Pentacyclic diindeno[1,2-b:2′,1′-d]thiophene () unit is a rigid and coplanar conjugated molecule. To the best of our knowledge, this attractive molecule has never been incorporated into a polymer and thus its application in polymer solar cells has never been explored. For the first time, we report the detailed synthesis of the tetra-alkylated molecule leading to its dibromo- and diboronic ester derivatives, which are the key monomers for preparation of -based polymers. Two donor–acceptor alternating polymers, poly(diindenothiophene-alt-benzothiadiazole) and poly(diindenothiophene-alt-dithienylbenzothiadiazole) , were synthesized by using Suzuki polymerization. Copolymer was also prepared by using Stille polymerization. Although is prepared through a manner of random polymerization, we found that the different reactivities of the dibromo-monomers lead to the resulting polymer having a block copolymer arrangement. With the higher structural regularity, , symbolized as (thiophene-alt-)0.5-block-(thiophene-alt-BT)0.5, shows the higher degree of crystallization, stronger π–π stacking, and broader absorption spectrum in the solid state, as compared to its alternating analogue. Bulk heterojunction photovoltaic cells based on ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al configuration were fabricated and characterized. /PC71BM and /PC71BM systems exhibited promising power-conversion efficiencies (PCEs) of 1.65 % and 2.00 %, respectively. Owing to the complementary absorption spectra, as well as the compatible structures of and , the PCE of the device based on the ternary blend //PC71BM was further improved to 2.40 %.