A pentacyclic indacenodiselenophene (IDS) arene was synthesized via intramolecular Friedel-Craft cyclization of the selenophene moieties. This IDS framework was used as a model system to investigate the alkyl/alkoxy side-chain effect by preparing IDS-OCH8 and IDS-C6, where the side chains on the sp3 carbon in the cyclopentadienyl ring are 4-octyloxyphenyl groups and 4-hexylphenyl groups, respectively. The Sn-IDS-OCH8 and Sn-IDS-C6 monomers were copolymerized with 4,7-dibromo-2,1,3-benzothiadiazole (BT), 4,7-diiodo-5,6-difluoro-2,1,3- benzothiadiazole (FBT) and 1,3-dibromothieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor monomers by Stille polycondensation to afford five new IDS-based donor-acceptor alternating copolymers, PIDSBT-OCH8, PIDSBT-C 6, PIDSFBT-OCH8, PIDSFBT-C6, and PIDSTPD-C 6. Despite the fact that the octyloxy and hexyl side chains play a negligible role in the optical and electrochemical properties of the resulting polymers, the solar cell performance is highly associated with the side chains of the polymers. Under similar device fabrication conditions, the PIDSBT-C 6 and PIDSFBT-C6-based devices showed much improved efficiencies than the corresponding PIDSBT-OCH8, and PIDSFBT-OCH 8-based devices (2.6% and 1.9% vs 3.8% and 3.9%). The improvement is mainly the result of much enhanced Jsc values. Consistently, PIDSBT-C6 and PIDSFBT-C6 exhibited much higher FET hole mobilites than the corresponding PIDSBT-OCH8, and PIDSFBT-OCH 8. These results clearly revealed that the 4-hexylphenyl group is a more suitable side chain than the 4-octyloxyphenyl group in the IDS system, and the side-chain dependent mobility of the polymers is the dominating factor to determine the photocurrents and efficiencies of PSCs. PIDSBT-C6 exhibited a high hole mobility of 0.08 cm2 V-1 s -1 and PIDSTPD-C6:PC71BM (1:4 in wt %)-based solar cell with 5 v% chloronaphthalene (CN) delivered a Voc of 0.92 V, a Jsc of 9.77 mA/cm2, an FF of 51%, and a highest PCE of 4.6%. This work not only discloses a new selenophene-containing ladder-type IDS structure and its copolymers but also provides useful insights into the alkyl/alkoxy side-chain effect for future design of conjugated polymers.