Increasing the Charge Transport of P(NDI2OD-T2) by Improving the Polarization of the NDI2OD Unit along the Backbone Direction and Preaggregation via H‑Bonding

The charge transport of donor–acceptor (D–A) conjugated polymer along the intramolecular main chain is faster than the intermolecular π–π stacking. The higher the backbone coplanarity is, the more efficient the charge transport is. Here, we proposed a strategy to improve the backbone planarization by increasing the polarization of the A unit along the main chain direction via hydrogen-bonding (H-bonding) interactions. This is enabled by adding ethylene glycol (EG) to poly­{[N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P­(NDI2OD-T2)] in chloroform solution, and H-bonding interactions are formed between the H atoms on the hydroxyl groups of EG and O atoms on the NDI2OD units. The relative intensity of the antisymmetric (b) and symmetric (a) CO stretching normal modes increased from 2.5 without EG to 3.0 with 3% EG (the optimal content) as demonstrated by Fourier transform infrared spectra (FT-IR). In addition, compared to without EG the conformation is transformed from the random coil chains to ordered interchain stacking after adding EG as revealed by cryogenic transmission electron microscopy (cryo-TEM). The fraction of aggregate in solution was increased. The solid-state films obtained by blade coating exhibited anisotropic molecular chain alignment with the orientation parameter (S 2D) increasing from 0.55 to 0.95 and noticeable (001) peaks in parallel and (100) peaks in perpendicular to the coating direction, respectively. Furthermore, we fabricated top-gate bottom-contact field-effect transistors, exhibiting average electron mobility has improved nearly 5-fold, from 0.25 cm2 V–1 s–1 without EG to 1.21 cm2 V–1 s–1 with 3% EG. Overall, this study provides an approach for improving the charge transport of D–A semiconducting polymers.