Publications by authors named "Chan Eon Park"

74 Publications

Strategy for Selective Printing of Gate Insulators Customized for Practical Application in Organic Integrated Devices.

ACS Appl Mater Interfaces 2021 Jan 28;13(1):1043-1056. Epub 2020 Dec 28.

Department of Advanced Organic Materials Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea.

Direct drawing techniques have contributed to the ease of patterning soft electronic materials, which are the building blocks of analog and digital integrated circuits. In parallel with the printing of semiconductors and electrodes, selective deposition of gate insulators (GI) is an equally important factor in simplifying the fabrication of integrated devices, such as NAND and NOR gates, and memory devices. This study demonstrates the fabrication of six types of printed GI layers (high/low- polymer and organic-inorganic hybrid material), which are utilized as GIs in organic field-effect transistors (OFETs), using the electrostatic-force-assisted dispensing printing technique. The selective printing of GIs on the gate electrodes enables us to develop practical integrated devices that go beyond unit OFET devices, exhibiting robust switching performances, non-destructive operations, and high gain values. Moreover, the flexible integrated devices fabricated using this technique exhibit excellent operational behavior. Therefore, this facile fabrication technique can pave a new path for the production of practical integrated device arrays for next-generation devices.
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http://dx.doi.org/10.1021/acsami.0c18477DOI Listing
January 2021

Direct Printing of Asymmetric Electrodes for Improving Charge Injection/Extraction in Organic Electronics.

ACS Appl Mater Interfaces 2020 Jul 14;12(30):33999-34010. Epub 2020 Jul 14.

Department of Advanced Organic Materials Engineering, Yeungnam University, Gyeongsan 38541, Korea.

Engineering the energy levels of organic conducting materials can be useful for developing high-performance organic field-effect transistors (OFETs), whose electrodes must be well controlled to facilitate easy charge carrier transport from the source to drain through an active channel. However, symmetric source and drain electrodes that have the same energy levels are inevitably unfavorable for either charge injection or charge extraction. In this study, asymmetric source and drain electrodes are simply prepared using the electrohydrodynamic (EHD)-jet printing technique after the careful work function engineering of organic conducting material composites. Two types of additives effectively tune the energy levels of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-based composites. These solutions are alternately patterned using the EHD-jet printing process, where the use of an electric field makes fine jet control that enables to directly print asymmetric electrodes. The asymmetric combination of EHD-printed electrodes helps in obtaining advanced charge transport properties in p-type and n-type OFETs, as well as their organic complementary inverters. This strategy is believed to provide useful guidelines for the facile patterning of asymmetric electrodes, enabling the desirable properties of charge injection and extraction to be achieved in organic electronic devices.
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http://dx.doi.org/10.1021/acsami.0c08683DOI Listing
July 2020

Side chain engineering in DTBDT-based small molecules for efficient organic photovoltaics.

Nanoscale 2019 Aug 12;11(29):13845-13852. Epub 2019 Jul 12.

Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang National University, Jinju 660-701, Republic of Korea.

A new small-molecule donor with a dithieno[2,3-d:2',3'-d']-benzo[1,2-b:4,5-b']-dithiophene (DTBDT) core and both alkyl and alkylthio substituents is designed and synthesized to improve the miscibility between DTBDT-based small molecules and [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). The alkyl substituent on the 4-position and the alkylthio substituent on the 5-position of the substituted thiophene are expected to improve intermolecular interactions and prevent severe aggregation of the small molecules. The new small molecule, DTBDT-S-C8-TTR, exhibits a homogenous blend morphology with small domains and edge-on-oriented crystalline structures in blends with PCBM, and give a maximum power conversion efficiency (PCE) of 8.43%. To recover the crystallinity of the DTBDT-S-C8-TTR small molecules weakened after being blended with PCBM, a solvent vapor annealing (SVA) treatment is performed. The SVA-treated blend films reveal well-developed crystalline domains with interconnected fibrillar structures. This blend morphology allows efficient charge carrier transport in blends and leads to increased PCEs. The maximum PCE of 9.18% achieved using DTBDT-S-C8-TTR suggests that substituting both alkylthio and alkyl groups into DTBDT can yield small-molecule-based organic photovoltaics (OPVs) displaying improved photovoltaic performances.
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http://dx.doi.org/10.1039/c9nr04427bDOI Listing
August 2019

Acene-Modified Small-Molecule Donors for Organic Photovoltaics.

Chemistry 2019 Sep 20;25(53):12316-12324. Epub 2019 Aug 20.

Department of Chemistry and RINS, Gyeongsang National University, Jinju, 660-701, Republic of Korea.

A series of acene-modified small molecules have been designed and synthesized, and their photovoltaic characteristics were studied by using the small molecules in organic photovoltaics (OPVs). Different cores were introduced to modulate the conjugation lengths of the small molecules and the bulk heterojunction (BHJ) morphologies. Three small-molecule donors were prepared, namely Ph-TTR, Na-TTR, and An-TTR, which have phenyl, naphthalene, and anthracene moieties, respectively, as conjugated cores. These donors were synthesized in a few steps and exhibited favorable BHJ morphologies, thereby giving promising power conversion efficiencies (PCEs). The donors showed excellent miscibility with the acceptor PC BM, and the use of the additive 1,8-diiodooctane (DIO) led to a remarkable increase in crystallinity, thereby increasing the PCEs of their OPVs. Of the three donors, Na-TTR showed the most efficient charge carrier generation and favorable molecular packing structures; hence, of the three types of devices tested, the Na-TTR:PC BM devices exhibited the highest PCE, specifically 6.27 %, without pre- or post-treatments. The promising PCEs achieved from these easily synthesized acene-modified small molecules suggested that acene-modified small molecules can be useful materials in OPVs.
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http://dx.doi.org/10.1002/chem.201902177DOI Listing
September 2019

Facile and Microcontrolled Blade Coating of Organic Semiconductor Blends for Uniaxial Crystal Alignment and Reliable Flexible Organic Field-Effect Transistors.

ACS Appl Mater Interfaces 2019 Apr 27;11(14):13481-13490. Epub 2019 Mar 27.

Department of Chemical Engineering , Pohang University of Science and Technology , Pohang 790-784 , Korea.

The ability to fabricate uniform and high-quality patterns of organic semiconductors using a simple method is necessary to realize high-performance and reliable organic field-effect transistors (OFETs) for practical applications. Here, we report the facile fabrication of chemically patterned substrates in order to provide solvent wetting/dewetting regions and grow patterned crystals during blade coating of a small-molecule semiconductor/insulating polymer blend solution. Polyurethane acrylate is selected as the solvent dewetting material, not only because of its hydrophobicity but also because its patterns are easily produced by selective UV irradiation onto precursor films. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) crystal patterns are grown on the line-shaped wetting regions of the patterned film, and the crystallinity of TIPS-PEN and alignment of molecules are found using various crystal analysis tools depending on the pattern widths. The smallest width of 5 μm yielded an OFET showing the highest field-effect mobility value of 1.63 cm/(V·s), which is much higher than the value of the OFET based on the unpatterned TIPS-PEN crystal. Notably, we demonstrate flexible and low-voltage-operating OFETs for practical use of the patterned crystals, and the OFETs show highly stable operation under sustained gate bias stress thanks to the patterned crystals.
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http://dx.doi.org/10.1021/acsami.8b21130DOI Listing
April 2019

Morphology Driven by Molecular Structure of Thiazole-Based Polymers for Use in Field-Effect Transistors and Solar Cells.

Chemistry 2019 Jan 6;25(2):649-656. Epub 2018 Dec 6.

Department of Chemistry and RINS, Gyeongsang National University, Jinju, 660-701, Republic of Korea.

The effects of the molecular structure of thiazole-based polymers on the active layer morphologies and performances of electronic and photovoltaic devices were studied. Thus, thiazole-based conjugated polymers with a novel thiazole-vinylene-thiazole (TzVTz) structure were designed and synthesized. The TzVTz structure was introduced to extend the π conjugation and coplanarity of the polymer chains. By combining alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) or dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) electron-donating units and a TzVTz electron-accepting unit, enhanced intermolecular interactions and charge transport were obtained in the novel polymers BDT-TzVTz and DTBDT-TzVTz. With a view to using the polymers in transistor and photovoltaic applications, the molecular self-assembly in and their nanoscale morphologies of the active layers were controlled by thermal annealing to enhance the molecular packing and by introducing a diphenyl ether solvent additive to improve the miscibility between polymer donors and [6,6]phenyl-C71-butyric acid methyl ester (PC BM) acceptors, respectively. The morphological characterization of the photoactive layers showed that a higher degree of π-electron delocalization and more favorable molecular packing in DTBDT-TzVTz compared with in BDT-TzVTz leads to distinctly higher performances in transistor and photovoltaic devices. The superior performance of a photovoltaic device incorporating DTBDT-TzVTz was achieved through the superior miscibility of DTBDT-TzVTz with PC BM and the improved crystallinity of DTBDT-TzVTz in the nanofibrillar structure.
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http://dx.doi.org/10.1002/chem.201804803DOI Listing
January 2019

Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor.

ACS Appl Mater Interfaces 2018 Oct 20;10(42):36037-36046. Epub 2018 Sep 20.

Department of Polymer Science & Engineering and Department of IT Convergence , Korea National University of Transportation , Chungju 380-702 , Republic of Korea.

To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two dithieno[2,3- d:2',3'- d']-benzo[1,2- b:4,5- b']dithiophene (DTBDT)-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the [6,6]-phenyl-C-butyric acid methyl ester (PCBM) fullerene acceptor and the O-IDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor-acceptor interface, whereas blends with PCBM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased J and fill factor values and unusual decrease in V values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared with that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.
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http://dx.doi.org/10.1021/acsami.8b14020DOI Listing
October 2018

Surface Modification of CdSe Quantum-Dot Floating Gates for Advancing Light-Erasable Organic Field-Effect Transistor Memories.

ACS Nano 2018 Aug 23;12(8):7701-7709. Epub 2018 Jul 23.

Department of Energy Engineering , Hanyang University , Seoul 04763 , Republic of Korea.

Photoresponsive transistor memories that can be erased using light-only bias are of significant interest owing to their convenient elimination of stored data for information delivery. Herein, we suggest a strategy to improve light-erasable organic transistor memories, which enables fast "photoinduced recovery" under low-intensity light. CdSe quantum dots (QDs) whose surfaces are covered with three different organic molecules are introduced as photoactive floating-gate interlayers in organic transistor memories. We determine that CdSe QDs capped or surface-modified with small molecular ligands lead to efficient hole diffusion from the QDs to the conducting channel during "photoinduced recovery", resulting in faster erasing times. In particular, the memories with QDs surface-modified with fluorinated molecules function as normally-ON type transistor memories with nondestructive operation. These memories exhibit high memory ratios over 10 between OFF and ON bistable current states for over 10 000 s and good dynamic switching behavior with voltage-driven programming processes and light-assisted erasing processes within 1 s. Our study provides a useful guideline for designing photoactive floating-gate materials to achieve desirable properties of light-erasable organic transistor memories.
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http://dx.doi.org/10.1021/acsnano.8b01413DOI Listing
August 2018

Three-Dimensional Observation of a Light-Soaked Photoreactant Layer in BTR:PCBM Solar Cells Treated with/without Solvent Vapor Annealing.

ACS Appl Mater Interfaces 2018 Jul 22;10(26):21973-21984. Epub 2018 Jun 22.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering , Pohang University of Science and Technology (POSTECH) , Pohang 790-784 , Republic of Korea.

A key challenge to the commercialization of solution-processed solar cells is a proper understanding of the morphological variations during long periods, particularly under light-soaking conditions. Many research groups have competitively reported solvent vapor annealing (SVA)-treated small-molecule devices with efficiency rates exceeding 11%; however, their light-soaking effects have been rarely studied. Here, we investigate the morphological changes in the light-soaked devices with/without SVA treatments depending on the illumination time via three-dimensional observations. From the results, we found that the trends of morphological variations differ in the surface and bulk parts of the active film and that the difference is closely related to the device performance capabilities. Therefore, our research will enhance the underlying knowledge of the light-soaking effect on active morphologies over long term.
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http://dx.doi.org/10.1021/acsami.8b02871DOI Listing
July 2018

All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors: Synthesis and Full Characterization.

ACS Appl Mater Interfaces 2017 Dec 13;9(51):44667-44677. Epub 2017 Dec 13.

Department of Polymer Science & Engineering and Department of IT Convergence, Korea National University of Transportation , Chungju 380-702, Republic of Korea.

A series of naphthalene diimide (NDI)-based small molecules were synthesized as nonfullerene acceptors and incorporated in all-small-molecule solar cells. Three NDI-based small molecules, NDICN-T, NDICN-BT, and NDICN-TVT, were designed with different linkers between two NDI units to induce the different conjugation length and modulate the geometric structures of the NDI dimers. The small NDI-based dimer electron acceptors with slip-stacked structures that facilitate π-π stacking interactions and/or hinder excessive aggregation exhibited different morphological behaviors, such as miscibility or crystallinity in bulk heterojunction blends with 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTS-F) electron donors. The photovoltaic devices prepared with NDICN-TVT gave the highest power conversion efficiency (PCE) of 3.01%, with an open-circuit voltage (V) of 0.75 V, a short-circuit current density (J) of 7.10 mA cm, and a fill factor of 56.2%, whereas the DTS-F:NDICN-T and DTS-F:NDICN-BT devices provided PCEs of 1.81 and 0.13%, respectively. Studies of the charge-generation properties, charge-transfer dynamics, and charge-transport properties for understanding the structure-property relations revealed that DTS-F:NDICN-TVT blend films with well-developed domains and well-ordered crystalline structures performed well, whereas an excessive miscibility between DTS-F and NDICN-BT disrupted the crystallinity of the material and yielded a poor device performance.
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http://dx.doi.org/10.1021/acsami.7b16004DOI Listing
December 2017

Direct Writing and Aligning of Small-Molecule Organic Semiconductor Crystals via "Dragging Mode" Electrohydrodynamic Jet Printing for Flexible Organic Field-Effect Transistor Arrays.

J Phys Chem Lett 2017 Nov 31;8(22):5492-5500. Epub 2017 Oct 31.

Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 790-784, Korea.

Patterning and aligning of organic small-molecule semiconductor crystals over large areas is an important issue for their commercialization and practical device applications. This Letter reports "dragging mode" electrohydrodynamic jet printing that can simultaneously achieve direct writing and aligning of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. Organic field-effect transistors (OFETs) based on the optimally printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm/(V·s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays.
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http://dx.doi.org/10.1021/acs.jpclett.7b02590DOI Listing
November 2017

Directionally Aligned Amorphous Polymer Chains via Electrohydrodynamic-Jet Printing: Analysis of Morphology and Polymer Field-Effect Transistor Characteristics.

ACS Appl Mater Interfaces 2017 Nov 3;9(45):39493-39501. Epub 2017 Nov 3.

POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 37673, Republic of Korea.

Electrohydrodynamic-jet (EHD-jet) printing provides an opportunity to directly assembled amorphous polymer chains in the printed pattern. Herein, an EHD-jet printed amorphous polymer was employed as the active layer for fabrication of organic field-effect transistors (OFETs). Under optimized conditions, the field-effect mobility (μ) of the EHD-jet printed OFETs was 5 times higher than the highest μ observed in the spin-coated OFETs, and this improvement was achieved without the use of complex surface templating or additional pre- or post-deposition processing. As the chain alignment can be affected by the surface energy of the dielectric layer in EHD-jet printed OFETs, dielectric layers with varying wettability were examined. Near-edge X-ray absorption fine structure measurements were performed to compare the amorphous chain alignment in OFET active layers prepared by EHD-jet printing and spin coating.
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http://dx.doi.org/10.1021/acsami.7b04316DOI Listing
November 2017

Markedly different molecular formation in DPP-based small-molecule solar cells probed by grazing-incidence wide-angle X-ray scattering.

Acta Crystallogr B Struct Sci Cryst Eng Mater 2017 Oct 19;73(Pt 5):916-922. Epub 2017 Sep 19.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea.

This study comprehensively explores the nanostructural properties of two diketopyrrolo[3,4-c]pyrrole-1,4-dione (DPP)-based small molecules with different alkyl side groups and their blends with the fullerene derivative PCBM, using grazing-incidence wide-angle X-ray scattering synchrotron techniques. Preferentially relative face-on orientation within the larger and more ordered stacking phase of SM1 with its shorter side group (ethylhexyl) was observed in the majority of both pristine and blend thin films, whereas SM2 crystals showed strictly perpendicular orientation. These contrasting crystalline characteristics led to significant differences in the results, from which crystalline structure-performance property correlations are proposed. Thus, the results not only demonstrate important scientific insights into the relationship between molecular structure and crystalline formation but also provide molecular design directions that will facilitate further improvement to the morphology and performance of DPP-based small-molecule solar cells.
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http://dx.doi.org/10.1107/S2052520617009933DOI Listing
October 2017

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures.

ACS Appl Mater Interfaces 2017 Aug 9;9(33):27839-27846. Epub 2017 Aug 9.

Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 790-784, Korea.

Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a "lying-down" pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.
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http://dx.doi.org/10.1021/acsami.7b05129DOI Listing
August 2017

Repurposing compact discs as master molds to fabricate high-performance organic nanowire field-effect transistors.

Nanotechnology 2017 May;28(20):205304

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

Organic field-effect transistors (OFETs) have been developed over the past few decades due to their potential applications in future electronics such as wearable and foldable electronics. As the electrical performance of OFETs has improved, patterning organic semiconducting crystals has become a key issue for their commercialization. However, conventional soft lithographic techniques have required the use of expensive processes to fabricate high-resolution master molds. In this study, we demonstrated a cost-effective method to prepare nanopatterned master molds for the fabrication of high-performance nanowire OFETs. We repurposed commercially available compact discs (CDs) as master molds because they already have linear nanopatterns on their surface. Flexible nanopatterned templates were replicated from the CDs using UV-imprint lithography. Subsequently, 6,13-bis-(triisopropylsilylethynyl) pentacene nanowires (NWs) were grown from the templates using a capillary force-assisted lithographic technique. The NW-based OFETs showed a high average field-effect mobility of 2.04 cm V s. This result was attributed to the high crystallinity of the NWs and to their crystal orientation favorable for charge transport.
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http://dx.doi.org/10.1088/1361-6528/aa6909DOI Listing
May 2017

Photoinduced Recovery of Organic Transistor Memories with Photoactive Floating-Gate Interlayers.

ACS Appl Mater Interfaces 2017 Apr 22;9(13):11759-11769. Epub 2017 Mar 22.

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 37673, Republic of Korea.

Optical memories based on photoresponsive organic field-effect transistors (OFETs) are of great interest due to their unique applications, such as multibit storage memories and flexible imaging circuits. Most studies of OFET-type memories have focused on the photoresponsive active channels, but more useful functions can be additionally given to the devices by using floating gates that can absorb light. In this case, effects of photoirradiation on photoactive floating-gate layers need to be fully understood. Herein, we studied the photoinduced erasing effects of floating-gate interlayers on the electrical responses of OFET-type memories and considered the possible mechanisms. Polymer/C composites were inserted between pentacene and SiO to form photoresponsive floating-gate interlayers in transistor memory. When exposed to light, C generated excitons, and these photoexcited carriers contributed to the elimination of trapped charge carriers, which resulted in the recovery of OFET performance. Such memory devices exhibited bistable current states controlled with voltage-driven programming and light-driven erasure. Furthermore, these devices maintained their charge-storing properties over 10 000 s. This proof-of-concept study is expected to open up new avenues in information technology for the development of organic memories that exhibit photoinduced recovery over a wide range of wavelengths of light when combined with appropriate photoactive floating-gate materials.
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http://dx.doi.org/10.1021/acsami.7b02365DOI Listing
April 2017

Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing.

ACS Appl Mater Interfaces 2016 Dec 6;8(50):34353-34359. Epub 2016 Dec 6.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 790-784, Republic of Korea.

Here we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of the rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made with DTBDT-Rho:PCBM blends showed an extremely high power conversion efficiency (PCE; 7.10%); notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.
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http://dx.doi.org/10.1021/acsami.6b11297DOI Listing
December 2016

Effect of Solvent-Assisted Dispersions of Clay/Epoxy Nanocomposites on Steel Passivation.

J Nanosci Nanotechnol 2016 Jan;16(1):981-6

Dispersion of clay in polymer matrices is important to improve their engineering performances. Here we report the effect of solvent on dispersion of montmorillonite (MMT) in an epoxy matrix by examining transmission electron micrographs and X-ray diffraction of MMT/epoxy composites prepared with solvents with different polarities. We found that N-metyl-2-pyrrolidone (NMP) used as a polar solvent exhibited the improved dispersion of MMT in the epoxy owing to positive interaction energies with components, which prevents the aggregation of MMT platelets. The solvent-assisted dispersion of MMT significantly increased the corrosion resistance of MMT/epoxy nanocomposites pre-coated onto steel plates.
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http://dx.doi.org/10.1166/jnn.2016.11105DOI Listing
January 2016

Isoindigo-based polymer photovoltaics: modifying polymer molecular structures to control the nanostructural packing motif.

Phys Chem Chem Phys 2016 Jul 21;18(27):17957-64. Epub 2016 Jun 21.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

Donor molecular structures, and their packing aspects in donor:acceptor active blends, play a crucial role in the photovoltaic performance of polymer solar cells. We systematically investigated a series of isoindigo-based donor polymers within the framework of a three-dimensional (3D) crystalline motif by modifying their chemical structures, thereby affecting device performances. Although our isoindigo-based polymer series contained polymers that differed only by their alkyl side chains and/or donating units, they showed quite different nanoscale morphological properties, which resulted in significantly different device efficiencies. Notably, blends of our isoindigo-based donor polymer systems with an acceptor compound, whereby the blends had more intermixed network morphologies and stronger face-on orientations of the polymer crystallites, provided better-performing photovoltaic devices. This behavior was analyzed using atomic force microscopy (AFM) and two-dimensional grazing incidence wide angle X-ray diffraction (2D-GIWAXD). To the best of our knowledge, no correlation has been reported previously between 3D nano-structural donor crystallites and device performances, particularly for isoindigo-based polymer systems.
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http://dx.doi.org/10.1039/c6cp03273gDOI Listing
July 2016

Impact of the Crystalline Packing Structures on Charge Transport and Recombination via Alkyl Chain Tunability of DPP-Based Small Molecules in Bulk Heterojunction Solar Cells.

ACS Appl Mater Interfaces 2016 05 13;8(20):12940-50. Epub 2016 May 13.

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) , Daejeon 305-600, Republic of Korea.

A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.
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http://dx.doi.org/10.1021/acsami.6b01576DOI Listing
May 2016

Nanowires: A Lattice-Strained Organic Single-Crystal Nanowire Array Fabricated via Solution-Phase Nanograting-Assisted Pattern Transfer for Use in High-Mobility Organic Field-Effect Transistors (Adv. Mater. 16/2016).

Adv Mater 2016 04;28(16):3034

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, South Korea.

S. H. Kim, S. G. Hahm, C. E. Park, and co-workers fabricate a 50 nm-wide organic single-crystalline nanowire array on a centimeter-sized substrate via a facile roll-to-plate process, as described on page 3209. Nanowire growth in a nano-confined space adopts a lattice-strained and single-crystalline packing motif, which can be harnessed for strong intermolecular electronic coupling. Thus, nanowire-based field-effect transistors show extremely high field-effect mobilities up to 9.71 cm(2) V(-1) s(-1) .
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http://dx.doi.org/10.1002/adma.201670107DOI Listing
April 2016

Following the nanostructural molecular orientation guidelines for sulfur versus thiophene units in small molecule photovoltaic cells.

Nanoscale 2016 Apr;8(14):7654-62

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

In bulk heterojunction (BHJ) organic photovoltaics, particularly those using small molecules, electron donor and/or electron acceptor materials form a distributed network in the photoactive layer where critical photo-physical processes occur. Extensive research has recently focused on the importance of sulfur atoms in the small molecules. Little is known about the three-dimensional orientation of these sulfur atom-containing molecules. Herein, we report on our research concerning the heterojunction textures of the crystalline molecular orientation of small compounds having sulfur-containing units in the side chains, specifically, compounds known as DR3TSBDT that contain the alkylthio group and DR3TBDTT that does not. The improved performance of the DR3TBDTT-based devices, particularly in the photocurrent and the fill factor, was attributed to the large population of donor compound crystallites with a favorable face-on orientation along the perpendicular direction. This orientation resulted in efficient charge transport and a reduction in charge recombination. These findings underscore the great potential of small-molecule solar cells and suggest that even higher efficiencies can be achieved through materials development and molecular orientation control.
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http://dx.doi.org/10.1039/c6nr00775aDOI Listing
April 2016

A Lattice-Strained Organic Single-Crystal Nanowire Array Fabricated via Solution-Phase Nanograting-Assisted Pattern Transfer for Use in High-Mobility Organic Field-Effect Transistors.

Adv Mater 2016 04 24;28(16):3209-15. Epub 2016 Feb 24.

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, South Korea.

A 50 nm-wide 6,13-bis(triisopropylsilylethynyl) pentacene nanowire (NW) array is fabricated on a centimeter-sized substrate via a facile nanograting-assisted pattern-transfer method. NW growth under a nanoconfined space adopts a lattice-strained packing motif of the NWs for strong intermolecular electronic coupling, and thus a NW-based organic field-effect transistor shows high field-effect mobility up to 9.71 cm(2) V(-1) s(-1) .
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http://dx.doi.org/10.1002/adma.201506062DOI Listing
April 2016

Photo-Patternable ZnO Thin Films Based on Cross-Linked Zinc Acrylate for Organic/Inorganic Hybrid Complementary Inverters.

ACS Appl Mater Interfaces 2016 Mar 16;8(8):5499-508. Epub 2016 Feb 16.

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology , Pohang, North Gyeongsang 790-784, Republic of Korea.

Complementary inverters consisting of p-type organic and n-type metal oxide semiconductors have received considerable attention as key elements for realizing low-cost and large-area future electronics. Solution-processed ZnO thin-film transistors (TFTs) have great potential for use in hybrid complementary inverters as n-type load transistors because of the low cost of their fabrication process and natural abundance of active materials. The integration of a single ZnO TFT into an inverter requires the development of a simple patterning method as an alternative to conventional time-consuming and complicated photolithography techniques. In this study, we used a photocurable polymer precursor, zinc acrylate (or zinc diacrylate, ZDA), to conveniently fabricate photopatternable ZnO thin films for use as the active layers of n-type ZnO TFTs. UV-irradiated ZDA thin films became insoluble in developing solvent as the acrylate moiety photo-cross-linked; therefore, we were able to successfully photopattern solution-processed ZDA thin films using UV light. We studied the effects of addition of a tiny amount of indium dopant on the transistor characteristics of the photopatterned ZnO thin films and demonstrated low-voltage operation of the ZnO TFTs within ±3 V by utilizing Al2O3/TiO2 laminate thin films or ion-gels as gate dielectrics. By combining the ZnO TFTs with p-type pentacene TFTs, we successfully fabricated organic/inorganic hybrid complementary inverters using solution-processed and photopatterned ZnO TFTs.
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http://dx.doi.org/10.1021/acsami.6b00259DOI Listing
March 2016

Optimization of Al2O3/TiO2 nanolaminate thin films prepared with different oxide ratios, for use in organic light-emitting diode encapsulation, via plasma-enhanced atomic layer deposition.

Phys Chem Chem Phys 2016 Jan;18(2):1042-9

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

Encapsulation is essential for protecting the air-sensitive components of organic light-emitting diodes (OLEDs), such as the active layers and cathode electrodes. Thin film encapsulation approaches based on an oxide layer are suitable for flexible electronics, including OLEDs, because they provide mechanical flexibility, the layers are thin, and they are easy to prepare. This study examined the effects of the oxide ratio on the water permeation barrier properties of Al2O3/TiO2 nanolaminate films prepared by plasma-enhanced atomic layer deposition. We found that the Al2O3/TiO2 nanolaminate film exhibited optimal properties for a 1 : 1 atomic ratio of Al2O3/TiO2 with the lowest water vapor transmission rate of 9.16 × 10(-5) g m(-2) day(-1) at 60 °C and 90% RH. OLED devices that incorporated Al2O3/TiO2 nanolaminate films prepared with a 1 : 1 atomic ratio showed the longest shelf-life, in excess of 2000 hours under 60 °C and 90% RH conditions, without forming dark spots or displaying edge shrinkage.
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http://dx.doi.org/10.1039/c5cp06713hDOI Listing
January 2016

A Mechanistic Understanding of a Binary Additive System to Synergistically Boost Efficiency in All-Polymer Solar Cells.

Sci Rep 2015 Dec 11;5:18024. Epub 2015 Dec 11.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

All-polymer solar cells are herein presented utilizing the PBDTTT-CT donor and the P(NDI2OD-T2) acceptor with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) binary solvent additives. A systematic study of the polymer/polymer bulk heterojunction photovoltaic cells processed from the binary additives revealed that the microstructures and photophysics were quite different from those of a pristine system. The combination of DIO and CN with a DIO/CN ratio of 3:1 (3 vol% DIO, 1 vol% CN and 96 vol% o-DCB) led to suitable penetrating polymer networks, efficient charge generation and balanced charge transport, which were all beneficial to improving the efficiency. This improvement is attributed to increase in power conversion efficiency from 2.81% for a device without additives to 4.39% for a device with the binary processing additives. A detailed investigation indicates that the changes in the polymer:polymer interactions resulted in the formation of a percolating nasnoscale morphology upon processing with the binary additives. Depth profile measurements with a two-dimensional grazing incidence wide-angle X-ray scattering confirm this optimum phase feature. Furthermore impedance spectroscopy also finds evidence for synergistically boosting the device performance.
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http://dx.doi.org/10.1038/srep18024DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676009PMC
December 2015

Schematic Studies on the Structural Properties and Device Physics of All Small Molecule Ternary Photovoltaic Cells.

ACS Appl Mater Interfaces 2015 Sep 15;7(38):21423-32. Epub 2015 Sep 15.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 790-784, Republic of Korea.

Although the field of ternary organic solar cells has seen much progress in terms of device performance in the past few years, limited understanding has restricted further development. For example, studies of the crystalline packing structure of ternary blends have rarely been reported in the solar cell field. Consequently, we chose two ternary blends of small molecules, two fullerene derivatives (small-molecule:PC71BM:PC61BM or small-molecule:PC71BM:ICBA), to investigate crystallization behavior and interactions among the three components. The crystalline structure of the ternary active blends was characterized using various techniques such as 2D-GIWAXS and AFM, and the relationship of the observed morphologies to device performance is discussed. Furthermore, the device physics associated with the charge generation, transport, and recombination dynamics of these ternary blend systems were investigated.
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http://dx.doi.org/10.1021/acsami.5b06365DOI Listing
September 2015

A new multi-functional conjugated polymer for use in high-performance bulk heterojunction solar cells.

Chem Commun (Camb) 2015 Jul 22;51(58):11572-5. Epub 2015 Jun 22.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea.

We report a new multi-functional copolymer, PDTP-DTBDT, containing DTP and DTBDT units. Surprisingly, the introduction of novel DTP and DTBDT units brings not only superior charge transfer properites but also charge transport characteristics for efficient bulk heterojunction solar cells.
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http://dx.doi.org/10.1039/c5cc03815dDOI Listing
July 2015

3D Hollow Framework Silver Nanowire Electrodes for High-Performance Bottom-Contact Organic Transistors.

ACS Appl Mater Interfaces 2015 Jul 26;7(26):14272-8. Epub 2015 Jun 26.

†POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk 790-784, Korea.

We successfully fabricated high performance bottom-contact organic field-effect transistors (OFETs) using silver nanowire (AgNW) network electrodes by spray deposition. The synthesized AgNWs have the dimensions of 40-80 nm in diameter and 30-80 μm in length and are randomly distributed and interconnected to form a 3D hollow framework. The AgNWs networks, deposited by spray coating, yield an average optical transmittance of up to 88% and a sheet resistance as low as 10 ohm/sq. For using AgNWs as source/drain electrodes of OFETs with a bottom-contact configuration, the large contact resistance at the AgNWs/organic channel remains a critical issue for charge injection. To enhance charge injection, we fabricate semiconductor crystals on the AgNW using an adsorbed residual poly(N-vinylpyrrolidone) layer. The resulting bottom-contact OFETs exhibit high mobility up to 1.02 cm(2)/(V s) and are similar to that of the top-contact Au electrodes OFETs with low contact resistance. A morphological study shows that the pentacene crystals coalesced to form continuous morphology on the nanowires and are highly interconnected with those on the channel. These features contribute to efficient charge injection and encourage the improvement of the bottom-contact device performance. Furthermore, the large contact area of individual AgNWs spreading out to the channel at the edge of the electrode also improves device performance.
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http://dx.doi.org/10.1021/acsami.5b02610DOI Listing
July 2015