Although　many　fully-conjugated　polymerized　small-molecule　acceptors　(PSMAs)　with　high　photovoltaic　performance　have　been　reported　recently,　most　of　them　exhibit　high　stiffness　and　are　very　brittle,　which　limits　their　applications　in　wearable　and　stretchable　electronics.　In　this　work,　we　show　that　PSMAs　using　ladder-type　heteroheptacene-cored　SMAs　(M-series　acceptors)　as　the　main　building　blocks　can　achieve　high　mechanical　ductility　while　maintaining　high　power　conversion　efficiencies　(PCEs).　To　finely　tune　molecular　stacking　and　miscibility　of　the　resulting　PSMAs　(PQ-HD　and　PQ-OD),　two　pendant　groups　with　different　alkyl　lengths　(namely,　2-hexyldecyl　and　2-octyldodecyl)　are　anchored　on　their　backbones.　With　the　shorter　side-chains,　a　more　intermixed　bulk-heterojunction　blend　morphology　as　well　as　highly　well-ordered　molecular　packing　are　achieved,　leading　to　simultaneous　enhancement　of　both　photovoltaic　and　mechanical　properties.　The　best-performing　PSC　based　on　PQ-HD　with　2-hexyldecyl　side-chains　shows　a　high　PCE　of　13.36%　and　excellent　mechanical　stretchability　with　a　crack-onset　strain　(COS)　of　17.5%,　both　of　which　are　superior　to　that　based　on　PQ-OD　with　2-octyldodecyl　side-chains　(PCE　=　10.87%　and　COS　=　12.8%).　Moreover,　the　PQ-HD-based　PSC　also　exhibits　excellent　thermal　stability.　These　outstanding　properties　in　combination　with　its　low　synthetic　complexity　shall　make　PQ-HD　a　promising　candidate　for　future　applications　in　flexible　and　wearable　devices.　©　2023　Elsevier　B.V.