Yeong Don Park

End-capping effect of a narrow bandgap conjugated polymer on bulk heterojunction solar cells.

Device performances of BHJ solar cells based on poly[(4,4-didodecyldithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzoxadiazole)-4,7-diyl]and PC₇₁BM improve by capping the chain ends with thiophene fragments. This structural modification yields materials that are more thermally robust and that can be used in devices with thicker films – important considerations for enabling the mass production of plastic solar cells.

Evaporation‐Induced Self‐Alignment and Transfer of Semiconductor Nanowires by Wrinkled Elastomeric Templates

The evaporation-induced self-alignment of semiconductor nanowires is achieved using wrinkled elastomeric templates. The wrinkled templates, which have a surface topography that can be tuned via changes in the mechanical strain, are used as both a template to align the nanowires and as a stamp to transfer the aligned nanowires to target substrates.

A polymer brush organic interlayer improves the overlying pentacene nanostructure and organic field-effect transistor performance

We investigated the crystalline nanostructures and film morphologies of pentacene films deposited onto a polymer brush organic interlayer in high performance organic field-effect transistors (OFETs). Polymer brushes were grafted onto the oxide substrates by spin-coating and thermal annealing. Pentacene FETs fabricated on top of the polymer brushes showed excellent device performance, with a field-effect mobility of 0.82 cm2 V−1s−1 and an on/off current ratio of 107. These properties were superior to those of devices using typical surface modification techniques, such as octadecyltrichlorosilane (ODTS) and hexamethyldisilazane (HMDS). The improvements in OFET performance appeared to be due to the pentacene layers crystalline nanostructure and grain interconnectivity, which formed during the submonolayer stage of film growth. This stage of growth is strongly correlated with the surface energy, morphology, and viscoelastic properties of the resulting gate dielectrics. The inclusion of a polymer brush dielectric surface modification is a significant step toward optimizing the nanostructures of organic semiconductors, which are directly linked to device performance enhancement, by engineering the interfaces in OFETs.

Surface Modification of the LiFePO4 Cathode for the Aqueous Rechargeable Lithium Ion Battery

The LiFePO4 surface is coated with AlF3 via a simple chemical precipitation for aqueous rechargeable lithium ion batteries (ARLBs). During electrochemical cycling, the unfavorable side reactions between LiFePO4 and the aqueous electrolyte (1 M Li2SO4 in water) leave a highly resistant passivation film, which causes a deterioration in the electrochemical performance. The coated LiFePO4 by 1 wt % AlF3 has a high discharge capacity of 132 mAh g-1 and a highly improved cycle life, which shows 93% capacity retention even after 100 cycles, whereas the pristine LiFePO4 has a specific capacity of 123 mAh g-1 and a poor capacity retention of 82%. The surface analysis results, which include X-ray photoelectron spectroscopy and transmission electron microscopy results, show that the AlF3 coating material is highly effective for reducing the detrimental surface passivation by relieving the electrochemical side reactions of the fragile aqueous electrolyte. The AlF3 coating material has good compatibility with the LiFePO4 cathode material, which mitigates the surface diffusion obstacles, reduces the charge-transfer resistances and improves the electrochemical performance and surface stability of the LiFePO4 material in aqueous electrolyte solutions.

Post-deposition dipping method for improving the electronic properties of a narrow bandgap conjugated polymer

By dipping into hexane, it is possible to efficiently eliminate the low MW component and also improve the molecular ordering of a conjugated polymer thin film. These changes improve the performance of field-effect transistors. The correlation between the nanoscalar structural features and the electrical properties enables us to determine both the appropriate dipping time and how the low MW component influences electronic properties.

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

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.

Marginal solvents preferentially improve the molecular order of thin polythiophene films

The crystalline order within π-conjugated polymer films prepared using solution processing methods determines the electrical properties of the film. A channels morphology is particularly important to device performance. The molecular order and morphology within a channel region near the underlying active layer have not yet been examined systematically. Here, we characterize the crystal order homogeneity as a function of the solvent penetration depth after applying simple solvent post-treatment. The morphological, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films could be profoundly improved by casting the films in methylene chloride solutions. The impact of the solvent application was most pronounced in the thin P3HT films, especially in the center of the film. During solvent casting, the central region of the film was exposed to methylene chloride for a longer period of time than the edge region of the film, thereby producing a thinner and more ordered film structure in the central region. Concomitant with the improved order, the charge carrier transport in the resulting field-effect transistors increased.

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

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.

Ultrasonication-Mediated Self-Assembly in Polythiophene Films via Control of Residual Solvent Evaporation

We have developed a facile ultrasonication method for the preparation of ordered polymer films. This method was tested by determining the structural, morphological, and electrical characteristics of poly(3-hexylthiophene) films ultrasonicated for various periods under various solvent vapor pressures. Ultrasonic irradiation for short periods under solvent vapor conditions intensifies the interactions between the polymer molecules and the solvent, which results in decreased polymer chain entanglement and in turn promotes polymer association and reorganization. The resulting improved molecular order and favorable morphologies were found to enhance the charge transport in organic transistors prepared with these films. The correlation between the molecular structural order and the electrical characteristics of the films was used to determine the appropriate ultrasonication time and solvent vapor pressure.