A wide bandgap polymer donor PBDTS-DTBTO is designed and synthesized, which shows an optical bandgap of 1.76 eV and strong π-π interaction. By using a low-bandgap acceptor ITIC, power conversion efficiency over 9% is achieved in the fullerene-free polymer solar cells (PSC) device. Furthermore, the PSC devices fabricated by PBDTS-DTBTO:ITIC show relatively small sensitivity to variations of the active layer thickness.