We highlight the influence of processing conditions on polymorphism and structure formation on the mesoscale for the family of PCPDTBT polymers with branched alkyl side chains. Direct correlations of morphology to the chemical structure and to transistor device performance are established. We found that up to four different packing motifs could be realized depending on the polymer derivative and the processing conditions: amorphous, pi-stacked, crosshatched and dimer-containing polymorphs. While C- and F-PCPDTBT display similar packing behavior organizing in pi-stacked and dimer-like structures, Si-PCPDTBT gives rise to cross-hatched structures upon simple deposition from solution. The observed differences in chain packing for C-/F-PCPDTBT versus Si-PCPDTBT are attributed to differences in backbone conformations and aggregation behavior in solution. The effect of polymorphism on charge transport is probed using field-effect transistors, in which both pi-stacked and cross-hatched polymer chain arrangements yield the highest hole mobilities. Mesoscopic morphology and mobility simulations rationalize our experimental findings by relating mobility to distributions of electronic coupling elements between the chains.