The structure and performance relationship in photovoltaic cells is still not fully understood, particularly in the case of controlling/optimizing the fill factor (FF). Here a pair of molecules DR2TDTCz and DR3TCz with similar backbone structures and varying conjugated central units were designed and synthesized, and their photovoltaic performance was studied and compared. The molecule DR2TDTCz, containing dithieno[3,2-b;6,7-b]carbazole (DTCz) as the central building block, with a carbazole ring in the center and two fused thiophene rings at the two sides of carbazole, exhibits improved solar light absorption and slightly narrow band gap, compared with the analogue system DR3TCz which has carbazole and two un-fused thiophene rings in the central building block. More importantly, it is found that introducing DTCz with thiophene fused 2,7-carbazole to replace 2,7-carbazole achieves a better molecular packing and favorable orientation, thus benefiting charge transport. As a result, the DR2TDTCz based device exhibits a power conversion efficiency (PCE) up to 7.03% with an impressively high FF of 75%, while the DR3TCz based device shows a PCE of 4.08% with a much lower FF of 54%. The results indicate that the FF can be tuned directly by the molecular structures and enlarged conjugation central core units could be beneficial to achieve high FF for the devices based on the acceptor–donor–acceptor (A–D–A) type small molecules.