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.