Light Outcoupling Layer Research Articles

Enhancing the Light Output‐Coupling of Inverted Top‐Emitting Organic Light‐Emitting Diodes by Using the Localized Surface Plasmon Resonance of Ag Nanoparticles

AbstractIn this study, a light out‐coupling layer (LOCL) composed of Ag nanoparticles (NPs) and MoOx is introduced on top of the inverted top‐emitting organic light‐emitting diode (OLED). The full width at half maximum (FWHM) is narrowed to 50 nm due to the micro‐cavity effect of the top‐emitting structure and the turn‐on voltage is as low as 2.6 V. The AgNPs in the LOCL significantly improves the light output‐coupling of the OLED owing to the localized surface plasmon resonance, and the MoOx in the LOCL effectively protects the water‐ and oxygen‐sensitive electron injection material. As a result, the performance of the OLED with the LOCL is significantly enhanced. The maximum current efficiency and external quantum efficiency of the device are increased by 53% and 30% to 28.27 cd A−1 and 13.09%. The enhanced electroluminescent performance of the OLED, together with the narrowed FWHM and the low turn‐on voltage, provides a new way to realize ultra‐high‐definition display and micro‐display.

High-tech functional polymers designed for applications in organic electronics

Several examples of polymeric materials specially developed for optoelectronic applications are highlighted demonstrating that by fine-tuning chemistry and architecture in highly aromatic polymers significant progress can be made in their performance. Thus, polymeric semiconductors of controlled architectures and high charge mobility had been prepared. The use of branching allowed to realize highly aromatic polymers of high solubility suitable for solution processing as dielectric materials in OFETs and as light-outcoupling layer of high refractive index in OLEDs.

Patternless light outcoupling enhancement method for top-emission organic light-emitting diodes

An increase of 65% in the luminous flux of a top-emission organic light-emitting diode (TE-OLED) was obtained by fabricating a stacked N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (0.2 µm)/CaF2 (2.5 µm) light outcoupling layer on the TE-OLED. The high-refractive-index NPB layer extracted the trapped light energy in the TE-OLED for input into the light outcoupling layer and protected the top cathode of the TE-OLED from damage due to the CaF2 layer. The surface morphology of the CaF2 layer had an irregular shape consisting of randomly dispersed pyramids; the irregular structure scattered the waveguide mode energy into air. By combining the effects of the NPB and CaF2 layers, the external quantum efficiency of the TE-OLED was increased significantly. The light outcoupling layer can be fabricated using a thermal evaporation process without patterning and, hence, provides a practical solution for the enhancement of TE-OLED light outcoupling using a patternless fabrication process.

Metal-anode-dependent spectra and efficiency in blue top-emitting organic light-emitting devices

In this study, the blue top-emitting organic light-emitting devices (TEOLEDs) with different metal anodes are fabricated. The effect of different anode materials on the spectra and efficiency of blue TEOLEDs is studied. We demonstrate that Al is a more suitable anode material for blue TEOLEDs due to its larger phase shift on reflectance (PSR) than the other common metal materials, such as Ag and Au. The influence of light outcoupling layer (LOL) on the transmittance and PSR of cathode is also investigated to obtain the optimum condition for devices. Angle-independent electroluminescence (EL) spectra are obtained in blue TEOLEDs for each metal anode but the device with Al anode possesses higher efficiency and much thicker organic layers, which is beneficial to the lifetime of the device. These results offer a practicable platform for the realization of TEOLEDs based full-color displays and lightings.

Top-emitting white organic light-emitting diodes based on a ZnS light outcoupling layer

Top-emitting white organic light-emitting diode (TEWOLED) has potential applications in lighting and full color displays. Microcavity effect in TEWOLED restrains the realization of the white emission with excellent optical and electric performances. In this paper, a ZnS film with a high refractive index used as a light outcoupling layer is introduced into the metal cathode to enhance its transmittivity to a maximal value in the blue light wavelength region. In addition, transfer matrix theory is utilized to optimize the thicknesses of the cathode and the ZnS outcoupling layer and the wide-angle interference is used to design the position of the blue emission layer inside the organic light-emitting diode. Based on the above work, the white light with relatively high luminous efficiency, good color purity, and small CIE coordinate change is acquired. The corresponding luminance and current efficicency are 9213 cd/m2 and 3 cd/A, respectively. The CIE coordinates belong to the white emission and are near the white light equal-energy point. The white emission also shows stable spectra with respect to the observation angle, with a limited CIE coordinate change of (0.02, 0) for a large observation angle change from 0 to 60.

Blue top-emitting organic light-emitting devices based on wide-angle interference enhancement and suppression of multiple-beam interference

Blue top-emitting organic light-emitting devices (TEOLEDs) based on blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C 2′] picolinate (FIrpic) are demonstrated. Instead of using microcavity effects to obtain the blue top emission in conventional reports, the blue emission with a good chromaticity is achieved in this paper through the suppression of the multiple-beam interference by introducing a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline light outcoupling layer onto the semitransparent Sm/Ag cathode. The electroluminescent performances of the blue emission is further improved based on the effective carrier transport and the consideration of the wide-angle interference at the emission peak of FIrpic with a maximum luminance of 8029 cd/m 2 and a luminous efficiency of 4.02 cd/A achieved at 14 V and 10 V, respectively. With a polarizer on the top cathode, a high pixel contrast ratio of 113:1 is realized under an ambient illumination of 140 lx and a pixel brightness of 1000 cd/m 2.

Nanopatterned aluminum nitride template for high efficiency light-emitting diodes

Nanopatterned aluminum nitride (NP-AlN) templates were used to enhance the light extraction efficiency of the light-emitting diodes (LEDs). Here, the NP-AlN interlayer between the sapphire substrate and GaN-based LED was used as an effective light outcoupling layer at the direction of bottom side and as a buffer layer for growth of GaN LEDs. The cross-sectional transmission electron microscopy (TEM) analysis showed that the formation of stacking faults and voids could help reduce the threading dislocations. Micro Raman spectra also revealed that the GaN-based epilayer grown on the NP-AlN template had smaller residual stress than that grown on a planar sapphire substrate. The normalized electroluminescence (EL) spectra at the top and bottom sides of device revealed that the enhancement of the bottom side emission of the LED with the NP-AlN interlayer was more notable than a planar sapphire substrate due to the graded-refractive-index (GRIN) effect of the NP-AlN.

Blue top-emitting organic light-emitting devices using a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline outcoupling layer

We fabricate a blue top-emitting organic light-emitting device (TEOLED) based on 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl. Different from the conventional TEOLEDs that use ITO as a thickness adjustment layer to make the cavity length matchable with the resonant wavelength of the blue light, we construct the blue emission TEOLED through suppressing multiple-beam interference and utilizing wide-angle interference to enhance the blue emission. With a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline light outcoupling layer to reduce the reflectivity of the Sm/Ag cathode, the blue emission can be obtained and its chromaticity is comparative to that from a bottom-emitting OLED. In addition, the blue emission shows stable spectra when the viewing angle alters from 0° to 75°.

Improvement of viewing angle and pixel contrast ratio in green top-emitting organic light-emitting devices

Green top-emitting organic light-emitting devices based on tris-(8-hydroxyl-quinoline) aluminum are fabricated with a low reflective Sm/Ag cathode and a comparative Al/Ag cathode. An additional 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP) layer, used as a light outcoupling layer, is deposited onto the SmAg cathode to improve device performances. The influence of different cathodes and the BCP layer on electrical and optical characteristics are investigated and discussed. It is worth mentioning that the introduction of the BCP layer not only enhances the brightness and the luminous efficiency but also improves the viewing angle and the pixel contrast ratio.

Effect of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline outcoupling layer on electroluminescent performances in top-emitting organic light-emitting devices

The organic compound, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is used as a light outcoupling layer in top-emitting organic light-emitting devices with tris(8-hydroxyquinoline) aluminum emission layer. It is found that, in addition to the improvement of electroluminescent (EL) intensity and luminous efficiency with a factor of 1.6, the BCP capping layer causes a blueshift of EL spectra and a larger full width at half maximum (FWHM). A larger FWHM at the BCP capping layer of 50nm is attributed to the reduction of cathode reflectivity. In the meantime, the blueshift of EL spectra is caused by the increase of the reflective phase shift of the cathode when a thicker BCP layer is deposited. Moreover, our calculation demonstrates that the best EL coincides with a maximum magnification coefficient.