Figure 11. (a) This diagram illustrates the changes in the
structure of DRTT-T:N3 blend films when subjected to one-step and
two-step TA. Reproduced from [134]. (b) AFM and TEM pictures with
pristine, TA, and DTA. Reproduced from [135].
In the TA process, the control of
parameters is also very important. Levitsky et al. studied the effect of
different annealing temperatures on the film-forming process.[136]
PCE11:PCBM blend films were annealed at different times at 80℃, 110℃,
125℃ and 140℃. Subsequently, these films were regularly examined using
optical microscopy (Figure 12a). Higher temperatures facilitate the
dispersion of PCBM molecules, leading to accelerated aggregate growth,
ultimately leading to decreased density and increased aggregate size.
The temperature of 150 ℃ is in closer proximity to the metastable
monotectic temperature, consequently resulting in a reduced driving
force for PCBM nucleation in comparison to 130 ℃. Due to the inverse
relationship between the nucleation barrier and the square of the
driving force, the barrier at 150 ℃ is significantly greater compared to
130 ℃. As a result, the rate of nucleation at a temperature of 150 ℃ is
significantly less than that at 130 ℃. The findings indicate that
following annealing at a temperature of 150℃, the film exhibits a
reduced presence of sizable PCBM particles, with the presence of
unaltered spinodal microstructure interspersed throughout. As a result,
after annealing at 150 °C, substantial crystalline PCBM aggregates have
already formed, while a bicontinuous spinodal-like phase separation
persists due to the slow nucleation rate of crystalline PCBM. The rate
of molecular diffusion can be influenced by temperature. Liang et al.
changed the TA temperature to investigate the effect of the molecular
diffusion rate onp -DTS(FBTTh2)2:EP-PDI blend
films.[112] As shown in Figure 12b, when the annealing temperature
is lower than 90 °C, EP-PDI andp -DTS(FBTTh2)2 are almost
uniformly mixed, and there is no obvious phase separation structure.
When the temperature is between 90 °C and 130 °C, the film forms a
nanoscale interpenetrating network structure, which not only facilitates
exciton separation, but also ensures efficient carrier transport. When
the annealing temperature was higher than 130 °C, the crystallinity of
EP-PDI increased significantly, which dominated the film morphology and
formed large phase separation. Figure 12c shows the fluorescence
spectra. The molecular diffusion rate of the blend system can be divided
into the following three stages, as shown in Figure 12d, When the
temperature is lower than 90 °C (blue area), the molecular diffusion
rate of both films is relatively slow, and the films have no obvious
phase separation structure. When the temperature is between 90 °C and
130 °C (purple region), the diffusion rate of EP-PDI molecules is faster
than that of p -DTS(FBTTh2)2molecules, and p -DTS(FBTTh2)2crystals form a network skeleton structure, which restricts the
diffusion of EP-PDI molecules. During the cooling process, EP-PDI formed
microcrystals and filled in thep -DTS(FBTTh2)2 crystal frame,
forming nanoscale interpenetrating network structure. When the
temperature exceeds 130 °C (yellow region), the molecular diffusion rate
of both films is faster, and EP-PDI breaks through the restriction, and
the film phase separation size further increases. Therefore, reasonable
control of molecular diffusion rate is an effective way to construct the
interpenetrating network structure of active layer.