# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 2 Crystallinity in Active Layers of OSCs  
In the early days of OSCs, active layers of polymer-based solar cells were thought of being primarily amorphous. However, the introduction of thiophene-based conjugated polymers, such as poly(3-hexylthiophene) P3 HT, demonstrated the importance of crystallinity in OSCs [21; 22]. Indeed, the need for an active layer annealing, in order to achieve improved current density-voltage (_I-V_) characteristics, which was initially mostly linked with phase segregation, was soon associated with polymer crystallisation [23; 24; 25; 26; 27; 28]. However, the concept of crystallinity for polymers is somewhat different from the crystallinity of small molecules since, in the former case, crystallinity is understood as the presence of ordered stacks of molecular fragments in an otherwise disordered matrix of polymer chains. These fragments stack over a sufficiently long range to form well-defined planes detected by X-ray diffraction, hence the association with crystallinity [29]. Although these stacks are ordered, the very nature of polymers, i.e., polydisperse mixtures of different chain lengths, limits the order and size of these domains. The condition for such stacking phenomena is the existence of relatively strong intramolecular interactions, generally, \(\pi\)\(-\pi\) stacking or hydrophobic interactions favourable over long distance ranges. Similar crystalline behaviour is observed in the case of liquid crystals, which contrary to polymers, consist of small molecules with discrete and well-defined structures. Their specific design--i.e., the extended conjugated core and long lateral alkyl chains--form ordered stacks of molecules in the mesophases (intermediate phase between crystalline and liquid phase) [30]. These disordered crystalline domains form generally from \(\pi\)\(-\pi\) stacked conjugated cores or alkyl chain intramolecular interactions. Therefore, these strong intermolecular interactions limit the degree of order to a similar extent to that of polymers. That is, despite consisting of small molecules of a discrete structure, their packing in the mesophases is not expected to adopt a perfect arrangement such as that of small molecules in the solid state. Indeed, the concept of crystallinity in the case of the latter molecules is more comprehensive. The well-defined structure of the molecules, together with their higher degrees of freedom resulting from more diverse types of weak intermolecular interactions, defines crystallinity as the perfect arrangement of molecules following Bravais lattices (from one of the 230 existing space groups) [31]. This type of packing habit is, in fact, identical to that of inorganic materials, however, the intramolecular interactions between organic molecules being weaker, the size of the crystals, i.e., the extent of the arrangement, is generally limited to small sizes compared to inorganic crystalline materials, which generally rely on strong ionic interactions. The crystallinity of either polymers or SMs is mainly studied using X-ray diffraction, which is one of the best methods to characterise materials encompassing different degrees of crystallinity and order. Indeed, the diffraction of ordered planes following Bragg's law can give direct information on the interplane distances of specific molecule stacks for polymers and liquid crystals [32]. However, in the case of small molecules, these interplane distances must be assigned to the diffraction planes of an arrangement of the molecule in a specific Bravais lattice. This statement implies that small molecules in a crystalline state consist of a perfect arrangement of packed units in one of the 230 existing space groups in the crystalline state. The space group constituting the crystal is first determined from a collection of diffraction peaks recorded in all directions of space. The molecular packing (so-called crystal structure) in that specific space group is later retrieved from the relative intensity of the diffraction peaks via diffraction data refinement.