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-γ 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-γ 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.