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 pi-pi 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 cm(2) V-1 s(-1) without EG to 1.21 cm(2) V-1 s(-1) with 3% EG. Overall, this study provides an approach for improving the charge transport of D-A semiconducting polymers.