Organic field-effect transistors based on dendrimers and small molecules

Mutkins, Karyn Elizabeth (2012). Organic field-effect transistors based on dendrimers and small molecules PhD Thesis, School of Mathematics and Physics, The University of Queensland.

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Author Mutkins, Karyn Elizabeth
Thesis Title Organic field-effect transistors based on dendrimers and small molecules
School, Centre or Institute School of Mathematics and Physics
Institution The University of Queensland
Publication date 2012
Thesis type PhD Thesis
Supervisor Paul Meredith
Paul L. Burn
George Vamvounis
Total pages 168
Total colour pages 9
Total black and white pages 159
Language eng
Subjects 020403 Condensed Matter Modelling and Density Functional Theory
100702 Molecular and Organic Electronics
0204 Condensed Matter Physics
1007 Nanotechnology
Formatted abstract
This thesis describes a body of work studying the physical, chemical and transport properties of solution-processable amorphous p type semiconducting dendrimers. The objective of this research was to understand whether these amorphous materials with isotropic conductivity could produce high field-effect mobilities when used as the active layer in organic filed-effect transistors (OFETs). These materials were designed to be deposited from solution without the requirement for post deposition treatments. The effect of molecular shape (dimensionality) on charge transport properties was studied through the use of dimensionally-rich and planar materials. In addition to OFET characterisation, density functional calculations were carried out to optimise the molecular geometries of the molecules. The resulting Kohn Sham orbitals demonstrated the delocalisation of the orbital density over each molecule in both the neutral and charged (cation for p-type, anion for n-type) states. This information was used to aid an understanding of how delocalisation of orbital density could potentially improve isotropic charge transport.

      Initially, molecules with triarylamine cores were tested as the active layer in OFETs to study the effect of dendronisation and increasing the number of thiophene units in the arms of the tripodal molecules. These results showed that dendronisation increased the solubility of the molecules but reduced the hole mobility. Increasing the number of thiophene units in the arms also increased the conjugation length and thus the mobility by enhancing intermolecular interactions.

      The structure-property relationships of three series of dendrimers utilising the same carbazole/fluorene-based dendrons with up to three generations in each series were also studied as hole transporting materials in OFETs. These dendrimers possessed either a 9,9’ spirobifluorene (SBF) or 9,9 di n propylfluorene (Fl) core with either two or four dendrons attached. For the SBF cored dendrimers with two dendrons attached, the mobility was seen to slightly decrease with increasing generation. However, all of the mobilities for this series were of the order of 10−5 cm2/Vs. The next series of dendrimers had a SBF-core with four dendrons attached. The decrease in mobility observed with increasing generation in the SBF dendrimers bearing four dendrons was more severe than the di substituted dendrimers with a decrease of an order of magnitude from 10−4 to 10−5 cm2/Vs. The highest mobility for this series of dendrimers of 4.1 × 10−4 cm2/Vs, for the first generation dendrimer, was the highest mobility reported for a solution processed p type dendrimer observed in this work. The decreasing mobility in these two series of dendrimers with generation was attributed to the localisation of the orbital density on the core with increasing generation. For the Fl-cored series of dendrimers there was an increase in the mobility with increasing generation with a maximum mobility of 1.5 × 10−4 cm2/Vs for the third generation dendrimer. This reversal in mobility trend relative to the SBF dendrimers was attributed to the planarity of the Fl core, allowing them to pack in closer proximity in thin films.

      The effect of altering the surface groups and branching units for first-generation SBF cored dendrimers was investigated to understand how substituting individual constituents of the dendrons while keeping the core structure the same affected their charge transport properties. Upon changing the surface groups from fluorene to carbazole and 2 ethylhexyloxy there was a decrease in mobility relative to the prototypical SBF-cored dendrimers. This was attributed to the increase in the length of the alkyl chains on the surface groups that resulted in an insulating shell around the chromophore. This made it more energetically unfavourable for charge carrier hopping between dendrimers in a film. Similarly changing the branching unit from carbazole to dithienopyrrole with fluorene surface groups decreased the hole mobility. These detrimental effects on mobility highlighted the importance of effective design criteria when choosing the components for SBF-cored dendrimers.

      Finally two n-type small molecules which crystallise at temperatures in the range 70 – 90 °C were investigated. The first small molecule, [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12) could be processed into crystalline films by both solution and vacuum-deposition methods with a maximum electron mobility of 2.4 × 10−3 cm2/Vs. A variation of K12 where the 9,9-di-n-propylfluorene functionality was replaced by a 4,4 di-n-propylsilolodithiophene unit (YF25) was used as the active layer in OFETs and found to have a maximum electron mobility of 3.6 × 10−3 cm2/Vs. When MoO3/Au source and drain electrodes were used instead of Al a maximum hole mobility of 1.6 × 10−4 cm2/Vs was recorded, demonstrating that YF25 displays both n and p type charge transport.

      In summary, this thesis addresses a number of issues regarding charge transport in organic materials of both an amorphous and crystalline nature. The work indicates the importance of understanding inter- and intra-molecular interactions in such films in guiding material design. The results described clearly demonstrate how difficult it is to achieve ultra high mobilities (> 1 cm2/Vs) with amorphous organic dendrimer films and highlights the effect of subtle changes in molecular structure on charge transport properties.
Keyword Organic field effect transistor
Density functional theory
Charge transport

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Created: Wed, 13 Mar 2013, 19:25:02 EST by Karyn Mutkins on behalf of Scholarly Communication and Digitisation Service