Synthesis and characterization of semi-crystalline n-type semiconducting molecules for organic field effect transistors and solar cells

다국어 초록 (Multilingual Abstract)

Organic semiconductors that can secure sufficient flexibility are very important for the portability, flexibility, weight reduction, and convenience of next-generation electronics, information, and display-related products. Compared to inorganic materials, organic semiconductors offer their inherent superior mechanical flexibility or elasticity. It is compatible with print-based manufacturing technology (low-cost, high-throughput roll-to-roll production processes) using low-temperature processes. This allows the use of a variety of flexible substrates such as paper, plastics, and fibers, providing electrical, mechanical, and industrial advantages for many applications. These organic semiconductors are used as key materials for organic field effect transistors (OFETs) and organic photovoltaics (OPVs). Various donor and acceptor monomers are used in the synthesis of organic semiconductors. The energy levels are primarily determined by the donor's highest occupied molecular orbital (HOMO) energy level and the acceptor's lowest unoccupied molecular orbital (LUMO) energy level, and the energy level can be adjusted by optimizing the donor and acceptor moiety. However, controlling the HOMO and LUMO energy levels individually and accurately is not a very simple process. Because parameters such as substituents, planarity, molecular weight, and intermolecular interactions influence and correlate the energy level. It is necessary to properly select the position of the substituent and carefully design it to have the appropriate energy level and crystallinity. Herein, this dissertation describes the synthesis and characterization of semi-crystalline n-type semiconducting molecules for OFET and OPV.
In chapter 2, three types of dicyanodistyrylbenzene (DCS)-based copolymers (PBDT-DCS, PT-DCS, and PNDI-DCS) were reported, which present highly balanced ambipolar charge transport characteristics in OFETs. The introduction of the DCS moiety in a polymer backbone not only lowers the LUMO level, but also increases the crystalline ordering via interchain dipole-dipole interactions. As a result, the LUMO levels for PBDT-DCS, PT-DCS, and PNDI-DCS were decreased to −3.76, −4.00, and −3.99 eV, respectively, which is beneficial for efficient electron injection from Au electrode for improving ambipolar charge transport. The determined hole/electron mobilities of the OFETs were 0.064/0.014, 0.492/0.181, and 0.420/0.447 cm2V–1s–1 for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively, after thermal annealing at 250 °C. By incorporating the electron-deficient naphthalene diimide (NDI) unit in the copolymers, the n-channel transport was enhanced, with decreasing frontier molecular orbitals with enhanced electron injection and impeded hole injection from the Au electrode. Therefore, PNDI-DCS provided completely symmetric output curves in the positive and negative drain voltage regions with almost equivalent hole and electron mobilities. Benefitting from the balanced ambipolar feature of the PNDI-DCS OFETs, a complementary inverter was successfully fabricated.
In chapter 3, a series of regioisomeric n-type small molecules were designed and synthesized, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by 2D NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed OFETs. Interestingly, a minor change in the substitution position of the octyl side-chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 ℃, the kinked DPP(EH)-INCNO1 structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ~0.6 cm2V–1s–1 compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 molecules become more crystalline with thermal treatments but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.
In chapter 4, two types of small molecule nonfullerene acceptors (IDICO1 and IDICO2) based on 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) are synthesized by attaching octyl side-chains onto terminal end groups. The alkyl substitution increases the LUMO (−3.81 to −3.86 eV) of the two acceptors, compared to that of IDIC (−3.94 eV). Interestingly, the IDICO1 and IDICO2 films have higher integrated absorption coefficients (1.49 × 107 cm−1) than the IDIC (1.29 × 107 cm−1) film. Also, the electron mobilities of IDICO1 and IDICO2 are approximately twice as high as that of IDIC. The terminal octyl substitution also improves the miscibility with a donor polymer (PBDB-T) to form well-intermixed blends with a decreased π–π stacking distance. As a result, their photovoltaic devices exhibit significant improvements in both the open-circuit voltage and short-circuit current density, compared to those of the reference PBDB-T:IDIC device, exhibiting maximum power conversion efficiencies of up to 9.64%, 20.4%, and 1.68% under 1-sun, 1000-lx LED, and halogen lamp illumination, respectively, which are significantly higher than those of PBDB-T:IDIC (7.2%, 11.7%, and 1.2%, respectively). It is worth noting that a maximum power density of 141.4 μW cm−2 is achieved for the PBDB-T:IDICO2-based device under a halogen lamp, which is the highest value report0ed to date among those achieved under indoor lighting conditions.

목차 (Table of Contents)

• 1. CHAPTER 1. GENERAL INTRODUCTION １
• 1.1. Semiconducting materials ２
• 1.1.1. Principle ２
• 1.2. Organic Field Effect Transistors (OFET) ６
• 1.2.1. History ６
• 1.2.2. Device configurations ８
• 1.2.3. Operating principle of OFET １０
• 1.2.4. Efficiency parameter １４
• 1.3. Organic Photovoltaic (OPV) １６
• 1.3.1. History １６
• 1.3.2. Device configurations ２１
• 1.3.3. Operating principle of OPV ２５
• 1.3.4. Efficiency parameter ２８
• 1.4. References ３２
• CHAPTER 2. DICYANODISTYRYLBENZENE-BASED COPOLYMERS FOR AMBIPOLAR ORGANIC FIELD-EFFECT TRANSISTORS WITH WELL-BALANCED HOLE AND ELECTRON MOBILITIES ３７
• 2.1. Introduction ３８
• 2.2. Results and Discussion ４１
• 2.2.1. Materials Synthesis and Characterization ４１
• 2.2.2. Optoelectronic Properties and Optimized Molecular Geometry ４３
• 2.2.3. Device Properties Based on TCBG OFET ４８
• 2.2.4. Film Morphologies and Packing Structure Analysis ５４
• 2.2.5. Complementary Inverter ６１
• 2.3. Conclusion ６３
• 2.4. Experimental Section ６４
• 2.5. Reference ７１
• CHAPTER 3 SYNTHESIS, MOLECULAR PACKING, AND ELECTRICAL PROPERTIES OF NEW REGIOISOMERIC N-TYPE SEMICONDUCTING MOLECULES WITH MODIFICATION OF ALKYL SUBSTITUENTS POSITION. ７６
• 3.1. Introduction ７７
• 3.2. Results and Discussion ８０
• 3.2.1. Material Synthesis and Characterization ８０
• 3.2.2. Optoelectronic Characteristics and Optimized Molecular Geometry ８４
• 3.2.3. Device Properties Based on TCBG OFETs ９０
• 3.2.4. Film Morphologies and Packing Structure ９５
• 3.2.5. Device Properties Based on BCTG OFETs １０３
• 3.3. Conclusion １０６
• 3.4. Experimental section １０７
• 3.5. Reference １１４
• CHAPTER 4 TERMINAL ALKYL SUBSTITUTION IN THE A-D-A-TYPE NONFULLERENE ACCEPTOR: SIMULTANEOUS IMPROVEMENTS IN THE OPEN-CIRCUIT VOLTAGE AND SHORT-CIRCUIT CURRENT FOR EFFICIENT INDOOR POWER GENERATION. １２０
• 4.1. Introduction １２１
• 4.2. Results and Discussion １２６
• 4.2.1. Material Synthesis and characterization １２６
• 4.2.2. Photovoltaic Characteristics under Outdoor and Indoor Conditions １３４
• 4.2.3. Film Morphologies and Packing Structure Analysis １４８
• 4.3. Conclusion １５８
• 4.4. Experimental Section １６０
• 4.5. Reference １６６