Planar backbone, locked conformation, and low lowest unoccupied molecular orbital level provide polymer F4BDOPV-2T with ultrahigh electron mobilities of up to 14.9 cm(2) V-1 s(-1) and good air stability. It is found that the nonlinear transfer curves can be tuned to near-ideal ones by changing fabrication conditions, indicating that film morphology largely contributes to the nonlinear transfer curves in high-mobility conjugated polymers.