T. M. Miller

Photophysics of phenylenevinylene polymers

Abstract We report steady-state and transient optical characterization of phenylenevinylene polymers and extract a picture of the photoexcitations and dynamics which may apply more generally to nondegenerate ground-state conjugated polymers. The most common excitation pathway involves formation of nonemissive interchain excitons, unless the polymer chains are separated so that intrachain excitons responsible for photoluminescence are formed preferentially. Oxidation defects can be introduced thermally or photochemically and lead to charge transfer quenching of intrachain excitons whose dynamics can be understood quantitatively. We have identified the defect site, quantified concentrations at which it becomes problematic and shown that we can essentially eliminate it with improved synthesis. The maximum electroluminescence efficiency is not simply related to the photoluminescence quantum yield Φ PL and might be several times higher than Φ PL /4 which we estimate to be only 4% for our material.

Physics and applications of organic microcavity light emitting diodes

The important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail. The photon density of states is redistributed such that only certain wavelengths, which correspond to allowed cavity modes, are emitted in a given direction. This enables us to realize color selectivity over a large wavelength (and color coordinate) range with broadband emitters such as 8‐hydroxyquinoline aluminum (Alq), and intensity enhancement in narrow band emitters. The intensity enhancement in Alq‐based cavity light emitting diodes (LEDs) is extensively evaluated both experimentally and theoretically. The design considerations for and device characteristics of a novel multiple emissive layer LED are also described.

Color variation with electroluminescent organic semiconductors in multimode resonant cavities

The range of possibilities in controlling the color of an organic semiconductor based light emitter diode (LED) by incorporating the active layers in a multimode Fabry–Perot cavity is demonstrated. The combination of carefully designed multimode microcavities and electroluminescent organic semiconductors makes possible the realization of mixed colors such as white, purple, etc. with a single LED. The parameters affecting the color include the total optical thickness of the device and the position of the electromagnetic‐field antinodes with respect to the location of the emitting dipoles. The electrical characteristics and quantum efficiency of such devices are also reported.

Microcavity effects in organic semiconductors

Microcavity structures containing hydroxyquinoline aluminum and diamine layers commonly used in electroluminescent devices are described. We show that it is possible to obtain emission at red, green, and blue wavelengths by changing the thickness of a polyimide filler layer in the cavity. The angular dependence of the emission wavelength and linewidth are reported and the implications for organic electroluminescent color displays are discussed.

Conductivity-type anisotropy in molecular solids

Thin polycrystalline films of perylenetetracarboxylic dianyhydride (PTCDA), an organic molecular solid, exhibits substantial anisotropies in its electronic transport properties. Only electrons transport in the directions along molecular planes, while mainly holes transport in the direction normal to molecular planes. A series of measurements on both field effect transistors with PTCDA active layers and light emitting diodes with PTCDA transport layers documents the anisotropy seen in the electronic transport in thin films of PTCDA.

Dependence of the electro-optical properties of polymer dispersed liquid crystals on the photopolymerization process

We have studied the dependence of the electro-optical properties of polymer dispersed liquid crystals (PDLC) on the ultraviolet (UV) cure of the solution of monomer and liquid crystal. The kinetics of UV polymerization and its effect on the morphology of the phase separated droplets of liquid crystal determine the switching voltage, response time, and luminance of the PDLC. Using a series of statistically designed experiments, we have mapped the dependence of these responses on the weight fraction of liquid crystal, the temperature of the cell during cure, and light intensity. Temperature and composition are strongly coupled parameters that influence switching voltage, luminance, and response times. Switching voltages are minimized at 4–5 V for an 8 μm cell gap over a large region of temperature-composition space. An abrupt transition line occurs through that space. On one side of the transition line, voltage increases linearly either as temperature increases or composition decreases, and on the other sid...