Chemistry: New Era for Organic Semiconducting Polymers
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Semiconducting Polymers, Polythiophenes
The discovery of poly(acetylene); a conjugated polymer in 1977 paved the pathway for a new era for organic semiconducting polymers.1, 2 Easy processability, low cost, mechanical flexibility, light weight, higher absorption coefficients of these conjugated organic semiconducting polymers made them promising candidates for the applications such as organic field effect transistors (OFETs), bulk heterojunction (BHJ) solar cells and organic light emitting diodes (OLEDs).1, 3 Over the past decades extensive research has been carried out to investigate new conjugated materials to achieve higher efficiencies in BHJ solar cells and higher charge carrier mobilities in OFETs. Among these polymeric materials the poly(alkylthiophenes) have been one of the vastly studied conjugated polymers.1, 4, 5
Over the years many methods have been utilized to synthesis poly(alkylthiophenes) such as, McCullough method,6 Rieke method,7 electro chemical oxidization, Stille coupling polymerization and Grignard metathesis (GRIM) polymerization8, 9 and poly(alkylthiophenes) with high molecular weights and higher regioregularities > 98% has been achieved. Furthermore the optoelectronic properties of theses poly(alkythiophenes) have been optimized by varying the molecular weights, the regioregularity, processing techniques and so far power conversion efficiency of 5% for BHJ solar cells and charge carrier mobility of 0.1 cm2/Vs in OFETs has been achieved. REF There have been many reviews published in the past decades about the evolution of photo electronic properties and the semiconducting applications of polyalkythiophenes. In this chapter, we will provide highlights about the synthesis of polyalkylthiophenes, charge transport properties and their applications in organic field effect transistors and bulk heterojunction solar cells.
Synthesis of Polythiophenes
Synthesis of non-substituted polythiophenes
The synthesis of polythiophene is believed to be the result of 2,5-coupling of thiophene monomers. The first attempt to synthesize a non substituted polythiophene was reported in the late 1960s by using a protic acid.10 In the early 1980s a metal-catalyzed polycondensation polymerization of 2,5-dibromothiophene was reported by Yamamoto et al. and Lin and Dudek to synthesis unsubstituted polythiophene (PT).11, 12
During the synthesis, the precipitation of the polymers under normal reaction conditions limits the formation of high molecular weight polymers. Also polythiophenes contain 1-3 % impurities as determined by elemental analysis. However the polythiophene obtained from these polymerization methods were lack of characterization details due to the low solubility. Further research was carried out by Wudl et al. who used diiodo derivatives for metal catalyzed polycondensation polymerization.13 Electrochemical synthesis of polythiophenes was performed on metal substrates such as iron, copper and silver electrodes in the presence of ionic salts.