Figure 4. Composition ratio across the vertical direction of the active layer film by FLAS base on BC film (a-c,e) and SqP film (d,f).
Next, we investigate the effect of SqP and SA on the vertical phase segregation of the active layer. To this end, film-depth-dependent light absorption spectra (FLAS) were measured on the BC and SqP films. By sequentially etching the film using plasma while monitoring the changes in absorption spectra in real-time (etching spectra shown inFigure S5 ), the vertical distribution profile (Figure 4 ), exciton generation profile in the vertical direction (Figure S6 ), and exciton generation rate curve in the vertical direction (Figure S7 ) were calculated using the transfer matrix model.[50-51]The vertical distribution profiles (Figure 4a-f ) show distinct differences in the final distribution due to the variations in film preparation methods as well as in material structures of the acceptors. In the PJ1 system, the introduction of the solid additive does not alter the vertical phase segregation in the BC films prepared with toluene, but it results in noticeable adjustments to the vertical morphology for BC films processed from chloroform and the SqP films from toluene: The addition of the SA make the bottom portion of the active layer (closer to the anode) exhibit a higher concentration of donor and a lower concentration of acceptor than those without the SA, which is beneficial for interfacial charge selectivity. The trends in the vertical phase segregation for these films are well consistent with the trend in their device electrical properties such as the TPV and TPC decay constants and fill factor. Similar observation has been made in the PYF-T-o -based system (Figure S9 ) with an additional effect provided by the SA: the incorporation of the SA leads to a more uniform distribution of donor and acceptor in the middle region of the active layer.
To evaluate the miscibility between the materials from a thermodynamics point of view, surface tensions of the materials are first analyzed. The contact angle of different solvents on the solid films are shown inFigure S8 . We calculated the surface tension (γ ) of each material or material combination based on Wu’s model and the interfacial tension between materials as an indication of miscibility. Generally, a lower interfacial tension value indicates a higher miscibility between the materials at thermal equilibrium. As presented in Table S5 , the interfacial tension value between PM6 and PYF-T-o is 0.03 mN/m, while that between PM6 and PJ1-γ is 0.67 mN/m. In comparison, the miscibility between PJ1 and PM6 is weaker, particularly when the solid additive is introduced (the interfacial tension increases to 2.08 mN/m). This suggests that compared to PM6:PYF-T-o , PJ1-γ has a higher tendency to phase separate from the donor during the film formation. Furthermore, the fact that the SqP reduces the intermixing time indicates that the SqP method could regulate the degree of phase separation to some extent.