POLYMER PHYSICS

Research Overview

LIST OF RESEARCHERS

Prof. Evan S. H. Hyunkoo Kang (07-14)

ORIGIN OF HIGH MOBILITY IN A SEMICONDUCTOR THIN FILM

We fabricated amorphous Dithienylcyclopentadieneone-Carbazole copolymer for organic thin-film transistors. X-ray diffraction analysis indicates an amorphous solid is formed. Atomic force microscopy (AFM) images also show an amorphous surface mophology regardless of the annealing temperature. Although there has been consensus that the lack of structural order in amorphous polymer devices results in poor mobility, relatively high mobility, of the order of 10-2 cm2 V-1 s-1, was found. Low temperature transport measurements were carried out to understand the origin of high mobility. Relatively low activation energy and low characteristic temperature were obtained, reflecting that localization of the charge carrier is not substantial in the film.

- Researcher: Prof. Evan S. Hyunkoo Kang

- Result: E. S. H. Kang et al, Journal of Materials Chemistry 20, 2759-2765 (2010)

CORRELATION BETWEEN MICROSTRUCTURE AND CHARGE TRANSPORT

We presented the effect of recrystallization rate on the microstructure and charge transport properties of high-performance poly(thieno-thiophene) (PBTTT) thin films. Differential scanning calorimetry (DSC) measurements indicate that the cooling rate directly influences the recrystallization process. In addition, atomic force microscopy (AFM) studies with an X-ray diffraction diagrams (2D GIWAXS) demonstrate that slow cooling leads to well-connected large domains with enhanced three-dimensionally ordered structures, whereas fast cooling results in misaligned small domains with rough surfaces. At various temperatures, transport characteristics show an increase in the charge carrier mobility and a reduction in the activation energy with decreasing cooling rate of the films. Cooling rates slower than 15K/min induce the saturation of mobility and activation energy. These results assert that the cooling rate is a crucial factor for ordering in a microstructure as well as for the high performance of polymer thin films.

- Researcher: Prof. Evan S. Hyunkoo Kang

- Result: E. S. H. Kang, and E. Kim, Organic Electronics 12, 1649-1656 (2011)

MULTI-BARRIER FIELD EMISSION AT LOW TEMPERATURE

We investigated the low-temperature transport mechanism for poly[2,5-bis(3-alkyl-thiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT). The temperature-dependent trans-port behavior was studied by varying the drain-source electric field and gate bias. The results suggest that low-temperature charge transport is dominated by direct tunneling at low electric fields, while field emission is prevailing for high electric fields with high carrier densities. However, the obtained barrier heights are remarkably greater than expected in a conventional field emission. We propose a simplified model of field emission through quasi-one-dimensional path with multiple barriers which shows good agreement with the results more clearly. Field emission across the domain boundaries may assist in overcoming the transport barriers induced by the interchain disorder, which results in the weak temperature dependence of conductivities and nonlinear current-voltage relation at low temperatures.

- Researcher: Evan S. Hyunkoo Kang

- Result: E. S. H. Kang and E. Kim, Scientific Reports 5, 8396 (2015)

COLLABORATORS

Prof. Shinuk Cho (Department of Physics, University of Ulsan)