We introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An average hole mobility of 2.00 cm(2) V-1 s(-1) (with a maximum mobility of 2.48 cm(2) V-1 s(-1)), was obtained from solution-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the reference polymer with a branched alkyl side chain gave an average hole mobility of 0.30 cm(2) V-1 s(-1) and a maximum mobility of 0.57 cm(2) V-1 s(-1). This is largely explained by the polymer packing: our new polymer exhibited a pi-pi stacking distance of 3.58 angstrom, while the reference polymer showed a distance of 3.76 angstrom.