It is known that poly(3-alkylthiophene) (P3AT) side-chain length notably influences the photovoltaic performances of relating devices. However, comprehensively study on its impact on the structures of P3ATs and their blends with [6, 6]-phenyl-C-61 butyric acid methyl ester (PCBM) is insufficient. By using solid-state NMR and FTIR techniques, four P3ATs and their PCBM blends are investigated in this work, focusing on the phase structures as modulated by side-chain length. Recently, we revealed multiple crystalline main-chain packings of packing a and b together with a mesophase in poly(3-butylthiophene) (P3BT) films (DOI: 10.1021/acs.macromol.6b01828). Here, the semicrystalline structures are investigated on poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT), and poly(3-dodecylthiophene) (P3DDT) with traditional form I modification, where packing a and the amorphous phase are probed. Furthermore, crystallized side chain within packing a is detected in both P3OT and P3DDT films, which shows a FTIR absorption at 806 cm(-1). Structural studies are also conducted on P3AT:PCBM blends. Compared with the pure P3ATs, the polymer crystallinities of the blends show reduction of about 40% for P3OT and P3DDT, whereas only about 10% for P3HT. Moreover, in P3BT:PCBM and P3HT:PCBM, the crystalline polymers and PCBM are phase separated, while in P3OT:PCBM and P3DDT:PCBM, blend components are mostly miscible. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 751-761