Yvette Kaminorz

New aromatic poly(1,3,4-oxadiazole)s for light emitting diodes

Abstract New aromatic poly(1,3,4-oxadiazole)s were synthesized having excellent film forming properties due to their solubility in common organic solvents. The investigated new polyoxadiazoles can be used as emission material in single layer LED. The polyoxadiazoles show an emission in the range of blue to yellow light. The external quantum efficiency as well as the turn-on voltage of the devices are influenced when blends of the polyoxadiazole with hole transport materials are used.

SENSITIVITY OF POLYTHIOPHENE PLANAR LIGHT-EMITTING DIODES TO OXYGEN

Surface light emitting diodes (SLEDs) with a polymer-on-top geometry were used to study the sensitivity of light emission to oxygen. In these devices, pre-fabricated electrodes were coated with a conjugated polymer, which was thus directly exposed to the environment. Oxygen caused an immediate ten-to hundred fold decrease in electroluminescence efficiency relative to that in nitrogen or argon. Above the voltage for light emission, there was a sharp increase in current. Removing the oxygen led to recovery of the light intensity over a period of minutes, but the current returned immediately to its lower, original level. The electroluminescence and photoluminescence spectra were identical and were unaltered in shape by oxygen exposure (only decreasing in size). However, photoluminescence was unaffected by oxygen alone. This result indicates that oxygen does not affect excitons directly, but rather influences an intermediate species on the path to exciton formation, one that is significant only in electroluminescence and not in photoluminescence. Under simultaneous exposure to oxygen and UV light, the photoluminescence irreversibly decreased, presumably due to photo-oxidation

Study of Related Poly(1,3,4-Oxadiazole-Amide)s Containing Silicon or Hexafluoroisopropylidene Groups in the Main Chain

Two series of related poly(oxadiazole-amide)s have been synthesized by solution polycondensation reaction of aromatic diamines containing preformed 1,3,4-oxadiazole rings with diacid chlorides having silicon or hexafluoroisopropylidene groups. All these polymers were soluble in polar amidic solvents, such as N-methylpyrrolidinone and dimethylformamide and gave thin transparent free-standing films on casting their solutions. The polymers exhibited high thermal stability, with the initial decomposition temperature being above 400 °C. Most of these polymers did show a glass transition, being in the temperature range of 238–275 °C. Very thin coatings, in the range of tens of nanometres, which were deposited onto silicon wafers exhibited smooth and pinhole-free surfaces in atomic force microscopy investigations. The study of photoluminescence properties revealed that some of these compounds emit blue light, in the range of 440–475 nm. All these characteristics are discussed and compared with those of conventional aromatic poly(1,3,4-oxadiazole)s.

Characteristics of polythiophene surface light emitting diodes

Abstract Surface light emitting diodes (SLEDs), in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings (250 nm and 10 or 20 μm) and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison (electrode separation 50 nm). In this study, the emitting layer was poly[3-(2′,5′-bis(1″,4″,7″trioxaoctyl)phenyl)-2,2′-bithiophene] (EO–PT), a conjugated polymer based on polythiophene with oligo(ethyleneoxide) side chains. The current–voltage ( I ( V )) and light–voltage ( L ( V )) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide (ITO)–aluminum, gold silicide–aluminum, and gold silicide–gold SLEDs, but the turn-on voltages were lowest with the ITO–aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO–PT increased the current and brightness, decreased the turn-on voltages, and made the I ( V ) characteristics symmetric; thus, a device with an electrode separation of 10 μm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.

Planar microfabricated polymer light-emitting diodes

Conjugated polymers are organic semiconducting materials that can emit light. These polymers have the advantages of being light, cheap and easy to process, and in addition the band gap can be tailored. We report the microfabrication of surface light-emitting diodes (SLEDs) on silicon substrates in which the electrodes are underneath the organic electroluminescent layer. Patterned electrodes are separated by a 2500 ? thick insulating layer of silicon oxide or are interdigitated with a separation of 10 or 20 m; the luminescent polymer is spin coated or solvent cast on top of the electrodes. This fabrication method is completely compatible with conventional silicon processing because the polymer is deposited last and the light is emitted from the upper surface of the diodes. Despite the large spacing between electrodes, and despite the absence of an evaporated top contact, the voltages required for light emission were not much greater than those used in conventional sandwich-type structures.

Synthesis and study of new silicon-containing polyoxadiazoles

A series of new poly-1,3,4-oxadiazoles has been synthesized by polycondensation reaction of hydrazine sulfate with a mixture of a dicarboxylic acid containing unsaturated bonds and a dicarboxylic acid containing silicon, by using methanesulfonic acid/phosphorus pentoxide as a reaction medium. These polymers were highly thermostable but they were only soluble in strong inorganic acids such as sulfuric or methanesulfonic ones. An alternative way was followed by using the corresponding dihydrazides containing unsaturated bonds and the corresponding diacid chloride containing silicon that reacted in N-methylpyrrolidinone (NMP) to give soluble silicon-containing unsaturated polyhydrazides, which were cyclodehydrated either by thermal or chemical treatment to give the corresponding polyoxadiazoles. Very thin coatings of polyhydrazides and polyoxadiazoles were deposited onto silicon wafers and they showed a very smooth surface, free of pinholes, when studied by atomic force microscopy (AFM). Some polyoxadiazole films showed strong blue photoluminescence.

OLEDs based on new oxadiazole derivatives

The absorption, photoluminescence and electroluminescence properties of systematically modified poly(1,3,4-oxadiazole)s are reported and compared with low molar mass compounds. Poly(1,3,4-oxadiazole)s are functionalised by introducing pendent alkyl side chains and tetraphenylsilane or hexafluoroisopropylidene group (6F) into the main chain, respectively. The photo-and electroluminescence of single layer devices was found to be in the blue and green spectral range. A further strategy was to start with the optimisation of the substituents of low molar mass compounds followed by bonding the optimised structure to a flexible PMMA main chain. It was demonstrated that it is possible to preserve the optical properties of the oxadiazole unit and at the same time to improve film forming properties of the final polymer.

Optical and electrical properties of substituted 2,5-diphenyl-1,3,4-oxadiazoles

New substituted 2,5-diphenyl-1,3,4-oxadiazoles are reported as luminescent materials in light emitting diodes (LEDs). The investigated new oxadiazoles show efficient blue and green emission in single layer devices. The combination with a hole transporting and red emitting polythiophene led to a white emission with higher quantum efficiency (QE).

Supramolecular Structures of VD Films based on 2,5-Diphenyl-1,3,4-Oxadiazole Derivatives

Well ordered films on Si(SiO 2 ) substrates are prepared by Vapor Deposition(VD) in vacuum of 10 -4 Pa using substituted diphenyl-1,3,4-oxadiazoles. Results of X-ray Specular Reflectivity(XSR) reveal a bilayer-by-bilayer film growth. The film periodicity in normal direction shows a linear dependence on the length of the substituted aliphatic chain. This dependence is used to draw conclusions regarding the tilt of the aromatic and aliphatic unit of the film with respect to the substrate normal direction.

Supramolecular structure and electro-optical properties of functionalized maleic anhydride copolymers

New amphiphilic maleic acid copolymers containing oxadiazole rings as pendent groups were synthesized. Most of them form excellent Langmuir-Blodgett films. The polymers were characterized by UV/Vis- and fluorescence spectroscopy and show electroluminescent behavior in the green-yellow light region.