UVM Theses and Dissertations
Format:
Print
Author:
Cour, Ishviene
Dept./Program:
Materials Science Program
Year:
2013
Degree:
Ph. D.
Abstract:
Molecular alignment and orientation in organic thin films significantly influence their usefulness as electronic materials, and control of this alignment is a central challenge for materials research in this area. The various factors that govern the molecular alignment in thin films are interface energies, nucleation process; kinetics of crystallization, alignment using template layers, etc. Better understanding of molecular alignment will ultimately lead to improved properties, including charge carrier mobility and exciton diffusion lengths.
We have switched the orientation of discotic phthalocyanine molecules from in-plane to homeotropic (perpendicular orientation of discotic molecules) alignment on open supported thin films. We find that the in-plane and perpendicular alignments have unequal interface energies and that the growth of nuclei is kinetically limited and it grows faster along the columnar direction. A two orders of magnitude increase in the charge carrier mobility is observed for films oriented in the vertical direction.
We further focus on the course of ordering during the complex process of solution deposition of small molecular organic semiconductor thin film via hollow capillary writing. We present real-time synchrotron x-ray scattering results combined with optical video microscopy, revealing the four distinct stages of ordering during the deposition of TIPS Pentacene. Limited long range ordering is observed in the first stage of initial crystallization with intermediate solid form. On a timescale of ~ 3-4 seconds the intermediate form gradually turns into a fully ordered solid state. For depositions above room temperature, buckling and delamination of film are observed.
Compressive stress originates from thermal transients related to solvent evaporation on timescales similar to the development of long range ordering. During cooling tthe heated samples to room temperature, the buckling direction is observed to rotate by 90 degrees which indicates an interchange of the direction of tensile and compressive strain and also leads to the formation of cracks.
Elimination of cracks and other structural defects has significantly improved the average charge carrier mobility in organic field-effect transistors. The large variation in mobility with the writing speed in the convective regime is proposed to be primarily due to the strain-induced defects. Enhancement of the intrinsic mobility due to variation of lattice strain evidently plays a much smaller role.
We have switched the orientation of discotic phthalocyanine molecules from in-plane to homeotropic (perpendicular orientation of discotic molecules) alignment on open supported thin films. We find that the in-plane and perpendicular alignments have unequal interface energies and that the growth of nuclei is kinetically limited and it grows faster along the columnar direction. A two orders of magnitude increase in the charge carrier mobility is observed for films oriented in the vertical direction.
We further focus on the course of ordering during the complex process of solution deposition of small molecular organic semiconductor thin film via hollow capillary writing. We present real-time synchrotron x-ray scattering results combined with optical video microscopy, revealing the four distinct stages of ordering during the deposition of TIPS Pentacene. Limited long range ordering is observed in the first stage of initial crystallization with intermediate solid form. On a timescale of ~ 3-4 seconds the intermediate form gradually turns into a fully ordered solid state. For depositions above room temperature, buckling and delamination of film are observed.
Compressive stress originates from thermal transients related to solvent evaporation on timescales similar to the development of long range ordering. During cooling tthe heated samples to room temperature, the buckling direction is observed to rotate by 90 degrees which indicates an interchange of the direction of tensile and compressive strain and also leads to the formation of cracks.
Elimination of cracks and other structural defects has significantly improved the average charge carrier mobility in organic field-effect transistors. The large variation in mobility with the writing speed in the convective regime is proposed to be primarily due to the strain-induced defects. Enhancement of the intrinsic mobility due to variation of lattice strain evidently plays a much smaller role.