C. Dridi

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.

Investigation of exciton photodissociation, charge transport and photovoltaic response of poly(N-vinyl carbazole):TiO2 nanocomposites for solar cell applications

The photogeneration of charge carriers in spin-coated thin films of nanocrystalline (nc-)TiO(2) particles dispersed in a semiconducting polymer, poly(N-vinylcarbazole) (PVK), has been studied by photoluminescence and charge transport measurements. The solvent and the TiO(2) particle concentration have been selected to optimize the composite morphology. A large number of small domains leading to a large interface and an improved exciton dissociation could be obtained with tetrahydrofuran (THF). The charge transport mechanism and trap distribution at low and high voltage in ITO/nc-TiO(2):PVK/Al diodes in the dark could be identified by current-voltage measurements and impedance spectroscopy. The transport mechanism is space charge limited with an exponential trap distribution in the high voltage regime (1-4xa0V), whereas a Schottky process with a barrier height of about 0.9xa0eV is observed at low bias voltages (<1xa0V). The current-voltage characteristics under white illumination have shown a dramatic increase of the short circuit current density J(sc) and open circuit voltage V(oc) for a 30% TiO(2) volume content corresponding to the morphology exhibiting the best dispersion of TiO(2) particles. A degradation of the photovoltaic properties is induced at higher compositions by the formation of larger TiO(2) aggregates. A procedure has been developed to extract the physical parameters from the J-V characteristics in the dark and under illumination on the basis of an equivalent circuit. The variation of the solar cell parameters with the TiO(2) composition confirms that the photovoltaic response is optimum for 30% TiO(2) volume content. It is concluded that the photovoltaic properties of nc-TiO(2):PVK nanocomposites are controlled by the interfacial area between the donor and the acceptor material and are limited by the dispersion of the TiO(2) nanoparticles in the polymer.

Optical and electrical properties of semi-conducting calix[5,9]arene thin films with potential applications in organic electronics

Indium tin oxide (ITO) substrates have been functionalized by several substituted calix[n]arene (n = 5 or 9) derivatives using spin coating to fabricate organic diode devices. The effect of rim size and side substituents has been investigated by UV–visible absorption spectrophotometry. The energy band gaps of these calixarene derivative thin films have been found in the 1.166–1.450 eV range. The electrical properties of ITO/calix[n]arene/Al diodes have been studied by current–voltage measurement showing an ohmic behaviour at low voltage. The I(V) characteristics could be modelled by a space-charge-limited current (SCLC) mechanism at high applied bias voltage. The ac electrical transport of calix[5,9]arene derivatives has been studied over a wide range of bias voltage and frequency by impedance spectroscopy. The device had been accurately modelled, for a frequency between 100 Hz and 10 MHz, by a single parallel resistor and capacitor network in series with a resistance. A dielectric relaxation time in the ms range and a transport mechanism controlled by an exponential trap distribution were deduced from the fit of the experimental data. The evolution of electrical parameters with chemical structure (rim size and substituent) has been discussed. The conductivity σ(ω) evolution with frequency and bias voltage was studied for ITO/calix[n]arene/Al devices. The dc conductivity σdc for these devices has been determined. The ac conductivity σac showed a variation in angular frequency as A.ωs with a critical exponent s < 1 suggesting a hopping conduction mechanism at high frequency and a microscopic picture of the relaxation and hopping processes has been proposed.

Electrical properties of ITO/benzylated cyclodextrins (β-CDs (Bz))/Al diode structures

Abstract Investigations of the electrical characteristics of benzylated cyclodextrins (β-CDs (Bz)) diodes are reported. We present current–voltage characteristics and impedance spectroscopy measurements performed on partially benzylated cyclodextrins β-CDs (Bz) thin films in sandwich structures ITO/b-CDs (Bz)/Al. The static electrical characterizations show a space charge limited conduction (SCLC) and a conductivity with power low frequency behavior characteristic of a hopping transport in disordered materials. The impedance spectra can be discussed in terms of an equivalent circuit model designed as a parallel resistor Rp and capacitor Cp network in series with resistor Rs. We extract numerical values of these parameters by fitting experimental data. Their evolution with bias voltages has shown that the SCLC mechanism is characterized by an exponential trap distribution. We estimated from the capacitance voltage characteristic an acceptor concentration of about 1011 cm−3 due to trap states.

A Highly Sensitive Miniaturized Impedimetric Perchlorate Chemical Sensor

In this paper, we have developed a miniaturized a chemical sensor based on a new nanostructured Co-phthalocyanine (Co(II)Pc-PAA) derivative functionalized Au microelectrodes for perchlorate ClO−4 detection. The morphological properties of the sensitive layer have been characterized by contact angle measurement. The response of the obtained sensor-based CoPc/Au microelectrodes has been investigated by electrochemical impedance spectroscopy measurements. The experimental impedance data of the sensor device were analyzed by an equivalent electrical circuit using a modified Randles model for better understanding the phenomena present at the sensing membrane/electrolyte interface. Therefore, under optimized working conditions in terms of polarization and frequency, best performances have been achieved when compared with those obtained in the literature for Au electrodes-based devices functionalized with the same molecule. The present chemical sensor has provided a lower detection limit (17.3 pM), the lowest achieved until now to our knowledge, with a larger linear range from 1.73 10−11 to 10−1 M. The selectivity of the sensor has been also studied by evaluating the response towards ClO−4 with other interfering anions. The measurement were stable after ten days of the chemical sensor storage at room temperature. This is very promising for environmental application using rapid analyses and low-cost chemical sensors. Perspectives for a potentiometric sensor at higher concentrations were also assessed.

A novel amperometric enzyme inhibition biosensor based on xanthine oxidase immobilised onto glassy carbon electrodes for bisphenol A determination

A novel and simple biosensor for the determination of bisphenol A (BPA) based on xanthine oxidase (XOD) enzymatic inhibition has been developed. The biosensor was prepared from xanthine oxidase immobilised by crosslinking with glutaraldehyde, with hypoxanthine as enzyme substrate, and was successfully applied to the determination of BPA using fixed potential amperometry. Biosensor performance was optimised with respect to the applied potential, influence of pH of the electrolyte solution, XOD loading and the substrate concentration. The enzyme inhibition mechanism was evaluated from Cornish-Bowden plus Dixon plots and was found to be reversible and competitive with an apparent inhibition constant of 8.15u202fnM. Under optimised conditions, the determination of BPA can be achieved in the linear range up to 41u202fnM with a detection limit of 1.0u202fnM, which is equal to the lowest reported in the literature, with very good repeatability and reproducibility. The selectivity of the biosensor was evaluated by performing an interference study and found to be excellent; and stability was investigated. It was successfully applied to the detection of BPA in mineral water and in river water.