京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20408号 / 工博第4345号 / 新制||工||1673(附属図書館) / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 赤木 和夫, 教授 古賀 毅, 教授 辻井 敬亘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Conjugated polymer architecture and morphology are two of the key factors that determine corresponding opto-electronic device performance. It is well-known that conjugated polymers display a variety of conformations and exhibit aggregation in their materials and even for individual polymer chains. The intrinsic structural heterogeneity of conjugated polymers strongly complicates the active layer morphology and phase separation, which are crucial for photoinduced charge generation and transport in polymer based bulk heterojunction-organic photovoltaics device (BHJ-OPVs). Aiming to probe the molecular level correlations between conjugated polymer architecture, morphology and optoelectronic properties, single molecule spectroscopy (SMS) and single particle spectroscopy (SPS) were employed. The molecular level folding properties of conjugated polymers were studied and correlated to the chemical architecture and rigidness of the polymer backbones by means of SMS and single molecule polarization anisotropy imaging. First, a block copolymer consisting of poly(3-hexylthiophene) (P3HT) and (60)fullerene (C₆₀) was investigated due to its potential for forming active layers in OPV devices that exhibit long-term phase stability and efficient exciton dissociation into free charge carriers. It was demonstrated that the grafting of the C₆₀-containing block does not significantly affect the conformation of the backbone of the P3HT block. Next, a series of thiophene based polymers showing different macroscale crystallization behavior were investigated. The rigidness of the conjugated polymer backbones was found to be correlated with the chemical architecture of the molecules. However, even the polymers that show no folding in their respective crystals and are thus expected to be the most rigid, still exhibit folding at the single molecule level. From this work it is clear that besides chemical architecture, intermolecular interactions in the crystal structure also need to be considered. For conjugated polymer materials, in this dissertation specifically the blends of conjugated polymers with fullerenes as found in the active layer of OPVs, the investigation of the molecular level correlations between conjugated polymer architecture, morphology and optoelectronic properties can be prohibitively complex due to the presence of a large number of molecules. Furthermore, in the research presented herein, as well as in the literature, it has been clearly shown that the polymer molecules themselves exhibit severe heterogeneity in their properties (chain morphology, aggregation, optical and electronic properties). Therefore, in order to simplify the structure-property investigations concerning nanodomains in BHJ-OPVs, we developed P3HT/PC₆₀BM (PC₆₀BM: (6,6)-phenyl-C₆₁-butyric acid methyl ester) composite nanoparticles (NPs). The size of the nanoparticles corresponds with a few polymer and fullerene domains when considering a similarly sized volume in the active layer of OPVs. Single particle spectroscopy combined with this unique nanoparticle material system reveals variations in molecular conformation and aggregation of the conjugated polymer chains upon doping with different weight percentages of fullerene. These newly developed NPs were embedded in a hole-injection device to study the exciton-hole polaron interactions and the charge transfer processes at the interface between a hole-transporting layer and the NPs. Pronounced charge trapping was observed for donor-acceptor blend NPs due to the large amount of photogenerated free charge carriers. Besides fundamental studies on morphology-property relations for thiophene based conjugated polymers, fabrication of BHJ-OPVs based on P3HT and PC₆₀BM was also completed. Low band gap polymer PTB-7 (poly((4,8-bis((2-ethylhexyl)oxy)benzo (1,2-b:4,5-b')dithiophene-2,6-diyl)(3-fluoro-2-((2-ethylhexyl)carbonyl)thieno(3,4-b) thiophenediyl))) and a near-infrared (NIR) small dye molecule were incorporated into active layers of these P3HT/PC₆₀BM BHJ-OPVs to expand the photoresponse of the devices. The effects of doping the P3HT/PC₆₀BM BHJ-OPVs with PTB-7 and NIR dye on the device performance and film morphology were investigated. The doping of PTB-7 can efficiently extend the photoresponse of the resultant devices into the NIR regime and improve the device performance with respect to the reference (undoped) devices, demonstrating an elegant and pragmatic approach in improving light-harvesting efficiency in BHJ-OPVs.

As the global economy searches for reliable, inexpensive and environmentally friendly renewable energy resources, energy conservation by means of photovoltaics has seen near exponential growth in the last decade. Compared to state-of-the-art inorganic solar cells, organic photovoltaics (OPVs) composed of conjugated polymers are particularly interesting because of their processability, flexibility and the potential for large area devices at a reduced fabrication cost. It has been extensively documented that the interchain and intrachain interactions of conjugated polymers complicate the fundamental understanding of the optical and electronic properties in the solid-state (i.e. thin film active layer). These interactions are highly dependent on the nanoscale morphology of the solid-state material, leading to a heterogeneous morphology where individual conjugated polymer molecules obtain a variety of different optoelectronic properties. Therefore, it is of the utmost importance to fundamentally study conjugated polymer systems at the single molecule or nanoparticle level instead of the complex macroscopic bulk level. This dissertation research aims to develop simplified nanoparticle models that are representation of the nanodomains found in the solid-state material, while fundamentally addressing light harvesting, energy transfer and interfacial charge transfer mechanisms and their relationship to the electronic structure, material composition and morphology of the nanoparticle system. In preceding work, monofunctional doped nanoparticles (polymer-polymer) were fabricated with enhanced light harvesting and Fӧrster energy transfer properties by blending Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at various MEHPPV doping ratios. While single particle spectroscopy (SPS) reveals a broad distribution of v optoelectronic and photophysical properties, time-correlated single photon counting (TC-SPC) spectroscopy displays multiple fluorescence lifetime components for each nanoparticle composition, resulting from changing polymer chain morphologies and polymer-polymer aggregation. In addition, difunctional doped nanoparticles were fabricated by doping the monofunctional doped nanoparticles with PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) to investigate competition between intermolecular energy transfer and interfacial charge transfer. Specifically, the difunctional SPS data illustrated enhanced and reduced energy transfer mechanisms that are dependent on the material composition of MEH-PPV and PC60BM. These data are indicative of changes in inter- and intrachain interactions of BPPV and MEH-PPV and their respective nanoscale morphologies. Together, these fundamental studies provide a thorough understanding of monofunctional and difunctional doped nanoparticle photophysics, necessary for understanding the morphological, optoelectronic and photophysical processes that can limit the efficiency of OPVs and provide insight for strategies aimed at improving device efficiencies.

Conjugated polymers are semiconducting materials that are currently being researched for numerous applications from chemical and biological sensors to electronic devices, including photovoltaics and transistors. Much of the novel research on conjugated polymers is performed in academic settings, where scientists are working to prepare conjugated polymers for commercially viable applications. By offering numerous advantages, inherent in macromolecular materials, conjugated polymers may hold the key to cheap and environmentally friendly manufacturing of future electronic devices. Mechanical flexibility, and solvent-based coating processes are two commonly cited advantages. Transitions in the backbone conformation of polythiophenes (PT) in organic solvents have been widely observed to influence thin-film morphology. However, conformational transitions of water-soluble PT derivatives, with respect to their intramolecular versus intermolecular origin, remain largely obscure. Here, conformational transitions of a water- soluble polythiophene in aqueous ionic surfactants are investigated by means of Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), polarizing optical microscopy (POM), ultraviolet-visible (UV-Vis) absorption and fluorescence spectroscopy, and various X-ray scattering techniques. As-prepared complexes exist as stable hydrogels. Upon dilution, a significant time-dependent chromism occurs spontaneously. A coil-to-rod conformational transition is identified in this mechanism and verified using small-angle x-ray scattering (SAXS). Study into the corresponding kinetics demonstrates an inverse first-order rate law. It is found that the conformational transition is thermally reversible and concentration-independent. The critical transition temperature is largely dependent on the surfactant formulation. A theoretical model is presented to explain this new phenomenon and the mechanisms behind its influence on the optoelectronic and solid-state morphological properties. A relationship between the dilute-solution processing with surfactants and the final properties of the system is substantiated.

The research presented in this dissertation focuses on the design and synthesis of novel hybrid conjugated polymer materials using two different approaches: 1) inorganic/organic hybrid semiconductors through the incorporation of carboranes into the polymer structure and 2) the modification of surfaces with conjugated polymers via grafting approaches. Hybrid conjugated polymeric materials, which are materials or systems in which conjugated polymers are chemically integrated with non-traditional structures or surfaces, have the potential to harness useful properties from both components of the material to help overcome hurdles in their practical realization in polymer-based devices. This work is centered around the synthetic challenges of creating new hybrid conjugated systems and their potential for advancing the field of polymer-based electronics through both greater understanding of the behavior of hybrid systems, and access to improved performance and new applications. Chapter 1 highlights the potential applications and advantages for these hybrid systems, and provides some historical perspective, along with relevant background materials, to illustrate the rationale behind this work. >Chapter 2 explores the synthesis of poly(fluorene)s with pendant carborane cages. The Ni(0) dehalogenative polymerization of a dibromofluorene with pendant carborane cages tethered to the bridging 9-position produced hybrid polymers produced polymers which combined the useful emissive characteristics of poly(fluorene) with the thermal and chemical stability of carborane cages. The materials were found to display increased glass transition temperatures and showed improved emission color stability after annealing at high temperatures relative to the non-hybrid polymer. The design and synthesis of a poly(fluorene)-based hybrid material with carborane cages in the backbone, rather than as pendant groups, begins in chapter 3. Poly(fluorene) with p-carborane in the backbone is synthesized and characterized, and the material is found to be a high MW, soluble blue emitter which shows a higher glass transition temperature and greater stability than a non-hybrid polymer. UV absorbance and fluorescence spectroscopy indicated some electronic interaction between the conjugated polymer and the cages, but they did not appear to be fully conjugated in the traditional sense. Chapter 4 describes the design, synthesis, and characterization of poly(fluorene) with o-carborane in the backbone. Profound changes in the behavior of the polymer, from its polymerization behavior to its emission characteristics, were observed and their origins are discussed. Experiments to explore the nature of the cage/polymer interactions were performed and possible applications which take advantage of the unique nature of the o-carborane hybrid polymer are explored and discussed. Hybrid conjugated polymer materials via grafting approaches to surfaces and surface modification are discussed starting in chapter 5. The synthesis of a dibromofluorene-based silane coupling agent for the surface functionalization of oxide surfaces is presented, and the surface directed Ni(0) dehalogenative polymerization of poly(dihexylfluorene) is explored. Chapter 6 focuses on the exploration of conjugated polymer/cellulose hybrid materials. Surface medication of cellulose materials with monomer-like anchor points is discussed. Grafting of the modified cellulose with conjugated polymers was explored and the grafting of three different repeat structures based on fluorene-, fluorenevinylene-, and fluoreneethynylene motifs were optimized to provide a general route to cellulose/conjugated polymer hybrid materials. Characterization and possible applications of such hybrid materials are discussed. Finally, chapter 7 is devoted to the simultaneous surface patterning and functionalization of poly(2-hydroxyethylmethacrylate) thin films using a silane infusion-based wrinkling technique. While not a conjugated polymer system, the spontaneous patterning and functionalization methods explored in this chapter produce hybrid organic/inorganic polymer thin films which have applications that range from optics, to adhesion, to polymer-based electronics, and the research compliments the other chapters. The spontaneous generation of complex patterns, of a small scale approaching 100nm feature size, over a large area with simultaneous control over surface chemistry is explored. Examples of complex, hierarchically patterned films which integrate lithographic processes such as nanoimprint lithography and electron beam lithography with spontaneous patterning via wrinkling are presented.

All-polymer solar cells (APSC) are a class of organic solar cells in which hole and electron transporting phases are made of conjugated polymers. Unlike polymer/fullerene solar cell, photoactive material of APSC can be designed to have hole and electron transporting polymers with complementary absorption range and proper frontier energy level offset. However, the highest reported PCE of APSC is 5 times less than that of polymer/fullerene solar cell. The low PCE of APSC is mainly due to: i) low charge separation efficiency; and ii) lack of optimal morphology to facilitate charge transfer and transport; and iii) lack of control over the exciton and charge transport in each phase. My research work is focused towards addressing these issues. The charge separation efficiency of APSC can be enhanced by designing novel electron transporting polymers with: i) broad absorption range; ii) high electron mobility; and iii) high dielectric constant. In addition to with the above parameters chemical and electronic structure of the repeating unit of conjugated polymer also plays a role in charge separation efficiency. So far only three classes of electron transporting polymers, CN substituted PPV, 2,1,3-benzothiadiazole derived polymers and rylene diimide derived polymers, are used in APSC. Thus to enhance the charge separation efficiency new classes of electron transporting polymers with the above characteristics need to be synthesized. I have developed a new straightforward synthetic strategy to rapidly generate new classes of electron transporting polymers with different chemical and electronic structure, broad absorption range, and high electron mobility from readily available electron deficient monomers. In APSCs due to low entropy of mixing, polymers tend to micro-phase segregate rather than forming the more useful nano-phase segregation. Optimizing the polymer blend morphology to obtain nano-phase segregation is specific to the system under study, time consuming, and not trivial. Thus to avoid micro-phase segregation, nanoparticles of hole and electron transporters are synthesized and blended. But the PCE of nanoparticle blends are far less than those of polymer blends. This is mainly due to the: i) lack of optimal assembly of nanoparticles to facilitate charge transfer and transport processes; and ii) lack of control over the exciton and charge transport properties within the nanoparticles. Polymer packing within the nanoparticle controls the optoelectronic and charge transport properties of the nanoparticle. In this work I have shown that the solvent used to synthesize nanoparticles plays a crucial role in determining the assembly of polymer chains inside the nanoparticle there by affecting its exciton and charge transport processes. To obtain the optimal morphology for better charge transfer and transport, we have also synthesized nanoparticles of different radius with surfactants of opposite charge. We propose that depending on the radius and/or Coulombic interactions these nanoparticles can be assembled into mineral structure-types that are useful for photovoltaic devices.

Single-walled carbon nanotubes (SWNTs) have excellent electronic, mechanical, and optical properties that make them promising materials for various applications. However, SWNT production methods produce a mixture of semiconducting and metallic species and non-SWNT impurities limiting their incorporation into devices. Among the different purification methods, conjugated polymer sorting has proven to be a scalable and cost-effective method. Conjugated polymers can easily be tuned to disperse SWNT species and obtain solubility in target solvents. They are multifunctional structures that enable the purification and extraction of specific SWNTs while simultaneously enhancing their processability. Therefore, they are suitable as purification methods for the fabrication of SWNT-based devices, particularly for printed electronics. However, the polymer backbone and the non-conductive side-chains negatively impacts the performance of SWNT devices by preventing good contact between the nanotubes. We first functionalized our polymer with thermally cleavable side-chains and demonstrated that the removal of the side-chains leads to a higher conductivity. We obtained stable dispersions in two green solvents compatible with inkjet printing. We also functionalized our polymer with photocleavable side-chains and showed efficient cleavage in solution. These investigations represent a proof-of-concept that could be used for the development of SWNT-based devices where the removal of the side-chains will improve the device performance. Lastly, we synthesized a fluorene-based polymer that contains a photocleavable ortho-nitrobenzylether unit and is functionalized with hydrophilic side-chains. We demonstrated the degradation of the polymer in organic and aqueous solvents. These investigations highlight the challenges of dispersing SWNTs in aqueous solvents using conjugated polymer. / Thesis / Doctor of Philosophy (PhD) / The objective of this thesis is to develop cleavable complexes between conjugated polymers and single-walled carbon nanotubes (SWNTs) to maximize the potential performance of printed devices post-processing. We functionalized a conjugated polymer with cleavable side-chains and investigated the impact on the conductivity after their removal. In addition, this work also focuses on dispersing SWNTs in green solvents that are compatible with printing processes such as inkjet printing. Lastly, we synthesized a degradable and water-soluble conjugated polymers to produce dispersant free-SWNTs.

The thesis work described herein explores two avenues of post-functionalization of conjugated polymers using ‘click’ chemistry. The first avenue utilizes the Strain-Promoted Alkyne-Azide Cycloaddition (SPAAC) and the second an Inverse Electron-Demand Diels-Alder (IEDDA). In the first part of this thesis, various azide moieties were SPAAC ‘clicked’ onto a dibenzocyclooctyne-containing polymer, such as small molecules like para-phenyl-nitroazide, as well as larger azide-terminated chains like polystyrene and polyethylene glycol. Host-guest chemistry and self-healing organogels were also explored. The synthesis of each component, including the cyclooctyne diamine monomer, dialdehyde comonomer, resulting polymer, various azide moieties, as well as the SPAAC click reactions, are all described in detail along with extensive characterization. Similarly, the second part of this thesis involved the synthesis and characterization of several components, including the tetrazine monomer, fluorene comonomer, resulting polymer, and various TCO derivatives for the post-polymerization IEDDA ‘click’ reactions onto the backbone. Some of the click reactions described include small molecule TCO derivatives, polymeric PEG TCO, and a difunctional linker to generate a crosslinked foam. / Conjugated polymers attract significant attention due to their interesting optoelectronic and physical properties. Over the past few decades, tremendous effort has been devoted to expanding the structural diversity and applications of this class of macromolecules. The pursuit of structural variability of conjugated polymers has resulted in a broad range of research to understand their structure-property relationships via functionalization. This functionalization is crucial for tailoring performance in any given application. Thus, the ability to synthesize a library of homologous polymers would prove very useful. Efficiency is of utmost importance when creating a library of homologous conjugated polymers, as the faster a library can by synthesized, the sooner said polymers can be screened for any desirable properties. Such an approach requires a post-polymerization functionalization strategy, whereby a progenitor polymer undergoes efficient reactions at each repeat unit of the backbone. The work presented in this thesis involves synthesizing a reactive conjugated polymer scaffold, followed by efficiently post-polymerization functionalization via “click” chemistry. Two elegant click reactions are described in this work; the Strain-Promoted Alkyne-Azide Cycloaddition (SPAAC) and Inverse Electron-Demand Diels-Alder (IEDDA). The SPAAC reaction allowed for rapid functionalization of triazole moieties on a dibenzocyclooctyne-containing polymer backbone, creating a small polymer library with a consistent degree of polymerization (DP). Grafting with polystyrene and polyethylene glycol azide-terminated polymers allowed the efficient syntheses of a series of graft-co-polymers with Mn values up to 800 kDa and varying solubilities. Secondly, The IEDDA reaction was applied to a poly(tetrazine-co-fluorene) conjugated polymer, which resulted in the rapid and quantitative functionalization of the polymer backbone with trans-cyclooctene derivatives. These reactive conjugated polymers were explored in a variety of applications, including supramolecular chemistry and gel formation. / Thesis / Doctor of Philosophy (PhD) / Conjugated polymers are a class of macromolecular materials that attract significant attention due to their interesting behaviors and properties. Under certain conditions, these polymers even display conductivities like that of metals. As such, they show promise in applications such as organic solar cells, chemical sensors, organic light-emitting diodes, and supercapacitors. Over the past few decades, tremendous effort has been devoted to expanding on the types of conjugated polymers as well as their structural diversity. This, of course, has resulted in polymers that exhibit vastly different behaviours depending on what they are made of. As certain applications (e.g.: solar cells) require polymers with very specific properties, being able to ‘tune’ a conjugated polymer to ‘match’ a required property would be extremely useful. This tuning of polymer properties can be successfully accomplished by attaching different structures onto the polymer chain by utilizing a reaction known as ‘post-polymerization functionalization’. In doing so, a starting reactive polymer can be transformed into an entirely different polymer with specific chemical properties and behaviors. The work presented in this thesis involves synthesizing two types of conjugated polymers and attaching various structures onto their backbones to yield different properties. The synthesis, characterization, and potential applications of said polymers are described herein.

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