Advancements　in　narrow　bandgap　organic　small　molecule　acceptors　(SMAs)　has　promoted　organic　solar　cell　(OSC)　efficiencies　beyond　20%.　Achieving　this　milestone　necessitates　precise　control　over　the　active　layer　morphology,　particularly　its　crystallinity　and　phase　distribution,　to　optimize　light　absorption,　charge　transport,　and　suppress　charge　recombination.　However,　controlling　SMA　morphology　remains　a　significant　challenge　due　to　their　strong　aggregation　tendency.　Existing　methods,　including　high-temperature　annealing,　and　introducing　high　boiling　point　additives,　frequently　yield　disordered　polymorphs　with　limited　scalability.　Here,　we　report　a　novel　approach　of　utilizing　4-bromochlorobenzene　as　a　volatile　solid　additive　to　induce　the　formation　of　a　highly　ordered　polymorph　of　BTP-eC9　through　mild　annealing　at　60　degrees　C.　This　marks　the　first　demonstration　of　such　an　ordered　SMA　polymorph,　exhibiting　optical　properties　comparable　to　ideal　crystals,　including　enhanced　anisotropy,　refractive　index,　and　extinction　coefficients.　The　specific　polymorph　further　enables　the　formation　of　a　well-organized　PM6　donor　arrangement,　establishing　an　optimal　bicontinuous　network　morphology.　Consequently,　the　OSCs　based　on　PM6:BTP-eC9　achieve　a　power　conversion　efficiency　of　19.53%,　which　further　increases　to　20.32%　with　the　addition　of　an　antireflection　layer.　This　work　provides　a　scalable　and　effective　strategy　for　enhancing　OSC　performance　and　highlights　the　critical　role　of　polymorphism　in　optimizing　photovoltaic　performance.