Rational design of ambipolar organic semiconductors: Is core planarity central to ambipolarity in thiophene-naphthalene semiconductors?

Herein, we report a new family of naphthaleneamidinemonoimide-fused oligothiophene semiconductors designed for facile charge transport in organic field-effect transistors (OFETs). These molecules have planar skeletons that induce high degrees of crystallinity and hence good charge-transport properties. By modulating the length of the oligothiophene fragment, the majority carrier charge transport can be switched from n-type to ambipolar behavior. The highest FET performance is achieved for solution-processed films of 10-[(2,2′- bithiophen)-5-yl]-2-octylbenzo[lmn]thieno[3′,4′:4,5]imidazo[2,1-b] [3,8]phenanthroline-1,3,6(2 H)-trione (NDI-3 Tp), with optimized film mobilities of 2à- 10 ^-2 and 0.7à- 10 ^-2 cm ² V ^-1 s ^-1 for electrons and holes, respectively. Finally, these planar semiconductors are compared with their twisted-skeleton counterparts, which exhibit only n-type mobility, in order to understand the origin of the ambipolarity in this new series of molecular semiconductors.