Enhanced Molecular Doping for High Conductivity in Polymers with Volume Freed for Dopants

We present two new polythiophene isomers, PQTC16-TT and PQTSC16-TT, with more separation and lower side chain density on the backbone compared with the typical polythiophenes, such as PQT(S)12 and PBTTT(S)12. The introduction of spacer subunits to separate the side chains improves the accommodation of dopant molecules close to the polymer backbone. Therefore, the lamellar structure and π–π stacking of polymers were negligibly disturbed by dopants; in fact, improved ordering and closer π-stacking were observed after chemical doping. Furthermore, the doping efficiency is significantly enhanced and the electrical conductivity is improved up to 330 S cm–1, still maintained at 200 S cm–1 at higher doping concentration, for the combination of PQTSC16-TT/NOBF4. We investigate how charge transfer is affected by the geometry of the dopant with respect to the polymer repeat unit. It is shown that the sulfurs in the side chains act as binding sites for dopants, resulting in higher thermal and environmental stability of alkylthio-substituted polymers upon doping.