4.4.1 Change the film-forming time
Because the boiling point of some additives is higher than the boiling point of the solvent, the additive often remains after the solvent evaporates, continuing to induce molecular crystallization, and extending the film-forming time. Liu et al. studied how additives with different boiling points affected the film-forming process.[41] They divided the additives into two categories, high boiling point (CF) and low boiling point (DCB, TCB, CN). The crystallization behavior without and with additives is shown in Figure 8a-f. The low boiling point additive (CF) reduces film-forming time, the domain size and crystallinity. The high boiling point additives (DCB, TCB) extended the film-forming time, and the crystallization process of P3HT and O-IDTBR was separated. However, too high a boiling point, such as CN, will lead to longer film-forming time and excessive phase separation. When using cosolvents with similar properties, those with higher boiling points require a smaller optimal content to achieve the desired morphology. Therefore, the reasonable selection of additives is very important. Zhan et al. investigated the crystallization kinetics of PM6: IT-4F film following the introduction of DIO.[117] The addition of DIO significantly prolonged the film-forming time, resulting in crystallization kinetics different from that of the film without DIO (Figure 8g, h). The solubility-driven first-step crystallization has a higher nucleation rate, resulting in an increased number of sites for polymer chains to grow. The trace solvent and DIO additive are key to starting the secondary crystallization process, providing ordered and stable fluidity for the PM6 chain. Therefore, a tightly fiber network with enhanced crystallinity is formed. Simultaneously, IT-4F molecules are being placed into this fiber network. This morphology optimization improves the mobility of holes and electrons, balances the transport, and thus PCE increased from 10.11% to 12.67%. Hernandez et al. investigated the effect of additives on P(T3-TPD):PC71BM mixtures.[118] According to Figure 8i and j, films made without DIO undergo a simple procedure where CF rapidly evaporates, and the film usually dries approximately 3.5 seconds after the blade passes. In contrast, films made with DIO undergo a drying process that consists of two steps, which considerably prolongs the time required for drying. The reflection spectra show two separate changes. The first change indicates the end of fast CF evaporation around 2.5 seconds after the blade passes, while the second change happens when DIO evaporation finishes, which usually takes a much longer time and is often not finished until 5500 seconds. After the evaporation of CF, a film swollen with DIO is left, in which the polymer is insoluble but the fullerene can dissolve. Throughout the following 90 minutes, despite the clear indication of DIO evaporation through changes in the reflection data, there is minimal observable change in the absorbance spectra. The results indicate that DIO has a part in increasing the density of nucleation in the early phases of film-forming, leading to a decrease in the domain size.Simultaneously, the prolonged period in which DIO forms a film encourages uninterrupted expansion of crystals, thereby enhancing the domain size. The reduced domain size led to an expansion of the surface area of the polymer fullerene, leading to a rise in FF from 44% to 61%. Consequently, the performance of OSCs was enhanced.