# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
## 3 X-ray Diffraction Characterisation Techniques  
The characterisation of thin films of active layers of solar cell devices has adapted to the materials composing the active layers of early solar cell devices, namely polymers. In the case of crystallinity, the choice of X-ray diffraction is evident, however the measuring setup must be adapted to the features of the active layers. Therefore, one of the most common X-ray diffraction systems used on the lab scale is the so-called Bragg-Brentano geometry, where the incident angle \(\omega\) is defined between the X-ray source and the sample. The diffracted angle, 20, is defined between the incident beam and the detector angle (Figure 1a). The incident angle \(\omega\) is always \(\frac{1}{2}\) of the detector angle 20. Although this setup can be used to characterise thin films, it is usually limited to films of very crystalline materials (e.g., inorganic materials such as perovskites, for example), the Bragg-Brentano geometry is generally most commonly used for crystalline powders [31,32]. The limited crystallinity of polymers--i.e., stacked polymer units over short ranges--together with the fact that OSCs' active layers are in the range of 50 to 200 nm in thickness at most, requires a deeper penetration of the X-ray beam for increased signal-to-noise ratios, since the intensity of the diffraction beam is proportional to the overall crystalline volume. In order to reach such irradiation depth, a so-called grazing incidence X-ray diffraction (GIXRD), or grazing incidence wide-angle scattering (GIWAXS) setup, has emerged as the benchmark geometry for analysing thin films of OSCs' active layers, where the incident X-ray beam is fixed to a very fixed low-angle (0.5 to 5\({}^{\circ}\)) with respect to the substrate normal, thus guaranteeing a maximum penetration of the X-rays through the layer [5,10,33]. The technique can be implemented in laboratory diffractometers; however, the high photon fluxes and high-brilliance X-ray beams offered by synchrotron light sources are essential, in order to produce well-defined diffraction patterns. It is worth mentioning that the outcome of a GIWAXS analysis is a 2D image allowing for the analysis of the orientation of the diffracting material with respect to the surface (so-called texture), while the Bragg-Brentano geometry provides a 1D diffractogram in which the information on the texture is missing. The resulting area images of GIWAXS patterns show typical diffraction planes in the out-of-plane direction (parallel to the substrate normal) and in-plane direction (perpendicular to the substrate normal). The diffraction peaks' position, shape and width can be directly related to the interplane distance (from Bragg's diffraction law), the orientation of the crystalline domains, and their size. A depiction summarising the attributes of the technique and typical scattering patterns expected for polymer-based thin films is shown in Figure 1b. The analysis of GIWAXS should first proceed with the assignment of peaks to specific reflections within the crystal lattice, if the latter is known. This is, however, usually only possible for single-crystal-like packing types, i.e., small molecules, therefore, the analysis generally consists in assigning the diffraction peaks to specific molecular or intermolecular d-spacings. The most common distances for polymers used in OSCs correspond to \(\pi-\pi\) stacking spacing at \(d\approx 3.5\) A. Additional peaks corresponding to a \(d\)-spacing in the range of 15\(-\)25 A are often observed, corresponding to a longitudinal ordering triggered by the intermolecular interactions of alkyl chains. Once diffraction peaks are assigned, the shape of the diffraction peaks allows one to determine the orientations of diffraction planes' directions with respect to the film surface. For example, diffraction peaks near the \(q_{z}\) axis will be caused by planes perpendicular to the film normal, whereas those near the \(q_{xy}\) axis will be caused by planes within the film plane.