2.3 The in situ GIWAXS and GISAXS
GIWAXS, is based on scattering X-rays to study the molecular arrangement and crystal structure of a sample. GIWAXS detector is close to the sample, the scattering angle range detected is large, and the corresponding q value is large. According to d =2π/q , it can be seen that the detected crystal plane spacing is small. Therefore, GIWAXS represents the crystallization and ordered arrangement of molecular scale.[76-78] For example, the aggregation state of organic semiconductor molecules and the evolution of crystal structure in solution can be monitored in real time by in situ GIWAXS. By monitoring the change of diffraction pattern, the growth rate of crystal, molecular orientation and the optimization of crystal structure can be revealed. Perez used in situ GIWAXS to study the effect of DIO on thep -DTS(FBTTh2)2:PCBM films.[79] If no DIO is added,p -DTS(FBTTh2)2 crystallinity is low. When DIO is used,p -DTS(FBTTh2)2 first forms an intermediate phase (liquid crystal phase) in the film, and then the liquid crystal phase gradually transforms into a crystalline phase in the middle and late stages of film-forming, and the crystallity of the film is improved. The scattering data for the sample treated with the additive reveals multiple distinct and intense peaks. The peaks’ sharpness, combined with the emergence of higher-order reflections at q ≈ 5.6 and 8.4 nm−1, show that the correlation length and quality of the crystals in the film treated with the additive are significantly improved. GISAXS can characterize the long-period structure, quasi-periodic structure and interfacial layer structure on a larger scale. GISAXS detector is far away from the sample, has a small range of detected scattering angles, a small corresponding q value, and a large distance between detected crystal faces. Therefore, GISAXS represents the ordered arrangement of phases and aggregates. Figure 1c shows the schematic diagram of GIWAXS and GISAXS. Russell et al. used GIWAXS and GISAXS to study the role of DCB in DPPBT:PC71BM film.[80] The change of intensity of peak (100) in GIWAXS of the DPPBT can explain the change of crystallinity. In addition, the change of scattering peak location in GISAXS can explain the degree of phase separation. The results show that when DCB content is 5%, the (100) peak intensity of GIWAXS of the DPPBT changes synchronously with the scattering peak position of GISAXS. When the DCB content is high (20% and 50%), the (100) peak intensity of GIWAXS increases earlier than the shift of GISAXS scattering peak location, indicating the crystallization signal of GIWAXS changes before the phase separation signal of GISAXS. This indicates that DCB promotes the crystallization of DPPBT.