Yvan Bonnassieux

Piezoelectric actuators and sensors location for active control of flexible structures

A method for optimal positioning of piezoelectric actuators and sensors on a flexible structure is presented. First, a two-dimensional (2-D) model of a piezoelectric actuator bonded to a plate is obtained. Then, a Ritz formulation is used to find a state model of the system in view of its control. To define an optimal positioning strategy, an energy based approach is developed. This leads quite naturally to the study of controllability and observability properties of the overall dynamical model. A new criterion based on energy assessment is proposed to locate actuators and sensors.

Capacitive behavior of pentacene-based diodes: Quasistatic dielectric constant and dielectric strength

The capacitive behavior of pentacene films was investigated in the metal-semiconductor-metal (MSM) diode structure. Impedance analysis of diodes with a thick pentacene layer up to 1012 nm showed a full depletion of the organic layer. This observation allowed us to regard the MSM diode as a parallel-plate capacitor in the reverse-bias regime without current flow. Under forward-bias, the diode was evaluated through frequency-dependent impedance measurements by using an equivalent circuit composed of a single parallel resistance-capacitance circuit. The analysis of the data in both the reverse and forward bias regime led us to electrical methods for quantifying dielectric properties of pentacene.

Fundamental Benefits of the Staggered Geometry for Organic Field-Effect Transistors

In this letter, decisive advantages of the staggered-type organic field-effect transistors (OFETs) over the coplanar type are elucidated by 2-D device simulation. It is found that the charge transport in the channel is not limited by the contact electrode in staggered OFETs, whereas coplanar OFETs show strongly contact-limited behavior. This dissimilarity originates from the continuity (staggered) or discontinuity (coplanar) of the carrier concentration at the channel ends, which is directly connected to the channel potential profile. The calculated current-voltage curves also support these arguments as the current in coplanar OFETs follows the contact-limited transistor model.

Compact DC Modeling of Organic Field-Effect Transistors: Review and Perspectives

In spite of impressive improvements achieved for organic field-effect transistors (OFETs), there is still a lack of theoretical understanding of their behaviors. Furthermore, it is challenging to develop a universal model that would cover a huge variety of materials and device structures available for state-of-the-art OFETs. Nonetheless, currently there is a strong need for specific OFET compact models when device-to-system integration is an important issue. We briefly describe the most fundamental characters of organic semiconductors and OFETs, which set the bottom line dictating the requirement of an original model different from that of conventional inorganic devices. Along with an introduction to the principles of compact modeling for circuit simulation, a comparative analysis of the reported models is presented with an emphasis on their primary assumptions and applicability aspects. Critical points for advancing OFET compact models are discussed in consideration of the recent understanding of device physics.

Selective electroless copper deposition on self-assembled dithiol monolayers

The paper reports the use of self-assembled monolayers (SAMs) of dithiols to induce electroless copper deposition on a gold substrate. The metallization catalyst, palladium nanoparticles, is bound on the dithiol SAM. The assembly process is followed by IR and X-ray photoelectron spectroscopies to confirm the formation of a monolayer with bound catalyst. Electroless metallization is then carried out with a steady deposition rate of 130 nm/min. Additionally, microcontact printing of the catalyst on the SAM by poly(dimethylsiloxane) stamps is used to localize copper deposits. Resulting metallization is selective and allows for a high resolution.

Charge Distribution and Contact Resistance Model for Coplanar Organic Field-Effect Transistors

We propose a theoretical description of the charge distribution and the contact resistance in coplanar organic field-effect transistors (OFETs). Based on the concept that the current in organic semiconductors is only carried by injected carriers from the electrodes, an analytical formulation for the charge distribution inside the organic layer was derived. We found that the contact resistance in coplanar OFETs arises from a sharp low-carrier-density zone at the source/channel edge because the gate-induced channel carrier density is orders of magnitude higher than the source carrier density. This image is totally different from the contact resistance in staggered OFETs, in which the contact resistance mainly originates from the resistance through the semiconductor bulk. The contact resistance was calculated through charge-distribution functions, and the model could explain the effect of the gate voltage and injection barrier on the contact resistance. Experimental data on pentacene OFETs were analyzed using the transmission-line method. We finally noticed that the gate-voltage-dependent mobility is a critical factor for proper understanding of the contact resistance in real devices.

A Compact Model for Organic Field-Effect Transistors With Improved Output Asymptotic Behaviors

Here, we propose an advanced compact analytical current-voltage model for organic field-effect transistors (OFETs), which can be incorporated into SPICE-type circuit simulators. We improved the output saturation behavior by introducing a new asymptotic function that also enables more precise low-voltage current and conductance fitting. A new expression for the subthreshold current was suggested to cover all operation regimes of OFETs. All model parameters were extracted by a systematic method, and the comparison of the modeled current with the experimental data on pentacene-based OFETs confirmed the validity of the model over a wide operation range.

All-printed infrared sensor based on multiwalled carbon nanotubes

This contribution deals with all-printed infrared sensors fabricated using multiwalled carbon nanotubes deposited on a flexible polyimide substrate. A high responsivity of up to 1.2 kV/W is achieved at room temperature in ambient air. We evidence a strong dependence of the device transduction mechanism on the surrounding atmosphere, which can be attributed to bolometric effect interference with water molecule desorption upon irradiation.

Modeling the low-voltage regime of organic diodes: Origin of the ideality factor

This paper investigates the physics of single-layer organic diodes in the low-voltage regime. A simple analytical model is developed to describe the current-voltage characteristics of the device. At variance with what is often reported in the literature, the operating mechanism of the organic diode is closer to that of the p-n junction than that of the conventional Schottky diode. The influence of an exponential distribution of traps is also analyzed. Alongside a drastic reduction of the current at above-diffusion-potential regime, traps introduce a substantial ideality factor in the low-voltage current. Two-dimensional physically based simulations are carried out in order to ascertain the validity of our model. By including trap effects, device simulation could fairly fit the experimental data of the organic diodes made of vacuum-evaporated pentacene.

Templating and Charge Injection from Copper Electrodes into Solution-Processed Organic Field-Effect Transistors

Solution-processed organic field-effect transistors (OFETs) using chemically modified copper electrodes are reported. The purpose of this study is to shed light on the use of inexpensive copper electrodes in bottom-contact OFETs, which is consistent with the major goal of organic electronics: the realization of low-cost electronics. 6,13-Bis(triisopropylsilylethynyl)pentacene was used for solution-processed hole-transporting molecular films and pentafluorobenzenethiol was used to form self-assembled monolayers (SAMs) on the contact metals. We conducted a comparative study on copper and gold contacts and realized that, under the same performance improvement schemes, via SAM treatment and controlled crystal growth, the copper electrode device experienced a more significant enhancement than the gold electrode device. We attribute the beneficial effects of SAMs to the improved charge injection and transport properties, which are critical double effects from the fluorinated aromatic SAM structure. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that templating property of SAMs promotes the crystallization of TIPS-pentacene films at the metal/organic interface. The presented result indicates that copper can be regarded as a promising candidate for reducing the use of gold in organic-based circuits and systems, where the cost-effective production is an important issue.