Kuen-Ru Chuang

White-light emission from electroluminescence diode with polyaniline as the emitting layer

Abstract A light-emitting diode (LED) with emeraldine base polyaniline (PAn) as the emitting layer, indium—tin oxide coated glass plate as the hole injector and deposited aluminum (or magnesium) thin film as the electron injector was fabricated, which can emit nearly white light covering the full range of visible light (380–750 nm). It is found that the white light is emitted from the phase with reduced repeat units (amine form), while the phase with oxidized repeat units (quinoid form) has no contribution to the emitting light. The turn-on voltage for eye-observable light intensity is 13 and 6 V for the LEDs with aluminum and magnesium electrodes, respectively, at a thickness of the emitting layer of 800 A. The range of the electroluminescence (EL) spectrum covering the range 300–800 nm is much broader than that of the photoluminescence (PL) spectrum, 350–510 nm.

Failure phenomena and mechanisms of polymeric light‐emitting diodes: Indium–tin–oxide damage

Indium–tin–oxide (ITO) coated on a glass plate as the transparent electrode in polymeric light‐emitting diodes (LEDs) generates some volcano‐like patterns during use especially at a higher applied electric field strength. Such an ITO damage phenomenon is independent of Joule heat, conjugation structure of the polymer, and light‐emission process, but only dependent on the applied electric field strength. The ITO damage, which results from a self‐decomposition reaction, can cause a reduction of lifetime of the device.

Temperature effect on the electronic spectra of poly(p-phenylenevinylene)

Abstract The experimental temperature-dependent absorption, phololuminescence and photoluminescence excitation spectra of poly ( p -phenylenevinylene) (PPV) are presented and a molecular theory is used to analyze the results. In order to model the complicated system such as PPV, the structural inhomogeneity has to be included. The structural inhomogeneity is represented by the different Huang-Rhys factors associated with different chain lengths. The theoretical spectra agree qualitatively with the experimental results.

Experimental and theoretical studies of absorption and photoluminescence excitation spectra of poly(p-phenylene vinylene)

In this paper, we present the experimental absorption spectra and photoluminescence excitation spectra of poly(p-phenylene vinylene) (PPV). We also present the theoretically calculated absorption and photoluminescence spectra of PPV. The singlet-to-singlet transition energies and moments calculated from the intermediate neglect of differential overlap with the spectroscopic parametrization (INDO/S) semi-empirical method were used to analyze the absorption spectra. The symmetrical vibrational modes calculated from the abinitio method were used to obtain the vibronic features in the spectra of PPV. It is demonstrated that the inhomogeneity observed in the absorption spectra of PPV can be modeled by summing up the absorption of PPV oligomers with different chain lengths. The difference between absorption and photoluminescence excitation spectra is explained.

The inhomogeneity in conjugated polymer blend films

Conjugated polymer blend films are studied by time-resolved fluorescence and near-field scanning optical microscopy. Phase separations were directly observed by NSOM. The domains are found to be composed of different fractions of the two constituent polymers. The composition of the domains and the domain structure alter as a function of blend ratio. Surface roughness and optical contrast exhibit similar trend at the change of blend ratio. At 4 : 1 ratio, both have higher contrasts. For low PdOPV ratio, the two polymers are mixed homogeneously. As the concentration of PdOPV increases, condensed phase of PdOPV is formed.

Effects of displacement and distortion of potential surfaces on absorption and emission spectra of MPV oligomer in solutions

We report here the vibrational modes for MPV oligomer in solvents of toluene as well as chloroform, obtained by studying the photo-luminescence and absorption spectra. In analyzing the absorption and emission spectra, it is found that both displacements and distortions of vibrational modes are important in fitting the observed spectra.

Nanometer scale mixing homogeneity in light emitting polymer blend thin films

Phase separation in polymer blend thin films is directly observed by near-field scanning optical microscopy. Time-resolved fluorescence spectra indicate that the phase domains are composed of different fractions of the two constituent polymers. The mixing homogeneity was found to vary with the blending ratio.

Phase separation in polyaniline with near-field scanning optical microscopy

We report the studies of conjugated polymers, polyaniline thin films, with a near-field scanning optical microscope. Because of the absorption variation in different oxidation states, transmission-mode near-field scanning optical microscope images were employed to map out the distribution of the oxidation states on a submicrometer scale. When the near-field wavelength is varied (between 632.8 and 543.5 nm), the phase separation between the oxidized and the reduced repeated units, with domain sizes on a nanometer-length scale, is observed.

Excitonic Energy Transfer in Light Emitting Polymers

A model including the migration and trapping of excitons on linear chains was proposed to understand the fluorescence dynamics of light emitting polymers. Analytic solution of the model was worked out. To verify the usefulness of the model, time-resolved PL experiments were performed on a series of materials. The characteristics in PL decay dynamics of different materials can be understood with the model qualitatively.

Aggregated luminescence from light-emitting polymer in dilute solution

The photo-physical properties of a PPV derivative. Poly(dioctyloxy phenylene vinylene) (PdOPV) in various states good solvent, poor solvent and thin films were investigated. The solubility of PdOPV in toluene exhibits dramatic variation at different temperature. Below room temperature, the solubility is decreased, and polymer forms aggregates, even in dilute phase. This decreasing of solubility also introduces a quenching effect. From compared the absorption and fluorescence of the film and the poor solution, it was concluded that the films are comprised of individual polymer and interchain aggregates, but the fluorescence only exhibits the behavior of aggregates. In addition, at low temperature, the fluorescence dynamics of the films exhibits a wavelength dependent behavior, which excludes the vibronic origin for the feature of fluorescence. In the films, the aggregates suffer additional quenching, which reduces the fluorescence efficiency.