# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 4 Crystallinity and Crystalline Packing in SM-BHJ
Despite their different crystallisation modes, the hole mobility measured from the corresponding active layers was very similar for all donors (in the range of 5 \(\times\) 10\({}^{-4}\) cm\({}^{2}\)/V\(\cdot\)s), further demonstrating that an extremely ordered crystalline packing is most probably not a determinant in obtaining devices with enhanced \(J\)-\(V\) characteristics. A contemporary class of SM donors to the above oligothiophene, based on the dithienosilde (DTS) core [71; 72; 73; 74; 75; 76; 77; 78; 79] first described by [PERSON] and co-workers [76], confirms the shift in crystallisation trend mentioned above. In the case of the most popular \(p\)-DTS (PTTh\({}_{2}\))\({}_{2}\), crystallisation kinetics were seen to be unusually fast, as active layers would display some degree of crystallinity directly after deposition (and therefore, some PV performance) [73]. Active layers were shown to acquire the optimum crystallinity upon processing with a small fraction of diiodooctane (DIO) additive in the processing solvent, with no further annealing required. In this particular case, the packing pattern is exclusively of the liquid-crystal type, an effect that may be enhanced by the fact that the DTS core contains alkyl chains in both the longitudinal and lateral parts of the conjugated core. This likely forces the packing towards a more disordered type, hence being favoured even under fast film drying conditions (Figure 8). The enhancement of PV properties was shown to have a clear dependency on the amount of DIO additive, and the best results were obtained when a small fraction of polystyrene was also added [80]. The additive was not shown to have an impact on the packing itself, which remains of the liquid-crystalline  
Figure 7: (**a**,**b**) Chemical structures of DRCN5T and DRCN6T, (**c**,**e**) GLXD patterns of pure films of DRCN5T and DRCN6T, respectively; (**d**,**f**), GLXD patterns of blend films of DRCN5T:PC\({}_{71}\)BM and DRCN6T:PC\({}_{71}\)BM, respectively. Adapted with permission from the American Chemical Society, copyright 2019 [67]. type, with \(\pi\)-stacked units along the out-of-plane direction, and lamellar in the in-plane direction. However, the DIO was key in adjusting the size of the crystalline domains to an optimum value that would maximise photoinduced processes. The effect of DIO has been rationalised by carrying out _in-operando_ GIWAXS studies, which showed that intermediate crystallites with different-plane spacing distances formed in the early moments of the active layer deposition, which rearrange upon the slow evaporation of the high boiling point DIO additive towards the final liquid-crystalline-type packing [77,78]. Donors of the DTS family have been studied extensively, and it is interesting to note that at least one close derivative of the original DTS (PTTh\({}_{2}\))\({}_{2}\) donor shows an intermediate crystalline habit in a very similar manner as the DRCNT donors described above. That is, in this particular example from [PERSON] et al. [79], the DTS (Oct)\({}_{2}\)-(2T-DCV-Me)\({}_{2}\) donor, which differs from DTS (PTTh\({}_{2}\))\({}_{2}\), amongst other parameters, by the length of the alkyl side chains, show off-meritional symmetrical diffraction peaks at \(q\) 0.47\({}^{-1}\) (Figure 8e,f). The authors suggest that this is evidence of high crystallinity and \"good molecular packing\" and attempted the resolution of the crystalline packing using molecular modelling tools. Here again, the experimental results demonstrate how the presence of shorter alkyl chains tips the balance towards the formation of single-crystalline-like packing rather than liquid-crystalline-like. The ADA (or ApDpA) approach, together with the addition of an extensive amount of alkyl chains to the conjugated structure of the SM, has undoubtedly had a major impact on devices' characteristics.