[Show abstract][Hide abstract] ABSTRACT: We examined the light diffusing effects of nano and microstructures
on microcavity designed OLEDs. The results of FDTD
simulations and experiments showed that the pillar shaped nano-structure
was more effective than the concave micro-structure for light diffusing of
microcavity OLEDs. The sharp luminance distribution of the microcavity
OLED was changed to near Lambertian luminance distribution by the nanostructure,
and light diffusing effects increased with the height of the nanostructure.
Furthermore, the nano-structure has advantages including light
extraction of the substrate mode, reproducibility of manufacturing process,
and minimizing pixel blur problems in an OLED display panel. The nanostructure
is a promising candidate for a light diffuser, resolving the viewing
angle problems in microcavity OLEDs.
[Show abstract][Hide abstract] ABSTRACT: A random scattering layer (RSL) consisting of a random nano-structure (RNS) and a high refractive index planarization layer (HRI PL) is suggested and demonstrated as an efficient internal light-extracting layer for transparent organic light emitting diodes (TOLEDs). By introducing the RSL, a remarkable enhancement of 40% and 46% in external quantum efficiency (EQE) and luminous efficacy (LE) was achieved without causing deterioration in the transmittance. Additionally, with the use of the RSL, the viewing angle dependency of EL spectra was reduced to a marginal degree. The results were interpreted as the stronger influence of the scattering effect over the microcavity. The RSL can be applied widely in TOLEDs as an effective light-extracting layer for extracting the waveguide mode of confined light at the indium tin oxide (ITO)/OLED stack without introducing spectral changes in TOLEDs.
[Show abstract][Hide abstract] ABSTRACT: In this study, we investigated the effect of a random nanostructure scattering layer (RSL) on the microcavity and light extraction in organic light emitting diodes (OLEDs). In the case of the conventional OLED, the optical properties change with the thickness of the hole transporting layer (HTL) because of the presence of a microcavity. However, OLEDs equipped with the an RSL showed similar values of external quantum efficiency and luminous efficacy regardless of the HTL thickness. These phenomena can be understood by the scattering effect because of the RSL, which suppresses the microcavity effect and extracts the light confined in the device. Moreover, OLEDs with the RSL led to reduced spectrum and color changes with the viewing angle.
[Show abstract][Hide abstract] ABSTRACT: The light extraction efficiency of OLEDs with a nano-sized random scattering layer (RSL-OLEDs) was analyzed using the Finite Difference Time Domain (FDTD) method. In contrast to periodic diffraction patterns, the presence of an RSL suppresses the spectral shift with respect to the viewing angle. For FDTD simulation of RSL-OLEDs, a planar light source with a certain spatial and temporal coherence was incorporated, and the light extraction efficiency with respect to the fill factor of the RSL and the absorption coefficient of the material was investigated. The design results were compared to the experimental results of the RSL-OLEDs in order to confirm the usefulness of FDTD in predicting experimental results. According to our FDTD simulations, the light confined within the ITO-organic waveguide was quickly absorbed, and the absorption coefficients of ITO and RSL materials should be reduced in order to obtain significant improvement in the external quantum efficiency (EQE). When the extinction coefficient of ITO was 0.01, the EQE in the RSL-OLED was simulated to be enhanced by a factor of 1.8.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t In this study, we demonstrated a nano-structured random scattering layer (RSL) as an internal light extraction method to improve the light extraction efficiency of organic light-emitting diodes (OLEDs). Using dewetted Ag droplets as a hard mask, we textured the glass surface to have a scattering layer of the random structure. OLEDs equipped with the RSL showed more that 50% improvement in the external quantum efficiency (EQE) and luminance efficacy (LE) compared to OLEDs without the RSL. This improvement can be understood by the scattering effect which reduces the optical loss at wave-guided modes. Also, by combining the RSL and an external light extraction micro-lens array (MLA), it was possible to achieve further improvements of 105.8% and 92.06% in the EQE and the LE, respectively.
[Show abstract][Hide abstract] ABSTRACT: We have evaluated high refractive index organic–inorganic hybrid films fabricated from a solution prepared
from triethylene glycol and titanium(IV) butoxide in N,N-dimethyl acetamide (DMAc). The organic–inorganic
hybrid solution in DMAc is homogeneous and stable. The solution was spin-coated on a silicon wafer and a
glass plate and dried under nitrogen to give a transparent film. The refractive index (n), transparency, and
thickness of the hybrid films fabricated from the solution varied as the film drying conditions and feeding
mole ratios between the organic and inorganic compounds were varied. The film drying condition was
observed to have a strong influence on the surface evenness of the films. The refractive indices of the
films were between 1.62 and 2.16 at a wavelength of ca. 600 nm. Scanning electron microscope (SEM)
and atomic force microscope (AFM) results showed that the films were very smooth and that the rootmean-
square values (Ra) of the surface roughness of the films were between 0.25 and 2.37 nm
depending on the film drying conditions. Values for n and Ra of the hybrid film were 2.06 and 0.25 nm,
respectively, when it was annealed at 250 �C. The film transparency and thickness increased with
decreasing the annealing temperature. We identify our organic–inorganic hybrid solutions as possible
good candidate materials for films with high refractive index and surface evenness.
Journal of Materials Chemistry C. 01/2014; 2:4468.
[Show abstract][Hide abstract] ABSTRACT: We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron transport layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications. Particular attention is paid to the surface morphology on the Ag film and its influence on the optical transparency and electrical conductivity. With the use of Cs-ETL, a smooth and continuous surface morphology of Ag film was achieved, leading to a high transmittance of ∼75% in TOLED with a low sheet resistance of 4.5 Ω/Sq in cathode film. We successfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparent OLEDs. Our approach suggests a new electrode structure for transparent OLED applications.
[Show abstract][Hide abstract] ABSTRACT: In this work, we develop a simulation method to predict a two-dimensional luminance distribution method using a circuitry simulation. Based on the simulation results, we successfully fabricate large area () transparent organic light-emitting diode panels with high luminance uniformity.
[Show abstract][Hide abstract] ABSTRACT: A linear stability model was formulated to analyze the perturbation of solid polymer surfaces. Surface energy and thermal stress were considered as the main variables. The surface tends to more unstable as the temperature increase. This is interpreted as the dominancy of the lattice vacancy diffusion over surface mass diffusion and the increase in thermal stress.
Materials Research Bulletin 10/2012; 47(10):2788–2791. · 1.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: As an innovative anode for organic light emitting devices (OLEDs), we have investigated graphene films. Graphene has importance due to its huge potential in flexible OLED applications. In this work, graphene films have been catalytically grown and transferred to the glass substrate for OLED fabrications. We have successfully fabricated 2 mm × 2 mm device area blue fluorescent OLEDs with graphene anodes which showed 2.1% of external quantum efficiency at 1000 cd/m2. This is the highest value reported among fluorescent OLEDs using graphene anodes. Oxygen plasma treatment on graphene has been found to improve hole injections in low voltage regime, which has been interpreted as oxygen plasma induced work function modification. However, plasma treatment also increases the sheet resistance of graphene, limiting the maximum luminance. In summary, our works demonstrate the practical possibility of graphene as an anode material for OLEDs and suggest a processing route which can be applied to various graphene related devices.
Materials Research Bulletin 10/2012; 47(10):2796–2799. · 1.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to improve the external quantum efficiency of organic
light-emitting diodes (OLEDs), we purpose an anode which has a structure
of indium tin oxide (ITO)/Ag/ITO, in which the Ag layer has a function
of micro-cavity inducing reflector. In order to maximize the microcavity
effect the thicknesses of the hole transport layers of blue and red
phosphorescent OLEDs (PhOLEDs) were deduced using simulations. By the
use of our optically designed anode, it was possible to achieve
approximately 50% improvements in the external quantum efficiency.
Japanese Journal of Applied Physics 09/2012; 51(9). · 1.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, we demonstrate fully uniform blue fluorescence graphene
anode OLEDs, which have an emission area of 10×7 mm2.
Catalytically grown multilayered graphene films have been used as the
anode material. In order to compensate the current drop, which is due to
the graphene's electrical resistance, we have furnished metal bus lines
on the support. Processing and optical issues involved in graphene anode
OLED fabrications are presented. The fabricated OLEDs with graphene
anode showed comparable performances to that of ITO anode OLEDs. Our
works shows that metal bus furnished graphene anode can be extended into
large area OLED lighting applications in which flexibility and
transparency is required.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t We report on white transparent organic light emitting diode (TOLED) with double layered capping layer (CL), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and ZnSe. By introducing the double layered CL, total efficiency was improved by 27% to have 90% of that of conventional bottom-emissive device. The achievement of highly improved efficiency was interpreted as the synergetic effect of constructive interference and maximized reflec-tance of CL. By adjusting the reflectance of the CL, it was possible to choose the emissions direction selectively and achieve spectral matching between top and bottom emissions as well as enhanced total efficiency in white TOLED. Ó 2012 Elsevier B.V. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: We have manufactured highly efficient OLED devices fabricated on chemically etched glass substrates. The external quantum efficiency of the OLED devices with the etched glass substrates was increased by 5-27% in comparison with the reference flat glass substrate. Surface morphology, such as indented patterns, significantly affected the external luminance efficiency. A clean surface and the presence of smooth bent edges of indented patterns were found to be important for improving the external luminous efficacy.
Journal of Nanoscience and Nanotechnology 04/2012; 12(4):3447-50. · 1.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, we report on blue organic light emitting devices (OLEDs), which have multilayered graphene as its anode. Our graphene films have been grown catalytically and transferred to the support. The fabricated blue OLEDs with graphene anode showed outstanding external quantum efficiency of 15.6% and power efficiency of 24.1 lm/W at 1000 cd/m2. Weak oxygen plasma treatments on graphene film surfaces improved the injection property between the anode and hole injection layer.
[Show abstract][Hide abstract] ABSTRACT: In transparent organic light-emitting diodes (TOLEDs), the asymmetry in the optical paths causes difference between the bottom and top emitting lights, both in emissions and spectral distributions. Capping layers (CLs) can be used as an optical functional to enhance the emissions and adjust the spectral distributions. Here, we report on the optical effects of an organic CL, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), on the characteristics of TOLEDs Both simulated and experimental results are presented. We demonstrate the possibility of improving the total emission and achieving spectral matching of bottom and top emissions by varying the CL thickness. The optical effects of CLs have been interpreted from interference perspectives. Finally, we have presented a guideline that is practically useful in designing high-performance TOLEDs with CLs.
[Show abstract][Hide abstract] ABSTRACT: We have developed the highly efficient OLEDs using the embossed glass substrate and the low yellowing TiO2/Epoxy nano composite layer. The nano composite can be applied to a scattering layer and a planarization layer on an embossed glass. The TiO2/Epoxy nano composite was effective for increasing the light extraction efficiency of OLEDs, however the yellowing degraded the luminous efficacy seriously. The luminous efficacy of the OLED with the nano composite layer on the embossed glass was 21.0 lm/W at 1000 cd/m2 and it increased by 36% compared to the conventional OLED.
[Show abstract][Hide abstract] ABSTRACT: We fabricated blue fluorescent organic light-emitting diodes (OLEDs) by using a laser patterned ITO anode. An indium tin oxide (ITO) layer was defined directly with an infrared laser system (wavelength = 1064 nm), which was carried out at an optimum laser focal position, 1 W of laser power and 500 m/s of laser scanning speed. As a result, the problems with the laser patterning method, such as a shoulder at the ITO/glass edge, crack on the surface, and ITO debris were minimized. Moreover, we found that the OLED with laser patterned ITO showed almost similar electrical and optical properties to that of ITO that was patterned by photo-lithography.
Journal of Nanoelectronics and Optoelectronics 07/2010; 5(2):161-164. · 0.37 Impact Factor