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.