Richard E. Gill

Effect of the top electrode work function on the rectification ratio of light-emitting diodes (LEDs) based on poly(3-octylthiophene)

Electroluminescence efficiency and current-voltage characteristics of conjugated polymer poly(3-octylthiophene) (P3OT) light emitting diodes (LEDs) are discussed in terms of the presently popular band model. Devices were studied in forward and reverse mode of operation, with different types of top contacts to show the dominance of the top electrode workfunction on device properties. Changing the workfunction of the top contact with only 0.9 eV (from Au to Al) results in a change in the preferred current direction, corresponding to a change in the rectification ratio of the diodes of effectively 6 orders of magnitude. When Au is used as a top electrode, no electroluminescence was observed, because the barrier for electron injection is too high. The observation of light emission in the reverse mode of operation for ITO/P3OT/Al is a result of direct carrier tunneling into the transport bands at high electric fields.

Light-emission in reverse bias operation from poly(3- octylthiophene)-based light-emitting-diodes

We report light emission from light emitting diodes with poly(3‐octylthiophene) (P3OT) as the active layer in both forward and reverse bias operation. The onset of electroluminescence (EL) of ITO/P3OT/Al devices occurs at current densities of 6.25×10−4 A/cm2 in both modes of operation; both cases show identical EL spectra. For a P3OT thickness of 100 nm the onset of electroluminescence and current occurs at 3 V in the forward bias mode, and at about 18 V in the reverse mode of operation, at which a completely different voltage dependence of the current is observed. In the reverse mode of operation, the data suggest that carrier injection is a tunneling process through a triangular barrier of 0.4 eV at the metal–polymer interface. In the forward bias a Schottky‐like behavior is seen.

Stimulated emission from vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

The influence of the morphology on the light emission properties of vacuum-deposited polycrystalline oligo(p-phenylene vinylene) thin films under intense laser excitation was studied. The morphology of the thin films was varied by annealing just below the first melting point (crystal-mesophase transition) and by crystallization from the isotropic melt. Amplified spontaneous emission occurred within the individual domains and was only observed when the domain size was increased by thermal treatment. The amplified spontaneous emission threshold for the melt-crystallized thin films was found to be comparable to that for conjugated polymer thin films.

Model compounds for light-emitting PPV's: Optical and structural data of substituted oligomers

Phenylene-vinylene oligomers are investigated as model compounds for the luminescent moiety of PPV-type homopolymers or copolymers having oligo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have been substituted at various positions on the vinylene linkages. For several compounds single crystals have been obtained and their structure has been resolved, which allows the spectral response to be related to molecular conformation and packing. Thin film spectra demonstrate that cyano substitution results in a considerable red shift of the emission: a more planar conformation, π-stacking and, possibly, dipolar interactions cooperate with the electronic effect of the cyano group. The use of a cyano-substituted five-ring compound in a multilayer LED was found to strongly increase the luminescence efficiency.

Optical properties of single crystals and vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

Abstract The optical properties and structural organization of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) have been investigated in the solid state. For this study solution-grown single crystals and vacuum-deposited thin films were used. The unit cell of the solution-grown single crystals was determined to be monoclinic, space group I2 a . The as-deposited oligomer films showed a polycrystalline structure. Annealing the film at a temperature of 120°C resulted in a morphological change, which manifested itself in an increase of crystalline domain size from approximately 4 μm to 20 μm. Both single crystals and thin films exhibit a high photoluminescence quantum yield of 0.5 and 0.7, respectively. In order to determine the relative importance of oxygen and water on photoluminescence quenching, we measured the luminescence decay of a vacuum-deposited film exposed to water and air. It was found that quenching is mainly due to contamination with oxygen from air rather than water and that this contamination is reversible to a great extent. We demonstrate that under intense laser excitation it is possible to obtain stimulated emission from both single crystals and thin films. Non-optimized single-layer light-emitting diodes prepared from the oligomer showed uniform green/yellow light emission with external electroluminescence efficiencies up to 0.15% using Ca as electron-injecting electrode. The electroluminescence efficiency is drastically increased by annealing.

Stimulated emission from films of conjugated polymers and oligomers

Oligomers and block copolymers structurally related to PPV were investigated under intense optical excitation. Their well-defined molecular structure allows a better control of emission properties than is usually feasible in semiconducting polymers. Stimulated emission is demonstrated in single crystals of PPV-type oligomers, and also in thin films obtained by spin casting of copolymers containing PPV- type blocks. Waveguiding is shown to provide the length of interaction required for mirrorless laser generation. Thin films of oligomers obtained through deposition from the vapor phase are polycrystalline, and the optical losses in the as-deposited films are too large for lasing to be achieved. These films show stimulated emission only after the domain size has been increased by annealing.Lasing occurs within individual crystalline domains with a threshold value comparable to that found for optically clear amorphous films of conjugated polymers.

COLOR TUNING OF POLY[(SILANYLENE)THIOPHENE] BLOCK-COPOLYMERS

Block copolymers are materials where control over luminescence can be achieved by means of adjusting the length of the active ((pi) conjugated) block. We report on electroluminescent devices from poly[(Silanylene)Thiophene] block copolymers.