Work-Function Engineering of Reduced Graphene Oxides via Direct Surface Functionalization

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Structure and Charge Transport in Organic Semiconductor Thin Films

DoctorOrganic semiconductor materials have attracted special attention in recent decades because of their potential applications for organic electronics, especially for organic field-effect transistors (OFETs). OFETs, often referred to organic thin-film transistors (OTFTs), are amplifying and switching devices in electronic circuits and are considered as the key element for realizing flexible printed electronics, including applications for wearable medical sensors and active matrices for bendable/foldable displays. However, compared to its inorganic counter parts, the poor device performance of OFETs, particularly their inferior charge transport characteristics, remains a serious obstacle to their commercial use. The charge carrier mobility of OFETs reflects the charge transport characteristics of organic semiconductor layers, and the value is affected by a numerous factors during OFET fabrication. Investigating the effects of the governing factors is an important challenge with regard to realizing practical, useful, and high-performance OFETs. This dissertation systematically studies the links among the chemical structures of small-/macro-molecular organic semiconductors, processing history of semiconductor layers, their solid-state ordering, and resultant device performance. Various polymers and organic small molecules are examined as active materials for p-type and n-type OFETs, and their thin-film microstructures are thoroughly investigated. Furthermore, various processing strategies are introduced for controlling the morphologies and crystalline microstructure of organic semiconductors to demonstrate high-mobility OFETs. This dissertation consists of 11 chapters. The background and motivation of the research are introduced in Chapter 1. Chapters 2 through 6 cover the chemical tailoring of molecular structures and its effects on crystalline microstructure, morphology, and charge transport characteristics. In Chapter 2, a series of diketopyrrolopyrrole-based copolymers designed by implementing the concept of intramolecular non-covalent conformational locks through the functionalization of polymer backbones with fluorine atoms or methoxy groups are studied and compared with their unfunctionalized analogue. In contrast to the bimodal texture of the unfunctionalized polymer, the thin films of the polymer with fluorine atoms exhibit predominantly edge-on texture with much improved crystalline ordering. The thin films of the polymer modified with methoxy groups have a principally face-on texture. These dramatic differences in thin-film texture are correlated with the solubilities of polymers. The improved crystalline ordering of these semiconductor polymers also enables the fabrication of high-performance OFETs. The hole mobility of the methoxy-modified polymer is reduced by half with respect to that of the unmodified polymer, whereas the hole mobility values of the fluorine-modified polymer are approximately up to six times higher at 1.32 cm2 V?1 s?1, and exhibit pronounced thermal stability. These results provide new guidelines for the molecular design of semiconducting polymers with non-covalent conformational locks. In addition to high hole mobility p-type semiconducting polymers in Chapter 2, high electron mobility n-type semiconducting polymers are studied in Chapter 3 through 5. In Chapter 3, oligo(ethylene glycol)-incorporated hybrid linear alkyl side chains that serve as solubilizing groups are designed and introduced into naphthalene-diimide-based n-type copolymers. The new side chains are designed by combining hydrophilic oligo(ethylene glycol) and hydrophobic alkyl chains. Such hybrid linear side chains result in polymer semiconductors with rigid backbones that have similar solubility in organic solvents as branched-alkyl analogues. Furthermore, when these polymers are processed, the attached side chains do not disrupt the rigidity of the polymer backbones in the thin films. The synthesized polymers exhibit a unipolar n-type operation with an electron mobility of up to 1.64 cm2 V?1 s?1, which demonstrates the usefulness of the hybrid side chains in polymer electronic applications. In Chapter 4, semi-fluorinated linear alkyl side chains are designed and incorporated into naphthalene diimide-based n-type copolymers. The strong self-organization of semi-fluoroalkyl chains in itself induces superior ordering of the resulting polymers, especially in enhanced backbone rigidity. The polymers exhibit a unipolar n-channel transport in field-effect transistors with extremely high electron mobilities of up to 6.50 cm2 V?1 s?1 with a high on?off current ratio of 105, which is among the highest values for n-type conjugated polymers. Chapter 5 presents an in-depth study of Chapter 3 and 4 through an exploration of the microstructural origins of charge traps inside OFET devices. The unique electrical properties of an n-type semiconducting polymer, poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2OD-T2)) are examined to study the correlation between the microstructures of polymer semiconductor thin films and the bias stability of an OFET. Although the charge carrier mobilities in a series of devices may be similar, the bias stress stabilities can differ significantly depending on the molecular orientations of the semiconducting thin films. A higher degree of bias stress stability was attained in the P(NDI2OD-T2) FETs prepared with face-on thin-film structures compared to the bias stress stability attained in the edge-on film structures. Further experimental evidence suggests that the aliphatic alkyl chains in edge-on-oriented P(NDI2OD-T2) films, in contrast with the face-on structured thin films, present a hurdle to vertical charge transport and induce large numbers of bipolarons during bias stress. Chapter 6 discusses the systematic investigation of the effects of substituted alkyl chain length on solution-processable organic semiconductors. A series of anthradithiophene (ADT) derivatives with an alkylphenylethynyl group (Cn-PEADT) is designed as model compounds to control their alkyl chain lengths (n = 4, 6, 8, 10). The Cn-PEADTs shows similar crystalline structures, but the relative positions between the conjugated layers are changed along with the side chain length. Slight structural changes lead to huge differences in the film formation properties of the compounds, thereby resulting in dramatically different electrical properties in solution-processed OFET devices. The physical structure of an organic solid is strongly affected by the surface of the underlying substrate. Controlling this interface is an important issue to improve device performance. Chapters 7 and 8 discuss the role of the underlying substrate surface and introduce two surface treatment methods for improving the quality of a vacuum deposited organic semiconductor thin film in a view of thin-film microstructures. The study in Chapter 7 systematically examines the effects of the grain structure of crystalline octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on the growth of organic semiconductor thin films on such monolayers. The electrical characteristics of the resulting semiconductor films are studied as well. The grain structure of the OTS monolayers is controlled by preparing the monolayers at different temperatures. The OTS monolayer prepared at the low temperature of ?30 °C showed larger crystalline grains and longer-range homogeneity of alkyl chain arrays compared with the monolayer prepared at a higher temperature. Pentacene films deposited on such OTS monolayers exhibit larger crystalline grains with higher degrees of crystallinity and lateral alignment compared to films deposited on OTS monolayers prepared at temperatures higher than ?30 °C. This outcome follows the surface characteristics of the underlying OTS monolayers. As a result, pentacene FETs prepared with ?30 °C OTS monolayers show lower charge trap densities and higher field-effect mobility values than devices fabricated using other OTS monolayers. These results demonstrate the enhanced quasi-epitaxial growth of pentacene films on OTS monolayers with large grains. Chapter 8 demonstrates a new approach that utilizes an organic heterointerface to improve the crystallinity and control the morphology of organic thin film. Pentacene was used as an active layer above, and m-bis(triphenylsilyl)benzene was used as the bottom layer. Sequential evaporations of these materials result in extraordinary morphology with far fewer grain boundaries and myriad nanometer-sized pores. These peculiar structures are formed by difference in molecular interactions between the organic layers and the substrate surface. The pentacene film exhibits high mobility up to 6.3 cm2 V?1 s?1, and the pore-rich structure improves the sensitivity of organic-transistor-based chemical sensors. This approach opens a new way for the fabrication of nanostructured semiconducting layers towards high-performance organic electronics. In contrast to the vacuum evaporation process used in Chapters 7 and 8, Chapters 9 through 11 cover solution-based printing processes instrumental in fabricating cost-efficient flexible printed electronic devices. The last three chapters introduce process optimization strategies for printed, high-mobility OFET devices. To this end, microstructures of organic semiconductor thin films are controlled. Chapter 9 presents the process optimization of spin-coating. Although spin-coating is used extensively in the fabrication of organic electronic devices, but the effects of its processing parameters are still not fully determined. Herein, the effects of spin-coating time on the microstructure evolution during semiconducting polymer solidification is systematically characterized. The short spin-coating time of a few seconds dramatically improve the morphology and molecular order in a conjugated polymer thin film. This outcome is attributed to the fact that π-π stacking structures formed by the polymer molecules grow slowly and with a greater degree of order owing to the residual solvent present in the wet film. The improved ordering is correlated with the improved charge carrier transport in the FETs prepared from these films. The chapter also demonstrates the effects of various processing additives on the resulting FET characteristics and the film drying behavior during spin-coating. The physical properties of the additives are found to affect the film drying process and the performance of the resulting device. In Chapters 10 and 11, a capillary pen drawing technique is demonstrated as a new patterning methodology for the large-area patterning and fabrication of organic electronics. This approach provides several advantages over conventional approaches: it is simple and versatile, has no restrictions on the patterning shapes that can be produced, and can be tailored to a variety of substrates. By exploiting the solvent additives studied in the previous chapter, the optimal ink formulation is suggested and the microstructural changes are observed according to the added additives and chemical structures of the substrate surface. In the chapter that follows, fully-drawn all-organic FET arrays on mechanically flexible substrates are demonstrated by using only the capillary-pen printing method. The sequential printing processes deposit a highly crystalline organic semiconductor, a very smooth insulating polymer, and conducting polymer layers from the solution. The OFETs prepared on a flexible substrate exhibit superior field-effect mobility, reproducibility, operational stability, and mechanical bendability. These results indicate that capillary pen printing is promising as a manufacturing technique for a wide range of large-area electronic applications based on various functional materials

Work-Funtion Engineering of Reduced Graphene Oxide Electrodes for Organic Thin Film Transistors

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Bias-Stress-Induced Charge Trapping in Organic Transistor

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Molecular Orientation-dependent Bias Stress Stability in n-Type Organic Transistors.

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Growth of Large-Grain Macroporous Organic Semiconductor Thin Films via Organic Heterointerface

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Development of Stretchable and Transparent Organic Semiconducting Material via Blending of Conjugated Polymer Nanowires and Elastomer

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Analysis of growth mechanism for self-assembled monolayer in organic field effect transistor

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