Organic semiconductors are envisioned to have widespread applications in flexible displays, radio-frequency identification (RFID) tags, bio- and chem-sensors, as well as organic solar cells. Polymer semiconductors are particularly suitable for the low-cost manufacture of organic electronics using printing techniques due to their excellent solution processability and mechanical properties. This work focuses on the development of two novel building blocks, IBDF and DTA, which can be used for the construction of high performance organic thin film transistors (OTFTs) and organic photovoltaics (OPVs). Two copolymers, P6-IBDF-T and P5-IBDF-T, and a homopolymer P6-IBDF were prepared using the IBDF building block. Copolymer P6-IBDF-T has been prepared via the Stille-coupling polymerization. This polymer exhibits a small band gap of 1.36 eV with HOMO/LUMO energy level of -5.69 eV/-4.43 eV. P6-IBDF-T showed stable electron transport performance in encapsulated thin film transistors and ambipolar transport performance in non-encapsulated TFTs. Balanced hole/electron mobilities of up to 8.2 ??10-3/1.0 ??10-2 cm2V-1s-1 was achieved in bottom-contact, bottom-gate organic thin film transistors. In addition, the broad absorption of the polymer over the UV-Vis range suggested that this polymer is suitable for applications in solar cells. The effect of conjugation on mobility and UV-vis spectra of the polymer was studied by comparing P5-IBDF-T with P6-IBDF-T. The ideal of indirect electron transition was proposed to explain the difference between UV-Vis light absorption spectra for these two polymers. DTA building block was used to construct four D-A copolymers, namely PDTA-T, PDTA-BT, PDAT-BTV, and PDTA-TT. These polymers were characterized by UV-Vis, CV, DSC, TGA, AFM and XRD. Device performance was also investigated on OTFTs. The device performance of DTA based polymer increased as the area of electron donor increase from T in PDTA-T to BTV in PDTA-BTV. PDTA-BTV exhibits hole mobility of 1.3??10-3 cm2 V-1 s-1 with Ion/Ioff value of ~103-4 in bottom-contact, bottom-gate organic thin film transistors. All DTA based copolymers exhibited small optical bandgaps (1.18 ??? 1.27 eV) and required none or moderate thermal treatment during fabrication process. These make them promising candidates for cost-effective OPV applications.

A systematic approach has been used to optimise the fabrication process of pentacene-based nonvolatile organic thin film memory transistors (OTFMTs) operating at low programming voltages. In the first part of this work, reliable, reproducible and hysteresis free organic metal-insulator-semiconductor (OMIS) devices and organic thin film transistors (OTFTs) were fabricated and characterised. All devices were based on poly(methyl methacrylate) (PMMA) and poly(vinyl phenol) (PVP) as the organic insulators. The second part of this work focused on optimising the evaporation parameters to fabricate high-performance pentacene-based devices. About 50 nm thickness of pentacene film with a deposition rate of 0.03 nm s-1 on ~ 300 nm of PMMA was found to produce large, uniform and condense grains leading to high quality devices. OTFTs with high mobility of 1.32 cm2 V−1 s−1, on/off current ratio of 106, and negligible hysteresis and leakage current were demonstrated. The effect of the environment on the OTFTs obehaviour was also investigated. The bias stress effect was also investigated in terms of threshold voltage shift ΔVT at various conditions and times. The results show ΔVT increases with the increase of stress voltage. A negligible hysteresis is evident between the forward and reverse direction of the transfer characteristics and the shape of the transfer characteristics does not change with the bias stress. Floating gate memory structures with thin layer of gold, gold nanoparticles (AuNPs) and single walled carbon nanotubes (SWCNTs) were fabricated and characterised during this investigation. Hysteresis in memory structures was a clear indication of the memory effect and charge storage in these devices. Also, the hysteresis was centred close to 0 V for SWCNTs-based structures, which indicate that a low operation voltage is needed to charge the devices. A memory window of about 40 V was observed for AuNPs-based memory devices based on PVP; while the memory windows for devices based on PMMA with thin layer of Au and AuNPs floating gates were 22 V and 32 V, respectively. The electrical properties of the OTFMTs were improved by the use of the Au nanoparticles as the floating gate compared with that of an Au thin film. Using appropriate negative or positive voltages, the floating gate was charged and discharged, resulting in a clear shift in the threshold voltage of the memory transistors. Negative and positive pulses of 1 V resulted in clear write and erase states, respectively. Additionally, these organic memory transistors exhibited rather high carrier mobility of about μ = 0.319 cm2 V-1 s-1. Furthermore the data retention and endurance measurements confirmed the non-volatile memory properties of the memory devices fabricated in this study.

A novel polymer semiconductor with side chains thermally cleavable at a low temperature of 200 °C was synthesized. The complete cleavage and removal of the insulating 2-octyldodecanoyl side chains were verified with TGA, FT-IR, and NMR data. The N-H groups on the native polymer backbone are expected to form intermolecular hydrogen bonds with the C=O groups on the neighboring polymer chains to establish 3-D charge transport networks. The resulting side chain-free conjugated polymer is proven to be an active p-type semiconductor material for organic thin film transistors (OTFTs), exhibiting hole mobility of up to 0.078 cm2V-1s-1. This thermo-cleavable polymer was blended with PDQT to form films that showed a higher performance than the pure individual polymers in OTFTs. MoO3 or NPB was used as a hole injection buffer layer between the metal electrodes and the polymer semiconductor film layer in OTFT devices. This buffer layer improved hole injection, while its use in the OTFT, improved the field-effect mobility significantly due to better matched energy levels between the electrodes and the polymer semiconductor.

A novel polymer semiconductor with side chains thermally cleavable at a low temperature of 200 °C was synthesized. The complete cleavage and removal of the insulating 2-octyldodecanoyl side chains were verified with TGA, FT-IR, and NMR data. The N-H groups on the native polymer backbone are expected to form intermolecular hydrogen bonds with the C=O groups on the neighboring polymer chains to establish 3-D charge transport networks. The resulting side chain-free conjugated polymer is proven to be an active p-type semiconductor material for organic thin film transistors (OTFTs), exhibiting hole mobility of up to 0.078 cm2V-1s-1. This thermo-cleavable polymer was blended with PDQT to form films that showed a higher performance than the pure individual polymers in OTFTs. MoO3 or NPB was used as a hole injection buffer layer between the metal electrodes and the polymer semiconductor film layer in OTFT devices. This buffer layer improved hole injection, while its use in the OTFT, improved the field-effect mobility significantly due to better matched energy levels between the electrodes and the polymer semiconductor.

Our group has previously shown that small changes to molecular structure result in large changes to device properties and stability in organic electronic applications. By functionalizing aromatic heteroacenes with group 14 and group 16 elements, it is possible to control morphology and improve stability for a variety of applications such as thin film transistors and solar cells. Functionalization within the heteroacene core led to changes in electronic structure as observed by electrochemistry and light absorption. By substituting down the periodic table, the carbon heteroatom bond length increased, leading to subtle changes in crystal packing. Absorption maxima were red-­‐shifted and stability to light decreased. Substitution of group 14 elements to the solubilizing ethynyl groups attached to the heteroacene also had an effect on crystallization and stability. Substitution of silicon with carbon decreased solubility as well as stability to light. Substitution with germanium also decreased stability to light, but close contacts within the crystal structure and solubility in nonpolar organic solvents increased.

Soluble organic semiconductors have attracted interest due to their potential in making flexible and cheap electronics. Though their use is being implemented in electronics today, the conduction mechanism is still under investigation. In order to study the charge transport, this study examines the position, voltage, and frequency dependence of charge induced changes in far infrared absorption in soluble organic semiconductors in thin-film transistor structures. Measurements are compared to a simple model of a one-dimensional conductor which gives insight into the charge distribution and timing in devices. Main results of the study are dynamic measurements of charge taken by varying the frequency of the applied gate voltage while observing signal at one position within the transistor; mobility values obtained from a comparison to the one-dimensional model compare well with standard current-voltage measurements. Two small molecule soluble organic semiconductors were studied: 6,13 bis(triisopropylsilylethynyl)-pentacene and fluorinated 5,11 bis(triethylsilylethynyl) anthradithiophene.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21884号 / エネ博第385号 / 新制||エネ||75(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 佐川 尚, 教授 萩原 理加, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(情報学) / 甲第24746号 / 情博第834号 / 新制||情||140(附属図書館) / 京都大学大学院情報学研究科通信情報システム専攻 / (主査)教授 佐藤 高史, 教授 橋本 昌宜, 教授 新津 葵一 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

Silicon phthalocyanines (R₂-SiPcs) are an emerging class of high-performance n-type or ambipolar organic semiconductors which have found application in organic electronic devices, including organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). Owing to their tetravalent silicon metal centre, R₂-SiPcs can be substituted with a range of axial ligands including phenols, carboxylic acids, and silanes to tune their intermolecular interactions, optical properties, electronic properties and solubility. While early reports of R₂-SiPcs have demonstrated promising results, the relationship between their structure and performance in OTFTs is poorly understood. Additionally, many OTFTs with R₂-SiPcs as semiconductor only demonstrate n-type behaviour under inert atmospheres due to their shallow lowest unoccupied orbital level below -4.1 eV making them susceptible to electron trapping by moisture and oxygen. This thesis presents developments in both the understanding of how R₂-SiPc structure influences performance, device engineering and exploration of these materials in ammonia sensors. First, I develop of structure-property relationships for a catalogue of fifteen R₂-SiPcs integrated into OTFTs including eleven materials used in OTFTs for the first time. I then explore the influence of dielectric surface chemistry on the texture of R₂-SiPc films and their resulting performance in OTFTs using silane self-assembled monolayers and para-sexiphenyl to understand the weak epitaxial growth behaviour of this class of materials. Next, I report eight novel peripherally fluorinated and axially substituted silicon phthalocyanines (R₂-FₓSiPcs) to investigate the influence of peripheral and axial fluorination on air-stable electron transport and determine the threshold for achieving air-stable n-type OTFTs. Finally, I integrate R₂-FₓSiPcs into organic heterojunction ammonia gas sensors to understand the influence of peripheral fluorination on the majority charge carrier in this device architecture.

Organic electronics have reached the level of commercialization and are important parts of our daily life. They are integrated into portable computers, cell phones, identification cards, television, cars, etc. The organic thin film transistors (OTFTs) are the most attractive organic electronic elements that have applications as electronic flexible paper, sensors, smart cards, erasable memory devices, RF-ID tags, and in backplanes for OLED displays. Their performance has already exceeded the performance of transistors based on the amorphous silicon (α-Si). Organic thin film phototransistors (OPTs) have attracted significant research attention as functional OTFTs due to the unique structure of OTFTs (three-terminal device) complemented with the light (fourth terminal). The OTFTs structure enables modulation and amplification of the output signal (the drain current) while light gives the functionality and enhances the performance. Compared to organic photodiodes, OPTs have higher sensitivity and lower noise due to the OTFT structure. Additionally, the advantage of OPTs over inorganic PTs lays in a variety of light responsive organic materials that can be used as active channel materials. Accordingly, use of organic compounds enabled OPTs fabrication from solution, melt, and printing, over large areas of plastic substrates with which they are compatible. So far, many researchers have reported high-performance OPTs. Typically, synthesis of the new light receiving/emitting semiconducting materials is the common approach for the OPT development. Another way is to engineer the device structure by introducing new layers with different functionalities. Often, synthesis is costly, complex, lengthy, and not industrially feasible. This thesis focuses on the development of new methods and materials for OPT performance enhancement to avoid lengthy synthesis and fabrication processes. According to the layers in a typical OPT, that is, from the top to the bottom: active channel, channel/gate dielectric interface, and the gate dielectric layer, the thesis has three major focuses: engineering of the active channel for high-performance OPTs using existing small molecule and existing or new dielectric polymeric materials (Chapters 3-5), interface engineering (Chapter 6), and engineering of the gate dielectric layer (Chapter 7). Utilizing blends of a UV-A responsive 2,7-dipentyl[1]benzothieno[3,2-b][1]benzothiophene (C5-BTBT) small molecule semiconductor and various dielectric polymers (polyesters, PMMA, PVAc, PS, and PC) we developed highly photoresponsive and photosensitive OPTs. Furthermore, we designed and synthesized a new polyimide that is soluble, thermally stable, with reduced deep coloration and more importantly with the strong electron withdrawing groups. High-performance and highly photosensitive OPTs were achieved with capabilities of the application as photo memory elements characteristic of fast switching and long retention times of the persistent photocurrent. We demonstrated that by simple channel/dielectric interface modification using organosilanes with different end groups, the drain photocurrent, and hole mobility could be modulated and enhanced under the UV light illumination. In the final part, we demonstrated that both active channel and dielectric layer engineering could synergistically enhance the performance of OPTs for potential fabrication as photo memory elements. This thesis contributed significantly to fundamental knowledge of photoresponsive organic electronic devices and application of OPTs in the area of printed and flexible electronics / Thesis / Doctor of Philosophy (PhD) / Organic electronics have become a part of our daily life since they are integrated into cell phones, computers, TVs, displays, etc. Their advantage is their versatility due to a variety of organic compounds that can be used as semiconductors for the specific applications, low-cost processing methods (solution, printing, and melt) and large-area flexible substrates that can be used for their fabrication. For the same reasons, organic phototransistors are very attractive for modern optoelectronics. Generally, in this study, we developed and demonstrated new strategies of developing an organic phototransistor and enhancing/optimizing its performance. Firstly, we developed semiconducting blends that are responsive to UV-A light when integrated into an organic phototransistor. Secondly, by channel/gate dielectric interface manipulation we demonstrated control over photoelectrical properties of the organic phototransistor and discovered mechanisms of the enhancement. Thirdly, we optimized and developed reliable, high-performance, and highly UV responsive organic phototransistors with potential application as a photo memory element