New thiophene based semi-conducting materials for applications in plastic electronics

Abstract

Organic semiconductors have several advantages over inorganic ones, such as their cheap solution processability and their tolerance for flexible substrates. On the other hand organic semiconductors are still lacking the performance of inorganics, which makes the large scale commercialisation challenging. This work focuses on synthesizing new fused thiophene based donor moieties and their subsequent incorporation into semiconducting polymers. At first the synthesis of the benzotrithiophene (BTT) chromophore was attempted. The large aromatic unit has a strong tendency to aggregate and its incorporation into semiconducting polymers, resulted initially in insoluble material. However by carefully adjusting the solubilising side chains on the BTT core and by judiciously choosing the electron accepting comonomer, it was possible to synthesize highly soluble polymers, both for organic field effect transistors (OFET) and organic photovoltaics (OPV). When designing the polymers for OFET applications, extra care was taken to minimize backbone bending, thus allowing high charge carrier mobilities of 0.22 cm²/Vs. A similarly good performance was achieved in solar cells when BTT containing low bandgap polymers where employed, increasing the efficiency from initially 1.4% to promising 5%, and this without the use of processing additives. A second part of this work focuses on the synthesis of extended π-conjugated ladder type monomers, which present many desirable features for optoelectronical applications. Two classes of such polymers will be synthesized, silaindacenodithiophene (SiIDT), respectively thieno[3,2-b]thienobis(silolothiophene) (Si4T) containing polymers. Overall thirteen different polymers were obtained with those new donor units, which allowed an extensive study of various parameters on polymer thin film morphology and the effects on device performance. By systematically adjusting the polymer’s energy levels, solubilising alkyl side chains and aromatic building blocks, it was possible to achieve power conversion efficiencies in excess of 6% in organic solar cells and hole mobilities of ~0.3 cm2/Vs in OFET devices. Furthermore this series of polymers was the ideal platform to investigate the influence of polymer purity on device performance, which is of great importance to industry, because it would potentially eliminate any batch-to-batch variation making the production more reproducible and reliable. According to our findings, polymer purity is a key requirement for high performing devices because the solar cell efficiencies could be increased by 30% after material purification.