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.