Fabricating organic solar cells (OSCs) in open-air and room-temperature environments is essential for cost-effective roll-to-roll printing, but its performance is limited by non-ideal block and surface morphology of active layer. Herein, we demonstrate a record power conversion efficiency (PCE) of 19.72% in open-air and as-cast processed devices by using a low-cost terpolymer donor (PTQ20-5) to optimize the block and surface morphology. Compared with the counterpart PTQ10, PTQ20-5 shows increased hydrophobicity, which can better prevent the intrusion of H₂O molecules in the film formation process in open-air environment, giving excellent humidity tolerance of corresponding devices. Moreover, the enhanced dielectric constant results in reduced exciton binding energy and improved charge transport of PTQ20-5, leading to improved charge separation and transfer, and suppressed carrier recombination in the devices. Besides, the PTQ20-5-based film has more appropriate block morphological features of balanced molecular self-assembly and phase separation, which simultaneously promotes the charge carrier transport and inhibits the carrier recombination at the donor/acceptor interface. This work is of great significance for promoting the industrialization of OSCs.