The effect of the length of solubilizing alkyl side chains, ranging from hexyl to pentadecyl, on the formation and structure of two distinct semi-crystalline semiconductor phases, ß1and ß2, of a single conjugated polymer is investigated for a low band gap poly(diketopyrrolopyrrole-alt-quaterthiophene). Compared to ß1, the ß2phase exhibits a distinct redshifted absorption and an associated near infrared photoluminescence. The length of the alkyl side chains controls the formation of the ß1and ß2phases. Intermediate length alkyl side chains (nonyl and dodecyl) can selectively provide the ß1or ß2phase in solution and in semi-crystalline thin films, depending on the nature of the solvent used. For short side chains (hexyl) the ß2phase forms more readily while for long side chains (pentadecyl) the ß1phase is predominant. The kinetics of ß2phase formation is investigated and reveals a reduced growth rate when long alkyl side chains are present. X-ray diffraction reveals a closer p-p stacking distance for ß2than for ß1, consistent with its redshifted absorption and its higher mobility in field-effect transistors. The polymer with hexyl side chains adopts an edge-on orientation in thin films, while the longer alkyl chains induce a face-on orientation. Photovoltaic devices exhibit an additional near infrared spectral contribution to the photocurrent for the ß2phase. The study shows that the formation of the two polymorphs ß1and ß2is controlled by the alkyl side chains and the solubility that arises from them. Shorter side chains (lower solubility) favor ß2and longer side chains (higher solubility) ß1, and at intermediate lengths both phases can be formed.