Here, taking a polythiophene derivative (PBDD4T) as a starting polymer, we tried to increase the rotation barrier and hence stabilize its backbone conformation by introducing fluorine into the beta and beta'-position of the alpha-linked bithiophene segments and then synthesized a new polymer named as PBDD4T-2F. Our results demonstrate that the rotation barrier between the a-linked bithiophene significantly increases after the fluorination, so PBDD4T-2F has a more stable backbone conformation than PBDD4T. Compared to PBDD4T, PBDD4T-2F shows stronger aggregation effect in solution state and more compact pi-pi stacking in solid thin film and also possesses deeper HOMO level. These properties make PBDD4T-2F being an ideal donor material in PSCs. When blended with PC71BM, a fullerene acceptor, the PBDD4T-2F-based device showed a power conversion efficiency (PCE) of 9.04%, which is 38% higher than that of the PBDD4T-based device; when blended with ITIC, a non-fullerene acceptor, the PBDD4T-2F-based device showed a PCE of 8.69%, which is almost 20 times higher than that of the PBDD4T-based device. What is more, the tandem cell, in which the blend of PBDD4T-2F:PC61BM was used for making the front subcell, exhibited a high PCE of 10.12%. The photovoltaic results indicate that the fluorination is an effective method to enhance interchain pi-pi interaction for the polythiophene and hence to tune its photovoltaic properties in PSCs, especially for the fullerene-free device based on ITIC.