A critical issue in polymer-based solar cells (PSCs) is to improve the power conversion efficiency (PCE) as well as the stability. Here, we describe the development of new semiconducting polymers consisting of thiophene, thiazolothiazole and naphthobisthiadiazole in the polymer backbone. The polymers had good solubility and thus solution-processability, appropriate electronic structure with narrow band gaps of ~1.57 eV and low-lying HOMO energy levels of ~-5.40 eV, and highly ordered structure with the favorable face-on backbone orientation. Solar cells based on the polymers and PC71BM exhibited quite high PCEs of up to 9%. More interestingly, the cells also demonstrated excellent stability as they showed negligible degradation of PCE when stored at 85˚C for 500 hours in the dark under nitrogen atmosphere. These results indicate that the newly developed polymers are promising materials for PSCs in the practical use.