AbstractTernary organic solar cells (OSCs) represent an efficient and facile strategy to further boost the device performance. However, the selection criteria and rational design of the third guest small molecule (SM) material still remain less understood. In this study, two new SM donor isomers, with α‐chlorinated thiophene (αBTCl) and β‐chlorinated thiophene (βBTCl) as side chains, are systematically designed, synthesized and incorporated as a third component in PM6:L8‐BO binary blends. It is noticed that introducing the SM donors guest has extended the absorption of photo‐active layer, induced desired component distribution vertically with enhanced crystallinity and reduced recombination process, leading to increased short‐circuit current (J_SC) and improved fill factor. Moreover, due to the synergetic suppressed nonradiative loss and preferable morphology, the ternary OSCs feature improves open‐circuit voltage (V_OC). Consequently, an impressive champion power conversion efficiency of 18.96% and 18.55% is achieved by αBTCl‐based and βBTCl‐based ternary OSCs, respectively. Furthermore, a record efficiency of 17.46% is obtained with a 330 nm thickness of αBTCl‐based ternary OSCs. This study demonstrates that molecular isomerization can be a promising design approach for SM donors to construct high‐performance ternary OSCs with simultaneous enhancement of all photovoltaic parameters.