The operation stability of polymer solar cells (PSCs) is one of the most important prerequisites for their practical use. In this work, we report a new acceptor-donor-acceptor (A-D-A) triad-type small molecule,5TRh-PCBM, as a compatibilizer for enhancing the thermal stabilities and mechanical properties of efficient PSCs while increasing their power conversion efficiencies (PCEs). This multifunctional5TRh-PCBMmolecule, consisting of an oligothiophene segment as the central core and fullerene derivatives as the end groups, is designed to enable strong interactions between the 5TRh-core with various types of efficient polymer donors containing thiophene or fused-thiophene units, while the end fullerene groups preferentially interact withPCBMacceptors. To examine the effectiveness of this molecular compatibilizer, PSCs with different donors (PTB7-Th,PBDB-T, andP3HT) have been fabricated and tested, with addition of various amounts of5TRh-PCBM. The addition of 5 and 10 wt% of5TRh-PCBMsignificantly enhances the thermal and mechanical stabilities of all tested PSCs. Importantly, unlike typical compatibilizers, the addition of5TRh-PCBMcan increase the PCEs of the PSCs due to its light harvesting capability. In particular, the PCE ofPTB7-Th:PCBM-based PSCs is increased from 9.37% to 10.09% with the 5 wt% addition of5TRh-PCBM. Our comprehensive investigations have revealed the effects of5TRh-PCBMon the optical, morphological, photovoltaic, and mechanical properties of molecularly engineered PSCs.