In this work, solution-processed organic solar cells with conjugated small molecules both as electron donors and electron acceptors were studied, where the influence of the chemical structures of the donor and acceptor on the device performance was systematically investigated. A small molecular donor incorporating binary electron-deficient units, diketopyrrolopyrrole and pentacyclic aromatic bislactam, was synthesized to provide a low band gap of 1.65 eV and low-lying energy levels. Three molecules, from a fullerene derivative to non-fullerene perylene bisimide-based acceptors, were selected as electron acceptors to construct organic solar cells. The results showed that fullerene-based solar cells provided power conversion efficiencies (PCEs) of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, with a photoresponse of up to 750 nm. Further analysis of the bulk-heterojunction systems between donors and acceptors revealed that the relatively low carrier mobilities of the non-fullerene acceptors and the large phase separations are mainly responsible for the less efficient solar cells. Our results demonstrate that molecules containing several electron-deficient units can effectively reduce the band gap of small molecules, and thus offer great potential for realizing high performance fullerene and non-fullerene solar cells.