The discovery of semi-conducting and conducting organic materials has opened new possibilities for electronic devices and systems. Applications, previously unattainable for conventional electronics, have become possible thanks to the development of conjugated polymers. Conjugated polymers that are both ion- and electron conducting, allow for electrochemical doping and de-doping via reversible processes as long as both forms of conduction remain available. Doping causes rearrangement of the -system along the polymer backbone, and creates new states in the optical band gap, resulting in an increased electronic conductivity and also control of the color (electrochromism). Doping can also occur by charge injection at a metal – semiconducting polymer interface. Electrochemical electronic devices and solid state devices based on these two types of doping are now beginning to enter the market. This thesis deals with organic based-devices whose working mechanism involves electrolytes. After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic conductivity, types, electric double layer and the electric field distribution) are briefly presented. Thereafter, a short review of the field of organic field effect transistors as well as a description of transistors that are gated via an electrolyte will be reviewed. Paper I present a novel technique to visualize the electric field within a two-dimensional electrolyte by applying the electrolyte over an array of electronically isolated islands of electrochromic polymer material on a plastic foil. By observing the color change within each polymer island the direction and the magnitude of the electric field can be measured. This technology has applications in electrolyte evaluation and is also applicable in bio-analytical measurements, including electrophoresis. The focus of paper II lies on gating an organic field effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon applying a potential to the gate, results in low operation voltages and fast response. This type of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is considered in paper III. The effect on the electronic performance of the transistor is studied as well by varying the humidity level.

<p>The discovery of semi-conducting and conducting organic materials has opened new possibilities for electronic devices and systems. Applications, previously unattainable for conventional electronics, have become possible thanks to the development of conjugated polymers. Conjugated polymers that are both ion- and electron conducting, allow for electrochemical doping and de-doping via reversible processes as long as both forms of conduction remain available. Doping causes rearrangement of the -system along the polymer backbone, and creates new states in the optical band gap, resulting in an increased electronic conductivity and also control of the color (electrochromism). Doping can also occur by charge injection at a metal – semiconducting polymer interface. Electrochemical electronic devices and solid state devices based on these two types of doping are now beginning to enter the market.</p><p>This thesis deals with organic based-devices whose working mechanism involves electrolytes. After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic conductivity, types, electric double layer and the electric field distribution) are briefly presented. Thereafter, a short review of the field of organic field effect transistors as well as a description of transistors that are gated via an electrolyte will be reviewed.</p><p>Paper I present a novel technique to visualize the electric field within a two-dimensional electrolyte by applying the electrolyte over an array of electronically isolated islands of electrochromic polymer material on a plastic foil. By observing the color change within each polymer island the direction and the magnitude of the electric field can be measured. This technology has applications in electrolyte evaluation and is also applicable in bio-analytical measurements, including electrophoresis. The focus of paper II lies on gating an organic field effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon applying a potential to the gate, results in low operation voltages and fast response. This type of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is considered in paper III. The effect on the electronic performance of the transistor is studied as well by varying the humidity level.</p>

Thesis (Ph. D.)--Ohio State University, 2007. / Full text release at OhioLINK's ETD Center delayed at author's request

Thienoacenes represent an intriguing class of organic semiconducting molecules with potential applications in organic electronics. Some of thienoacenes have been reported with high charge carrier mobility in organic field-effect transistors (OFET). OFETs based on naphtho[2,1-b:3,4-b’]dithiophene (NDT) exhibited moderate device performance and low-band gap donor-acceptor copolymers based on NDT showed a promising solar power conversion efficiency. In this thesis, four novel series of thienoacenes based on naphtho[2,1-b:3,4-b’]dithiophene backbone were designed and synthesized for OFET applications. Firstly, a novel series of p-type semiconducting naphthodithieno[3,2-b]thiophene derivatives (NDTT-n) composed of six-fused aromatic rings were designed and synthesized (Figure 1). The OFETs based on NDTT-10, and NDTT-12 fabricated by vacuum deposition showed a hole mobility of 0.22 and 0.13 cm2/(Vs), respectively with Ion/Ioff above 107 after annealing at 80 oC. Secondly, the derivatives of NDT fused with benzene rings at the flanks of thiophene, namely NBBT-n (Figure 2) were also designed and synthesized. OFETs based on NBBTF-10 fabricated by vacuum deposition exhibited a hole mobility of 0.35 cm2/(Vs) with a current on/off ratio of 106 107 after annealing at 160 oC. Further extension of π-conjugation of NDTT by incorporating with fused thiophenes leading to a new NBTBT-n series was also developed (Figure 3). The OFETs fabricated by NBTBT-10 showed the hole mobility up to 0.25 cm2/(Vs) with a current on/off ratio of 105 106 after annealing at 220 oC. Lastly, two dimensionally π-extended, butterfly-shaped thienoacenes (Figure 4) were also synthesized. The OFETs based on SMB-10 fabricated by spin-coating showed the best performance in this series with an average mobility of 0.027 cm2/(Vs) for five devices and the highest mobility of 0.038 cm2/(Vs) with a current on/off ratio of 106 107 by from chloroform. Key words: organic semiconducting molecules, organic field-effect transistor, thienoacene, charge carrier mobility.

Over the past few decades, metal phthalocyanines (MPcs) have been thoroughly investigated as active materials in organic field-effect transistors (OFETs) towards the commercialization of flexible integrated circuits and displays. One of several advantages to MPcs as building blocks for OFETs is the high degree of functionality, from which the choice of metal ion, substituents along with the phthalocyanine framework and axially bound ligands can synergistically tune the physical and self-assembly properties of the material. Recent interest has been directed to the introduction of main-group elements as the central ion of MPcs as an avenue to install both hole and electron transport properties and improve device performance. To prepare materials that are suitable to be employed as the semiconducting active layer in organic field-effect transistors, a family of novel silicon phthalocyanine derivatives was prepared. The synthesis and optoelectronic properties of those new axially disubstituted silicon phthalocyanines are detailed in this work. Axial ligand variation mainly includes alkylsiloxy derivatives. The emphasis of the current thesis, however, is on tailoring the Pc backbone, which includes replacing the four benzene units with pyrazine moieties, extending the degree of conjugation with naphthalene, and introducing substituents on their peripheral positions. Several metal-containing tetra-2,3-pyrazinoporphyrazines are also described, but their applications are limited due to the difficulty of purification. Specifically, Chapter 1 serves as a comprehensive review of main-group phthalocyanines and their use as active materials in organic field-effect transistors. In Chapter 2, silicon tetra-2,3-pyrazinoporphyrazine complexes are explored. The isosteric substitution of CH groups in Pc macrocycle for nitrogen atoms leads to an obvious hypsochromic shift in their main absorption band, and their relatively low energy levels make them promising air-stable n-type organic semiconducting materials for OFETs. The synthesis and characterization of silicon tetra(tert-butyl)-2,3-naphthalocyanine complexes are described in Chapter 3. The extension of π-conjugation leads to obvious bathochromic shifts in the main absorption band. In addition, the introduction of tert-butyl groups on the periphery of the molecule reduces the tendency of the naphthalocyanine molecules to aggregate, thereby increase their solubility. Chapter 4 covers the synthesis and characterization of zinc tetra-2,3-pyrazinoporphyrazine and cobalt tetra-2,3-pyrazinoporphyrazine, whereas more future works are expected. The fifth chapter provides a conclusion to this work, and possible future directions of the research conducted herein.

<p> In this thesis, the current-voltage (I-V) behaviour of Liquid Organic Field-Effect Transistor (LOFET) was systematically studied with respect to the gate voltage, channel length and channel fluid. LOFETs in both internal and external gate modes were successfully fabricated in four-probe configuration. </p> <p> It was discovered that the effect of gate voltage on the source-to-drain current of LOFETs was significant. The drain current clearly increased when the gate voltage increased. This phenomenon was found in all LOFETs samples with different channel fluids and channel lengths. In addition, it was also proved that anions are the majority carriers in LOFETs. The concentration of anions inside the LOFET channel increased while applying a larger voltage to the gate, resulted in an increase of the drain current. This achievable gate modulation set up a solid foundation for further research on the manipulation of ionic and molecular species. </p> <p>It was also obtained that the drain current was changed with variable channel lengths. The current through the LOFET channel decreased while the channel length increased. At the same time, the difference between drain current in various channels evidently increased when the gate voltage increased from 0 to 5V. This was found to be due to the anion concentration change with varying gate voltages. </p> <p> The drain currents through LOFET channels filled with fluids of different polarities were also measured. It was observed that when the polarity of the molecule increased from that of 4,4'-Dihydroxybiphenyl to that of 2-Amino-4 Phenylphenol, the drain current increased significantly. At the same time, the difference between drain current in specific solutions was also more significant, when applying higher voltage to the gate. Combining these results with the gate modulation above, there is great potential of developing new sensing techniques and even logic operation in the future. </p> <p> This work represents a step towards a new group of cheap and effiecient electronic components of LOFETs. Guided by systemic observations from the effects of gate voltage, channel length and fluid structure, there is no doubt that LOFET will become a more attractive research topic because of its promising advantages, such as easy fabrication, low cost and its highly sensitive response. </p> / Thesis / Master of Applied Science (MASc)

Organic electronics based on π-conjugated semi-conductor raises new technology, such as organic film transistors, e-paper, and organic photovoltaic cells that can be implemented cost-effectively on large-area applications. Currently, the device performance is limited by low charge carrier mobility. Poly(3-hexylthiophene) (P3HT) and organic field effect transistors (OFET) is used as a model to investigate morphology of the organic film and corresponding electronic properties. In this thesis, processing parameters such as boiling points and solubility are controlled to impact the micro- and macro-morphology of the film to enhance the charge transport of the device. Alternative approach to improve ordering of polymer chains and increase in charge transport without post-treatment of P3HT solution is studied. The addition of high boiling good solvent to the relatively low boiling main solvent forms ordered packing of π-conjugated polymers during the deposition process. We show that addition of 1% of dichlorobenzene (DCB) to the chloroform based P3HT solution was sufficient to improve wetting and molecular structures of the film to increase carrier mobility. Systematic study of solvent-assisted re-annealing technique, which has potential application in OFET encapsulation and fabrication of top-contact OFET, is conducted to improve mobility of OFET, and, to suggest a cost-effective processing condition suitable for industrial application. Three process parameters: boiling point, polarity, and solubility are investigated to further understand the trend of film response to the solvent-assisted technique. We report the high boiling non-polar solvents with relatively high RED values promote highest improvement in molecular packing and formulate crystalline structure of the thin film, which increases the device performance.

Conjugated polymers have attracted much interest as promising alternatives to inorganic semiconductors, due to their low-temperature, solution-based processability, which may provide for low-cost, large-area electronic device fabrication. However, commercialization of polymer-based electronic devices has been restricted owing to low device performance of solidified thin-films. In order to enhance charge transport of polymer semiconductor thin-films, the self-organization of organic polymer semiconductors into ordered supramolecular assemblies has been achieved by tuning a range of process parameters including film deposition method (spin vs. drop cast), solvent boiling point (low vs. high boiling point), polymer-dielectric interface treatment, and post-deposition processing (solvent vapor or thermal annealing). However, these strategies give rise to limitations for large-scale high-throughput processing due to associated pre- and/or post semiconductor deposition steps. Therefore, in this thesis, we identify alternative processing parameters (i.e., hydrogen bonds between good and poor solvents, UV irradiation to polymer precursor solutions, and combination of sonication and subsequent UV irradiation to polymer precursor solutions) which can contribute to enhancement in charge transport of a model polymer semiconductor, poly(3-hexylthiophene) (P3HT), eliminating the additional pre- and/or post-steps mentioned above. Further, we understand of how the processing parameters effect intra- and intermolecular interactions of the polymer chains, micro- through macroscopic morphologies, and charge transport characteristics of the resultant films.

The π-conjugated organic and polymeric semiconducting materials have attracted much attention in the past years due to their significant potential in applications to electronic and optoelectronic devices including organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic light-emitting diodes (OLEDs), etc. Yet, organic and polymeric semiconductors still have challenges associated with their relatively low charge carrier (hole and electron) transport mobilities and ambient stability in OFET applications. This dissertation discusses the molecular engineering on backbones and side-chains of π-conjugated semiconducting polymers to enhance the hole and electron field-effect mobilities. Three donor-acceptor copolymers, the hole transport (p-type) poly(hexathiophene-co-benzo- thiazole) (PBT6), the hole transport poly(thiophenes-benzothiadiazole-thiophenes-diketopyrrolo- pyrrole) (pTBTD), and the electron transport (n-type) poly(dithieno-diketopyrrolopyrrole-bithiazole) (PDBTz) have been developed. Besides, the effect of polymer side chains on polymer solution-processability and charge carrier transport properties was systematically investigated: a side chain 5-decylheptadecyl having the branching position remote from the polymer backbone merges the advantages of the improved solubility from traditional branched side chains in which the branch chains are close to polymer backbone and the effective π-π intermolecular interactions commonly associated with linear side chains. This indicates the potential of side chain engineering to facilitate the charge carrier transport performance of organic and polymeric semiconductors. Additionally, PDBTz solution-processing to OFETs based on non-halogenated solvents (xylenes and tetralin) was studied. The resultant thin-film OFET devices based on non-halogenated solvents exhibited similar film morphology and field-effect electron mobilities as the counterparts based on halogenated solvents, indicative of the feasibility of developing high mobility OFET devices through more environmentally-benign processing.

Organic π-conjugated Donor-Acceptor copolymers are emerging as potential candidate materials for organic field effect transistor (OFET) and organic photovoltaic (OPV) applications. The electron-deficient benzothiadiazole group coupled with an electron-rich oligothiophene to form donor-acceptor copolymers has attracted significant attention. These low optical band gap materials absorb photons in the range of 400-800 nm and exhibit good thermal stability. In particular, poly(benzothiadiazole-sexithiophene) (PBT6) exhibits excellent performance in optoelectronic devices and high thermal stability. Here, we present the chemical synthesis and characterization of the polymer, PBT6. Three samples of PBT-6 with differing molecular weights in the range of Mn 18000-45000 Da were synthesized. Each polymer was characterized with respect to its photophysical, thermal properties and field-effected mobility was determined. Devices were prepared by drop-casting polymer solutions in 1,2-dichlorobenzene (DCB) onto an OFET (bottom gate/bottom contact) substrate and the devices were used to examine the charge transport properties of each polymer system. The optimal solvent to be used for processing technique was determined and surface techniques using OTS-8 and OTS-18 were compared through contact angle measurements. The measured charge carrier mobilities were in the range of 0.45-0.6 cm² / V.s. Polymer films prepared via drop-casting and which were thermal annealed exhibit mobilities as high as 0.825 cm² / V.s. This work examines the effect of molecular weight on the charge carrier transport properties and demonstrates the correlation of performance with molecular ordering. Drop-casted films of PBT-6 exhibit highly ordered crystalline lamellar structure with high degree of π- π stacking with edge-on orientation on the substrate. The longer conjugation lengths promote intrachain charge transfer. This high degree of molecular ordering in high MW samples of PBT6 improves the interchain and intrachain charge transfer leading to enhanced mobilities. The increased molecular weight (MW) facilitates in forming more uniform thin films which is vital in processing and application of polymer thin film technologies. These results and observations clearly demonstrate the potential of PBT-6 as a semiconducting material for Optoelectronic devices.