Figure 3. In situ UV-Vis absorption spectra to show the formation of active layer using the the BC (a-c, e) and SqP (d, f).
To further evaluate the film formation process in the active layer, we conducted in-situ UV measurements, which monitors the variation of the absorption spectra during the active layer film-formation process. The change in raw optical density with respect to deposition time is shown in Figure S4 . Figure 3 is the mapping of the evolution of absorbance at each wavelength for the BC and SqP films during their liquid-to-solid transitions. First, in the BC method, the film-formation time (from the deposition of the blend solution to the time when the UV-vis density stops changing) for the toluene (Tol) solution (>5 s) is much longer than the chloroform solution (<0.7s), as expected. The addition of the SA in the BC method causes an increase in the film-formation time when the solvent is chloroform but a decrease in the toluene-based solution. In the SqP method, the deposition of the acceptor solution make the donor’s absorption resemble that of the liquid state, suggesting a high degree of swelling. Then, the donor and acceptor precipitates during the evaporation of toluene, and the drying times for the without and with SA solution are 1.706 and 1.790, respectively. The time for the acceptor being co-dissolved with the donor in the solution is obviously much shorter than the BC method, which has a higher possibility to make the domain of the acceptor or donor purer by reducing the intermixing time. It is worth noting that in the best-performing BC (with chloroform) and SqP (with toluene) systems, the addition of the solid additive increases the film formation time, by 18 ms for the PJ1 system for the BC film and by 84 ms for the SqP film.