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.
We describe chemically sensitive organic transistors in which the semiconductor film consists of a base layer of a high
mobility p- or n-channel molecular solid, and an overlayer contains analogous compounds terminated with hydroxy
functional groups. Such devices respond to dimethyl methylphosphonate at concentrations on the order of 100 ppb and
on time scales <1 minute. Semiconductor cores include diphenylbithiophene and naphthalenetetracarboxylic diimide,
with OH end groups in some cases. End groups include both alkyl and phenolic OH. Devices are as thin as four
monolayers. Sensitivity is highly gate dependent, and means of ensuring the gate setting for maximum response are
proposed. Contrasting response to dinitrotoluene, a component of nitroaromatic explosive vapors, is reported. Finally,
the influence of the channel versus near-contact regions on the vapor-induced changes in resistance is evaluated.
Recent progress in the field of organic electronics is due to a fruitful combination of both innovative molecular design and promising low-cost material/device assembly. Targeting the first strategy, we present here the general synthesis of fluoroarene-containing thiophene-based semiconductors and the study of their properties with respect to the corresponding fluorine-free hole-transporting analogues. The new compounds have been characterized by elemental analysis, mass spectrometry, and 1H- and 19F NMR. The dramatic influence of fluorine substitution and molecular architecture has been investigated by solution/film optical absorption, fluorescence emission, and cyclic voltammetry. Single crystal data for all of the oligomers have been obtained and will be presented. Film microstructure and morphology of this new class of materials have been studied by XRD and SEM. Particular emphasis will be posed on the solution-processable oligomers and polymers.
Recent research on organic and polymeric semiconductors is directed towards highly ordered molecular structures in solid states. Through molecular design and engineering, it has been shown possible to control the molecular orientation and processing conditions of these materials as well as fine tuning their energy levels and color emissions. Thin film field-effect transistors (FETs) have been used as testing structures for evaluating the semiconducting properties of new organic semiconducting materials. Performance similar to amorphous-Si can now be realized with some organic materials. Large-scale integration of organic transistors has been demonstrated. In addition, several low cost novel non-lithographic patterning methods have been developed, which resulted in the first flexible electronic paper. The field-effect transistor device structure can also be utilized as a means to induce a great amount of charge carriers in organic thin films through the gate field. Using this type of structure, superconductivity was observed in a highly ordered conjugated regioregular poly(3-hexylthiophene).
The early demonstrations of field-effect transistors based on organic semiconductors with high dynamic range utilized single devices incorporating p-channel materials such as thiophene oligomers and pentacene. Through end-group substitution and use of electron-deficient cores, we have built a library of semiconductors with a variety of attributes, including low off-current, n-channel function and chemical sensitivity. Devices containing these materials can be assembled to form complementary circuits, pixel drivers, and chemoselective sensors. Specific properties of these materials have led to a new approach to a polymer-based memory element.
A new class of photodefinable polymers based on phosphonic acid esters has been developed. Photogenerated acid catalysts convert the esters to phosphonic acids in the exposed regions of films during post-exposure bake. Those phosphonic acids, in addition to providing the base-solubility necessary for positive-tone development, are also uniquely capable of binding metal ions and cations from solution. Preliminary lithographic evaluations indicate that these polymers generally show high contrast (approximately 10), good sensitivity, low volume loss (< 15 percent) and the potential for submicron resolution. More importantly, the patterned deposition of refractory metal ions has also been demonstrated which could be useful for at-the- surface imaging and circuit fabrication applications.
We summarize some of our past work in the field on optimizing molecules for second order and third order nonlinear optical applications. We also present some previously unpublished results suggesting a particular optimization of the popular cyano- and nitrovinyl acceptor groups. In addition we provide some new quadratic electro-optic results which serve to further verify our choice of a restricted three-level model suitable for optimizing third order nonlinearities in molecules. Finally we present a new squarylium dye with a large third order optical nonlinearity (-9.5 X 10-34 cm7/esu2; EFISH (gamma) at 1906 nm).
Multilayer assembly of organic chromophores with inorganic interlayers is a facile technique for the preparation of nonlinear optical materials with well-defined architectures. Recent reports of T. M. Mallouk (University of Texas-Austin) have indicated that zirconium organodiphosphonates form particularly stable surface multilayers that can be constructed straightforwardly one monolayer at a time. We have synthesized a series of chromophores with shapes and functional groups designed for layerwise deposition as zirconium phosphonates. These include electron donating thiophene oligomers, electron accepting quinodimethanes, and dipolar azo dyes. Cohesive, electrically insulating, thermally stable films of these compounds as layered phosphonates on various substrates were prepared with predictable thicknesses and, in some cases, polar order. Second harmonic intensity from the polar azo dye films was proportional to the thickness squared, consistent with theory. The dielectric and optical responses of a variety of other kinds of samples are discussed.