Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation...

The development of high-performance unipolar n-type semiconducting polymers still remains a significant challenge. Only a few examples exhibit a unipolar electron mobility over 5 cm² V^-1 s^-1. In this study, a series of new poly(benzothiadiazole-naphthalenediimide) derivatives with a high unipolar electron mobility (μ_e) up to 7.16 cm² V^-1 s^-1 in thin-film transistors are reported. The dramatically increased μ_e is achieved by finely optimizing the coplanar backbone conformation through the introduction of vinylene bridges, which can form intramolecular hydrogen bonds with the neighboring fluorine and oxygen atoms. The hydrogen-bonding functionalities are fused to the backbone to ensure a much more planar conformation of the conjugated π-system, as demonstrated by the density functional theory (DFT)-based calculations. The theoretical prediction is in good agreement with the experimental results. As the coplanarity is promoted by the hydrogen bonding, the thin-film crystallinity and molecular packing strength are also improved, which is evidenced by the synchrotron two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) measurements. Notably, the GIWAXS measurements reveal an extremely short π-π stacking distance of 3.40 Å. Overall, this study marks a significant advance in the unipolar n-type semiconducting polymers and offers a general approach for further increasing the electron mobility of semiconducting polymers in organic electronics.