Conjugated polymers based on the dithienogermole (DTG) units showed promising properties for the applications in polymer solar cells (PSCs), so that the investigations of the natures and photovoltaic properties of the DTG-based polymers with varied backbone structures would be of great interest. In this work, four DTG-based polymers named as PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP were synthesized and characterized. The results indicate that the DTG-based polymers show varied absorption bands and molecular energy levels. In X-ray diffraction measurements, these polymers show different laminar packing and pi-pi stacking distances in solid films. The PSC devices based on the four DTG-based polymers were fabricated, and their photovoltaic properties were characterized. The results show that different device fabrication conditions are needed to get optimal photovoltaic performance of these four polymers. The device of PDTG-BDD shows a PCE of 6.3% with a high V-oc 0.935 V, a FF of 65.0%, and a J(sc) of 10.3 mA/cm(2), which is the highest one in these four polymers; the devices of PDTG-DPP showed a miserably low J(sc) of 3.19 mA/cm(2) due to the unfavorable morphologies of the polymer:PC71BM blend. Overall, the comparisons among these four polymers provide fundamental information for understanding the correlations among molecular structures and photovoltaic properties of the DTG-based polymers, and how to control or modulate the bandgaps, molecular energy levels, and morphologies of the DTG-polymers will be the key to fully explore their potentials as photovoltaic materials.