# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 4 Crystallinity and Crystalline Packing in SM-BHJ
Thereafter, a quantitative analysis of the \(\Delta G_{V}\) contribution was derived from the free energy change of crystallisation (\(\Delta_{\text{crys}}G\)) for derivatives **1** and **2** from the enthalpy of fusion (\(\Delta_{\text{HusH}}\)) of pure crystalline powder samples of **1** and **2** isolated as pure P-1 and P21/c phases, respectively. The experimental values of \(\Delta_{\text{crys}}G\) show that the formation of crystallites of **2** in the P21/c phase is thermodynamically more favourable ((\(\Delta_{\text{crys}}G\)(2-P21/c) \(<\Delta_{\text{crys}}G\)(1-P-1)) than the formation of crystallites of **1** in a P-1 phase by unit of mole. Their volumetric counterparts (\(\Delta G_{V}\)) were calculated from the unit cell volume derived for each crystalline phase from the X-ray diffraction single crystal structure. We then used these values as a relative indicator to draw a hypothetical energy profile of the nucleation process (Figure 6e). The higher \(\Delta G_{V}\) contribution compensates for the higher interfacial energy experienced by nuclei of derivative **2** during the early step of nucleus formation. This results in \(r^{*}_{2}\) being inferior to \(r^{*}_{1}\), and in the formation of nuclei of **1** being kinetically favoured with respect to **2** (\(\Delta G^{*}_{1}<\Delta G^{*}_{2}\)). This model showed good agreement with the experimental crystallisation habits of each derivative. Although limited to certain types of SM systems, the above analysis shone some light on the molecular packing of SMs and the energetics of crystallisation, when SMs (in this case, donors) adopt a well-ordered single-crystal-like packing. The advent of the DAD-type donors, particularly oligothiophene-, dithioside (DTS)-, benzodithiophene (BDT)-based SM donors, marked the start of a new era in SM-BHJ in a way that the crystallinity, and more importantly the packing of molecules, shifted to a liquid-crystalline-like mode. The transition between single-crystalline-like and liquid crystalline-like behaviour is well exemplified by the oligothiophenes of the DRCNT library first described by [PERSON] and co-workers [66; 67; 68; 69; 70]. They consisted of a library of oligothiophenes comprised of 5 to 9 iophenes, which pioneered the manufacture of SM-BHJ devices approaching 10% PCE. The characterisation of their typical active layers by GIWAXS  
Figure 6: **(a,b)** X-ray crystal structures of molecules **1** and **2**, showing the unit cells; (**c,d**) Stacked diffractograms of active layers, “powdered” active layer diffractogram and superimposed refined diffractogram from the X-ray single-crystal structure, and calculated diffractogram (at 100 K) from the X-ray single-crystal structure of 1 and 2 shown in **(a,b)**; (**e**) hypothetical energetic profile of donors’ nucleation in active layers of derivatives **1** and **2**. Adapted with permission from Wiley VCH, copyright 2017 [64]. showed what appeared to be a transition between the two packing modes. The 5- and 7-thiophene analogues seem to adopt a well-defined crystalline packing of the single-crystal type or complex liquid-crystalline type, respectively, while the 6 and 9 analogues seem to form simplified \(\pi\)\(-\)\(\pi\) stacks of molecules in the out-of-plane direction very reminiscent of crystalline polymers. Figure 7 shows the GIWAXS area images of DRCN5T and DRCN6T, with diffraction peaks located mainly in the out-of-plane direction for DRCN6T, while a multitude of off-meridional well-defined diffraction spots at very low \(q\) value can be seen in the data corresponding to DRCN5T. Although the authors attributed the corresponding \(d\)-spacing to a lamellar stacking distance (18-19 A), these peaks likely correspond to a single-crystal-like packing. Interestingly, the derivatives leading to higher PCEs were those of the highly ordered nature (5- and 7-thiophene units), although the PCEs were in a rather similar range.