Benzotrithiophene Copolymers: Influence of Molecular Packing and Energy Levels on Charge Carrier Mobility

significance statement

Organic electronics have a huge potential to revolutionize the current market of silicon based semiconductors and could open-up new routes towards more versatile and flexible electronics. One of the main advantages of organic semiconductors are the flexible deposition techniques that could be employed. Contrary to the energy intensive production of electronic grade silicon, organic semiconductors based on polymers could be formulated into inks, thus allowing large area deposition using common printing techniques at substantially lower cost. Furthermore, physical properties like solubility, material crystallinity and electronic properties are often intimately related to the molecular structure and can be tuned via subtle changes to the polymer structure.

In the work presented herein, we focussed on semiconducting polymers, incorporating our previously developed benzotrithiophene (BTT) moiety. The planar benzotrithiophene unit allows for extraordinary strong pi-pi interactions between adjacent polymer chains, however at the cost of significantly lower solubility and processability. Therefore we investigated on how we could substitute solubilising alkyl side chains along the polymer backbone without disturbing the molecular packing of the material or its electronic performance. By increasing the steric hindrance between alkyl side chains, the polymer solubility could be significantly enhanced, thereby reducing the previously high hole mobilities. By arranging the alkyl chains along the conjugated polymer backbone in a less impeding way, long range order was induced in the polymer films and enhanced hole mobilities were observed, but at the same time the processability of the material was rendered more demanding.

 

Benzotrithiophene Copolymers: Influence of Molecular Packing and Energy Levels on Charge Carrier Mobility

Macromolecules, 2014, 47 (9), pp 2883–2890.

Bob C. Schroeder *†, Stephan Rossbauer ‡, R. Joseph Kline §, Laure Biniek †, Scott E. Watkins ∥,Thomas D. Anthopoulos ‡, Iain McCulloch †, andChristian B. Nielsen †

 Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ,U.k and

 Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ,U.K and

§Material Science and Engineering Division,National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, United States and

 CSIRO Materials Science and Engineering, Melbourne, VIC 3169, Australia

 

Abstract

 

The planar benzotrithiophene unit (BTT) was incorporated into four different donor polymers, and by systematically changing the nature and positioning of the solubilizing alkyl side chains along the conjugated backbone, the polymers’ frontier energy levels and optoelectronic properties were controlled. Reducing the steric hindrance along the polymer backbone lead to strong interchain aggregation and highly ordered thin films, achieving hole mobilities of 0.04 cm2/(V s) in organic thin film transistors. In an attempt to increase the polymer’s processability and reduce chain aggregation, steric hindrance between alkyl side chains was exploited. As a result of the increased solubility, the film forming properties of the polymer could be improved, but at the cost of reduced hole mobilities in OFET devices, due to the lack of long-range order in the polymer films.

Copyright © 2014 American Chemical Society

 

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