLEDs) have drawn considerable attention for use in active-matrix OLED (AMOLED) displays because of their easy integration with n-channel metal-oxide-based thin film transistors (TFTs). The most crucial issue for IOLEDs is the poor electron injection caused by the bottom cathode. According to previous reports, the turn-on voltages of FIrpic-based IOLEDs are within a range from 4 to 8 V. In this study, we focus on developing bottom-emission IOLEDs with low operating voltages through the use of adequate-charge injection materials. We successfully demonstrate a turn-on voltage as low as 3.7 V for blue phosphorescent IOLEDs. The effective electron injection layers (EIL) were constructed by combining an ultrathin aluminum layer, an alkali metal oxide layer and an organic layer doped with alkali metal oxide, allowing for the effective adjustment of the carrier balance in IOLEDs. The peak efficiencies of the IOLEDs reached 15.6%, 31.8 cd/A and 23.4 lm/W. An external nanocomposite scattering layer was used to further improve light extraction efficiency. The IOLEDs equipped with the SiO2 nanocomposite scattering layer respectively provided performance improvements of 1.3 and 1.5 times that of pristine blue phosphorescent IOLEDs at practical luminance levels of 100 cd/m2 and 1000 cd/m2. Through sophisticated EIL and external light-extraction structures, we obtained blue phosphorescent IOLEDs with satisfactory efficiency and low operation voltages, thereby demonstrating the great potential of nanocomposite film for application in IOLEDs.
For the lighting purpose, white organic light-emitting devices (OLEDs) need to be operated at a high current density
to ensure an ample flux, which will lead the limited lifespan of the device. This situation could be improved through
diversified light-extraction methods. In this study, transparent photoresist mixed with titanium oxide (TiO2)
nanoparticles of different sizes could be utilized to form an internal extraction structures between the indium-tin-oxide
and glass substrate and thereby the out-coupling efficiency of white OLEDs could be significantly improved by this
sophisticated device architecture engineering. The high refractive index of TiO2 is essentially operative for increasing the
refractive index of nanocomposite film and thus diminishing the total internal reflection between the interfaces. In
addition, the nanoparticles served scattering function to multiply the ratio of the substrate and radiation modes. By
employing nanocomposite substrate with mixed dual-sized nanoparticles, we obtained external quantum efficiencies of
the white phosphorescent OLEDs that were about 1.6 times higher than that of the control device at the high
luminescence of 104 cd/m2.
We have developed an effective approach based on wavelength-selective mirrors to implement three-peak WOLEDs that
have EL spectra matching better with transmission spectra of typical color filters and thus give much enhanced color
gamut for full-color OLED display applications. The wavelength-selective mirror used here is highly compatible with
OLED fabrication.
Optical characteristics of microcavity organic light-emitting devices (OLEDs) having two mirrors are examined.
Analyses show that a high-reflection back mirror and a low-loss high-reflection exit mirror are essential for such
microcavity devices to obtain luminance enhancement relative to conventional noncavity devices. The capping layer in
the composite mirror plays the role of enhancing reflection and reducing absorption loss, rather than enhancing
transmission. In addition, by setting the normal-direction resonant wavelength around the peak wavelength of the
intrinsic emission, one obtains the highest luminance enhancement along the normal direction and hardly detectable
color shift with viewing angles, yet accompanied by highly directed emission and lower external quantum efficiency. On
the other hand, the highest enhancement in external quantum efficiencies and the most uniform brightness distribution
are obtained by setting the normal-direction resonant wavelength 20-40 nm longer than the peak wavelength of the
intrinsic emission, yet with noticeable color shift over viewing angles. Due to the tradeoffs between different emission
characteristics in choosing the resonant wavelength, the exact design of microcavity devices would depend on actual applications.
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