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.