Deformable & solution processable polymer semiconductors for plastic electronics

Conjugated polymers have evolved significantly the past decade and proven to be more than poorly conducting plastics. Instead, improved understanding of charge transport mechanism has resulted in respectable charge carrier mobilities and power conversion efficiencies achieved by various donor-acceptor type semiconducting polymers that rival traditional amorphous silicon devices. This opens a window of opportunity for next generation wearable and implantable electronics that possess unique properties such as light-weight, self-healing, stretchability and bio-degradability. However, their advantages in mechanical deformability as well as solution processability seemed to have conflicting molecular design requirements from those for high charge carrier transporting properties. Highly rigid polymer backbones with ordered morphologies are desired for efficient charge transport but often translates to brittle materials with poor processability. It is therefore a challenge to enhance the mechanical compliance and solubility of semiconducting polymers suitable for stretchable device applications and low-cost fabrication while maintaining good electric properties. In this thesis, the use of covalent crosslinks and flexible alkyl linkers to address the stretchability and processability of high-performance polymer semiconductors is explored. The first portion of my research introduces crosslinking as a post-polymerization strategy to enhance device extensibility and cyclability with an emphasis on the chemistry and structural design of the crosslinker additives. In our first design, a linear oligo-siloxane is covalently bonded to the alkyl side-chains of conjugated polymers through crosslinking to improve the elasticity of the semiconductor. We discover that the soft crosslinker has a plasticizing effect, which suppresses polymer crystallinity and enhances the overall film ductility. To further investigate the mechanism in which crosslinking manipulates semiconductor morphology and the role of crosslinker crystallinity, perfluorophenyl azide-based alkyl crosslinkers with different amounts of hydrogen bonded amide groups and alkyl branching are synthesized to tune crosslinker packing. We find that crosslinker crystallinity can have a profound impact on film morphology. Highly crystalline linear crosslinkers with hydrogen bonding readily phase separates and recrystallize in polymer network to form crosslinked domains that limits strain dissipation in the polymer network. Fully amorphous branch crosslinkers on the other hand exhibit excellent miscibility with the polymer semiconductor leading to an evenly crosslinked network for good cyclability. The second portion of this work focuses on the deposition and solubility aspect of high-performing polymer semiconductors which are to date mostly processed using toxic halogenated solvents and uneconomical coating methods such as spin-coating during device fabrication. To address this, alkyl flexible linkers with branched tertiary carbon atoms are inserted to a high-mobility polymer backbone to disrupt molecular packing. We find the addition of branch linkers to be an effective way to increase polymer free volume, decrease aggregation in solution and promote polymer-solvent interactions. This is evident in the enhanced solubility in non-polar and polar industrial solvents as well as green solvents such as 2-methyltetrahydrofuran and o-methyl anisole. The polymer can be further printed and patterned by solution-shearing and inkjet-printing at high concentrations to give aligned films with high charge carrier mobilities. Overall, this thesis contributes new strategies to achieve stretchable and solution processible polymer semiconductors, and in the process addresses the structural and morphological contradictions of low-cost, deformable and high-performing plastic electronics.

Keywords: Chemistry.
Advisor: Bao Zhenan 
Dauskardt R. H. (Reinhold H.) 
Xia Yan