Abstract
Spin-coating is an important process in the preparation of polymer thin films for functional applications. In previous studies, the focus lies mainly in the spin-off stage where fluid flow dominates. Here we pay more attention to the possible presence of a skin layer at the air-liquid interface starting from the relatively early stage of spin-coating. The film formation process during spin-coating of poly(3-hexylthiophene) (P3HT) solutions is examined by means of normal- incidence optical reflectivity (OR) and in-situ grazing incidence small/wide-angle X-ray scattering (GISWAXS). Straight-forward analysis (assuming a single liquid layer) of OR results indicates generally that there are 3 stages in the film-forming process: (1) the spin-off or flow-dominated stage, where the film-thinning rate decreases very quickly according to the Meyerhofer equation, (2) the evaporation-dominated stage of plateaued film-thinning rate, and (3) the freezing-in stage where the film-thinning rate drops to essentially zero. More sophisticated analysis (allowing for multi-layer stratification) suggests the emergence of a thin (ca. 30 nm in thickness) “epidermis” layer of high refractive index at the air/film surface during transition toward evaporation-dominated stage, which coincides approximately with the GIWAXS-detected emergence of P3HT crystallites. The GIWAXS results further indicate that both the relative crystallinity and crystallite size quickly reach saturated values and show only minor changes during the subsequent evaporation-dominated and freezing-in stages. Concurrent GISAXS results indicate the presence of oblate domains with equatorial diameter ≈ 200 nm and polar diameter ≈ 10 nm, within which P3HT crystallites presumably reside. The number of these oblate domains in the skin layer increases moderately with time before entering the freezing-in stage, but the size remains essentially the same.