Functional conjugated polymers: from design to devices

© 2015 Dr. Aaron Zhifan SongConjugated polymers are among the most promising materials for future technologies. These organic materials combine many of the properties of conventional polymers with the behaviour of inorganic semiconductors. The low-cost and solubility of many conjugated polymers make them attractive materials for use in future electronic and optoelectronic devices, such as field-effect transistors (FETs), polymer light-emitting diodes (PLEDs) and organic solar cells (OSCs). However, both design and synthesis of conjugated polymers with well-defined structures involve many intellectual and technological challenges. Although conjugated polymers of simple aromatic monomers have been studied in great detail, to date there is no sound methodology in rationalised design and synthesis of functional conjugated polymers. Therefore, controlling and fine-tuning the properties of conjugated polymers remains rather difficult to achieve, and thus limits their applications in devices. The purpose of this research is to develop a coherent strategy for the design and synthesis of functional conjugated polymers with well-controlled structures and properties, from simple aromatic molecules. The history, background and applications of conjugated polymers are reviewed before introducing the central concept in this work - modular design and synthesis of new materials. The approach adopted seeks to develop “greener” pathways to target materials, by avoiding many highly toxic solvents, highly corrosive reagents, and expensive organometallic catalysts. Some simple reactions in water or ethanol were utilised to synthesise early building blocks in large quantities, while Horner-Wadsworth-Emmons (HWE) olefination and Kumada coupling reactions were employed to construct carbon-carbon double and single bonds, respectively. A group of three conjugated polymers were first synthesised to investigate the practicality of the modular approach. These polymers were based on the alternating copolymer of phenylenevinylene (PV) with (2-ethylhexyloxy-4-methoxy)phenylenevinylene (MEHPV), or alt-co-MEH-PPV. Each polymer consists of three types of aromatic monomers in an alternating pattern. The properties of the novel polymers were compared to their parent polymers to reveal effects of inserting an additional type of monomer. Subsequent work investigated the synthesis and properties of conjugation confined MEHPPV polymers. The target polymers differ from polymers of similar type studied by other groups in both synthesis and properties. Regioregular, bifunctional oligomers of MEHPV were first synthesised, then polymerised with 4,4’-bis(methylenediethylphosphonate)-1,1’-biphenyl (bis(MDEP)-biphenyl) to yield poly(MEHPV-oligomer-alt-co-bp) polymers. These polymers have a higher density of chromophores, and demonstrated a clear structure-property correlation in their photophysical properties. Furthermore, a simple, empirical quantitative model was developed to describe the basic absorption and emission characteristics of the polymers. Successes in developing an efficient synthesis of bifunctional MEHPV oligomers paved the paths towards conjugated polymers with increased structural complexity. Similar schemes were developed to synthesis bifunctional oligomers of 3,4-dihexylthienylenvinylene (DHTV). Using a combinatorial matrix approach, varying the combination of the MEHVP and DHTV oligomers modules produced 16 conjugated polymers with well-defined structures. Basic photophysical properties of these polymers were studied in some detail and a number of simple quantitative, empirical models were discovered from data analysis, which described their absorption and emission characteristics to a high degree of accuracy. Photovoltaic devices were fabricated with selected polymers as the active layer in blends with PC60BM and an efficiency of 2.0% was obtained. The device performance was superior compared to devices utilising MEH-PPV or PTV. The combined combinatorial-modular approach was then used to synthesise another type of copolymers of interest, those of MEHPV and regioregular 3-hexylthiophene (3HT) oligomers. MEH-PPV and P3HT were chosen as they are among the most intensely investigated conjugated polymers. An efficient, palladium free synthesise of 3HT oligomers was developed, and the bifunctional modules were then combined with MEHPV modules to produce 12 alternating copolymers of MEHPV and 3HT oligomers with well-defined structures. The absorption and emission characteristics showed the limitations of the empirical model developed in the other synthesised polymers, demonstrating the complex role of polymer chain morphology in solutions and the solid state polymer photophysics. At Finally, 4 polymers consisting of DHTV and 3HT oligomers were synthesised and their basic photophysical properties were determined