We present herein efficient bulk heterojunction (BHJ) solar cells via mixing poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and 6,6-phenyl C61 butyric acid methyl ester (PCBM) with variable weight ratios. The photo-physics and morphology of F6T2:PCBM blend films and the electrical characteristics of their corresponding single cells were studied in details by changing PCBM concentration. The complete photoluminescence quenching of F6T2 emission occurs with only a small fraction of PCBM blended, demonstrating effective photoinduced charge transfer between F6T2 and PCBM. Morphology images from atomic force microscopy and scanning electron microscopy (SEM) reveal that the phase separation in F6T2:PCBM blend films becomes pronounced with the increase of PCBM concentration, resulting in the increased fill factor from 25.2% (1:1) to 56.9% (1:6). A SEM image also shows the phase separation is within the range of 10 - 20 nm. With the optimized F6T2:PCBM weight ratio (1:2), the single cell exhibits a highest power conversion efficiency of 2.46% due to the balance of light absorption and charge transport. Finally, the polymer-small molecule tandem cells are constructed using F6T2:PCBM BHJ as the bottom cell and copper phthalocyanine (CuPc):fullerene (C(60)) as the top cell. The open-circuit voltage (V(oc)) of tandem cell (1.27 V) is equal to the summation of the V(oc) values of the bottom cell (0.86 V) and the top cell (0.43 V).