Guhyun Kwon

Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors

Electron-deficient π-conjugated small molecules can function as electron-transporting semiconductors in various optoelectronic applications. Despite their unique structural, optical, and electronic properties, the development of BODIPY-based organic semiconductors has lagged behind that of other π-deficient units. Here, we report the design and synthesis of two novel solution-proccessable BODIPY-based small molecules (BDY-3T-BDY and BDY-4T-BDY) for organic thin-film transistors (OTFTs). The new semiconductors were fully characterized by (1)H/(13)C NMR, mass spectrometry, cyclic voltammetry, UV-vis spectroscopy, photoluminescence, differential scanning calorimetry, and thermogravimetric analysis. The single-crystal X-ray diffraction (XRD) characterization of a key intermediate reveals crucial structural properties. Solution-sheared top-contact/bottom-gate OTFTs exhibited electron mobilities up to 0.01 cm(2)/V·s and current on/off ratios of >10(8). Film microstructural and morphological characterizations indicate the formation of relatively long (∼0.1 mm) and micrometer-sized (1-2 μm) crystalline fibers for BDY-4T-BDY-based films along the shearing direction. Fiber-alignment-induced charge-transport anisotropy (μ∥/μ⊥ ≈ 10) was observed, and higher mobilities were achieved when the microfibers were aligned along the conduction channel, which allows for efficient long-range charge-transport between source and drain electrodes. These OTFT performances are the highest reported to date for a BODIPY-based molecular semiconductor, and demonstrate that BODIPY is a promising building block for enabling solution-processed, electron-transporting semiconductor films.

Design, synthesis, and characterization of α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene molecular semiconductors. Enhancement of ambipolar charge transport through synthetic tailoring of alkyl substituents

A series of indeno[1,2-b]fluorene-6,12-dione-thiophene derivatives with hydrocarbon substituents at α,ω-positions as side groups have been designed and synthesized. The new compounds were fully characterized by 1H/13C NMR, mass spectrometry, cyclic voltammetry, UV-vis absorption spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and melting point measurements. The solid state structure of the indeno[1,2-b]fluorene-6,12-dione acceptor core has been identified based on single-crystal X-ray diffraction (XRD). The structural and electronic properties were also studied using density functional theory calculations, which were found to be in excellent agreement with the experimental findings and provided further insight. The detailed effects of alkyl chain size and orientation on the optoelectronic properties, intermolecular cohesive forces, thin-film microstructures, and charge transport performance of the new semiconductors were investigated. Two of the new solution-processable semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, were deposited as thin-films via solution-shearing, drop-casting, and droplet-pinned crystallization methods, and their morphologies and microstructures were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The solution-processed thin-film transistors based on 2EH-TIFDKT and 2OD-TIFDKT showed ambipolar device operations with electron and hole mobilities as high as 0.12 cm2 V−1 s−1 and 0.02 cm2 V−1 s−1, respectively, with Ion/Ioff ratios of 105 to 106. Here, we demonstrate that rational repositioning of the β-substituents to molecular termini greatly benefits the π-core planarity while maintaining a good solubility, and results in favorable structural and optoelectronic characteristics for more efficient charge-transport in the solid-state. The ambipolar charge carrier mobilities were increased by two–three orders of magnitude in the new indeno[1,2-b]fluorene-6,12-dione-thiophene core on account of the rational side-chain engineering.

Ultralow bandgap molecular semiconductors for ambient-stable and solution-processable ambipolar organic field-effect transistors and inverters

The design and development of novel ambipolar semiconductors is very crucial to advance various optoelectronic technologies including organic complementary (CMOS) integrated circuits. Although numerous high-performance ambipolar polymers have been realized to date, small molecules have been unable to provide high ambipolar performance in combination with ambient-stability and solution-processibility. In this study, by implementing highly π-electron deficient, ladder-type IFDK/IFDM acceptor cores with bithiophene donor units in D–A–D π-architectures, two novel small molecules, 2OD-TTIFDK and 2OD-TTIFDM, were designed, synthesized and characterized in order to achieve ultralow band-gap (1.21–1.65 eV) semiconductors with sufficiently balanced molecular energetics for ambipolarity. The HOMO/LUMO energies of the new semiconductors are found to be −5.47/−3.61 and −5.49/−4.23 eV, respectively. Bottom-gate/top-contact OFETs fabricated via solution-shearing of 2OD-TTIFDM yield perfectly ambient stable ambipolar devices with reasonably balanced electron and hole mobilities of 0.13 cm2 V−1 s−1 and 0.01 cm2 V−1 s−1, respectively with Ion/Ioff ratios of ∼103–104, and 2OD-TTIFDK-based OFETs exhibit ambipolarity under vacuum with highly balanced (μe/μh ∼ 2) electron and hole mobilities of 0.02 cm2 V−1 s−1 and 0.01 cm2 V−1 s−1, respectively with Ion/Ioff ratios of ∼105–106. Furthermore, complementary-like inverter circuits were demonstrated with the current ambipolar semiconductors resulting in high voltage gains of up to 80. Our findings clearly indicate that ambient-stability of ambipolar semiconductors is a function of molecular orbital energetics without being directly related to a bulk π-backbone structure. To the best of our knowledge, considering the processing, charge-transport and inverter characteristics, the current semiconductors stand out among the best performing ambipolar small molecules in the OFET and CMOS-like circuit literature. Our results provide an efficient approach in designing ultralow band-gap ambipolar small molecules with good solution-processibility and ambient-stability for various optoelectronic technologies, including CMOS-like integrated circuits.

Multifunctional Organic-Semiconductor Interfacial Layers for Solution-Processed Oxide-Semiconductor Thin-Film Transistor

The stabilization and control of the electrical properties in solution-processed amorphous-oxide semiconductors (AOSs) is crucial for the realization of cost-effective, high-performance, large-area electronics. In particular, impurity diffusion, electrical instability, and the lack of a general substitutional doping strategy for the active layer hinder the industrial implementation of copper electrodes and the fine tuning of the electrical parameters of AOS-based thin-film transistors (TFTs). In this study, the authors employ a multifunctional organic-semiconductor (OSC) interlayer as a solution-processed thin-film passivation layer and a charge-transfer dopant. As an electrically active impurity blocking layer, the OSC interlayer enhances the electrical stability of AOS TFTs by suppressing the adsorption of environmental gas species and copper-ion diffusion. Moreover, charge transfer between the organic interlayer and the AOS allows the fine tuning of the electrical properties and the passivation of the electrical defects in the AOS TFTs. The development of a multifunctional solution-processed organic interlayer enables the production of low-cost, high-performance oxide semiconductor-based circuits.