# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 3 X-ray Diffraction Characterisation Techniques
This information is often used to determine whether the orientation of \(\pi-\pi\) stacking is in-plane (\"edge-on\") or out-of-plane (\"face on\") (Figure 1c). Note that comprehensive details on GIWAXS can be found elsewhere [4; 6; 9; 10; 33; 34]. Consequently, GIWAXS has been critical in determining the impact of crystallinity in polymer-fullerene solar cells on the \(J\)-\(V\) characteristics of solar cell devices. If the GIWAXS diffraction patterns of polymer-based active layers are rather straightforward to rationalise  
Figure 1: (**a**) Schematics of a Bragg–Brentano geometry X-ray diffraction setup. (**b**) Schematics of a GIWAXS geometry diffraction setup. (**c**) Archetypical GIWAXS images showing the correlation between diffraction peaks in the out-of-plane and in-plane directions of space and the orientation of polymer stacks. Randomly oriented crystallites would give rise to Debye diffraction rings (**left**) instead of well-defined spots (**right**). Adapted with permission from the American Chemical Society, copyright 2012 [34]. due to the simplified crystallisation modes of polymers, i.e., peaks corresponding to a very limited number of diffraction planes, the case of small molecules can be significantly more complex depending on their crystallisation habits. As an illustrative example, although the fullerene component (i.e., a small molecule) of polymer-based OSCs generally displays somewhat limited crystallinity in active layers when blended with polymers, in some rare cases the PCBM was shown to form a well-defined, highly ordered packing reminiscent of single-crystalline-like small molecules [35]. In these particular cases, the interplane distances corresponding to the diffraction peaks cannot be related to a direct molecular spacing. Instead, they should be related to a packing pattern from a matching X-ray diffraction single-crystal structure of PCBM (Figure 2a). In a more recent study from [PERSON] et al., a blend of a polymer and a non-fullerene SM acceptor showed a very similar trend in which, depending on the active layer deposition conditions, the CITIQ-4F non-fullerene acceptor (NFA) can form crystallites with an extremely well-arranged pattern. Figure 2b shows the GIWAXS image of films composed of pristine CITIQ-4F, indicative of single-crystal-like packing patterns of a pure or polymorphic form, as suggested by the authors [36]. This crystallisation habit is maintained when CITIQ-4F is blended with the polymer (PTB7-th), however the texture (crystal orientation) decreases, as seen by the appearance of Debye rings instead of well-defined diffraction spots. Interestingly, this behaviour is only prevalent when a slight amount (2%) of additive is added to the processing solvent. Indeed, if pure chlorobenzene is used, CITIQ-4F forms disordered liquid-crystalline-like stacks of individual molecules, as seen by the single diffraction peak in the out-of-plane and in-plane direction in the GIWAXS image corresponding to \(d=3.5\) A and \(d=15.3\) A (Figure 2c). These interplane distances are straightforwardly linked to face-on p-p stacking planes and edge-on lamellar stacking. Likewise, a slight modification of CITIQ-4F chemical structure, namely adding an ether group on one of the alkyl chains, shows again a liquid-crystalline-like behaviour. In this case, the extremely ordered single crystal-like packing is not observed even using additives [36]. These experiments are capital in demonstrating  
Figure 2: **(a,b)** GIWAXS diffraction patterns of P3 HT:PCBM films post-treated with chlorobenzene and toluene vapours, respectively. The dominant Bragg diffraction peaks corresponding to PCBM packed in a single crystal-like fashion are marked as black, yellow and red circles. Adapted with permission from the Royal Society of Chemistry, copyright 2016 [35].