Influence of bank geometry on the electrical characteristics of printed organic field-effect transistors

The electrical characteristics of organic field-effect transistors (OFETs) based on small-molecule organic semiconductors (OSCs) have been significantly improved by employing various fabrication techniques in solution processes to enhance the OSC crystallinity. However, complicated fabrication and inhomogeneity of OFETs remain as challenges before commercialization. In this work, we have efficiently controlled the size and orientation of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) crystalline domains by tuning the Cytop bank dimension, in which OSC inks are printed, to improve the device performance. The optimized bank pattern forms uniform thin film morphology and well-aligned TIPS-pentacene crystalline domains along the charge transport direction, resulting in four-fold increase in field-effect mobility and one third reduction in relative standard deviation.11Ysciescopu

Highly Oriented Liquid Crystal Semiconductor for Organic Field-Effect Transistors

We report a mesogenic compound which introduces nematic liquid crystal (LC) ordering into the benzothienobenzothiophene (BTBT) family of LCs, creating a new class of LC semiconducting materials which respond in a facile way to anisotropic surfaces, and can, thereby, be effectively processed into highly oriented monodomains. Measurement on these domains of the electrical conductivity, with in situ monitoring of domain quality and orientation using LC birefringence textures in electroded cells, brings a new era of precision and reliability to the determination of anisotropic carrier mobility in LC semiconductors

Precise orientation control of a liquid crystal organic semiconductor via anisotropic surface treatment

We report a three-dimensional (3D) molecular orientation control of a liquid crystal organic semiconductor (LC-OSC) based on the long-range ordering characteristic of an LC material. To this end, a synthetic LC-OSC molecule, MeOPh-BTBT-C8, with a fluidic nematic (N) phase that is essential for alignment control over a large area and a smectic E (SmE) phase showing high ordering, was prepared. A simple flipping of a sandwich cell made of the LC-OSC material between the top and bottom substrates that have uniaxial-planar degenerated alignment as well as crossed rubbing directions responds to the given surface anchoring condition and temperature gradient. Optical observation of the alignment-controlled LC-OSC was carried out by polarized optical microscopy (POM), and the corresponding charge carrier mobility was also measured by fabricating organic field-effect transistors (OFETs). Our platform offers a facile approach for multidirectional and multifunctional organic electronic devices using the stimulus-response characteristics of LC materials

Tailored growth of graphene oxide liquid crystals with controlled polymer crystallization in GO-polymer composites

Graphene Oxides (GOs) have been frequently employed as fillers in polymer-based applications. While GO is known to nucleate polymer crystallization in GO-polymer composites reinforcing the mechanical properties of semicrystalline polymers, its counter effect on how polymer crystallization can alter the microstructure of GO has rarely been systematically studied yet. In this work, we study the GO nematic liquid crystal (LC) phase during polymer crystallization focusing on their hierarchical structures by employing in situ small/wide-angle X-ray scattering/diffraction (SAXS/WAXD) techniques. We found that GO LC and polymer crystals co-exist in the GO/polymer complex, where the overall liquid crystallinity is influenced by polymer crystallization. While polymer crystallizes in bulk or at the interface depending on the cooling rate, the interfacial crystallization of poly(ethylene glycol) (PEG) on GO improves both GO alignment and orientation of PEG crystal. This work provides an opportunity to develop a hierarchical structure of GO-based crystalline polymer nanocomposites, whose directionality can be controlled by polymer crystallization under proper cooling rates

Parylene copolymer gate dielectrics for organic field- effect transistors

A double-layer gate dielectric has been used to overcome the drawbacks of organic field-effect transistors with a single-layer gate dielectric. However, the double-layered dielectrics require additional fabrication processes, resulting in increasedmanufacturing cost and complexity. Here, we present parylene copolymer gate dielectrics fabricated by in situ codeposition of two different families, parylene C and parylene F while maintaining double-layer structures. The effect of the copolymer dielectric on device performance is systematically investigated by evaluating dielectric properties and electrical characteristics. The results show that an organic transistor with a codeposited parylene dielectric exhibits high performance and stable operation without increasing the manufacturing complexity

Conjugated Random Copolymers Consisting of Pyridine- and Thiophene-Capped Diketopyrrolopyrrole as Co-Electron Accepting Units To Enhance both JSC and VOC of Polymer Solar Cells

One of the effective strategies to enhance the photovoltaic performance of polymer solar cells (PSCs) is to synthesize random copolymers composed of one electron donating unit and two different electron accepting units, if the absorptions of two electron accepting units are complementary to each other. To this end, we synthesized a new series of conjugated random copolymer composed of bithiophene (electron donating unit) with thiophene-capped diketopyrrolopyrrole (TDPP) and pyridine-capped diketopyrrolopyrrole (PyDPP) (co-electron accepting units). The random copolymers show broad light absorption and face-on orientation on the substrate, which is beneficial to achieving high short circuit current. The open circuit voltage of the random copolymer can also be controlled systematically by varying the ratio of PyDPP to TDPP in the copolymer, since the HOMO energy level becomes deeper as the PyDPP content in the random copolymer is increased. Consequently, the solar cell device made of the random copolymer with the ratio of 3:1 (TDPP:PyDPP) shows higher PCE (8.11%) than those made of corresponding homopolymers, PTDPP2T (6.70%) and PPyDPP2T (4.14%).OAIID:oai:osos.snu.ac.kr:snu2015-01/102/0000001236/11ADJUST_YN:NEMP_ID:A004558DEPT_CD:445CITE_RATE:5.8FILENAME:8. conjugated random copolymers consisting.pdfDEPT_NM:재료공학부SCOPUS_YN:YCONFIRM:

Controllable liquid crystal defect arrays induced by an in-plane electric field and their lithographic applications

We control the shape and arrangement of various kinds of liquid crystal (LC) defects in nematic (N) and smectic A (SmA) phases using an in-plane electric field. Periodic zigzag disclination line defects generated in the N phase with a winding number of -1/2 are transformed to arrays of focal conic domains (FCDs) in the SmA phase upon cooling under the electric field. Notably, reconfigurable arrays of FCDs such as hexagonal, alternating, and face-to-face arrangements are generated by varying the applied voltage, and are analyzed using polarized optical microscopy (POM) and fluorescence confocal polarized microscopy (FCPM). For the molecular scale analysis, in situ grazing incidence X-ray diffraction (GIXD) experiments are carried out to determine the internal layer and molecular configurations. To demonstrate the versatility of our platform, defect arrays produced in the SmA phase are used as lithographic templates to generate distinct quantum dot (QD) arrays. Our approach can provide reconfigurable lithographic templates for functional materials

Parylene-Based Double-Layer Gate Dielectrics for Organic Field-Effect Transistors

We demonstrate high-performance and stable organic field-effect transistors (OFETs) using parylene-based double-layer gate dielectrics (DLGDs). DLGDs, consisting of parylene C as the upper layer and F as the lower layer, are designed to simultaneously provide good interface and bulk gate dielectric properties by exploiting the advantages of each gate dielectric. The structural effects of DLGDs are systematically investigated by evaluating the electrical characteristics and dielectric properties while varying the thickness ratio of each gate dielectric. The OFET with the optimized DLGD exhibits high performance and operational stability. This systematic approach will be useful for realizing practical electronic applications

Controlling Packing Structure of Naphthalenediimide-Based Polymer Acceptors for High-Performance All-Polymer Solar Cells

Packing structure of semiconducting polymers at donor/acceptor interface plays a critical role in determining the performance of all-polymer solar cells (all-PSCs). Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalenediimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights (Mn) is reported. A series of P(NDI2HD-T2) polymers with different Mn of 13.6 (PL), 22.9 (PM), and 49.9 kg mol???1 (PH) were prepared by changing the amount of end-capping agent (2-bromothiophene) during polymerization. Increasing the Mn values of P(NDI2HD-T2) polymers led a remarkable shift of dominant lamellar crystallite textures from edge-on (PL) to face-on (PH). In addition, the portion of face-on oriented crystallites was dramatically increased from 21.5% and 46.1%, to 78.6% for PL, PM, and PH polymers. These different packing structures in terms of the molecular orientation greatly affected the charge dissociation efficiency at the donor/acceptor interface and thus the short-circuit current density of the all-PSCs. All-PSCs with PTB7-Th as electron donor and PH as electron acceptor showed the highest efficiency of 6.14%, outperforming those with PM (5.08%) and PL (4.29%

Molecular orientation in organic light emitting devices

The research on molecular orientation in organic light emitting device(OLEDs) has been working because it is known as one of key factor to OLED efficiency by considering the outcoupling enhancement as well as charge mobility, which are mainly analyzed using angular dependent photoluminescence spectra or X-ray diffraction. However, the two dimensional grazing incidence X-ray diffraction(2D-GIXD) anlysis using scttered X-rays has not been widely used to clarify the molecular orientation in OLEDs. The 2D-GIXD analysis with synchrotron light offers the unique opportunity to study the diffraction from weakly scattering of thin films. In this study, we found that instead of about 1, 6, and 1.8 cd/A for an isotropic emitter(mixed orientation), thus, the OLED efficiency with horizontal emitter orientation can be improved over 5, 11, and 12 cd/A due to increased light outcoupling, as shown in Fig. 1(a), (b), and (c). To clarify the molecular orientations with various parameters in synthesized materials, we analyzed the scattered X-ray intensity in the 2D-GIXD patterns of emitting materials with spectra of scttering in-plane and out-of-plane to the plane and azimuthal angle scan. In summay, we have discussed the relationship between molecular orientation and device efficiency in the synthesized FL, PH, and TADF-based emitting, which can be determined by the substituent group or linker position.1