Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors

The preparation of uniform large-area highly crystalline organic semiconductor single crystals remains a challenge in the field of organic field-effect transistors (OFETs). Crystal densities in the channel regions of OFETs have not yet reached sufficiently high values to provide efficient charge transport, and improving channel crystal densities remains an important research area. Herein we fabricated densely well-aligned single crystal arrays of the 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS_PEN) semiconductor using a straightforward scooping-up (SU) methodology to quickly produce a large-area self-assembled semiconductor crystal layer. The resulting crystalline TIPS_PEN strip arrays obtained using the SU method revealed a packing density that was 2.76 times the value obtained from the dip-coated channel, and the mean interspatial distance between the crystal strips decreased from 21.5 to 7.8 μm. The higher crystal packing density provided efficient charge transport in the FET devices and directly yielded field-effect mobilities as high as 2.16 cm²/(V s). These field-effect mobilities were more than three times the values obtained from the OFETs prepared using dip-coated channels. Furthermore, the contact resistance between the source/drain electrodes and the TIPS_PEN crystals decreased by a factor of 2. These contributions represent a significant step forward in improving semiconductor crystal alignment for the fabrication of large-area high-performance organic electronics

Gate-Bias Stability Behavior Tailored by Dielectric Polymer Stereostructure in Organic Transistors

Understanding charge trapping in a polymer dielectric is critical to the design of high-performance organic field-effect transistors (OFETs). We investigated the OFET stability as a function of the dielectric polymer stereostructure under a gate bias stress and during long-term operation. To this end, <i>iso</i>-, <i>syn</i>-, and atactic poly­(methyl methacrylate) (PMMA) polymers with identical molecular weights and polydispersity indices were selected. The PMMA stereostructure was found to significantly influence the charge trapping behavior and trap formation in the polymer dielectrics. This influence was especially strong in the bulk region rather than in the surface region. The regular configurational arrangements (isotactic > syntactic > atactic) of the pendant groups on the PMMA backbone chain facilitated closer packing between the polymer interchains and led to a higher crystallinity of the polymer dielectric, which caused a reduction in the free volumes that act as sites for charge trapping and air molecule absorption. The PMMA dielectrics with regular stereostructures (<i>iso</i>- and <i>syn</i>-stereoisomers) exhibited more stable OFET operation under bias stress compared to devices prepared using irregular <i>a</i>-PMMA in both vacuum and air

Comparison of changes in etiologic microorganisms causing early-onset neonatal sepsis between preterm labor and preterm premature rupture of membranes

<div><p></p><p><i>Objective</i>: To investigate changes in the etiologic microorganisms causing early-onset neonatal sepsis (EONS) in preterm labor (PTL) or preterm premature rupture of membranes (pPROM) cases over the past 16 years and to analyze the associated factors.</p><p><i>Methods</i>: We included consecutive singleton pregnancies delivered before 34 weeks due to PTL or pPROM. The etiologic microorganisms causing EONS in PTL and pPROM cases were compared between period 1 (1996–2004) and period 2 (2005–2012).</p><p><i>Results</i>: There was no difference in the incidence of Gram-positive bacteria causing EONS between period 1 and 2, either in PTL (2.0% versus 2.1%, <i>p</i> = 1.0) or in pPROM (1.5% versus 1.6%, <i>p</i> = 1.0). However, the incidence of EONS caused by Gram-negative bacteria was significantly increased in pPROM (0.6% versus 2.7%, <i>p</i> = 0.040) during period 2, compared to period 1; but not in PTL (0.3% versus 1.2%, <i>p</i> = 0.211). Multivariable analysis revealed that a prolonged ROM-to-delivery interval (>7 d) was significantly associated with EONS caused by Gram-negative bacteria in pPROM (odds ratio: 6.6, 95% confidence interval: 1.4–31.8, <i>p</i> = 0.018).</p><p><i>Conclusions</i>: The etiologic microorganisms causing EONS have changed over the past 16 years in pPROM cases but not in PTL cases.</p></div

Thermal Gradient During Vacuum-Deposition Dramatically Enhances Charge Transport in Organic Semiconductors: Toward High-Performance N‑Type Organic Field-Effect Transistors

A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from <i>N</i>,<i>N</i>′-bis­(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis­(dicarboxyimide) (PDI-CN2), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature one-dimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the PDI-CN2 film. The morphological and crystalline structures of the PDI-CN2 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the PDI-CN2-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm² V^–1 s^–1, more than a factor of 2 higher than the mobility of 0.25 cm² V^–1 s^–1 submitted to conventional thermal annealing and the mobility of 0.29 cm² V^–1 s^–1 from the horizontally applied temperature gradient

Branched Segments in Polymer Gate Dielectric as Intrinsic Charge Trap Sites in Organic Transistors

Charge traps in polymer gate dielectrics determine the electrical stability of organic field-effect transistors (OFETs), and polar alkoxy groups are well-known extrinsic charge traps. However, the actual location of intrinsic charge traps in nonpolar polymer gate dielectrics has been poorly understood yet. Here, we demonstrate that the skeletal structure of polymer chain plays an important role in determining the electrical stability. To verify it, we prepared linear and branched polystyrene (<i>l</i>-PS and <i>b</i>-PS) and blended them, in which branched segments provide much larger free volume than the other segments. The current-insulating performance and field-effect mobility increased with decease of <i>b</i>-PS portion. In particular, the bias-stress stability was remarkably varied according to the change of <i>b</i>-PS portion even though all measurements excluded reactive components such as oxygen and water; the increase of <i>b</i>-PS resulted in time-dependent decay of mobility and threshold voltage under bias stress. This indicates that the branched segments in <i>b</i>-PS provide intrinsic and metastable charge trap sites. Our result suggests that the skeletal structure of polymeric chains in gate dielectric is one of the important factors affecting intrinsic long-term operational stability of OFET devices

Babinet-Inverted Optical Yagi–Uda Antenna for Unidirectional Radiation to Free Space

Nanophotonics capable of directing radiation or enhancing quantum-emitter transition rates rely on plasmonic nanoantennas. We present here a novel Babinet-inverted magnetic-dipole-fed multislot optical Yagi–Uda antenna that exhibits highly unidirectional radiation to free space, achieved by engineering the relative phase of the interacting surface plasmon polaritons between the slot elements. The unique features of this nanoantenna can be harnessed for realizing energy transfer from one waveguide to another by working as a future “optical via”