Rapid progress in photorefractive polymers includes new materials with improved response rates. We briefly review recent developments in this rapidly evolving area and report grating formation with a 2-millisecond time-constant, the fastest reported to date in any photorefractive polymer.

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal transient features for different device configurations which can be reproduced using our phenomenological numerical model. We find that the temporal evolution of the electroluminescence can be characterised by five main features: i) a delay followed by; ii) fast initial rise at turn-on followed by; iii) a slow rise (slower by at least one order of magnitude); iv) fast modulation (<15ns, unresolved) at turn-off followed by v) a long-lived exponential tail. We suggest a method for extracting mobility values which is found to be compatible with CW drive schemes. Mobilities for holes and electrons are extracted for a poly(p-phenylenevinylene) co-polymer and poly(di-octyl fluorene).

We study the photovoltaic response of inorganic/organic composite devices containing the conjugated polymer, MEH-PPV, and a semi-transparent layer of titanium dioxide (TiO2) nanoparticles, which is similar to that employed in the Grätzel photoelectrochemical cell. Comparison of the photovoltage obtainable under illumination with either a Au, Ag, Al or Ca back electrode suggests that the built in potential is determined largely by the difference between the quasi Fermi level of the TiO2 under illumination and the work function of the back electrode. A qualitative model for charge generation and transport through the device is presented.

In this report, we describe our initial synthesis and characterization of mono-functional and bi-functional dibromoalkyl oligothiophenes to achieve amphiphilicity and telechelic functionality. Oligothiophenes are an important class of organic materials for opto-electronic devices and display applications. We have mono-functionalized oligothiophenes by the synthesis of a quinquethiophene bromoalkyl derivative. A bi-functional sexithiophene was derived primarily by the symmetrical coupling of terthiophene derivatives. Both were synthesized using Grignard coupling and lithiation reaction methodologies. UV-Vis, IR, NMR, MALDI-TOF-MS, and DSC confirmed the structure and physical properties of the oligomers. In addition, we have also synthesized an amphiphilic diamine derivative from the reaction of hexamethylenediamine with a bromoalkyl terthiophene derivative. Using photoluminescence, the photophysical properties of the oligomers were found to be that of typical oligothiophenes. Processing as ultrathin films for devices is currently being investigated.

Hexaphenyl thin films (HTF) are widely used as an electro-active organic medium in blue light emitting diodes. The optical parameters of the HTF-based devices significantly depend on their microstructural properties.

HTF of different types are produced by physical vapor deposition on glass substrates applying specific sample preparation conditions. The microstructural properties of HTF are characterized using X-ray diffraction line profile analysis and atomic force microscopy (AFM). Diffraction peaks representing three different types of preferred growth in HTF are analyzed, namely textures with (00λ), (223) and (203) net planes oriented parallel to the substrate. No additional line-broadening (compared to silicon powder used as standard) is observed in the case of a film prepared at high substrate temperature of 170 °C. On the other hand, considerable broadening is detected in a film with the substrate kept at room temperature. Multiple line analysis documents that the crystallite size and lattice strain for the sample is 150 nm and 3×10−4, respectively. Single line analysis performed on the other reflections reveal size-induced broadening for a crystallite size in the range 40 to 50 nm. From AFM data we obtained that the maximum roughness of the surface is about 40 nm. The results indicate that the deposition temperature significantly influences the microstructural properties and that higher substrate temperature promotes a higher mobility of the molecules on the substrate enabling growth of larger crystallites with lower strain.

Thin films of p-sexiphenyl (6P) were doped with increasing amounts of potassium in situ, and the change in the valence electronic structure of 6P upon the alkali metal deposition was followed with ultraviolet photoelectron spectroscopy. We observe the evolution of new intragap emissions, which are attributed to the formation of bipolarons, even for very low doping concentrations. The low binding energy intra-gap emission exhibits a pronounced asymmetric lineshape, in contrast to the findings when cesium is used as dopant. In order to investigate whether this lineshape is due to different emissive electronic species in the bulk and on the surface of the 6P film the take-off angle for the photoelectrons was varied. As no change in the lineshape is found when going from normal to near-grazing emission we can exclude that charged 6P molecules in the bulk and on the surface yield different valence electronic spectra. Therefore, the characteristic lineshape of the low binding energy emission is proposed to be related to the interaction of the doped organic molecule with the different counterions.

We studied the EL behaviors of benzoheterocyclic derivatives as emitting and hole transport layers for organic light-emitting-diodes (OLEDs). First, we studied the benzoheterocyclic derivatives having stylyl group, triphenylamine group and benzoxazole or benzothiazole group as an emission layer. These devices emitted a blue-green light. The current densities of the OLED having these benzoheterocyclic derivatives as an emission layer were higher than that of the Alq3 OLED at same applied voltage. However, these devices did not have a high EL efficiency (maximum 0.1 lm/W). From these results, in these benzoheterocyclic derivatives having a triphenylamine group, we thought that holes could flow out from hole transport to cathode. We newly synthesized dimer-type benzoheterocyclic derivatives without a triphenylamine group, which have benzoheterocycle at both sides. The current densities of the OLEDs having dimer-type benzoheterocyclic derivatives was more strongly suppressed than that of the OLEDs having benzoheterocyclic derivatives with a triphenylamine group at same applied voltage, but the EL efficiency could not be improved by dimerization and eliminating of a triphenyldiamine group obtained. Next, we studied the TPD derivatives having benzoxazole, benzothiazole and stylyl groups as hole transport layer. In new TPD derivatives, the EL efficiency the OLEDs having the TPD derivatives with stylyl groups was the best efficiency of all. The EL efficiency of ITO/a TPD derivative with stylyl groups/Alq3/AlLi is 1. 1 lm/W (max. luminance 12000cd/m2).

The morphological stability of evaporated films of aluminum(III) 8-hydroxyquinoline (Alq3) was investigated. Films which were found to be non-crystalline by x-ray diffraction upon deposition, crystallized rapidly upon annealing, especially where defects were present. Blends of Alq3with aluminum(III) 5-methyl-8-hydroxyquinoline were proposed for thermally stable amorphous emitting layers in light-emitting diodes. Films coevaporated at a 1:1 ratio did not show evidence of crystallization or phase separation even after long annealing periods at temperatures as high as 160°C.

We study the current-voltage characteristics of both double and single carrier polymer light emitting devices in the high intensity electrical excitation regime. Single layer devices are investigated with the orange-emitting MEH-PPV as the active polymer. Performing very low duty (≈0.001 %) pulsed electroluminescence measurements with electric fields up to 1*109 V/m, we observe for the hole-dominated devices a space charge limited current with a saturating mobility and a saturating external quantum efficiency of 1%. For double carrier devices, the current starts to saturate in the high-field regime, appoaching Ohmic-like behavior. We report current densities of 100 A/cm2and brightnesses of 7*105 cd/m2. The spectral features are monitored below the onset of degradation.

Conjugated polymers have been used as solid-state materials for a variety of opto-electronic applications. Thus, their processability in a number of device fabrication protocols is an important consideration. In this report, we indicate our results on the synthesis of conjugated poly-p-phenylene (PPP) materials derived from poly(1,3-cyclohexadiene) (PCHD). The precursor PCHD polymers were synthesized by living anionic polymerization to produce homo-and block copolymer configurations with polystyrene. We have used a variety of initiators, solvent, and temperature conditions to determine the right parameters for obtaining narrow MWD polymers. The conditions for polymerization determined the ratio of 1,2 and 1,4 isomers in the microstructure. We then proceeded with conversion to PPP derivatives using a dehydrogenation reaction with chloranil. Our results indicate a systematic conversion to a conjugated polymer with increased solubility and photoluminescence properties. The microstructure, MW and block copolymer composition affecting the processability and conversion properties dramatically.

We investigate the electrical properties of Polymer Light Emitting Diodes (LED's). The experimental data consists of steady state current-voltage characteristics and radiance as a function of temperature.

The basic LED structure is Anode/MEH-PPV (2-methoxy,5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene)/Cathode, with a polymer film 120-140 nm thick. We use different anode/cathode pairs to study transport and light emission properties. Measurements of external quantum efficiency of bipolar and monopolar devices are presented from 200 K to 300 K. The electron and hole mobilities are derived in the trap-free limit and at high voltages.

This contribution describes the use of self-limiting siloxane chemisoroption processes to self-assemble building blocks for the modification of vacuum-deposited organic LED (OLED) devices. One approach consists of the use of self-assembling OLED hole transport materials for application in hybrid self-assembled + vapor deposited two-layer devices. Another approach involves the application of self-limiting, chemisorptive self-assembly techniques to introduce thin dielectric films between the anode and hole transport layer of a vapor deposited two-layer OLED device.

Organic light emitting diodes have been fabricated using erbium tris(8-hydroxy-quinoline) as the emitting layer and N, N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) as the hole transporting layer. Room temperature electroluminescence was observed at 1.5 νdue to intra-atomic transitions between the 4I13/2; and 4I15/2; levels in the Er3+, ion. These results make the possibility of producing silicon compatible 1.5 νm technology based on such devices a reality.

The delayed fluorescence of poly(p-phenylene vinylene) (PPV) and poly(p-phenylene ethynylene) (PPE) derivative solids and frozen solutions at 20 K is described. It provides strong evidence for triplet-triplet annihilation to singlets excitons accounting for up to ∼3% of the total emission in PPV films and ∼1.5% in PPE powder. It also yields triplet lifetimes of 70 and 110 μs in PPV films and frozen solutions, and ∼200 and ∼500 μs in PPE powder and frozen solutions, respectively.