•The 40 nm DNTT films grown on para-sexiphenyl (p-6P) monolayer and bare SiO2 show same topographies.•The growth behavior, especially the initial stages of these two films are different.•Highly ordered DNTT films have been obtained on p-6P monolayer showing carrier mobility of 1.9 cm2/V s.The initial stage of organic film growth is considered to be vital for the carrier transport in organic thin-film transistors with bottom gate configuration. The same topographies of 40 nm dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) films on para-sexiphenyl (p-6P) monolayer and bare SiO2 exhibited quite different field-effect mobilities, 1.9 and 0.1 cm2/V s, respectively. The further investigation indicated there were different growth behaviors at their initial stages of film growth. Column islands with high density were observed on SiO2, while lamina islands on p-6P monolayer due to the good diffusion ability and their good epitaxial relationship. The latter is beneficial to obtain high quality film with less boundaries and defects. The work demonstrated that the initial stage of film growth is an important factor to determine the device performance of organic transistors, which is significant to improve the device fabrication and optimize the device performance.

•Large-area continuous α phase thin films of F32 and F64 were prepared by melt crystallization.•The crystallization of films is obtained by the radial growth to form spherulite-like structure composing of nanofibrils.•The molecules backbone chains remain extended in nanofibrils.•The melt-crystallized phases change from β′ phase (F16) to α phase (F32 and F64) as the molecular weight increases.Large-area continuous α phase thin films of monodisperse poly(9,9-dioctylfluorene) (PFO), with 32 or 64 fluorene repeating units, named as F32 or F64, respectively, are prepared by melt crystallization. A spherulite-like structure, composing of crystalline nanofibrils, is formed via radial growth after sufficient isothermal treatment. In these nanofibrils, the polymer chains remain extended conformation, which is demonstrated by the similarity between nanofibril widths and extended chain lengths. Comparing this discovery on F32/F64 with our previous observations on F16 (monodisperse PFO consisting of 16 repeating units) leads us conclude that the melt-crystallized phases change from β′ phase (F16) to α phase (F32 and F64) as the molecular weight increases. It is speculated that as the number of repeating unit goes up, the backbone planarity of PFO decreases, leading to the formation of different melt-crystallized phases at different molecular weights.

•A novel β′ phase crystal of monodisperse PFO was prepared by melt crystallization.•The β′ phase crystal has lamellar structure.•The unit cell parameters and space group of β′ phase were given.•The β′ phase crystals are more thermal stable than the β phase crystals.A novel β′ phase crystal with lamellar structure of monodisperse poly(9,9-dioctylfluorene) (PFO) has been prepared by means of melt crystallization, demonstrating similar optical properties with β phase crystals. A preliminary determination of unit cell dimensions and molecular packing (orthorhombic, a = 1.30 nm, b = 2.22 nm, c = 3.36 nm, 4 chains, with space group Pmc21 and density 1.06 g/cm3) was made by combined analysis of the selected-area electron diffraction (SAED) and X-ray diffraction (XRD) patterns. In the lamellar crystals, the molecular backbone chains are parallel to the lamellar plane with the alkyl side chains interdigitation in the thickness of lamella.