4.4.2 Change molecules interactions
Appropriate additives can change the interactions between molecules to
optimize the morphology. Manley et al. investigated the impact of three
distinct additives (DIO, CN, and DPE) on the process of
film-forming.[119] For pure solvents, the formation of crystallites
occurs quickly, usually happening within 3 s after the thinning
transition of the film. This timing is primarily influenced by the
boiling point of the solvent. Nevertheless, the length of this
transitional period demonstrates minimal connection with the boiling
point of the solvent and might even indicate a slight opposite
association. The introduction of an additive requires significantly
longer periods for the formation of crystallites, which can range from
minutes to almost 1.5 h. These extended formation times, when additives
are present, highlight the substantial influence of particular
interactions between the additive and the polymer. These interactions
are dictated by the molecular properties of the additive and their
affinity for different regions of the polymer structure. DIO has a
smaller boiling point than CN and DPE, but it results in a longer film
formation time (Figure 9a). In the solution phase, it was inferred that
DIO molecules establish robust connections with the PTB7 side chains,
which gradually detach as the drying period progresses. CN and DPE have
similar boiling points. In the case of 1% and 3% additives, the two
additives have similar membrane morphology evolution rates. It is
possible that self-folding properties and the saturated side chains of
PTB7 prevent the CN and DPE molecules from contacting the conjugate
polymeric backbone chains. By utilizing this information, people can
effectively adjust the fabrication conditions to attain customized film
morphologies that are ideal for particular applications. By adding DIO,
the active layer morphology can be precisely controlled, which is
conducive to the formation of fine fiber structure. Xu et al. precisely
manipulated the active layer of PM6:ITIC blend by adding DIO to
it.[93] Following the
introduction of DIO, a decrease in the PM6 (001) CCL from 12.5 to 2.5 nm
is detected through in situ GIWAXS analysis. Additionally, there is an
increase in the PM6 (100) CCL, indicating a length scale reduction in
the backbone and a rearrangement of the alkyl side chains. In addition
to this, in the third stage, the residual DIO continues to induce
crystalline formations, filling in the fiber network. The presence of a
small amount of solvent and DIO additive during the continuous
crystallization process leads to the formation of lamellae-ordering
length scales and a reduction in the coherence length of the polymer
backbone. This causes the fragmentation of the polymer backbone into
smaller fragments. The fragmentation occurs as a means to adapt to
internal stress arising caused by chain ordering in other directions and
the neighboring ITIC crystallites. The findings suggest that DIO has the
effect of weakening the planarization of the polymer backbone while
strengthening the interaction of the side chains of PM6. This causes
local crystal fragmentation and the formation of fiber structures
through connecting chains. At the same time, ITIC is distributed evenly
across the tightly interpenetrated fiber network (Figure 9b). In the
end, device PCE increased from 8.77% to 9.59%.