Chapter 5. Study of Material–Solvent Interaction Leading to the Evolution of
5.2 Results and Discussion
5.2.5 Film Morphology and Microstructure
The influence of the processing solvents on the morphology and microstructure of the blend films was further studied by a combination of techniques, including fluorescence microscopy, high-resolution
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transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDAX), and grazing incidence wide-angle X-ray scattering (GIWAXS).
Figure 5.4. HR-TEM images. (a) In TOL. (b) In OXY. (c) In MES. (d) A zoomed view of the cubic crystal observed in MES, where the background is from the center of the crystal and the highlighted area is from the interface. Inset images (bottom right) in each figure are the corresponding diffractograms.
As shown in the Figure 5.17 of section 5.5, in the full excitation scan, the fluorescence microscopy images for the blend films processed from both TOL and OXY showed a featureless finely structured intermix, whereas the film processed from MES exhibited large dark bulk clusters of a cubic crystal structure, as clearly visualized by scanning electron microscopy (SEM) (see the section 5.5, Figure 5.18). To further analyze such crystal structures, the red, green, and blue channels were also recorded using appropriate excitation filters (~639 nm, ~555 nm, and ~488 nm, respectively), which produced similar phenomena as that observed in full light. Figure 5.4 shows the HR-TEM images of films cast with different solvents. All films presented a fibril morphology and exhibited phase separation on the nanometer scale, which was also
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observed in both the topography and phase AFM images (see the section 5.5, Figure 5.19). Note that the cubic crystals were also pre-sent in the TEM image of the MES-processed film, which is consistent with the SEM image above.
Figure 5.5. Elemental mapping by EDAX analysis along with STEM images of ternary blends. (a) In TOL.
(b) In OXY. (c) In MES. Nitrogen and fluorine represent DR3TSBDT and PTB7-Th, respectively, and sulfur indicates the absence of PC71BM.
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The morphological differences induced by processing solvents were further supported by EDAX elemental mapping, where red, green, blue, and pink indicate nitrogen, fluoride, sulfur, and carbon signals, respectively, as shown in Figure 5.5. The elemental analysis of both TOL- and OXY-processed films clearly revealed well-dispersed dots with a homogenous distribution throughout the whole area, whereas the MES- processed film showed highly pink cubic crystals. This implies that the crystal is mainly composed of PC71BM crystallites in which some PTB7-Th chains are embedded into the PC71BM domains, as evidenced by the green and blue dots within the crystal.
In addition, the surface energy of PTB7-Th is 23.76 mJ m-2, which is far closer to that of PC71BM (23.75 mJ m-2) than that of DR3TSBDT (28.38 mJ m-2) (see the section 5.5, Figure 5.20). This fact explains how PTB7-Th molecules can be easily located in the PC71BM domains, well supporting the above observation by EDAX. This was further evidenced by optical microscopy images of the pure and binary films of PC71BM with either PTB7-Th or DR3TSBDT (see the section 5.5, Figure 5.21), where such a crystal is only observed in PTB7-Th:PC71BM. We also investigated the effect of the NMP additive on the formation of the crystals in MES-processed films. For all the cases, the similar crystals were observed, implying that they are independent of the presence of NMP in the MES films (see the section 5.5, Figure 5.22). Note also that in order to obtain a complete picture of the bimolecular crystal structure in the solid state via single crystal X-ray diffraction study, unfortunately, it is difficult to separate the pure cubic-like crystal in powder state. This is due to the presence of the amorphous PTB7-Th and PC71BM fractions in the solution (see the section 5.5, Figure 5.23).
For both TOL- and OXY-based processing systems, the GIWAXS patterns exhibited a similar bimodal texture of mixed edge-on and face-on orientations, with (100) lamellar and (010) π–π stacking peaks in both out-of-plane and in-plane directions (see Figure 5.6a and the section 5.5, Table 5.7 for the corresponding lattice parameters), which is quite similar to what was previously observed in the CB/DIO-based processing system.1d However, the GIWAXS patterns of the MES-processed films were significantly different due to the presence of the cubic crystals; as shown in Figure 5.6b, for the given MES-processed film sample, in addition to the broad (100) and (010) peaks, many scattering spots with irregular spacing were found in the GIWAXS image, and several distinct narrow peaks in the line-cut profiles were found in both out-of-plane and in-plane directions. Note that we clearly observed a strong and sharp peak at ~1.4 Å-1 as-signed to PC71BM in the in-plane direction, reflecting the favored perpendicular orientation relative to the substrate.
Additionally, we calculated the crystallite correlation length (CCL) of PC71BM lattice planes along the in- plane axis by using Debye Scherrer’s equation.16 Both TOL- and OXY-processed films had nearly similar small CCL values in the range 7 ̶ 8 nm, whereas a very large CCL of 230 nm was obtained in the MES- processed film (see the section 5.5, Figure 5.24). To further clarify the crystals, we also measured GIWAXS
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for the substrate side of the MES-processed film by using a float-off technique. The detailed process is given in the Methods and graphical illustration (see the section 5.5, Figure 5.25).
Figure 5.6. (a) GIWAXS pattern for ternary systems. (b) Out-of-plane and in-plane line cut profiles obtained from GIWAXS data processed in TOL, OXY, and MES. Insets in the line cuts are exemplary curve fittings in the corresponding higher q region. In MES, the surface and substrate sides of the films are exposed to beamline for in-depth analysis of the formed cubic structure throughout the volume. Different color bars represent intensity variance.
Compared to the surface-side measurement, the peak patterns of the substrate side showed some similarity, but the intensities of the narrow peaks assigned to the crystals were very different. Although as-signing the lattice structure of the crystals is difficult at this point in time, based on all the above results, one can conclude that PTB7-Th:PC71BM intercalating bimolecular crystals with an anisotropic lattice are grown on the MES-processed films.17 We also envisage that the low performance is likely due to the intermolecular deconstruction between the bimolecular crystals (see Supporting Figure S11 and S15), leading to the inefficient charge transfer, and ultimately low FF and JSC values. However, the size and thickness of the bimolecular crystals can be controllable by the kinetics of crystal growth process. This maybe create a new possibility of increased polymer-chain-mobility within thick nanocomposite thin-films, due to a combination of the increased connectivity and anisotropic lattice structure. Therefore, such features can
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make a good candidate for applications in p-n crystal junction-based optoelectronics and ambipolar organic transistor.