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, beta(1) and beta(2), of a single conjugated polymer is investigated for a low band gap poly(diketopyrrolopyrrole-alt-quaterthiophene). Compared to beta(1), the beta(2) phase exhibits a distinct redshifted absorption and an associated near infrared photoluminescence. The length of the alkyl side chains controls the formation of the beta(1) and beta(2) phases. Intermediate length alkyl side chains (nonyl and dodecyl) can selectively provide the beta(1) or beta(2) phase in solution and in semi-crystalline thin films, depending on the nature of the solvent used. For short side chains (hexyl) the beta(2) phase forms more readily while for long side chains (pentadecyl) the beta(1) phase is predominant. The kinetics of beta(2) phase formation is investigated and reveals a reduced growth rate when long alkyl side chains are present. X-ray diffraction reveals a closer pi-pi stacking distance for beta(2) than for beta(1), 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 beta(2) phase. The study shows that the formation of the two polymorphs beta(1) and beta(2) is controlled by the alkyl side chains and the solubility that arises from them. Shorter side chains (lower solubility) favor beta(2) and longer side chains (higher solubility) beta(1), and at intermediate lengths both phases can be formed.