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Controlling crystallinity and molecular packing at nano- and macroscopic length scales in conjugated polymer thin films is vital for improving the performance of polymer-based electronic devices. Herein, the inherent amphiphilicity of rigid donor-acceptor copolymers used in high performance polymer electronics is leveraged to allow the formation of highly ordered lyotropic mesophases. By increasing the length and branching of solubilizing chains on cyclopentadithiophene-alt-thiadiazolopyridine-based alternating copolymers, amphiphilicity can be increased, and lyotropic liquid crystalline mesophases are observed in selective solvents. These lyotropic mesophases consist of chain extended polymers exhibiting close, ordered π-stacking. This is evidenced by birefringent solutions and red-shifted absorbance spectra displaying pronounced excitonic coupling. Crystallinity developed in solution can be transferred to the solid state, and thin films of donor-acceptor copolymers cast from lyotropic solutions exhibit improved crystalline order in both the alkyl and π-stacking directions. Because of this improved crystallinity, transistors with active layers cast from lyotropic solutions exhibit a significant improvement in carrier mobility compared to those cast from isotropic solution, reaching a maximum value of 0.61 cm² V-1 s-1. This approach of rational side chain design bridges the gap from solution structure to solid state structure and is a promising and general approach to allow the expression of lyotropic mesophases in rigid conjugated polymers. [ABSTRACT FROM AUTHOR]