published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Abstract Organic semiconductors (OSCs) and graphene are two classes of conjugated materials that hold promise to create flexible electronic displays, high speed transistors, and low-cost solar cells. Crucial to understanding the behavior of these materials is understanding the effects metallic contacts have on the local charge environment. Additionally, characterizing the charge carrier transport behavior within these materials sheds light on the physical mechanisms behind transport. The first part of this thesis examines the origin of the low-temperature, high electric field transport behavior of OSCs. Two chemically distinct OSCs are used, poly-3(hexylthiophene) (P3HT) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene. Several models explaining the low-temperature behavior are presented, with one using the Tomonaga-Luttinger liquid (TLL) insulator-to-metal transition model and one using a field-emission hopping model. While the TLL model is only valid for 1-dimensional systems, it is shown to work for both P3HT (1D) and TIPS-pentacene (2D), suggesting the TLL model is not an appropriate description of these systems. Instead, a cross-over from thermally-activated hopping to field-emission hopping is shown to explain the data well. The second part of this thesis focuses on the interaction between gold and platinum contacts and graphene using suspended graphene over sub-100 nanometer channels. Contacts to graphene can strongly dominate charge transport and mobility as well as significantly modify the charge environment local to the contacts. Platinum electrodes are discovered to be strong dopants to graphene at short length scales while gold electrodes do not have the same effect. By increasing the separation distance between the electrodes, this discrepancy is shown to disappear, suggesting an upper limit on charge diffusion from the contacts. Finally, this thesis will discuss a novel technique to observe the high-frequency behavior in OSCs using two microwave sources and an organic transistor as a mixer. A theoretical model motivating this technique is presented which suggests the possibility of retrieving gigahertz charge transport phenomena at kilohertz detection frequencies. The current state of the project is presented and discrepancies between devices made with gold and platinum electrodes measured in the GHz regime are discussed.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

No description available.

In this work pentacene adsorption on both flat and stepped Ag(111) surfaces were investigated by using Density Functional Theory within Projected Augmented Wave method. On the flat Ag(111) surface favorable adsorption site for a single pentacene molecule was determined to be the bridge site with an angle of 60&amp / #9702 / between pentacene molecular long axis and [011] lattice direction. Potential energy surface was found to be flat, especially along lattice directions. Diffusion and rotation barriers for pentacene on this surface were found to be smaller than 40 meV indicating the possibility of a two dimensional gas phase. Calculated adsorption energies for the flat surface indicate a weak interaction between molecule and the surface indicating physisorption. On the flat surface monolayer case is found to have lower adsorption energy than the isolated case due to pentacene&amp / #8722 / pentacene interactions. On the stepped Ag(233) surface, close to the step edge, adsorption energy increased significantly due to the stronger interaction between pentacene molecule and low coordinated silver step atoms. On the terraces of this surface, far from step edges, however a flat potential energy surface was observed similar to the case of flat Ag(111) surface. On the stepped surface pentacene found its favorable configuration as parallel to the step with a tilt angle similar to the observed thin film phase of pentacene on Ag(111) surface. Pentacene molecule showed small distortions on stepped surface and are closer to the silver step atoms 1 &Aring / more than the case of flat surface, hinting a chemical interaction as well as van der Waals interactions. However on Ag(799) surface, the perpendicular orientation of the pentacene molecule to the step direction showed no strong interaction due to less matching of carbon atoms with silver step atoms.

Thin-film transistors (TFTs) capable of low-voltage and high-frequency operation will be required to reduce the power consumption of next generation electronic devices driven by microelectronic components such as inverters, ring oscillators, and backplane circuits for mobile displays. To produce high performance TFTs, transparent oxide-semiconductors are becoming an attractive alternative to hydrogenated amorphous silicon (a-Si:H)- and organic-based materials because of their high electron mobility vlaues and low processing temperatures, making them compatible with flexible substrates and opening the potential for low production costs. Practical electronic devices are expected to use p- and n-channel TFT-based complementary inverters to operate with low power consumption, high gain values, and high and balanced noise margins. The p- and n-channel TFTs should yield comparable output characteristics despite differences in the materials used to achieve such performance. However, most oxide semiconductors are n-type, and the only high performance, oxide-based TFTs demonstrated so far are all n-channel, which prevents the realization of complementary metal-oxide-semiconductor (CMOS) technologies. On the other hand, ambipolar TFTs are very attractive microelectronic devices because, unlike unipolar transistors, they operate independently of the polarity of the gate voltage. This intrinsic property of ambipolar TFTs has the potential to lead to new paradigms in the design of analog and digital circuits. To date, ambipolar TFTs and their circuits, such as inverters, have shown very limited performance when compared with that obtained in unipolar TFTs. For instance, the electron and hole mobilities typically found in ambipolar TFTs (ATFTs) are, typically, at least an order of magnitude smaller than those found in unipolar TFTs. Furthermore, for a variety of circuits, ATFTs should provide balanced currents during p- and n-channel operations. Regardless of the selection of materials, achieving these basic transistor properties is a very challenging task with the use of current device geometries. This dissertation presents research work performed on oxide TFTs, oxide TFT-based electronic circuits, organic-inorganic hybrid complementary inverters, organic-inorganic hybrid ambipolar TFTs, and ambipolar TFT-based complementary-like inverters in an attempt to overcome some of the current issues. The research performed first was to develop low-voltage and high-performance oxide TFTs, with an emphasis on n-channel oxide TFTs, using high-k and/or thin dielectrics as gate insulators. A high mobility electron transporting semiconductor, amorphous indium gallium zinc oxide (a-IGZO), was used as the n-channel active material. Such oxide TFTs were employed to demonstrate active matrix organic light emitting diode (AMOLED) display backplane circuits operating at low voltage. Then, high-performance hybrid complementary inverters were developed using unipolar TFTs employing organic and inorganic semiconductors as p- and n-channel layers, respectively. An inorganic a-IGZO and pentacene, a widely used organic semiconductor, were used as the n- and p-channel semiconductors, respectively. By the integration of the p-channel organic and n-channel inorganic TFTs, high-gain complementary inverters with high and balanced noise margins were developed. A new approach to find the switching threshold voltage and the optimum value of the supply voltage to operate a complementary inverter was also proposed. Furthermore, we proposed a co-planar channel geometry for the realization of high-performance ambipolar TFTs. Using non-overlapping horizontal channels of pentacene and a-IGZO, we demonstrate hybrid organic-inorganic ambipolar TFTs with channels that show electrical properties comparable to those found in unipolar TFTs with the same channel aspect ratios. A key characteristic of this co-planar channel ambipolar TFT geometry is that the onset of ambipolar operation is mediated by a new operating regime where one of the channels can reach saturation while the other channel remains off. This allows these ambipolar TFTs to reach high on-off current ratios approaching 104. With the new design flexibility we demonstrated organic-inorganic hybrid ambipolar TFT-based complementary-like inverters, on rigid and flexible substrates, that show a significant improvement over the performance found in previously reported complementary-like inverters. From a materials perspective, this work shows that future breakthroughs in the performance of unipolar n-channel and p-channel semiconductors could be directly transposed into ambipolar transistors and circuits. Hence, we expect that this geometry will provide new strategies for the realization of high-performance ambipolar TFTs and novel ambipolar microelectronic circuits.

The work presented in this thesis is primarily concerned with the synthesis and characterization of large, linear heteroacenes and their derivatives. We have been able to significantly expand on the types of materials available for application in organic electronic device architectures. In particular, the work focused on solution processible and novel derivatives of thiadiazoles, diazatetracenes, diazapentacenes, tetrazapentacences, and N,N-dihydrotetraazaheptacene. Extensive computational studies have been performed in order to better understand the optoelectronic properties of these materials. Although no devices have been fabricated that show appreciable hole or electron mobility, the properties of these materials are very promising. Besides our work on organic electronic materials for application in optoelectronic devices, we have also been able to develop, via the Click reaction, a series of aqueous metal sensors for copper (II), nickel (II), and silver (I) based upon fluorescence quenching. The use of a modified Stern-Volmer equation was necessary to fit the data in order to obtain binding constants. The exploration of new materials and their properties in the area of organic electronics is an exciting field for the synthetic organic chemist, as the goals associated with this work strive to impact humanity in a positive manner by reducing energy costs.

In this dissertation novel work is presented showing the performance and device physics of Organic Field Effect Transistors (OFETs) and bulk heterojunction Organic Photovoltaic (OPV) devices fabricated using novel acceptor small molecules. Pentacene and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN₂) were used as the active layer in p-channel and n-channel Organic Field Effect Transistors (OFETs), respectively, and novel pulsed voltage transient measurements were developed in order to extract transient mobilities and carrier velocities from the transistor response of the device, which were well correlated with the corresponding DC OFET characteristics. A distributed RC network was used to model the OFET’s channel and the transient and DC characteristics of the devices were successfully reproduced. Temperature dependent studies of the DC field effect mobilities and transient mobilities of these two materials were carried out and the results used to extract information on charge carrier transport in the materials at varying time scales. Open-circuit voltages of the OPV devices are correlated with the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) levels various acceptor small molecules and donor polymers comprising the active layers of the devices. / text

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