# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 4 Crystallinity and Crystalline Packing in SM-BHJ  
The crystalline features in SM-BHJ are, as introduced above, more difficult to rationalise than polymer-based BHJ OSCs. The crystallisation habit of SMs in thin films, particularly in active layers composed of D/A blends, can be divided into two distinct classes: a single-crystal-like packing following one of the possible Bravais lattices, or a more disordered liquid-crystalline-like behaviour. Early SM donors were exclusively mixed with fullerene derivatives, most commonly with [6,6]-phenyl-C61-butyric acid methyl ester (PC\({}_{60}\)BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC\({}_{70}\)BM), and featured a conjugated backbone often reminiscent of polymer monomers, functionalised with alkyl chains to increase solubility [37, 38, 39, 40, 41, 42, 43, 44, 45]. Later SMs relied on acceptor-donor alternating units (A-\(\pi\) bridge-D), which proved very effective in increasing the overall absorption coefficient of the molecule, in addition to shifting the onset of absorption towards the infrared part of the visible spectrum [46, 47, 48, 49, 50, 51, 52, 53, 54, 55]. The presence of relatively short alkyl chains on the donor's structure and the rather amorphous nature of PCBM suggested consideration of the crystallinity of SMs in a single-crystalline-like fashion. As such, early studies attempted, in a rather speculative manner, to link the characteristics of PV devices, with the packing of the SM donor derived from a known X-ray crystal structure of that SM. Although the approach is sensible, it relied on the hypothetical assumption that the SM donor would adopt the identical polymorphic crystalline phase in the active layer, let alone that the active layer would be crystalline [56]. This critical limitation was partially dealt with in later studies. The active layer was characterised via conventional XRD using either Bragg-Brentano or GIWAXS configurations, and the experimental diffractograms were compared to the calculated powder diffractograms of polymorphic single-crystal structures, however, with limited accuracy [57, 58, 59, 60, 61, 62]. Figure 3 shows such an attempted comparison where the scarcity of diffraction peaks in the active layers--presumably due to the very small crystallites' size, the low crystalline volume and the high degree of texture--render any comparison inaccurate. Indeed, in this early study from [PERSON] and co-workers [60], a library of oligothiophenes was reported, from which single-crystal structures were obtained of at least one polymorph for each oligothiophene derivative. The authors recorded a GIWAXS 1D diffractogram of an active layer made from a blend of a dicyanovinyl (DCV)-quaterthiophene substituted derivative (DCV4T-Me, Figure 3a) and fullerene-C60, deposited via thermal evaporation. The diffractogram shows three very broad distinct peaks, which the authors attributed to semi-amorphous C60, and to DCV4T-Me packed in an identical phase as that of the single-crystal structure obtained from DCV4T-Me (Figure 3b). Such assumptions were made from the comparison of the active layer's diffractogram (black line, Figure 3c) with a powder diffractogram of pristine C60 (blue line, Figure 3c), and with the calculated powder diffractogram of DCV4T-Me from the single crystal structure (red line). The authors assigned the highest-intensity diffraction peak of the blend to the highest-intensity calculated diffraction peak at 12\({}^{\circ}\) of DCV4T-Me (red line). The weak Bragg reflection at 25.95\({}^{\circ}\) (\(d\) = 3.43 A) in the experimental diffractogram was assigned to the calculated diffraction peak of DCV4T-Me (26.69\({}^{\circ}\), \(d\) = 3.34 A). Although they assign the shift between the peaks from the experimental diffractogram and the calculated one to an increase in the unit-cell dimensions, it is evident that such comparison entails an additional set of somewhat speculative assumptions, such as a high degree of texture (preferential orientation), and the existence of one stable polymorphic structure for DCV4T-Me.