Thomas J. Dawidczyk

Response diversity and dual response mechanism of organic field-effect transistors with dinitrotoluene vapor

We employ an assortment of n- and p-channel organic semiconductors to fabricate organic field-effect transistors (OFETs) that respond in different ways to dinitrotoluene (DNT) vapor. Different responses are obtained from devices with various semiconductor materials and receptor functionality. Differences are noted in the magnitude and direction of current changes, and also the device parameters (mobility and threshold voltage) affected. The pattern of responses includes sufficient diversity to set it apart from other vapors that might give similar responses to some OFETs in the set, but not all of them. This gives rise to the possibility of distinguishing DNT from other vapors in a manner that could be further developed using mechanistic principles elucidated in this work.

Naphthalenetetracarboxylic Diimide Layer-Based Transistors with Nanometer Oxide and Side Chain Dielectrics Operating below One Volt

We designed a new naphthalenetetracarboxylic diimide (NTCDI) semiconductor molecule with long fluoroalkylbenzyl side chains. The side chains, 1.2 nm long, not only aid in self-assembly and kinetically stabilize injected electrons but also act as part of the gate dielectric in field-effect transistors. On Si substrates coated only with the 2 nm thick native oxide, NTCDI semiconductor films were deposited with thicknesses from 17 to 120 nm. Top contact Au electrodes were deposited as sources and drains. The devices showed good transistor characteristics in air with 0.1-1 μA of drain current at 0.5 V of V(G) and V(DS) and W/L of 10-20, even though channel width (250 μm) is over 1000 times the distance (20 nm) between gate and drain electrodes. The extracted capacitance-times-mobility product, an expression of the sheet transconductance, can exceed 100 nS V(-1), 2 orders of magnitude higher than typical organic transistors. The vertical low-frequency capacitance with gate voltage applied in the accumulation regime reached as high as 650 nF/cm(2), matching the harmonic sum of capacitances of the native oxide and one side chain and indicating that some gate-induced carriers in such devices are distributed among all of the NTCDI core layers, although the preponderance of the carriers are still near the gate electrode. Besides demonstrating and analyzing thickness-dependent NTCDI-based transistor behavior, we also showed <1 V detection of dinitrotoluene vapor by such transistors.

Reducing leakage currents in n-channel organic field-effect transistors using molecular dipole monolayers on nanoscale oxides.

Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.

Visualizing and quantifying charge distributions correlated to threshold voltage shifts in lateral organic transistors.

Lateral organic field-effect transistors (OFETs), consisting of a polystyrene (PS) polymer gate material and a pentacene organic semiconductor (OSC), were electrically polarized from bias stress during operation or in a separate charging step, and investigated with scanning Kelvin probe microscopy (SKPM) and current-voltage determinations. The charge storage inside the polymer was indicated, without any alteration of the OFET, as a surface voltage with SKPM, and correlated to a threshold voltage (VT) shift in the transistor operation. The SKPM method allows the gate material/OSC interface of the OFET to be visualized and the surface voltage variation between the two gate material interfaces to be mapped. The charge distribution for three samples was derived from the surface voltage maps using Poissons equation. Charge densities calculated this way agreed with those derived from the VT shifts and the lateral gate-OSC capacitance. We also compared the behavior of two other polymers with PS: PS accepted the most static charge in its entire volume, poly(2-trifluoromethylstyrene) (F-PS) had the most stability to bias stress, and poly(methyl methacrylate) (PMMA) showed the most leakage current and least consistent response to static charging of the three polymers. This work provides a clear demonstration that surface voltage on a working OFET gate material can be related to the quantity of static charge responsible for bias stress and nonvolatility in OFETs.

Kelvin probe microscopic visualization of charge storage at polystyrene interfaces with pentacene and gold

Charge carriers trapped in polystyrene (PS) were investigated with Kelvin probe microscopy (KPM) and thermally stimulated discharge current (TSDC). Lateral heterojunctions of pentacene/PS were scanned using KPM, effectively observing polarization along a side view of a lateral nonvolatile organic field-effect transistor dielectric interface. TSDC was used to observe charge migration out of PS films and to estimate the trap energy level inside the PS, using the initial rise method.

Effects of pulsing and interfacial potentials on tellurium-organic heterostructured films.

Polycrystalline thin films of tellurium and organic semiconductor molecules are paired in heterostructured field-effect transistors built on Si/SiO2 substrates. While charge carrier mobilities can exceed 1 cm(2)/(V s), there is only a limited gate voltage range over which the current is modulated. We employ continuous and pulsed measurements on transistors to explore the influence of charge equilibration time on device behavior, finding that pulsed gating improves output characteristics. We also use surface potential measurements to investigate the interfacial vacuum level offset between materials, and we modify the interlayer potential profile by interposing statically charged dielectric layers on the silicon dioxide. We show that interfacial fields determine the gate voltage range over which Te shows a field effect in heterostructures with organic semiconductors and that modification of these fields can extend this range.

Negative polarity of phenyl-C61 butyric acid methyl ester adjacent to donor macromolecule domains

Interfacial fields within organic photovoltaics influence the movement of free charge carriers, including exciton dissociation and recombination. Open circuit voltage (Voc) can also be dependent on the interfacial fields, in the event that they modulate the energy gap between donor HOMO and acceptor LUMO. A rise in the vacuum level of the acceptor will increase the gap and the Voc, which can be beneficial for device efficiency. Here, we measure the interfacial potential differences at donor-acceptor junctions using Scanning Kelvin Probe Microscopy, and quantify how much of the potential difference originates from physical contact between the donor and acceptor. We see a statistically significant and pervasive negative polarity on the phenyl-C61 butyric acid methyl ester (PCBM) side of PCBM/donor junctions, which should also be present at the complex interfaces in bulk heterojunctions. This potential difference may originate from molecular dipoles, interfacial interactions with donor materials, and/or equi...

Design, synthesis, and static charge tuning of organic semiconductors for sensing applications

Organic and polymeric semiconductors are among the alternatives to silicon being considered for sensing devices and circuitry. Their synthesis is now well established, and some performance metrics such as charge carrier mobility and optoelectronic quantum yield exceed those of inorganic counterparts such as amorphous silicon. The best fit for organic semiconductors is in applications where inherent capabilities such as rational modification of carrier energy levels and covalent connection between charge channels and surface receptors are leveraged. This presentation will describe newly synthesized organic molecular solids and polymer films where these attributes are emphasized. For example, addition of a borane to a semiconductor enhances response to ammonia, and introduction of highly electron donating tetrathiafulvalenes into moderately electron-rich polymers enhances response to electron-poor analytes (for example, TNT), for the development of chemical sensors. Carrier energy levels are markedly and predictably altered by static charge embedded in polystyrene films adjacent to organic semiconductors, for multiple device activities to be obtained from a single device layout using one semiconductor, and also the avoidance of powering gate electrodes to set optimal sensor sensitivities during operation.

Organic semiconductors (OSCs) for electronic chemical sensors

This chapter discusses an assortment of organic semiconductors used as sensors. Both p- and n-channel semiconductors are used to create architectures such as field-effect transistors. Active layers of small molecules, polymers, carbon nanotubes and graphene are included. Depending on the analyte and organic semiconductor used in the active layer, different levels of responses are achieved, allowing for better selectivity. Receptors have been incorporated into the sensors to further enhance the response and selectivity to specific analytes.

Printed Organic Electronic Sensors

There has been great progress recently in the use of organic and carbon-based materials as the active conductors in electronic sensors for chemical species (analytes). Three principal classes of such materials are conjugated oligomers/polymers, carbon nanotubes, and molecularly imprinted polymers. These materials may be equipped with receptor subunits for analyte binding specificity, and show changed conductances when analytes bind or adsorb. There has been further advancement in the assembly of devices based on these materials into circuit elements that provide output suitable for data processing and networking. Examples of sensors based on these principles, and the mechanisms by which they transduce chemical to electrical information, are reviewed in this chapter.