The effects of the spatial arrangement of the conjugated side chains of two-dimensional polymers on their optical, electrochemical, molecular-packing, and photovoltaic characteristics were investigated. Accordingly, novel polythiophenes with horizontally (PBTTTV-h) and vertically (PBTTTV-v) grafted terthiophene–vinylene (TTV) conjugated side chains were synthesized that display two and one UV-vis peaks, respectively; the difference is due to the different constitutions of the conjugated side-chains. Because the spatial arrangement affects the molecular self-assembly, PBTTTV-h shows stronger crystallinity than PBTTTV-v, which enhances the charge mobility in devices. Moreover, PBTTTV-h has a lower HOMO energy level (−5.49 eV) than PBTTTV-v (−5.40 eV). Bulk heterojunction solar cells fabricated from PBTTTV-h/PC71BM and PBTTTV-v/PC71BM exhibit power conversion efficiencies of 4.75% and 4.00%, respectively, and Voc values of 800 and 730 mV, respectively, under AM1.5G illumination (100 mW cm(−2)). Thus, the architecture of the TTV conjugated side chains affects the optical, electrochemical, and photovoltaic properties; this study provides more ideas for improving 2-D conjugated polymers for semiconductor devices.