A fused donor, thienobenzo indacenodithiophene ( ), was designed and synthesized using a novel acid-promoted cascade ring closure strategy, and then copolymerized with a benzothiadiazole ( ) monomer. The backbone of is an expansion of the well-known indacenodithiophene ( ) unit and was expected to enhance the charge carrier mobility by improving backbone planarity and facilitating short contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm V s , lower than the performance of (∼1.5 cm V s ). Mobilities extracted from time-resolved microwave conductivity measurements were consistent with the trend in hole mobilities in organic field-effect transistor devices. Scanning tunneling microscopy measurements and computational modeling illustrated that exhibits a less ordered microstructure in comparison to . This reveals that a regular side-chain packing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that , despite containing a larger, planar unit, showed less stabilization of planar backbone geometries in comparison to . This is due to the reduced electrostatic stabilizing interactions between the peripheral thiophene of the fused core and the unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with -type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors.