Honggi Min

Directly Drawn Organic Transistors by Capillary Pen: A New Facile Patterning Method using Capillary Action for Soluble Organic Materials

A capillary pen drawing technique, developed as a new patterning methodology for the large-area patterning and fabrication of organic electronics, provides several advantages over conventional approaches: the method is simple and versatile, there are no restrictions on the patterning shapes that could be produced, and the method can be tailored to a variety of substrates.

Microstructural Control over Soluble Pentacene Deposited by Capillary Pen Printing for Organic Electronics

Research into printing techniques has received special attention for the commercialization of cost-efficient organic electronics. Here, we have developed a capillary pen printing technique to realize a large-area pattern array of organic transistors and systematically investigated self-organization behavior of printed soluble organic semiconductor ink. The capillary pen-printed deposits of organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), was well-optimized in terms of morphological and microstructural properties by using ink with mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). Especially, a 1:1 solvent ratio results in the best transistor performances. This result is attributed to the unique evaporation characteristics of the TIPS_PEN deposits where fast evaporation of CB induces a morphological evolution at the initial printed position, and the remaining DCB with slow evaporation rate offers a favorable crystal evolution at the pinned position. Finally, a large-area transistor array was facilely fabricated by drawing organic electrodes and active layers with a versatile capillary pen. Our approach provides an efficient printing technique for fabricating large-area arrays of organic electronics and further suggests a methodology to enhance their performances by microstructural control of the printed organic semiconducting deposits.

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, iso-, syn-, 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 (iso- and syn-stereoisomers) exhibited more stable OFET operation under bias stress compared to devices prepared using irregular a-PMMA in both vacuum and air.

Enhancement of Solvent-Resistance by Forming Interpenetrating Network for High-Performance Polymer Field-Effect Transistors

To enhance the solvent-resistance of polymer semiconductor film in organic field-effect transistors, bis(trichlorosily)hexane (BTH) as a cross-linkable agent was mixed with polymer semiconductors, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) and poly(3-hexylthiophene) (P3HT). The solvent-resistance was dramatically enhanced in both the F8T2/BTH and P3HT/BTH cases, even for the 1% addition of BTH. However, clear differences in the field-effect mobilities with increasing BTH-blend ratio were observed between the F8T2 and the P3HT cases. For the F8T2-FETs, the field-effect mobility was maintained by level of 90% at the 1% BTH-blend ratio, and decreased gradually above 1% blend ratio. In contrast, the field-effect mobilities of P3HT-FETs were dramatically decreased by blending the BTH, although the solvent-resistance was increased. This obvious difference is a result of the difference in crystalline properties between the amorphous F8T2 and the crystalline P3HT. This approach to improve the solvent-resistance of polymer films provides a facile method for the enhancement of the environmental stability in response to humidity and oxygen.