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).