Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
Teddy Salim1 2 Pio John Buenconsejo1 2 Yeng Ming Lam1 2

1, Nanyang Technological University, Singapore, , Singapore
2, Nanyang Technological University, Singapore, , Singapore

Conjugated organic small molecule and polymer-based electronics are attractive alternatives to the conventional silicon-based inorganic devices largely owing to the low-temperature and solution processibility. This family of materials also holds considerable potential for solid-state lighting and photovoltaic applications due to the possibility of employing roll-to-roll technology for large-scale manufacturing ultimately aiming to develop lightweight, flexible and cost-effective devices. One of the most-studied conjugated polymers in organic electronics is regioregular poly(3-hexylthiophene) (rr-P3HT), which has been ubiquitously used in the research of organic transistors and photovoltaics. Side-chain functionalization is a major synthetic strategy to impart chemical functionalities to conjugated polymers. The incorporation of free carboxylic acid side groups in polythiophene, including rr-P3HT, has been demonstrated to not only improve its solubility in polar solvent – a desirable feature towards “green” processing, but also encourage molecular-level structural ordering via hydrogen bonding. Besides, the carboxylic side groups can also serve as anchoring sites on oxide surface. In this work, we demonstrate a thermally activated self-assembly of conjugated regioregular poly[3-(5-carboxyhexyl)thiophene-2,5-diyl] (rr-P3CHT) in polar solvent. We developed a novel facile technique involving a rapid exposure of the polymer solution to flame (>800°C), resulting in the self-assembly of the polymer molecules. By using small angle X-ray scattering (SAXS), the temporal evolution during the self-assembly process as a function of polymer concentration could be monitored. The bathochromic shift in the absorption spectra – the darkening of the solution, generally associated with an enhancement in the π-conjugation length, is a beneficial feature in achieving effective photoabsorption. In addition, techniques such as electron microscopy, infrared spectroscopy, thin-film X-ray diffraction and atomic force microscopy have also been employed to gain insights into the nature of the self-assembly. The characteristics of the interaction between the self-assembled polymer and metal oxide substrate at the interface could also be probed with X-ray photoelectron spectroscopy (XPS) depth profiling. Finally we applied the mixture of the self-assembled rr-P3CHT and a fullerene (C60) derivative to form the photoactive layer for organic photovoltaic devices.

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