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%.