J. Davenas

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.

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.

Characterization of copper ion sensing thiacalix[4]arene films evaporated on semiconductor substrates

Abstract Thermal evaporation under vacuum of thin thiacalix[4]arene on electrolyte-insulator-semiconductor (EIS)- and ion-sensitive-field-effect-transistor (ISFET)-based structures allows to obtain sensitive and selective sensors for copper metal ions. The sensitivity of such devices, varying from a Nernstian response (30 mV/decade) to about 18 mV/decade was strongly dependent upon the film thickness. Such a film thickness influence onto the sensors behaviour was correlated to the morphology and the composition of the thiacalix[4]arene layer investigated using atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and diffraction of X-ray techniques. Furthermore, the complexation of copper ions by the thiacalix[4]ane layer, reported by electrochemical measurements, has been confirmed by Rutherford backscattering spectroscopy (RBS).

NIR spectrophotometry characterization of ITO electronic property changes at the interface with a PPV derivative

The electronic effects at polymer/indium tin oxide (ITO) interface play a crucial role in light-emitting devices as controlling charge injection. Near-infrared spectrophotometry has been used to characterize the modifications of the ITO electronic properties induced by the deposition of a PPV derivative. Near-infrared (NIR) spectra have been analyzed using Drude free electron theory providing information on the free carriers density and local conductivity. The optical analysis reveals a 10% reduction of the ITO free carrier concentration at the contact with the PPV derivative, which has been attributed to the formation of a polarization layer at the interface. Comparison has been done between polymer deposited directly on ITO or on ITO functionalized with a phosphonic acid layer (SAM) to improve the electronic/polymer interface. The NIR study indicates that charge transfers occur with the first deposited layer, providing confirmation of an interface effect.

Electrochemical and electromechanical properties of fully hydrolyzed polyacrylamide for applications in biomimetics

This paper focuses on the development of fully hydrolyzed polyacrylamide (PAAM) hydrogel for applications in biomimetics. We present an analysis of the motion of actuators based on PAAM hydrogel in order to obtain the elementary background needed for the design of actuating devices based on this material, which has a high compatibility with living tissues. The gel properties are investigated, the electroactivity of the hydrogel is shown and a qualitative–quantitative study demonstrating the basics of motion of such actuators is presented.