T. Östergård

Langmuir—Blodgett light-emitting diodes of poly(3-hexylthiophene) : electro-optical characteristics related to structure

Poly(3-hexylthiophene) Langmuir-Blodgett (LB) films have been used as the emitting layer in light-emitting diodes (LEDs). In order to develop further the device and increase the quantum efficiency by balancing the number of injected holes and electrons, two different approaches have been taken. Following the more conventional way, LB films of hole- and electron-transporting materials have been used in a separate layer and/or in a blend with the emitting material. Insulating polyaniline LB films at the electrode interfaces have also been used in order to control the operation. A simple model relating the electro-optical characteristics of the LED to its structure has been proposed.

Polymeric light-emitting diodes from molecularly thin poly(3-hexylthiophene) Langmuir-Blodgett films

Light‐emitting diodes based on molecularly thin films of a conjugated polymer have been fabricated. A few Langmuir–Blodgett layers of poly(3‐hexylthiophene) have been used as the active emitting material of the devices. Even the thinnest devices with only 3 monolayers, each having a thickness of ≊3 nm, yield the same luminance as thicker ones. The emission results from excitons in the polymer chains, and has a spectrum similar to that of photoluminescence, with a maximum at about 2.0 eV.

The role of interfaces in polymeric light-emitting diodes

Langmuir–Blodgett films of poly(3-hexylthiophene) and poly(methyl methacrylate) have been used to prepare organic dc and ac light-emitting diodes (LEDs). The transient as well as steady state properties of the LEDs have been studied. In order to explain electroluminescence (EL), charge transport mechanisms, and the role of interfaces in organic LEDs, transient EL, luminance–voltage and luminance–current measurements have been performed both for single layer and multilayer devices. The transient measurements were performed by applying a single square-wave voltage pulse or a square-wave ac voltage. We have shown that the time delay observed in transient EL measurements of single layer LEDs is mainly due to accumulation of charge carriers and not due to carrier transit times. The interfaces were found to have a major role on the operation of organic LEDs.

Langmuir-Blodgett films of conjugated polymers: electroluminescence and charge transport mechanisms

This paper reviews and reports our work on the development of polymeric light-emitting diodes (LEDs) based on Langmuir-Blodgett (LB) films. We have used LB deposition technique as a tool to fabricate dc and ac LEDs with a precise thickness ranging from a few molecular layers to tens of layers. In de LEDs, we have shown that as few as three LB layers of active polymer can yield the same luminance as the thicker ones. With the advantage of LB deposition technique to fabricate heterostructure (multilayer) devices, we have used carrier transporting materials and carrier blocking LB films to control and balance the charge injections in dc LEDs. The frequency dependence of (multilayered) ac LEDs has been studied and moderately high-frequency (20 kHz) electroluminescence (EL) intensity has been obtained. From the transient EL measurements, the role of interfaces in polymeric LEDs has been emphasized and the operation mechanism of LEDs has been discussed.

Electrochemically prepared light-emitting diodes of poly(para-phenylene)

Abstract Poly( para -phenylene) films have been used as the emitting material in organic light-emitting diodes. The poly( para -phenylene) films were electrochemically polymerized directly on indium tin oxide coated glass substrates. The indium tin oxide served as the anode while aluminum, vacuum evaporated on top of the poly( para -phenylene) film, formed the cathode of the final diode structure. The current-voltage characteristics and the luminance vs. current density for the diodes were measured as well as the electroluminescence spectra. A quantum efficiency of 0.35×10 −3 % was calculated for 2.5 A/cm 2 , the maximum current density used.

Light-emitting diodes of poly(p-phenylene vinylene) films electrochemically polymerized by cyclic voltammetry on ITO

Abstract Poly(p-phenylene vinylene) (PPV) films have been used as the emitting material in organic light-emitting diodes. The PPV films were polymerized directly on indium-tin oxide (ITO) by potential cycling. The electroluminescence (EL), luminance-current (L-I) and current-voltage (I-V) characteristics of the LEDs have been measured. The effect of aqua-regia treatment of the working electrode, ITO, as well as annealing of the ITO/PPV structure has been studied and compared to non-treated devices. A scanning electron microscope (SEM) has been used to study the morphology of the organic films.

Photoluminescence and electroluminescence in Langmuir–Blodgett films of poly(3-decylmethoxythiophene)

Abstract The photoluminescence (PL) and electroluminescence (EL) of poly(3-decylmethoxythiophene) have been studied for devices fabricated using both the spin-coating, and Langmuir–Blodgett (LB) technique. Strong thermochromic effects in the PL, correlated to a thermal transition in the polymer, are observed for the spin-coated films while negligible thermal dependence of the PL is observed in the LB films. A blue shift of the EL with respect to PL is observed for the LB films and is associated to partial polymer degradation. By analysing the time evolution of EL during the first minutes of device operation, the processes related to annealing and Al diffusion at the electrode/polymer interface are discussed.

Light-emitting diodes of poly(3-hexylthiophene) Langmuir-Blodgett films

Poly(3-hexylthiophene) Langmuir-Blodgett films have been used as emitting layers in light-emitting diodes. The effect of the film thickness and additional carrier-transport layers on current-voltage characteristics and quantum efficiency were studied, and the electroluminescence spectra measured. Hole transporting poly(9-vinylcarbazole) was used both as a separate layer in a heterostructure device and in a blend with emitting material. The electron-transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was used in a blend with the emitting material. The connections between the diode structure and the electro-optical properties as well as operation mechanisms are discussed.