Staf Borghs

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta2O5 as gate dielectric material for organic thin-film transistors. Ta2O5 has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta2O5 thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (r=21), Ta2O5 facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta2O5 gate dielectric are discussed.

Field-effect detection of chemical species with hybrid organic/inorganic transistors

In this letter, we demonstrate that organic-based transistors are able to detect charged/uncharged chemical species in aqueous media via the field effect. The chemical sensitivity of the organic transistors is illustrated for protons and glucose. The electrochemical potential developed at the solution/dielectric interface depends on the proton concentration in solution and it modifies the current flowing in the transistor according to this concentration. Glucose can also be detected if an enzymatic layer is coupled to the dielectric surface. An inorganic proton-sensitive dielectric, Ta2O5, is used in the transistor structure in order to reduce the operational voltages. The field-effect detection of chemical species allows us to realize sensors for a variety of ions/molecules if layers with a specific functionality are added to the transistor. Organic-based sensing devices show great potential for low-cost, health-related, and environmental applications because of their very simple fabrication process, whi...

Extraction of bulk and contact components of the series resistance in organic bulk donor-acceptor-heterojunctions

Abstract Basic solar cell characteristics are examined in bulk donor-acceptor-heterojunction devices. Therefore, spin-coated organic blends of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are used as active material sandwiched between a transparent IndiumTinOxide (ITO)-electrode and an Al backside contact. A comparison is made between cells with or without an extra interfacial layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. Furthermore, the effect of the thickness of the active layer on the photovoltaic performance of the devices is studied. It is seen that applying this extra PEDOT/PSS layer results in an important increase of the contact component of the series resistance of the cells. At the same time, open circuit voltage improved for devices with an interfacial layer while the fill factor was higher for cells with no PEDOT/PSS film. For thinner active layers, in both cases the bulk component of the series resistance decreased. Nevertheless, a decrease of short circuit current is seen at reduced illumination. Shunt resistance shows a slight increase resulting in an improvement of open circuit voltage. Also, the fill factor increases for thinner active layers. For the thinnest standard devices a power conversion efficiency of over 3% could be measured under AM1.5 conditions.

Demonstration of a monolithic multichannel module for multi-Gb/s intra-MCM optical interconnects

In this letter, we report on the demonstration of a 2.48-Gb/s multichannel optical data-link for intramultichip module interconnects. The optical module was fabricated in polymethylmethacrylate (PMMA) by deep proton lithography and monolithically integrates micromirrors and cylindrical lenses. With the same technology, we have also fabricated a single-channel optical bridge and used this component to demonstrate a proof-of-principle optical intrachip interconnect by establishing a digital data-link between optoelectronic transceivers integrated on the same chip.

Chaotic motion and supression of commensurability effects in an Andreev antidot billiard

We investigated antidot lattices with Andreev reflecting boundaries both in experiment and theory. The samples consist of arrays of niobium-filled antidots in an InAs-based heterostructure. The quality of the Nb–InAs contact could be controlled by an in situ Ar+ sputter cleaning step. In samples with a high transparency contact, we find a strong reduction of the resistance around B=0 and a suppression of the commensurability peaks. These observations can be explained in a classical Kubo approach by including Andreev reflection and assuming a magnetic-field-dependent Andreev reflection probability.

A Simple pH Detector Based on an Organic Field-Effect Transistor

Field-effect transistors realized with organic semiconductors present several potential advantages for sensor technology, mainly in terms of cost, compatibility with plastic substrates and biocompatibility. Within the main goal of developing low-cost, disposable transducers for biochemical applications, we have fabricated a simple device that enables pH monitoring, based on an organic-based field-effect transistor. We have also developed an analogue interface that takes into account the particularities of the organic transistor in order to illustrate the pH response. The device concept, electrical characteristics, pH sensitivity and stability are discussed.