Jui-Hung Hsu

Experimental and theoretical investigations of absorption and emission spectra of the light-emitting polymer MEH-PPV in solution

Abstract We report the absorption and photoluminescence spectra of the MEH-PPV polymer in a chloroform solvent. In the analysis, we assume that the polymer consists of short conjugated segments which are called oligomers with different lengths. The appearance of each chain length is controlled by a distribution function. Based on the conformational disorder caused by torsional motion, we found that the distribution function should take a Gaussian form with its center and width being adjustable parameters. While the energy level of the emissive S1 state of the oligomer and the corresponding oscillator strength are calculated from exciton theory. In addition, the associated vibrational motions are considered in determining the spectral shape in which both displacement and distortion of the potential surfaces are taken into account. With this information, the calculation result can fit the experiment reasonably well.

White-light electroluminescence from soluble oxadiazole-containing phenylene vinylene ether-linkage copolymer

We report a promising oxadiazole-containing phenylene vinylene ether-linkage copolymer, which can emit nearly white light from a single-layer light-emitting diode. The emission spectrum is composed of a red component originating from the new excited dimer in addition to the blue-green component from an individual lumophore and excimer. This excited dimer is formed under a strong electric field inside the diode and cannot be produced by photoexcitation, which is different from the excimer or exciplex that is often found both in photoluminescence and electroluminescence, and it is termed the “electromer.”

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.

Aggregated states of luminescent conjugated polymers in solutions

Abstract In order to study the electronic properties of the interchain state in light-emitting polymers, we have performed spectroscopic experiments for 2,5-dioctyloxy PPV (DOO-PPV) in both good (chloroform) and poor (2-methyl-tetrahydrofuran) solvents. The concentration-dependent absorption, photoluminescence (PL) spectra and dynamics of time-resolved PL can be explained by the coexistence of an intrachain and one kind of aggregated states in poor solution. The estimated lifetime and transition moment of the aggregated state lead to significant quantum efficiency of PL which is about 0.75 times that of the intrachain state. The PL spectrum for the aggregated state resembles that of a thin film, implying that the emissive species in thin film is more likely to be an aggregated state.

Programmable organic light-emitting devices

In this letter, we report a promising type of electrically programmable, i.e., reconfigurable, organic light-emitting devices (OLEDs) incorporating a thin carrier-blocking layer as the sacrificial fusing layer. In such devices, the carrier-blocking layer has a lower glass transition temperature than neighboring layers. By raising the internal temperature of the device above the transition temperature of the carrier-blocking layer with a large enough current, interdiffusion between organic layers could occur through such a layer. As a consequence, neighboring layers are fused and a new path for carrier transport is formed, bypassing the carrier-blocking property and altering the device characteristics. A device that emits blue light as fabricated but can be transformed into a green-emitting one is demonstrated. Such a type of device may be used for color pixels in OLED displays, user-programmable OLED applications, and nonvolatile memory devices.

Observation of multiple step de-excitation in luminescent single conjugated polymers

The fluorescence intensity fluctuation and polarization modulation of spin-cast single fluorescent conjugated polymers of 2,5-dioctyloxy p-phenylene vinylene (DOO-PPV) with length of the order of the persistence length have been studied. The results indicate that this type of polymer behaves like a collection of semi-independent excitons, with dipoles aligned predominately in the same direction. This suggests that this type of polymer can be considered as a linearly extended rigid structure (i.e., a straight rod). The fluorescent transient exhibits a multiple-step-like behavior with the count rate at each step being a multiple of a common step-size. The fluorescence variation follows the excitation polarization with a DC background observable in some molecules. This DC background indicates that the chromophores in some polymers are not aligned in the same direction. Variations in the Huang–Rhys factor at a single polymer level are also observed.

Thickness dependence of tunneling magneto-resistance effect in granular Fe-Al/sub 2/O/sub 3/ films

The magneto-transport properties of the granular Fe/sub 0.5/(Al/sub 2/O/sub 3/)/sub 0.5/ solids with various film thickness have been investigated. The temperature dependence of the resistivity shows that all samples follow a tunneling type equation /spl rho/(T)=/spl rho//sub 0/exp(T/sub 0//T)/sup 1//spl alpha// with /spl rho//spl ap/0.1/spl sim/1 /spl Omega/cm at 300 K and /spl alpha/=2. The results of magneto-resistance at 300 K indicate that the value is about 4.5% for thick films. It increases slowly with decreasing film thickness. At about 150 nm, a peak value of TMR is found, which is determined to be 5%. Then TMR drops sharply with further reducing film thickness. It becomes only 3% at t=20 nm. At low temperatures, the TMR effect becomes larger and shows similar thickness dependence. However, the values of T/sub 0/ that relate to the geometry of the tunneling barrier show behavior of opposite thickness dependence. A small T/sub 0/ is associated with a large TMR effect. In this report, the correlation of the film microstructure and the TMR effect are discussed in details.

Effect of initial stress/strain state on formation of (001) preferred orientation in L10 FePt thin films

The stress state of FePt thin films deposited at room temperature was controlled within the range from 1.01 GPa compressive to 0.18 GPa tensile before taking rapid thermal annealing (RTA). After the order–disorder transformation triggered by RTA at temperatures (Ta) from 650 to 800 °C for 5 min, the tensile-stressed films exhibit significant preferred orientation of (001) of L10 structure. However, the compressive-strained ones show isotropic texture. Strong (001) texture with high Lotgering orientation factor of 0.9 is obtained at Ta = 800 °C, resulting in enhanced perpendicular magnetic anisotropy. The results provide direct evidence of stress-induced (001) texture, which could be significant for future applications.

Magnetic properties and microstructure of low ordering temperature L10 FePt thin films

Polycrystalline Fe52Pt48 alloy thin films were prepared by dc magnetron sputtering on preheated natural-oxidized silicon wafer substrates. The film thickness was varied from 10 to 100 nm. The as-deposited film was encapsulated in a quartz tube and postannealed in vacuum at various temperatures for 1 h, then furnace cooled. It is found that the ordering temperature from as-deposited soft magnetic fcc FePt phase to hard magnetic fct L10 FePt phase could be reduced to about 350 °C by preheating substrate and furnace cooling treatment. The magnetic properties measurements indicated that the in-plane coercivity of the films was increased rapidly as annealing temperature is increased from 300 to 400 °C, but it decreased when the annealing temperature is higher than 400 °C. X-ray diffraction analysis shown that the as-deposited FePt thin film was a disorder fcc FePt phase. The magnetic measurement indicated that the transformation of disorder fcc FePt to fct L10 FePt phase was started at about 350 °C, which is c...

Decay dynamics of interchain excited states in luminescent conjugated polymer CN-PPV

Abstract We report the investigation of CN-PPV in poor solution. The fluorescence spectrum and the accompanying red-shifted absorption spectrum suggest that the polymer forms interchain state in poor solution. As the concentration increases, the intrachain emission becomes weaker, while the interchain species emission becomes stronger. The fluorescence of CN-PPV film exhibits similar behavior to that in concentrated poor solution. A two-state model is proposed to describe fluorescence dynamics of polymers in films and solution.