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.