Unconventional Molecular Weight Dependence of Charge Transport in the High Mobility n-type Semiconducting Polymer P(NDI2OD-T2)

The charge transport and microstructural properties of five different molecular weight (MW) batches of the naphthalenediimide-thiophene copolymer P(NDI2OD-T2) are investigated. In particular, the field-effect transistor (FET) performance and thin-film microstructure of samples with MW varying from M-n = 10 to 41 kDa are studied. Unlike conventional semiconducting polymers such as poly(3-hexylthiophene) where FET mobility dramatically drops with decreasing molecular weight, the FET mobility of P(NDI2OD-T2)-based transistors processed from 1,2-dichlorobenzene is found to increase with decreasing MW. Using a combination of grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, atomic force microscopy, and resonant soft X-ray scattering, the increase in FET mobility with decreasing MW is attributed to the pronounced increase in the orientational correlation length (OCL) with decreasing MW. In particular, the OCL is observed to systematically increase from <100 nm for the highest MW samples to approximate to 1 mu m for the lowest MW samples. The improvement in OCL and hence mobility for low MW samples is attributed to the lack of aggregation of low MW chains in solution promoting backbone ordering, with the pre-aggregation of chains in 1,2-dichlorobenzene found to suppress longer-range liquid crystalline order.