New selenophene-based semiconducting copolymers for high performance organic thin-film transistors

A series of new selenophene-based organic semiconducting copolymers, poly(5,5'-bis(3-dodecylthiophen-2-yl)-2,2'-biselenophene) (PDT2Se2) and poly(5,5'-bis(4,4'-didodecyl-2,2'-bithiophen-5-yl)-2,2'-biselenophene) (PDT4Se2), were successfully synthesized by Stille and oxidative coupling reactions. Our aim was to investigate the effects of the selenophene units and inserted dodecylthiophenes on the optical and electrochemical properties of these copolymers, their intermolecular ordering in the film state, and hence their thin-film transistor (TFT) performance. X-ray-diffraction (GIXRD and XRD) and theoretical calculations for models of these polymers were used to show that PDT2Se2 films have well organized interlayer packing and pi-pi stacking, whereas the films of PDT4Se2, which contain regioregularly inserted additional dodecylthiophenes next to the repeat units of PDT2Se2, have a long-range amorphous structure. The TFT characteristics of these polymers are strongly dependent on the intermolecular ordering of the polymer chains. PDT2Se2 exhibited a high hole transporting mobility of 0.02 cm(2) V-1 s(-1) due to its excellent intermolecular ordering, whereas PDT4Se2 exhibited a very poor mobility of 1.4 x 10(-5) cm(2) V-1 s(-1) due to its amorphous characteristics, which result from the repulsion between the additional dodecylthiophenes. These results confirm that it is important to consider intermolecular ordering in the design of semiconducting materials for high performance OTFTs.