A. Ltaief

Injection modifications by ITO functionalization with a self-assembled monolayer in OLEDs

Abstract Operating conditions of OLEDs are very sensitive to the surface properties of indium tin oxide (ITO). Wettability measurements have been performed to characterize ITO surface properties and their modification upon deposition of a self-assembled monolayer. Contact angle measurements demonstrate that ITO surface is basic. Upon grafting with phosphonic acid, the surface becomes acid. The I ( V ) characteristics of light-emitting diodes (LEDs) based on a soluble PPV derivative: poly(2-octoxy-5-methoxy-1,4-phenylenevinylene) (POMX) by such functionalized ITO show a reduction of the onset voltage and a rectification ratio enhancement. The electrical characteristics follow a space–charge limited conduction (SCLC) behavior. Such modified ITO electrodes lead to main improvements of the diode properties through operating voltage reduction and stability increase.

Study of organic thin film transistors based on nickel phthalocyanine: effect of annealing

Abstract Conducting organic materials offer a unique combination of interesting properties that make them more attractive than inorganic materials currently used in microelectronics. It is advantageous to use organic thin film transistors (OTFT) when large display areas are required. This needs the enhancement of transistor performances by increasing the Ion/Ioff ratio and the field effect mobility. In this report, the effects of annealing on the electrical properties of nickel phthalocyanine (NiPc) OTFTs were studied. Our work is based upon an analysis of different carrier-exchange mechanisms occurring at metal/organic material interface.

Facile synthesis and characterization of polypyrrole-multiwalled carbon nanotubes by in situ oxidative polymerization

Polypyrrole-multiwall carbon nanotube (PPy-MWCNT) nanocomposites were chemically synthesized via in situ oxidative polymerization of pyrrole. Ammonium peroxydisulfate and p-toluenesulfonic acid were used as an initiator and surfactant dopant, respectively. The molar ratio of monomer unit to initiator and dopant was 1:1:1, and the percentage of MWCNT in PPy varied from 1 to 10 wt.%. PPy-MWCNT nanocomposites were characterized to study chemical structure, morphology, thermal, electrical, and surface properties. To accomplish this, the samples have been characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, four probe resistivity method, and atomic force microscopy. The results showed that PPy-MWCNT nanocomposites were successfully synthesized via in situ oxidative polymerization method, and also, electrical conductivity of nanocomposites was increased when the content of MWCNT increase.

Charge Transport in Carbon Nanotubes-Polymer Composite Photovoltaic Cells

We investigate the dark and illuminated current density-voltage (J/V) characteristics of poly(2-methoxy-5-(2’-ethylhexyloxy)1-4-phenylenevinylene) (MEH-PPV)/single-walled carbon nanotubes (SWNTs) composite photovoltaic cells. Using an exponential band tail model, the conduction mechanism has been analysed for polymer only devices and composite devices, in terms of space charge limited current (SCLC) conduction mechanism, where we determine the power parameters and the threshold voltages. Elaborated devices for MEH-PPV:SWNTs (1:1) composites showed a photoresponse with an open-circuit voltage Voc of 0.4 V, a short-circuit current density JSC of 1 µA/cm² and a fill factor FF of 43%. We have modelised the organic photovoltaic devices with an equivalent circuit, where we calculated the series and shunt resistances.

Effect of functionalisation of MWCNTs on optical and morphological properties of MEH–PPV/MWCNTs nanocomposites

The effect of functionalisation of Multi–Walled CNTs (MWCNTs) as a way to ameliorate their dispersion has been quantified. Two kinds of nanocomposites have been elaborated, using a spin–coating as deposition technique: poly (2–methoxy–5–(2–ethyhexyl–oxy)–p–phenylenevinylene) (MEH–PPV) as an electron donor, pristine MWCNTs and polystyrene–functionalised MWCNTs (PS:MWCNTs) as electron acceptors. MEH–PPV/MWCNTs and MEH–PPV/PS:MWCNTs nanocomposites are characterised by photoluminescence spectroscopy and atomic force microscopy to study the charge transfer efficiency and morphological properties of these structures. An efficient charge transfer in MEH–PPV/MWCNTs and MEH–PPV/PS:MWCNT nanocomposites has taken place, but is broken when reaching a critical concentration of 0.5 and 1 wt.%, respectively. Once these critical concentrations are reached and supported by AFM images, aggregates of MWCNTs and PS:MWCNTs have been observed.

Optical and Structural Properties of MEH-PPV: C60-Based Structures

Structural evolution and optical properties in blends of poly[2-methoxy,5-(20-ethylhexyloxy)-1,4,-phenylene-vinylene] and fullerene (C60) upon thermal treatment at different temperatures have been investigated using X-ray diffraction, absorption, and photoluminescence measurements. From the XRD results, the as-cast film exhibited three broad diffraction peaks at around 2 Θ = 6°, 12° and 21.7° accompanied with two well-defined diffraction peaks at wide angle at about 2 Θ = 30.7° and 35.5°. The diffraction peaks at around 2 Θ = 6° and 21.7 were attributed to MEH-PPV while the diffraction peaks at around 2 Θ = 12°, 30.7° and 35.5° were originated from the effect of the incorporation of C60 molecules in the polymer matrix. As the temperature is increased to 125°C, the intensities of the diffraction peaks at wide angles increased and the peak at around 2Θ = 12° decreased progressively then at 125°C it disappeared completely. When the temperature is increased further to 180°C this peak reappears again and the diffraction peaks at 2 Θ = 30.7° and 35.5° dissipated. These results suggested the occurrence of a crystalline transition in the composite film at about 125°C. In addition to that it revealed that the amorphous domain was increased with annealing at elevated temperatures. The optical properties of MEH-PPV: C60 composites showed a strong dependence with the structural evolution with thermal treatment.