4.3 Adjusting the processing temperature
The effect of temperature on the morphology is mainly achieved by
controlling the interaction between polymer and small molecule and the
molecular diffusion rate in the active layer. The ordered aggregation or
deaggregation of molecules in the solution can be promoted by adjusting
the temperature of the solution. [109-111] In general, appropriately
raising the temperature helps to enhance interactions, increase the rate
of molecular diffusion, and facilitate processes such as phase
separation and self-assembly. All factors lead to better morphology.
The temperature has effect on the
molecular solubility. Liang et al. control the nucleation rate and grain
growth rate by adjusting the content of ordered aggregates in the
PffBT4T-2OD:N2200 blending system.[112] At low temperature (25-65
°C), PffBT4T-2OD reaches saturated concentration, the nucleation barrier
is low, and the solution contains many crystal nucleuses of PffBT4T-2OD.
However, the high viscosity of the solution obstructs the molecular
chain movement, resulting in low crystal growth rate, and the rate of
nucleation does not match the rate of growth. The slope of the curve
Ka=0.42 was calculated by the UV-vis absorption spectrum
in situ, which can reflect the crystallization rate. When the
temperature rises (65-100 °C), the molecular chain starts to move,
resulting in an increase in the rate of crystallization
(Ka = 1.86), and the nucleation and growth rate become
synchronized. With a subsequent rise in temperature (100-120 °C), the
elevated temperature of the solution leads to an escalation in the
nucleation barrier, resulting in a rapid crystallization rate
(Ka = 19.21) that does not align with the rate of
crystal nucleation and growth. After the creation of a solution at 95 °C
(Figure 7a-c), the film exhibited the highest level of crystallinity.
In addition, the temperature also
affects the aggregation state of the solution. Zhou et al. studied the
solution aggregation behavior at different temperatures.[113] As
shown in the Figure 7d, the areas
between dashed lines indicates the nano network structure can be formed
at the current ratio of donor/acceptor, and the squares and circles
indicate the domain size. As the temperature rises, the domain size
decreases significantly. The insensitivity ofp -DTS(FBTTh2)2 to temperature
causes a significant portion of P(NDI2OD-T2) aggregates to transform
into amorphous molecules as temperature increases. Before substantial
aggregation of p -DTS(FBTTh2)2occurring, The majority ofp -DTS(FBTTh2)2 molecules are
enclosed by the P(NDI2OD-T2) framework. Excitons are more likely to
diffuse to the donor/acceptor interfaces. When the temperature is 80 °C,
PL intensity of the film after solidification decreased by a quarter,
indicating significant enhancement of exciton dissociation (Figure 7e).