4.2 Selecting the processing solvent
Different solvents have different effects on the interaction between
polymer and small molecules, solvent volatilization rate and molecular
diffusion rate, which results in different morphologies. Generally,
there are many interaction forces between solvent and solute, including
van der Waals force, electrostatic interaction, hydrogen bond, etc.,
which can stretch and contract polymer chain, or stabilize polymer chain
structure, thus changing the structure and arrangement of crystal.
Strong solvent-polymer interactions contribute to the formation of
interspersed nanonetwork structures, while weak interactions tend to
lead to the formation of condensed phases. In addition, the choice of
solvents also needs to consider its environmental protection and cost.
Therefore, these factors should be considered comprehensively when
optimizing the morphology of active layer.
The specific solvents can shorten film-forming times, which can prevent
the larger phase separation structure caused by high crystallinity. Zhu
et al. replaced CB with 2-methyltetrahydrofuran (MTHF) to shorten the
film-forming time of PTzBI-Si: N2200 blend film from 11.7 s CB to 3.7
s.[104] Due to the low boiling point of MTHF, the solution
evaporates quickly, which can greatly shorten the film formation time.
The shortened film-forming time leads to a decrease in crystallinity,
which inhibits the formation of large-scale structures. And the diameter
of PTzBI-Si crystal is reduced from 400 nm to 35 nm (Figure 5a). The
smaller phase separation structure enhances exciton dissociation,
leading to increase the J sc from 2.76 mA
cm−2 to 15.41 mA cm−2 and resulting
in an increase in PCE from 1.01% to 9.01%. Rational use of mixed
solvents can also promote vertical phase separation. Sun et al. used
chlorobenzene/tetralin to prepare P3HT:PCBM films to regulate the
film-forming process.[105] During film formation, chlorobenzene
evaporates rapidly, leaving tetralin with a high boiling point in the
film. Since PCBM has a much better solubility than P3HT, most PCBM
dissolve in tetralin, while P3HT begins to precipitate. At the later
stage of film formation, the slow evaporation rate of
tetrahydronaphthalene prolonged the film-forming time, PCBM molecules
have more time to move and can diffuse upward with tetralin evaporation.
Based on the X-ray photoelectron
spectroscopy results (Figure 5b), the PCBM to P3HT quality ratio
(mPCBM:mP3HT) experienced an increase from 0.1 to 0.72 in close
proximity to the film surface, consequently leading to the formation of
a vertical phase separation structure. The PCE is 1.5 times better than
that of devices without tetralin.