# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 4 Crystallinity and Crystalline Packing in SM-BHJ
Indeed, as seen in Figure 3c, the scarcity of diffraction peaks in the active layer's diffractogram implies that the crystalline phase assignment is a more suggestive analysis rather than an accurate determination. A unique solution to the limitations expounded above was reported in a comprehensive study by our group [63], in which the crystalline phase adopted by a squaraine (SQ) donor in an active layer was accurately identified using a combination of experimental methods and powder diffraction refining methods. As such, Figure 4a shows the diffractograms (Bragg-Brentano 0-0 configuration, out-of-plane incident beam) of spin-coated thin-film active layers of SQ:PC\({}_{70}\)BM blends. A diffraction peak at a 20 angle of 8.01 can be seen, as well as a faint peak at 20 angle of 16.1 in some cases. Additional GIWAXS images of the same films provided clear evidence of a highly textured film (Figure 4b). Therefore, the two peaks appearing in the diffractograms, as shown in Figure 4a, could be unambiguously attributed to reflections of the same (hkl) family of diffraction planes. More importantly, those peaks appeared to match the expected diffraction pattern of SQ packed into a P21/n phase with a high degree of texture, and thus with the (011) plane parallel to the substrate (Figure 4a). This identification was possible from the calculated diffractogram of SQ obtained from a single crystal structure (P21/n polymorphic phase) with preferential orientation along the <011> direction (Figure 4a, red line). Figure 3: (**a**) Molecular structure of oligothiophene 2; (**b**) X-ray single-crystal structure of derivative DCV4T-Me **2** showing the molecular packing along the ab plane of the unit cell. Derivative **2** exhibits a layer arrangement in which molecules form rows interacting via four weak CH\(\cdots\)**NC** hydrogen bonds between vinyl and methyl hydrogens and the dicyanovinyl groups (red dotted lines); (**c**) GLXRD diffractograms of powder C\({}_{60}\) (blue line), active layer blend of 2 and C\({}_{60}\), and calculated powder diffractogram of 2 from X-ray crystal structure (**b**). Adapted with permission from the Wiley VCH, copyright 2012 [60]. However, similarly to the above example, the scarcity of diffraction peaks induced by the high degree of texture and the low diffraction volume limits the analysis to assumption. To circumvent this limitation, we developed an experimental turnaround consisting of recording a diffractogram of a so-coined \"powdered\" active layers [63]. The \"powdered\" active layer technique consists in depositing thin films of SQ: PC\({}_{70}\)BM onto large area (\(>\)50 \(\text{ cm}^{2}\)) glass substrates using similar conditions to those used for processing active layers in OSC devices, and subsequently submitting them to solvent vapour annealing, SVA, (the annealing method used for OSCs devices fabrication of that type) for an extended period of time, in order to allow for extended crystallite growth. The resulting thin films were scraped off the glass substrate, and the solid was used for X-ray powder diffraction analysis. This somewhat simple method allowed for the acquisition of the missing information from the direct measurement of active layers, and provided an improved signal-to-noise ratio as a result of greater crystalline volume. Consequently, the appearance of several additional diffraction peaks in the diffractogram of the powdered sample confirmed the hypothesis mentioned above, whereby the experimental diffractogram appears to match, both in 20 angle value and in relative intensity, the diffractogram calculated from the P21/n structure (Figure 5). The calculated diffractogram was further fitted to the experimental one using Rietveld refinement methods, in order to provide further quantitative evidence. Importantly, this study further demonstrated that the SQ could pack in a different space group (Pbcn) (identified using identical methodology) or a combination of the two, depending on the deposition conditions.