4.1.1 Spin-coating
Due to the ease of use and safety, spin-coating is often used in laboratory. Franeker et al. used a variety of measurement methods to study film-forming process of spin-coating.[89] Figure 3b shows the PS:PC70BM:o-xylene phase diagram calculated with Flory-Huggins theory. Figure 3a shows four stages of the film-forming process, at initial stage, scattered GISAXS strength decreases with the decrease of film thickness. During the commencement of laser scattering (t = tonsetLS), the solution undergoes an internal phase separation, resulting in the formation of domains enriched with PC70BM and PS, which are extensively swollen (average φ solv = 70%). Subsequently, the separation of the L-L phase takes place, and the signal strength increased. The scattering intensity for GISAXS (t = tonsetGISAXS) significantly increases, primarily because o-xylene has a strong attraction to PS, causing the PC70BM-rich domains to have a lower solvent content compared to the PS-rich matrix. Therefore, the PC70BM-rich domains will dry earlier. The peak intensity rapidly increases. Eventually, the solvent evaporates, forming a film. Reichenberger et al. studied the effect of spin-coating on aggregates at a critical temperature T c.[94] Figure 3c compares the absorption of PCE11 aggregates above and below Tc. When spin-coating at 22 °C, compared to the collective absorption spectrum observed at 70 °C, the collective absorption spectrum exhibits an absorption peak at a slightly reduced energy and has a larger peak ratio of 0-0/0-1. The observed behavior in P3HT is comparable and indicates a stronger delocalization of electronic states throughout the polymer chain. This increased delocalization is accompanied by a reduction in inter-chain coupling. They believe that once the deposited solution approaches T c in the first few seconds, the molecules form aggregates. In the film, residual solvents persist, and they can assist in generating planarized chromophores with robust intra-chain connection. These planarized chromophores might serve as nuclei in the subsequent transformation process. As a result, for both P3HT and PCE11, spin-coating onto a substrate belowT c doesn’t substantially affect the quantity of aggregates formed. However, it does alter the nature or characteristics of these aggregates.