Joon Hak Oh

Two-dimensional polyaniline (C3N) from carbonized organic single crystals in solid state

Significance Two-dimensional (2D) polyaniline (PANI) has been realized for the first time, to our knowledge, by direct solid-state reaction of organic single crystals. The 2D PANI framework consists of six nitrogen atoms that periodically surround a phenyl ring. Pristine 2D PANI (undoped) has electrical conductivity of 0.72 S/cm, which is 1010 times higher than its linear analog (undoped, 6.28 × 10−11 S/cm). When it is doped by hydrochloric acid (HCl), its conductivity jumps to almost 1,960 times (1.41 × 103 S/cm). Due to its highest conductivity among organic materials, we very strongly believe that this well-defined 2D PANI and its heterogeneity with C and N elements will open up a new research field of layered 2D materials beyond linear PANI and other organic/inorganic 2D materials. The formation of 2D polyaniline (PANI) has attracted considerable interest due to its expected electronic and optoelectronic properties. Although PANI was discovered over 150 y ago, obtaining an atomically well-defined 2D PANI framework has been a longstanding challenge. Here, we describe the synthesis of 2D PANI via the direct pyrolysis of hexaaminobenzene trihydrochloride single crystals in solid state. The 2D PANI consists of three phenyl rings sharing six nitrogen atoms, and its structural unit has the empirical formula of C3N. The topological and electronic structures of the 2D PANI were revealed by scanning tunneling microscopy and scanning tunneling spectroscopy combined with a first-principle density functional theory calculation. The electronic properties of pristine 2D PANI films (undoped) showed ambipolar behaviors with a Dirac point of –37 V and an average conductivity of 0.72 S/cm. After doping with hydrochloric acid, the conductivity jumped to 1.41 × 103 S/cm, which is the highest value for doped PANI reported to date. Although the structure of 2D PANI is analogous to graphene, it contains uniformly distributed nitrogen atoms for multifunctionality; hence, we anticipate that 2D PANI has strong potential, from wet chemistry to device applications, beyond linear PANI and other 2D materials.

Boosting the Performance of Organic Optoelectronic Devices Using Multiple-Patterned Plasmonic Nanostructures

Multiple-patterned nanostructures prepared by synergistically combining block-copolymer lithography with nano-imprinting lithography have been used as back reflectors for enhancing light absorption in organic optoelectronic devices. The multiple-patterned electrodes have significantly boosted the performance of organic photovoltaics and photo-transistors, owed to the highly effective light scattering and plasmonic effects, extending the range of their practical applications.

Flexible Organic Phototransistor Array with Enhanced Responsivity via Metal–Ligand Charge Transfer

Phototransistors based on organic photoactive materials combine tunable light absorption in the spectral region from ultraviolet to near-infrared with low-temperature processability over large areas on flexible substrates. However, they often exhibit low photoresponsivity because of low molar extinction coefficient of photoactive components. We report a simple, yet highly efficient solution method for enhancing the performance of organic phototransistors using ruthenium complex 1 (Ru-complex 1). An air-stable n-type organic semiconductor, N,N-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), has been deposited on a silicon wafer and a transparent polyimide (PI) substrate via thermal evaporation under vacuum. The BPE-PTCDI phototransistors functionalized with Ru-complex 1 exhibit ∼5000 times higher external quantum efficiency (EQE) than that of pristine BPE-PTCDI phototransistors, owing to the metal-ligand charge transfer (MLCT) from Ru-complex 1 to the active component of the device. In addition, a large 10 × 10 phototransistor array (2.5 × 2.5 cm(2)) has been prepared on a transparent PI substrate, showing distinct light mapping. The fabricated phototransistor array is highly flexible and twistable and works well under tensile and compressive strains. We believe that our simple method will pave a viable way for improvements in the photoresponsivity of organic semiconductors for applications in wearable organic optoelectronic devices.

Effect of the alkyl spacer length on the electrical performance of diketopyrrolopyrrole-thiophene vinylene thiophene polymer semiconductors

For systematic investigation of the structure–property relationship, a series of diketopyrrolopyrrole-thiophene vinylene thiophene (DPP-TVT) polymers, ranging from 25-DPP-TVT to 32-DPP-TVT, with branched alkyl groups containing linear spacer groups from C2 to C9, has been synthesized. The electrical performance of these polymers is clearly dependent on the length of the spacer group and shows the odd–even alterations in chemical and electronic properties induced by the different alkyl chain spacers. Spacer groups with even numbers of carbon atoms exhibit higher charge-carrier mobilities than those with odd numbers of carbon atoms for the linear spacer groups from C2 to C7. Furthermore, the optimal side chain geometry in the DPP-TVT system for the most efficient charge transport contains the C6 spacer between the branching point and the backbone, showing the maximum hole mobility of 8.74 cm2 V−1 s−1 (at VGS, VDS = −100 V). The results obtained herein demonstrate the intriguing odd–even effects induced by the length of the side chain alkyl spacers for DPP-TVT polymers, and provide insight into the side chain engineering for the most efficient charge transport in DPP-based polymer semiconductors.

Chemically Robust Ambipolar Organic Transistor Array Directly Patterned by Photolithography

Organic ambipolar transistor arrays for chemical sensors are prepared on a flexible plastic substrate with a bottom-gate bottom-contact configuration to minimize the damage to the organic semiconductors, for the first time, using a photolithographically patternable polymer semiconductor. Well-balanced ambipolar charge transport is achieved by introducing graphene electrodes because of the reduced contact resistance and energetic barrier for electron transport.

High-Performance Furan-Containing Conjugated Polymer for Environmentally Benign Solution Processing

Developing semiconducting polymers that exhibit both strong charge transport capability via highly ordered structures and good processability in environmentally benign solvents remains a challenge. Given that furan-based materials have better solubility in various solvents than analogous thiophene-based materials, we have synthesized and characterized furanyl-diketopyrrolopyrrole polymer (PFDPPTT-Si) together with its thienyl-diketopyrrolopyrrole-based analogue (PTDPPTT-Si) to understand subtle changes induced by the use of furan instead of thiophene units. PTDPPTT-Si films processed in common chlorinated solvent exhibit a higher hole mobility (3.57 cm2 V-1 s-1) than PFDPPTT-Si films (2.40 cm2 V-1 s-1) under the same conditions; this greater hole mobility is a result of tightly aggregated π-stacking structures in PTDPPTT-Si. By contrast, because of its enhanced solubility, PFDPPTT-Si using chlorine-free solution processing results in a device with higher mobility (as high as 1.87 cm2 V-1 s-1) compared to that of the corresponding device fabricated using PTDPPTT-Si. This mobility of 1.87 cm2 V-1 s-1 represents the highest performances among furan-containing polymers reported to the best of our knowledge for nonchlorinated solvents. Our study demonstrates an important step toward environmentally compatible electronics, and we expect the results of our study to reinvigorate the furan-containing semiconductors field.

Supramolecular Nanostructures of Chiral Perylene Diimides with Amplified Chirality for High‐Performance Chiroptical Sensing

Chiral supramolecular nanostructures with optoelectronic functions are expected to play a central role in many scientific and technological fields but their practical use remains in its infancy. Here, this paper reports photoconductive chiral organic semiconductors (OSCs) based on perylene diimides with the highest electron mobility among the chiral OSCs and investigates the structure and optoelectronic properties of their homochiral and heterochiral supramolecular assemblies from bottom-up self-assembly. Owing to the well-ordered supramolecular packing, the homochiral nanomaterials exhibit superior charge transport with significantly higher photoresponsivity and dissymmetry factor compared with those of their thin film and monomeric equivalents, which enables highly selective detection of circularly polarized light, for the first time, in visible spectral range. Interestingly, the heterochiral nanostructures assembled from co-self-assembly of racemic mixtures show extraordinary chiral self-discrimination phenomenon, where opposite enantiomeric molecules are packed alternately into heterochiral architectures, leading to completely different optoelectrical performances. In addition, the crystal structures of homochiral and heterochiral nanostructures have first been studied by ab initio X-ray powder diffraction analysis. These findings give insights into the structure-chiroptical property relationships of chiral supramolecular self-assemblies and demonstrate the feasibility of supramolecular chirality for high-performance chiroptical sensing.

Ultra-narrow-bandgap thienoisoindigo polymers: structure–property correlations in field-effect transistors

From a structural point of view, the newly conceived thienoisoindigo (TIIG) moiety can serve as an ideal building block for the synthesis of high-performance polymers. To expand the range of available TIIG-based conjugated polymers, herein we report the synthesis and characterization of two new TIIG-based donor–acceptor polymers (PTIIG-TT and PTIIG-TVT), containing either the thieno[3,2-b]thiophene (TT) or the (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) moiety. In addition, we conducted a systematic investigation on the relationship between the microstructure of the polymer film and charge transport in organic field-effect transistors (OFETs) fabricated using these polymers. It was observed that the incorporation of a TVT moiety into the TIIG backbone imparts higher crystallinity and increases the molecular packing density, leading to an increased hole mobility (∼0.45 cm2 V−1 s−1) in PTIIG-TVT, compared with PTIIG-TT. When an Al electrode is used instead of a Au electrode in the OFET devices, both polymers exhibit outstanding ambipolar characteristics. This study advances the understanding of the structural features of TIIG-based polymers, which will potentially accelerate the improvement in the mobility of TIIG-based polymers.

In situ FT-IR spectroscopic investigation on the microstructure of hyperbranched aliphatic polyesters

The synthetic process of hyperbranched aliphatic polyesters based on 2,2-bis(hydroxymethyl)propionic acid (AB2-type monomer) and 2ethyl-2-(hydroxymethyl)-1,3-propanediol (B3-type core) was investigated with in situ Fourier transform infrared (FT-IR) spectroscopic analysis. The experimental parameter was the stoichiometric ratio between monomer and core moiety corresponding to theoretical first, second and third generation. In the theoretical first generation, monomers were directly combined with a core molecule. In the theoretical second and third generation, oligomers derived from the reaction between monomers were gradually combined with a core moiety as the reaction time increased. The microstructure of the main product was surmised from the peak integration of in situ FT-IR spectra, the intensity of each repeating unit on 13 C NMR spectra and the number average molecular weight. q 1999 Elsevier Science Ltd. All rights reserved.

Toward Environmentally Robust Organic Electronics: Approaches and Applications

Recent interest in flexible electronics has led to a paradigm shift in consumer electronics, and the emergent development of stretchable and wearable electronics is opening a new spectrum of ubiquitous applications for electronics. Organic electronic materials, such as π-conjugated small molecules and polymers, are highly suitable for use in low-cost wearable electronic devices, and their charge-carrier mobilities have now exceeded that of amorphous silicon. However, their commercialization is minimal, mainly because of weaknesses in terms of operational stability, long-term stability under ambient conditions, and chemical stability related to fabrication processes. Recently, however, many attempts have been made to overcome such instabilities of organic electronic materials. Here, an overview is provided of the strategies developed for environmentally robust organic electronics to overcome the detrimental effects of various critical factors such as oxygen, water, chemicals, heat, and light. Additionally, molecular design approaches to π-conjugated small molecules and polymers that are highly stable under ambient and harsh conditions are explored; such materials will circumvent the need for encapsulation and provide a greater degree of freedom using simple solution-based device-fabrication techniques. Applications that are made possible through these strategies are highlighted.