Electron Transport Physics of n-Type Semiconducting Polymers: Effects of chain topology and chain length

How chain topology and chain length influence electron transport in n-type conjugated polymers are investigated. The electron mobility of semi-flexible polymers is found to reach a maximum within a narrow critical range of degree of polymerization (DP_c), which is in stark contrast to the linear dependence of electron mobility on DP observed in rigid-rod ladder polymers. The physics underlying electron transport in each class of polymers is elucidated in terms of structural disorder and electronic disorder. The decreased electron mobility below DP_c in semi-flexible polymers originates from reduced inter-crystallite connectivity while intrachain twisting, interchain entanglements, and intra-crystallite limitations dominate electron transport above DP_c. The linear growth of electron mobility seen in rigid-rod polymers is explained by increased electron delocalization along torsional-free polymer backbones, lower trap densities, and reduced paracrystallinity disorder. Our results provide important new insights to the electron transport physics of two topological classes of n-type semiconducting polymers while enabling design approaches to high electron-mobility conjugated polymers.