Polymer solar cells are fabricated by a novel solution coating process, roller painting. The roller-painted film – composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) – has a smoother surface than a spin-coated film. Since the roller painting is accompanied by shear and normal stresses and is also a slow drying process, the process effectively induces crystallization of P3HT and PCBM. Both crystalline P3HT and PCBM in the roller-painted active layer contribute to enhanced and balanced charge-carrier mobility. Consequently, the roller-painting process results in a higher power conversion efficiency (PCE) of 4.6%, as compared to that for spin coating (3.9%). Furthermore, annealing-free polymer solar cells (PSCs) with high PCE are fabricated by the roller painting process with the addition of a small amount of octanedi-1,8-thiol. Since the addition of octanedi-1,8-thiol induces phase separation between P3HT and PCBM and the roller-painting process induces crystallization of P3HT and PCBM, a PCE of roller-painted PSCs of up to 3.8% is achieved without post-annealing. A PCE of over 2.7% can also be achieved with 5 cm2 of active area without post-annealing.