5.2 Solvent vapor annealing
Another approach to control the morphology is SVA. Following the casting process, the film is promptly transferred to a chamber containing a greater concentration of solvent in SVA. Throughout this procedure, the solvent vapor infiltrates the film, influencing its structure. The vapor of the solvent has the ability to enhance the movement of molecules, providing them with additional time to rearrange and enhancing the quality of the film’s structure.[137] One benefit of SVA is its ability to be carried out at lower temperatures. Research has shown that it is effective even without an inert atmosphere, thereby verifying the possibility of its execution and enhancement.[138] The impact of a specific solvent on the active substance relies on various elements, encompassing its ability to dissolve, polarity, and experimental factors. The aggregation of PCBM and the crystallization of the polymer can be influenced by the use of highly volatile annealing solvents in polymer-fullerene systems.[69, 139]
The choice of solvent is the most important part of solvent annealing. Different solvents will affect the optimum annealing time. Christina et al. found that the optimal annealing time was inversely related to the donor solubility.[140] They used four solvents (CHCl3, THF, CS2, and C3H6O2), each showing a different solubility to the donor. By analyzing Figure 13a, the entire SVA process can be divided into three stages. During the initial stage, the solvent infiltrates the film and causes additional phase separation in the finely grained structures present in the as-cast state. During the second stage, these structures begin to grow. Because this growth is facilitated by diffusion mechanisms, diverse nanomorphologies develop under different solvent vapor atmospheres. Therefore, the rate of the diffusion is directly proportional to the solubility of the solvent being used. Following this linear growth regime, the DRCN5T fibers enter the subsequent sublinear growth phase characterized by the maturation and saturation of the structures in the final phase. In summary, if the solubility of the donor is low, the growth of the fiber will be slower and the annealing time will be longer. Appropriate solvents help to reduce domain size. Engmann et al. improved the phase size by adding THF to BTR:PC71BM blends.[32] During the initial phases of the SVA, when the film is exposed to CF, it leads to the dissolution of small imperfect crystals. The enhancement in film crystallization and phase purity is not apparent until the CF concentration in the vapor of the annealing chamber is decreased. The dissolution at the beginning leads to a decrease in nucleation density, resulting in the growth of a small number of crystals to a larger size, approximately 60 nm in domain size. When using a moderate solvent such as THF, the ability to dissolve tiny crystals decreases, leading to a comparatively higher density of nucleation. Nevertheless, the size of the crystals in general stays small, and the occurrence of excessive film coarsening is avoided. In all instances, an elevation in scattering intensity is noted during the annealing process, which signifies an enhancement in phase purity. This increase is accompanied by a noticeable expansion of characteristic length scales, indicating the coarsening of domains (Figure 13b). After the addition of THF, the phase region size is the smallest, about 30 nm. Han et al. induced phase separation of PC71BM and PCDTBT side chains by mixing solvents.[141] THF can effectively promote the crystallization of fullerene molecules, while CS2 can effectively increase the molecular motivity and promote the aggregation of polymer molecules. As shown in Figure 13c, On the basis of increasing molecular migration and diffusion ability, mixed steam treatment can promote the aggregation of fullerene molecules and induce phase separation between polymer side chain and fullerene. Fullerene aggregates react with polymers, reducing the degree of entanglement between polymers. Finally, the self-organizing ability of the polymer is improved, the crystal size is increased, and the interpenetrating network structure is formed.
Selecting the appropriate annealing method is also an effective strategy to optimize the morphology. The combination of TA and SVA can change the molecular crystallization sequence. Liang et al. prepared a highly crystalline network by adjusting the sequence of TA and SVA.[142] Films I and II showed more pronounced absorption near 635 nm compared to the original films, indicating increased crystallinity of PBDB-T. However, only Film I exhibits a noticeable rise in absorption at 700 nm, suggesting that the crystallinity of ITIC may have been improved during the TSA-I process (Figure 13d). The results of crystallization kinetics show that the crystallization of ITIC is significantly enhanced when the crystallization of ITIC occurs before the establishment of the PBDB-T crystal network. This is primarily because the diffusion of ITIC molecules is less constrained under these conditions. However, if PBDB-T crystallize preferentially, the fusion of ITIC is limited by the PBDB-T crystal network, leading to a lower crystallinity of ITIC. The results show that the performance of the device is improved from 8.02% to 10.95%. Xie et al. also used the combined method of SVA and TA to treat P3HT:PCBM films.[143] They treated the film with DCB as solvent vapor. The absorption spectra show that the absorption spectra of P3HT have a redshift, an increase in intensity, and a new peak in long wave strength (Figure 13e). This shows that the length of the P3HT conjugated chains increases and that an ordered structure is formed between the chains. PCBM aggregates appeared in large quantities and formed phase separation structure after annealing at 150 °C.