(327d) Chain Length Dependent Electron Transport Properties of Rigid-Rod Semiconducting Ladder Polymer

Several key factors, including polymer chain length, polymer crystallinity, and molecular orientation have been demonstrated to influence the charge transport properties of semi-flexible p-type and n-type semiconducting polymers, leading to quantitative design rules for developing high mobility p-type and n-type conjugated polymers. In contrast, fundamental understandings of factors that limit efficient charge transport in rigid-rod π-conjugated ladder polymers are currently lacking despite their unique advantages over their semi-flexible counterparts such as large persistence length, zero conformational disorder, liquid crystallinity, efficient intrachain electron delocalization, and excellent photochemical/thermal stability.

In this talk, I will discuss the chain length dependent electron transport properties and thin film microstructures of n-type rigid-rod ladder polymers by using poly(benzimidazobenzophenanthroline) (BBL) as a prototype system. The series of BBL ladder polymers synthesized via liquid crystalline phase polycondensation has a broad range of degree of polymerization (DP) spanning from 90 to 258. The electron mobility of BBL characterized by organic field-effect transistors (OFETs) is found to scale linearly with the logarithm of DP whereas the threshold voltage exhibits an exponential decay with respect to DP. The dependence of these electron transport parameters on the polymer chain length is indeed exclusive to the class of rigid-rod ladder polymers and to be starkly contrasted to that of semi-flexible polymers. A complement of characterization techniques is employed to probe the trap density and thin-film microstructure and provide mechanistic understandings of the underlying physics that govern electron transport in rigid-rod π-conjugated ladder polymers. The linear growth of electron mobility with respect to DP can be explained by a combination of increased electron delocalization along torsional-free polymer backbones, lower electron trapping frequency due to reduced chemical defects density, tighter π-π stacking distance, enhanced crystallinity, and reduced paracrystallinity disorder. The decay of threshold voltage as a function of DP is rationalized in terms of the decreased trap density, which also suggests that increasing the polymer chain length will result in OFET devices with faster switching behavior. These results have elucidated factors that limit electron transport in rigid-rod π-conjugated ladder polymers, provided design guidelines to achieve high mobility n-type semiconducting polymers, and contributed important device engineering strategies to develop high-performance organic field-effect transistors.