The effect of alkyl side chain length on the formation of two semi-crystalline phases in low band gap conjugated polymers
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, β1 and β2, of a single conjugated polymer is investigated for a low band gap poly(diketopyrrolopyrrole-alt-quaterthiophene). Compared to β1, the β2 phase exhibits a distinct redshifted absorption and an associated near infrared photoluminescence. The length of the alkyl side chains controls the formation of the β1 and β2 phases. Intermediate length alkyl side chains (nonyl and dodecyl) can selectively provide β1 or β2 phase in solution and in semi-crystalline thin films, depending on the nature of the solvent used. For short side chains (hexyl) β2 phase forms more readily while for long side chains (pentadecyl) β1 phase is predominant. The kinetics of β2 phase formation is investigated and reveals a reduced growth rate when long alkyl side chains are present. X-ray diffraction reveals a closer π-π stacking distance for β2 than for β1, 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 β2 phase. The study shows that the formation of the two polymorphs β1 and β2 is controlled by the alkyl side chains and the solubility that arises from it. Shorter side chains (lower solubility) favor β2 and longer side chains (higher solubility) β1, at intermediate lengths both phases can be formed.