4.1.2 Blade-coating
Compared with spin-coating,
blade-coating is more suitable for industrial applications because it is
comparable with large area film fabrication.[95] Peng et al. studied
the effects of the blade-coating speed and the third component on the
film-forming process.[96] When the coating speed increases, the
thickness of the film also increases.[97] A relatively prolonged
transition process from solution to film may promote an increase in
crystallinity. As a result, at a medium speed of 20 mm/s, polymers and
small molecules demonstrated appropriate pre-aggregation and underwent a
comparatively extended process of crystallization and molecular
arrangement. In addition, the introduction of components also changes
the film-forming kinetics (Figure 3d). In the first stage, as the
solvent evaporates, some polymers or small molecules begin to aggregate
in an ordered fashion. During the second phase, the incorporation of the
third component hinders the formation of crystal nuclei, leading to the
simultaneous crystallization of both the polymer and small molecules.
During the third phase, the residual solvent allows for further
rearrangement of the polymer chains or small molecules, leading to the
achievement of optimal crystallization. Stage four, the drying process
is complete. As a result, the ternary membrane exhibits a greater degree
of molecular organization and a reduced size of phase separation. In
addition, appropriate sequential deposition process methods can also
improve the crystallization of donor/acceptor. Wang et al. deposited
PBDB-T:FOIC onto PBDB-T:IT-M films by sequential-blade cast (seq-blade
cast), thereby improving donor and acceptor crystallization.[90] The
seq-blade cast method can induce varying levels of dissolution,
blending, and phase separation between the upper and lower layers. This
phenomenon facilitates the efficient mixing of the donor and acceptor
components to obtain more ideal nanostructures and morphologies. The
PBDB-T:FOIC in the seq-blade cast system creates a fresh structure, and
the PBDB-T:IT-M bottom layer supplies numerous crystal nuclei. The
competitive process of growth encouraged the formation of PBDB-T
crystals and restricted the clustering of FOIC, resulting in minimal
phase separation in the film. The binary films take a relatively long
time to crystallize, approximately 18 seconds, whereas the PBDB-T:IT-M
layer and PBDB-T:FOIC layer of seq-blade cast films reach their peak at
14 seconds and 8 seconds, respectively (Figure 3e). In the case of
seq-blade cast film (Figure 3f), the PBDB-T aggregation rate rose from
0.0428 to 0.0574. The results show that the seq-blade device
demonstrates the highest OSC performance, with a PCE of 11.91%. Yuan et
al. studied the film-forming dynamics of patterned blade
coating.[98] The use of
micro-structured pattern printing blades allows for the adjustment of
flow conditions to enhance the alignment of molecules and facilitate
mass transfer in the crystallization process. The fluid can be
facilitated to have both extensional and shear flow simultaneously by
these arrays of micro-structured blades. Experimental evidence has
demonstrated that extensional flow can align conjugated polymer chains,
while shear flow has the potential to induce elongation and modify the
structure of these polymers. During the subsequent evaporation process,
the tight packing and arrangement of PM6 polymer chains, influenced by
both extensional and shear flow, leads to effective crystallization of
Y6 molecules. As shown in Figure 3g and h, among the four films, the
film prepared with a low-speed patterning blade (PBC-LS) has the highest
CCL, indicating excellent crystallinity.
Simultaneously, PBC-LS films exhibit
a smaller π-π stack distance of 0.36 nm and a lamellar stack distance of
2.1 nm, suggesting a molecular structure that is densely packed. These
films demonstrate superior crystallinity and well-defined phase
separation, which is attributed to the patterned blade’s ability to
minimize disturbances of the solution. Finally, the PCE of PM6: Y6 film
prepared by PBC-LS was 15.93%, while the PCE of ordinary blade was
14.55%.