Two polymer donors, FTAZ and J71, and two fused-ring electron acceptors, ITIC1 and ITIC2, are used to investigate the effects of conjugation dimension on the performance of organic solar cells (OSCs). FTAZ and J71, ITIC1 and ITIC2 share the same molecular backbone, respectively, while J71 and ITIC2 possess conjugated thienyl side chains. The addition of conjugated side chains slightly red-shift the absorption spectra and lower the bandgap due to the extended 2D conjugation. Conjugated side chains on acceptor induce the self-aggregation of acceptors, while conjugated side chains on donor increase the miscibility of donors and acceptors, thus optimize the morphology of active layers. The blends based on mixed combinations, namely 1D donor/2D acceptor and 2D donor/1D acceptor, show better performance relative to 1D donor/1D acceptor and 2D donor/2D acceptor.