Universal Dependence of Carrier Mobility on Polymer Chain Length in n-Type Semiconducting Polymers*

The dependence of electron mobility of a n-type semiconducting polymer on polymer chain length is investigated and thereby factors limiting efficient electron transport are pinpointed. Both field-effect and space-charge limited current electron mobilities are found to reach maxima at a critical degree of polymerization (DP_c). This trend is shown to be similar for other n-type semiconducting polymers, suggesting a nearly universal dependence of electron mobility on polymer chain length. The underlying physics are deconvoluted: the bottleneck for DP<DP_c is intercrystallite transport due to insufficient domain connectivity whereas the limiting factor for DP>DP_c is intracrystallite transport due to intrachain charge localization, poor interchain hopping rate, and intracrystallite disorder. Our results argued that the basis to achieve efficient multiscale electron transport in n-type semiconducting polymers lie in the control of polymer chain length to concurrently optimize intracrystallite and intercrystallite charge transport.