High light extraction performance using evanescent waves for top emission OLED applications with thin film encapsulation.

We report high light extraction from the top emission OLED (TEOLED) device structure by improving mainly the waveguide mode loss in the atomic layer deposition processed thin film encapsulation (TFE) layer. A novel structure incorporating the light extraction concept using evanescent waves and the hermetic encapsulation of a TEOLED device is presented here. When the TEOLED device is fabricated using the TFE layer, a substantial amount of generated light is trapped inside the device due to the difference in refractive index (RI) between the capping layer (CPL) and the aluminum oxide (Al2O3) layer. By inserting a low RI layer at the interface between the CPL and Al2O3, the direction of the internal reflected light is changed by the evanescent waves. The high light extraction with the low RI layer is attributed to the presence of evanescent waves and an electric field in the low RI layer. The novel fabricated TFE structure, CPL/ low RI layer/ Al2O3/ polymer/ Al2O3, is reported here. The current efficiency of the fabricated blue TEOLED device using this low RI layer is improved by about 23% and the blue index value is enhanced by about 26%. This new approach for light extraction will be applicable to future encapsulation technology for flexible optoelectronic devices.

An accurate measurement of the dipole orientation in various organic semiconductor films using photoluminescence exciton decay analysis.

In this study, we report an accurate and more reliable approach to estimate the dipole orientation of emitters especially phosphorescence, fluorescence and even thermally activated delayed fluorescence. The dipole orientation measurements are performed by examining the variation of the photoluminescence (PL) exciton decay rate from time-resolved PL and optical analysis. Our anisotropic dipole orientation results are consistent with those of previous reports. The studied measurement approach is very reliable and accurate to estimate the dipole orientation of any organic semiconductor materials regardless of whether they are doped or neat films.

Next generation smart window display using transparent organic display and light blocking screen.

Transparent organic light emitting diodes (TOLED) have widespread applications in the next-generation display devices particularly in the large size transparent window and interactive displays. Herein, we report high performance and stable attractive smart window displays using facile process. Advanced smart window display is realized by integrating the high performance light blocking screen and highly transparent white OLED panel. The full smart window display reveals a maximum transmittance as high as 64.2% at the wavelength of 600 nm and extremely good along with tunable ambient contrast ratio (171.94:1) compared to that of normal TOLED (4.54:1). Furthermore, the performance decisive light blocking screen has demonstrated an excellent optical and electrical characteristics such as i) high transmittance (85.56% at 562nm) at light-penetrating state, ii) superior absorbance (2.30 at 562nm) in light interrupting mode, iii) high optical contrast (85.50 at 562 nm), iv) high optical stability for more than 25,000 cycle of driving, v) fast switching time of 1.9 sec, and vi) low driving voltage of 1.7 V. The experimental results of smart window display are also validated using optical simulation. The proposed smart window display technology allows us to adjust the intensity of daylight entering the system quickly and conveniently.

Efficient micro-cavity top emission OLED with optimized Mg:Ag ratio cathode.

Micro-cavity top-emitting organic light emitting diodes (TEOLEDs) are now receiving prominence as a technology for the active matrix display applications. The semi-transparent metal cathode plays the crucial role in realizing TEOLEDs structure. Here, we report the optimization results on Mg:Ag ratio as the semitransparent cathode deposited by vacuum thermal evaporation. The optimized Mg:Ag cathode with 1:10 ratio (wt %) shows a sheet resistance value as low as 5.2 Ω/□, an average transmittance of 49.7%, reflectance of 41.4%, and absorbance of 8.9% over the visible spectral region (400~700 nm). The fabricated red TEOLEDs device implemented using LiF (1nm)/Mg:Ag (1:10) cathode shows the voltage value of 4.17 V at a current density of 10.00 mA/cm2, and current efficiencies variation from 55.3 to 50.1 cd/A over the brightness range 2,000 - 12,000 cd/m2. The electroluminescence (EL) spectrum displays the light emission at 608 nm wavelength with a half width of 29.5 nm. The narrow half-width of red light emission is attributed to the micro-cavity effects due to the semitransparent cathode.

Hydrophobic properties of polytetrafluoroethylene thin films fabricated at various catalyzer temperatures through catalytic chemical vapor deposition using a tungsten catalyzer.

Using the catalytic chemical vapor deposition (Cat-CVD) method, polytetrafluoroethylene (PTFE) thin films were fabricated on Si(100) substrates at various catalyzer temperatures, using a tungsten catalyzer, and Fourier transform infrared (FTIR) spectroscopy and X-ray photoemission spectroscopy (XPS) were used to confirm the fabrication of the films. An atomic-force microscope (AFM) and a scanning electron microscope (SEM) were employed to study the correlation between the wettability and surface morphology of the samples. It was found that the wettability of the PTFE thin films fabricated via Cat-CVD is strongly correlated with the sizes of the film surfaces' nanoprotrusions, and that superhydrophobic PTFE thin-film surfaces can be easily achieved by controlling the sizes of the nanoprotrusions through the catalyzer temperature. The comparison of the wettability values and surface morphologies of the films confirmed that nanoscale surface roughness enhances the hydrophobic properties of PTFE thin films. Further, the detailed analysis of the films' surface morphologies from their AFM images with the use of the Wenzel and Cassie models confirmed that the nanoscale surface roughness enhanced the hydrophobic property of the PTFE films. Further, the variations of the wettability of the PTFE thin films prepared via Cat-CVD are well explained by the Cassie model. It seems that the increase in the trapping air and the reduction of the liquid-solid contact area are responsible for the superhydrophobicity of the PTFE thin films prepared via Cat-CVD.

Study of nanoscale photopolymerized fullerene clusters in solution droplets using an ultrasonic nebulizer unit.

In a designed and developed ultrasonic nebulizer system for obtaining macroscopic-quantity photopolymerized fullerene (C60) clusters, a C60 solution was vaporized to several micro-sized droplets in vacuum, resulting in the formation of C60 aggregates by evaporating the solvent (toluene). The system was invented to produce nanoscale photopolymerized carbon clusters through the irradiation of ultraviolet (UV) light on the C60 aggregates in vacuum. The products, photopolymerized C60 clusters obtained from the system using UV-visible (UV-Vis) absorption and high-performance (or high-pressure) liquid chromatography (HPLC) spectra, were characterized. Compared with the non-irradiating C60 solution, the UV-Vis absorption spectrum of the irradiated C60 solution was drastically decreased, especially at lambda = 335 nm and in the visible region from lambda = 450-650 nm. As such, the UV-Vis absorption spectra provide information about the polymerization of C60 molecules. These photopolymerized C60 clusters can be detected as having a heavy molecular mass order through the HPLC system, and the C60 and photopolymerized C60 cluster can be extracted from the trapped solution on the molecular mass. Although there is a possibility that the products include various forms of C60 clusters, the results suggest that the products obtained from the system using a vaporizer establish a new method of obtaining macroscopic-quantity C60 clusters.

Degradation of OLED performance by exposure to UV irradiation.

Organic light-emitting diode (OLED) displays are highly susceptible to the harsh environmental conditions found outdoors, like exposure to direct sunlight as well as UV radiation and storage temperature, resulting in a loss of luminance and lifespan, pixel shrinkage, and permanent damage and/or malfunction of the panel. Here, we fabricated top emission OLEDs (TEOLEDs) using Yb : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) and Mg : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) cathode units and the performances of the devices were investigated by subjecting them to UV radiation. A fabricated red TEOLED (control device), employing a standard Mg : LiF (1 : 1, 2 nm) electron injection layer (EIL) and an Ag : Mg (16 nm) cathode, showed a rapid decrease in luminance and a fast increase in driving voltage at 10 mA cm-2 over time after UV irradiation for 300 h. However, a cathode unit comprising a Yb : LiF (1 : 1, 2 nm) EIL and an Ag : Mg (10 : 1, 16 nm) cathode showed no loss of luminance or increase in driving voltage at 10 mA cm-2 over time after UV irradiation for 300 h. Therefore, we investigated the changes occurring in both cathode units due to UV irradiation using the lift-out FIB-TEM technique and EDS mapping. With UV irradiation for 300 h, Ag atoms migrated toward the center of the cathode, Mg atoms migrated toward the CPL, and no Mg atoms were observed in the EIL area. In contrast, we observed (i) no substantial migration of Ag atoms and they were located at the center of the cathode, (ii) no migration of Mg atoms toward the CPL layer, and (iii) no movement of Yb atoms after UV irradiation. Furthermore, the UV irradiated red TEOLED with an Mg : LiF (1 : 1, 2 nm) EIL showed (i) deterioration in electron injection into the emissive layer (EML) and an increase in the EIL/metal interface resistance, and (ii) a remarkable shift of the J-V curve to the higher voltage side, while almost no such changes were observed in the TEOLD with a Yb : LiF (1 : 1, 2 nm) EIL. Also, an almost identical RGB pixel emitting area was noticed in the Yb : LiF (1 : 1, 2 nm) based devices after UV irradiation for 300 h. These results suggest that Yb could become a good candidate for the cathode unit, providing better device stability against harsh environmental conditions as well as excellent electron injection properties.

Electrical characterization of N- and P-doped hole and electron only organic devices.

We have fabricated a series of hole only devices with tungsten oxide (WO3) and molybdenum oxide (MoO3) n-doping materials in N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB) hole transport layer, and electron only devices with CsF and Cs2CO3 p-doping materials in 4,7-diphenyl-1,10-phenanthroline (Bphen) electron transport layer. Current-voltage characteristics and conductivity of these devices are investigated. The optimal conditions for ohmic injection and low resistance properties, and process margins of each dopant are reported in this paper.