Figure 5. (a) Analysis of in situ GIXD results. Peak fitting was employed for the (100) and (010) peaks to determine various parameters. Reproduced from [104]. (b) Spectra of X-ray photoelectron spectroscopy (XPS). Reproduced from [105].
The appropriate solvent can change the crystallization of molecules and the aggregation state of the solution. Jiang et al. used CF instead of CB to balance the crystallization of molecules in PDTBT2T-FTBDT:BTP-4F blends.[106] They observed that PDTBT2T-FTBDT exhibited a tendency to primarily form H-aggregates in the solvent, while BTP-4F had a preference for forming J-aggregates. The subpar performance of devices coated using slot-die printing with CB can be ascribed to the excessive J-aggregation of BTP-4F, which leads to an imbalance in donor and acceptor crystals and a less ordered molecular alignment. After the addition of CF, the aggregation of BTP-4F was inhibited, and the crystallization was more balanced. As shown in Figure 6a-d, with the passage of time, the number of face-on oriented molecules in the blend film added with CF gradually exceeds the number of isotropic molecules, which does not exist in CB. Compared with CB blending films, the blending films using CF have better morphology, smoother surface and more obvious crystal orientation. In the end, the PCE increased to 13.2%. Zhou et al. changed the solvent from CN and o-DCB to CB, successfully changing the orientation of the P(NDI2OD-T2) molecules.[107] When CB is used instead of CN, the molecular orientation of P(NDI2OD-T2) will change to face-on (Figure 6e, f). This is because CB can induce the aggregation of P(NDI2OD-T2) molecules and prolong the film-forming time, thus leading to the change of P(NDI2OD-T2) molecular orientation. And this phenomenon also occurs in PTB7-th: P(NDI2OD-T2) blends. Due to the consistent orientation of donor and acceptor molecules, resulting in a pronounced built–in electric field that enhances exciton dissociation. The reason for this is that CB has the ability to cause the clustering of P(NDI2OD-T2) compounds and extend the duration of film-forming, ultimately resulting in the alteration of P(NDI2OD-T2) molecular orientation. Furthermore, this occurrence is also observed in blends of PTB7-th and P(NDI2OD-T2). The optimization is credited to the aligned positioning of donor and acceptor molecules, leading to a significant pronounced electric field that boosts the dissociation of excitons.
The mixing solvent can change the crystallization sequence and optimize the phase separation structure of the blends. Zhang et al. studied the effect on microstructure by changing the order of crystallization of donor and acceptor in the P3HT:N2200 blend.[108] When N2200 had a molecular weight lower than 50.0 kDa, both polymers crystallized simultaneously in a solution of CB. However, when Ani/Tol was employed as the co-solvent, P3HT exhibited a preference for crystallization over N2200. When the average molecular weight is greater than 72.0 kDa, the crystallization of N2200 happens faster than P3HT, completing preferentially within a similar time frame. As shown in Figure 6g, they discovered that when the two components crystallize at the same time, they form larger size structures. When P3HT preferentially crystallizes, the microstructure is dominated by P3HT fiber crystals can be formed, and the molecular orientation tends to be edge-on orientation. The microstructure is mainly composed of fibrous crystals of N2200 when it crystallizes preferentially, and the molecular orientation tends to be face-on orientation, increasing the likelihood of forming the interpenetrating network structure. The findings demonstrate that the order in which crystallization occurs is a critical factor in shaping the microstructure of polymer blends.