Showing papers by "Yeong Don Park published in 2014"

Organic Semiconductor/Insulator Polymer Blends for High-Performance Organic Transistors

Abstract: We reviewed recent advances in high-performance organic field-effect transistors (OFETs) based on organic semiconductor/insulator polymer blends. Fundamental aspects of phase separation in binary blends are discussed with special attention to phase-separated microstructures. Strategies for constructing semiconductor, semiconductor/dielectric, or semiconductor/passivation layers in OFETs by blending organic semiconductors with an insulating polymer are discussed. Representative studies that utilized such blended films in the following categories are covered: vertical phase-separation, processing additives, embedded semiconductor nanowires.

Electrical performance of organic solar cells with additive-assisted vertical phase separation in the photoactive layer

Abstract: Understanding the vertical phase separation of donor and acceptor compounds in organic photovoltaics is requisite for the control of charge transport behavior and the achievement of efficient charge collection. Here, the vertically phase-separated morphologies of poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend films are examined with transmission electron microtomography, dynamic secondary ion mass spectroscopy, and X-ray photoelectron spectroscopy. The 3D morphologies of the processed films are analyzed and how the solvent additive causes vertical segregation is determined. The photocurrent–voltage characteristics of the vertically segregated blend films are strongly dependent on the 3D morphological organization of the donor and acceptor compounds in the photoactive layer. This dependence is correlated with asymmetric carrier transport at the buried interface and the air surface in the vertically segregated blend films.

Sequential solvent casting for improving the structural ordering and electrical characteristics of polythiophene thin films

Abstract: We developed a facile post-deposition method for preparing high-performance organic transistors using direct solvent exposure. The morphological, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films were profoundly influenced by the solubility of P3HT in a solvent. Exposure to an optimized binary solvent mixture comprising methylene chloride and toluene efficiently improved the morphology and molecular ordering in a conjugated polymer thin film. The improved ordering was correlated with improved charge carrier transport in the field-effect transistors (FETs) prepared from the films. The correlation between the thin films' structural features and the electrical properties of the films guided the identification of an appropriate binary solvent mixing ratio and characterized the influence of the physical properties on the electronic properties of solvent-exposed P3HT films in an FET.

Charge transport behaviors of end-capped narrow band gap polymers in bottom-contact organic field-effect transistors

Abstract: This report examines the influence of narrow band gap polymer end groups (bromine or thiophene) on the performance of field-effect transistors prepared using these polymers. The conjugated polymer chains that were capped with thiophene units enhanced the intermolecular packing structure and decreased device hysteresis by removing charge traps. The presence of the end-capping groups increased the hole mobility by a factor of 2 or 4, depending on the molecular weight of the polymer, in the bottom contact field-effect transistors.

The Molecular Structures of Poly(3-hexylthiophene) Films Determine the Contact Properties at the Electrode/Semiconductor Interface

Abstract: The contact properties between gold and poly(3-hexylthiophene) (P3HT) films having either of two distinct molecular orientations and orderings were investigated. Thermal treatment increased the molecular ordering of P3HT and remarkably reduced the contact resistance at the electrode/semiconductor interface, which enhanced the electrical performance. This phenomenon was understood in terms of a small degree of metal penetration into the P3HT film as a result of the thermal treatment, which formed a sharp interface at the contact interface between the gold electrode and the organic semiconductor.

Built-in water resistance in organic transistors modified with self-assembled monolayers

Abstract: We systematically investigated the effects of a self-assembled monolayer (SAM), prepared on the gate dielectric, on the performances of bottom-gate organic field-effect transistor (OFET) devices under various humid environments. OFETs prepared with gate dielectrics modified by depositing a hydrophilic SAM display large variations in their carrier mobilities and on/off ratios when operated under dry or humid conditions. By contrast, the performances of OFETs with a hydrophobic SAM remain relatively constant, regardless of the humidity level. The stability conveyed by the hydrophobic SAM in the presence of humidity is closely related to the water resistance of the SAM, which is based on the hydrophilic and hydrophobic characteristics of the modified gate dielectric.

Low Spin-Casting Solution Temperatures Enhance the Molecular Ordering in Polythiophene Films

Abstract: High-crystallinity poly(3-hexylthiophene) (P3HT) thin films were prepared by aging the precursor solutions, prepared using a good solvent, chloroform, at low temperatures prior to spin-casting. Lower solution temperatures significantly improved the molecular ordering in the spin-cast P3HT films and, therefore, the electrical properties of field-effect transistors prepared using these films. Solution cooling enhanced the electrical properties by shifting the P3HT configuration equilibrium away from random coils and toward more ordered aggregates. At room temperature, the P3HT molecules were completely solvated in chloroform and adopted a random coil conformation. Upon cooling, however, the chloroform poorly solvated the P3HT molecules, favoring the formation of ordered P3HT aggregates, which then yielded more highly crystalline molecular ordering in the P3HT thin films produced from the solution.

Electrical Characteristics Enhancement of Conjugated Polymer Thin Film Transistor by Using Dipping Method

Abstract: In this work, it is possible to simply improve the molecular ordering of a conjugated polymer thin film by dipping into poor solvent. The structural order, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films were profoundly influenced by dipping time and solubility of solvent. Especially the dipping time in methylene chloride was controlled to efficiently improve the molecular ordering of the P3HT. The correlation between the structural order and the electrical properties was used to optimize the dipping time in the appropriate solvent.