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.