Effects of a newly synthesized additive, 2,2'5,5'-tetrathiophene-3,3'-hexyldithiophene (TTH), on the performance of polymer solar cells (PSCs) based on poly(3-hexylthiophene):[6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) bulk-heterojuction were investigated. The PSC with a non-annealed P3HT:PCBM:TTH blend film showd an increase in the short-circuit current and the fill factor, resulting in a significant enhancement in the power conversion efficiency, compared to that of a reference cell without the TTH additive. It was revealed that the addition of 5wt% TTH to the P3HT:PCBM blend film facilitated the crystallization (ordering) of P3HT chains without even post-thermal annealing, leading to a higher absorbance and a larger crystal size of P3HT, and thereby increasing the short-circuit current and the fill factor.