2.6. Morphology properties
The surface and bulk morphologies of blend films based onOC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8 are investigated using the atomic force microscope (AFM) and transmission electron microscopy (TEM) (Figure 6 and Figure S2). The root-mean-square (RMS) roughness values of the D18:OC4-4Cl-Ph , D18:OC4-4Cl-Thand D18:OC4-4Cl-C8 blend films are 1.67, 1.23 and 0.98 nm, respectively. Similarly, for TEM images, the OC4-4Cl-C8 based film can form nanoscale phase separation, which is favorable for charge transport. The crystallinity and molecular packing of neat and blend films are analyzed by GIWAXS measurements. As depicted in Figure 7, many strong diffraction rings can be observed in the 2D graphs ofOC4-4Cl-Ph , OC4-4Cl-Th and OC4-4Cl-C8 neat films, indicating their higher crystallinity. For the blend films, D18:OC4-4Cl-Ph , D18:OC4-4Cl-Th and D18:OC4-4Cl-C8blend films all exhibit distinct (010) diffraction peaks in the out-of-plane direction, with π–π stacking distances of 3.78 Å (1.66 Å−1), 3.74 Å (1.68 Å−1) and 3.67 Å (1.71 Å−1), respectively. Furthermore, the crystal coherence lengths (CCLs) of (010) diffraction peaks are estimated to be 29.11, 34.50, and 37.29 nm for D18:OC4-4Cl-Ph , D18:OC4-4Cl-C8 , and D18:OC4-4Cl-Th blend films, respectively, using the equation: CCL=2π/FWHM. The D18:OC4-4Cl-C8 blend film have smaller stacking distance and higher CCL value suggest the development of closer molecular packing and higher crystallinity, which can promote charge transport and endow better photovoltaic performance.