Y. Kawabe

Highly efficient and bright organic electroluminescent devices with an aluminum cathode

The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a quinacridone doped organic layer. Devices with an Al/LiF cathode demonstrated a luminance in excess of 20 000 cd/m2 and an external quantum efficiency of 3%, which is comparable to devices with a Mg/LiF cathode. These devices show maximum luminance of 45 000 cd/m2 prior to failure in continuous bias operation. For the same LiF thickness, the operating voltage for devices with Al/LiF was lower than the corresponding operating voltage for devices with Mg/LiF or Mg alone. Tunneling theory is used to explain this enhancement.

Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode

We report a two-layer, blue organic light-emitting diode with a 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of 3000 cd/m2. Insertion of the thin LiF layer results in a 50-fold increase in the device efficiency compared to a device with an aluminum only cathode, and eliminates the need for an electron-transporting layer, such as tris(8-hydroxyquinoline)aluminum. This results in a device with excellent color purity with an emission peak at 476 nm and a full width at half maximum of 78 nm. Using ultraviolet photoelectron spectroscopy, we find that the effective work-function of aluminum decreases dramatically with sub-monolayer amounts of LiF deposited on the surface.

Whispering-gallery-mode microring laser using a conjugated polymer

We observed laser emission in whispering gallery modes using a microring composed of a semiconducting polymer poly[2,5-bis-(2′-ethylhexyloxy)-p-phenylenevinylene coated on an etched fiber under transient and quasisteady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 μJ/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasisteady-state pumping shows the possibility to develop electrically pumped polymer lasers.

A model for the current–voltage characteristics and the quantum efficiency of single-layer organic light emitting diodes

A model for the current–voltage characteristics and the quantum efficiency of single-layer organic light emitting diodes is presented. With variables such as the electric field, the hole, and electron carrier densities, a set of coupled nonlinear differential equations is derived by using classical electrostatics and by assuming Fowler–Nordheim injection. Numerical calculations for different carrier mobility and barrier height conditions show that low barrier height at both electrodes leads to higher efficiency and higher carrier mobility leads to higher brightness. We find that for applications that require high current injection such as lasers, materials with high mobility are desired to reduce space charge effects.

Comparison of the χ(3) values of crystalline and amorphous thin films of 4‐butoxy‐carbonyl‐methyl‐urethane polydiacetylene at 1.06 and 1.3 μm

The third‐order optical nonlinear susceptibilities of single‐crystalline and spin‐coated amorphous 4‐butoxy‐carbonyl‐methyl‐urethane polydiacetylene (red form) thin films were measured by third‐harmonic generation. The third‐harmonic intensities were calculated as a function of incident angle by solving the propagation equation of the electromagnetic field for a two‐layer system with one nonlinear media. From the best fit of the calculated and experimental data, the ( χ(3)) values of the spin‐coated amorphous film (χ(3)p) were found to be (9.6±1.0)×10−12 esu and (2.0±0.2)×10−11 esu at 1.064 and 1.319 μm, respectively, while the χ(3) values of the crystalline ( χ(3)pc) were (4.9±0.5)×10−11 esu and (1.0±0.07)×10−10 esu at 1.064 and 1.319 μm, respectively. A factor of five increase in the χ(3) of the crystalline film is attributed to the orientation effect of the polymer chains. The enhancement of the nonlinearity at 1.319 μm may be due to either three‐photon or two‐photon resonant effects.

Light amplification and laser emission in conjugated polymers

Seppo Honkanen, MEMBER SPIE Mahmoud Fallahi, MEMBER SPIE Bernard Kippelen Nasser Peyghambarian, MEMBER SPIE University of Arizona Optical Sciences Center Tucson, Arizona 85721 Abstract. We study optical gain and laser emission from semiconducting conjugated polymers after strong optical excitation with femtosecond laser pulses. Stimulated emission from few-hundred-nanometer thin films results in a sharp increase in emission intensity and emission line narrowing. Very large optical gain due to the stimulated emission is measured directly by pump-probe spectroscopy. Using the large light amplification in these conjugated polymers, we demonstrated planar and ring lasers of very small sizes that are suitable for applications in integrated optics. Vertical cavity surface emitting lasers, distributed feedback through surface relief grating, and whispering-gallery mode emission from ring resonators are demonstrated.

Novel techniques in fabricating more efficient and brighter organic electroluminescent devices

In this paper, we present two techniques for fabricating efficient and bright organic light emitting devices. The first technique allows for an enhancement in the electron injection process. This is accomplished through inserting a layer of LiF with appropriate thickness between the cathode and a quinacridone doped organic layer. Devices with an Al/LiF cathode demonstrated a luminance in excess of 20,000 cd/m2 and an external quantum efficiency of 3 percent, which is comparable to devices with a Mg/LiF cathode. These devices show maximum luminance of 45,000 cd/m2 prior to failure in continuous bias operation. In the second technique, partially ionized beam deposition was utilized in the fabrication process of organic electroluminescent devices. Preliminary results indicate that devices fabricated with this technique are more efficient and brighter than similar devices fabricated with the traditional thermal evaporation process.

Rhodamine-6G-doped fluorophosphate glasses

A high optical quality Rhodamine 6G doped low melting temperature fluorophosphate glass was obtained. The glass transition temperature, crystallization temperature, melting temperature, refractive index, and chemical durability in the water were examined. The absorption spectra, excitation spectra, and emission spectra of the Rhodamine 6G in the low melting temperature fluorophosphate glasses were investigated. The effect of melting time of Rhodamine 6G in the glass on the spectral properties was investigated. The results indicate that this low melting temperature glass is a promising host for laser dyes. The aggregation and degradation of Rhodamine 6G were significantly diminished in this low melting temperature glass.

Advanced Organic Materials for Optoelectronic Integrated Devices, Interconnects, and Packaging

Organic materials complement many commonly used inorganic materials in optical devices due to their improved linear and nonlinear optical properties. Research has progressed on several fronts, investigating the electrooptic, photorefractive, and light-emitting properties of organic materials, and their mechanical and chemical stability for commercial applications. We present some of our achievements which demonstrate the attractive performance of the organic materials, and show that practical optical devices using organic materials are closer to commercial development.

Semiconducting conjugated polymers: light amplification and lasing

We study the emission properties of various laser cavities under pulsed optical excitation of the active semiconducting conjugated polymer material. Physical origin, magnitude, and dynamics of optical gain in these novel active laser materials are discussed leading to a selection of suitable cavity configurations for laser applications. We demonstrate laser action for various planar and ring resonator configurations that can be achieved in the regimes of transient inversion and quasi stationary excitation of the polymer material pumping with femtosecond and nanosecond pulses, respectively.