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Effect of Molecular Structure on the Optoelectronic Properties of Isoindigo-based Semiconductors



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Organic electronics have developed rapidly over the past few decades. Despite this progress, device performance must be further improved before organic photovoltaic devices and organic field effect transistors can be widely commercialized. Therefore, both the design of new organic semiconductors and understanding how molecular structure affects the optoelectronic properties of these semiconductors are critically important. In this context, this thesis discusses the effect of molecular structure on the optoelectronic properties (e.g., oscillator strength, frontier orbital energies, optical band gap, and charge carrier mobilities) of organic semiconductors. The thesis deals with two different types of structural modifications to isoindigo-based semiconductors. The first section focuses on the effect of linear-conjugation and cross-conjugation on the oscillator strength of molecular organic semiconductors based on a ring-expanded isoindigo derivative. Compared to isoindigo analogues, extending the conjugation of the -system through ring fusion substantially red-shifted the absorption maximum of the HOMO to LUMO transition; however, the cross-conjugation significantly reduced the oscillator strength. As a result, the photocurrent was significantly lower for organic photovoltaic devices made with cross-conjugated materials compared to linearly conjugated materials. The second section discusses the effect of torsional strain in two new polymeric semiconductors, namely poly(ethynyl-alt-isoindigo) and polybisisoindigo, on their electron mobilities. The electron mobilities are compared to the control polymer polyisoindigo, which exhibited a relatively poor electron mobility because of a large torsion angle. In the ethynyl-alt-isoindigo copolymer, the sterically unencumbered ethynyl spacer helps reduce the torsion angle between adjacent isoindigo units and conserve planarity in the polymer backbone. In polybisisoindigo, every second isoindigo unit was fused together, replacing a single bond between repeating units. This lowers the overall degree of torsional strain in the polymer chain. The increased planarity improved the electron mobility of these new polymers relative to polyisoindigo. Out of the two new polymers, thin-film transistors fabricated using polybisisoindigo displayed the highest electron mobility of 1.26 10-3 cm2/V·s. This was attributed to the extended conjugation of the ring-fused bisisoindigo-based system which increased intra-chain electron transport along the polymer backbone. In addition, the larger size of this ring-fused -system might have made polybisisoindigo less sensitive towards positional disorder. In conclusion, a substitution pattern leading to linear conjugation is essential for high oscillator strengths. This increases the absorption coefficient of the material, resulting in a higher photocurrent when incorporated in photovoltaic devices. A minimized torsional strain in the semiconductor backbone is also crucial to improve the charge carrier mobilities.



organic solar cell, organic thin-film transistors, oscillator strength, torsion angle



Doctor of Philosophy (Ph.D.)






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