5.1 Thermal annealing
TA plays a crucial role in optimizing the morphology, and it has been demonstrated to have a positive impact on charge transport, structure development, and device performance.[127] The film is heated and evaporated for several minutes, allowing it to reshape and self-aggregate in the process.[128-130] This can enhance the crystallinity of molecules, and enhance phase purity, which leads to improved molecular orderliness and phase separation in the material.[131, 132] Since temperature can promote molecular movement, an appropriate increase in temperature during the annealing process can promote molecular movement, improve crystallization and phase separation, but too high a temperature will lead to too much phase separation.[112] In addition, the annealing time is also very important, a shorter annealing time will lead to insufficient movement of the molecules, the degree of phase separation is small, and a longer annealing time will lead to a larger phase separation.[133]
Different TA methods lead to different film-forming kinetics. Two-step TA is beneficial to improve crystallinity and phase separation. Cheng et al. used a two-step TA method to achieve DRTT-T:N3 blend films with highly ordered molecular stacking.[134] The materials are annealed at T c,onset of DRTT-T to induce nucleation and crystallization of the donor material. Subsequently, they undergo annealing at 130 °C for a very brief duration. This second annealing step promotes the formation of more ordered packing for both the acceptor and donor molecules simultaneously but does not lead to coarsening due to the donor nucleus being limited by diffusion. The donor nucleate in specific areas when annealing atT c,onset, as depicted in Figure 11a. Simultaneously, the acceptor aggregate around these crystal nuclei. Following this, the process of heating above theT c,onset promotes the acceptor and donor molecules to adopt a more organized arrangement and attain a suitable phase segregation. Hence, the TA method with two steps can enhance device efficiency, enhance molecular arrangement, and achieve suitable phase segregation. The PCE of devices with a two-step TA process achieves 13%, surpassing the efficiency of devices annealed with a one-step process. The upside-down thermal annealing (DTA) is also an effective method to adjust the morphology. Zhang et al. propose a novel upside-down thermal annealing (DTA) method for conditioning PffBT4T-2OD:PC71BM thick films.[135] After undergoing the DTA treatment, there was a noteworthy 15% enhancement in PCE, when compared to the traditional method of thermal annealing. The enhanced performance can be credited to the better organized π-π stacking of the PffffBT4T-2OD structure and the rearrangement of PC71BM compounds. The AFM findings indicate that applying DTA treatment can restrict the excessive clustering of PffffBT4T-2OD on the surface, leading to an enhanced interface between the donor and acceptor (D/A) that promotes charge transportation (Figure 11b). It can also be seen from the SEM image that PffBT4T-2OD:PC71BM blend film has more obvious fiber structure. This is because PffBT4T-2OD molecular packing increase after the DTA process, resulting in PC71BM molecular redistribution. This phenomenon plays a role in establishing continuous pathways for carrier transport within the mixed region, leading to the enhancement of phase separation in thick films. The likelihood of excitons reaching the D/A interface is enhanced by these findings, which aids in charge transport and electron collection, ultimately resulting in higherJ sc and FF.