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%.