X-ray methods are very powerful tools and are nearly always used when investigation of materials is concerned (either in thin films or bulk shape) for any photovoltaic applications. This analysis is nondestructive and provides quantitative information on the crystalline phases. With a suitable choice of X-ray source and optical geometry, X-ray diffraction (XRD) analysis can be applied to a very wide range of scientific material problems, such as crystallinity, phase determination, crystallite size, surface and interface roughness, texturation (orientations), etc. Many interesting books derive X-ray physics with fundamental and applied aspects for a wide community of material scientists, chemists, biologists or physicists (see for instance [Cullity and Stock, 2011]; [Als-Nielsen and McMorrow, 2001]; [Willmott, 2011]; [Guo, 2011]; [Pavlinsky, 2008]; [Warren, 1990]). In the following, essential features are reported concerning the main characteristics of X-ray techniques as well as a few related examples. Generally speaking, it is important to keep in mind that only elastic scattering from electrons is generally considered. The amplitude of the signal is very dependent upon Z, the atomic number. The order of magnitude of X-ray energy used for scattering experiments strongly depends on the material under investigation and should in principle be lower than the material absorption energy to avoid fluorescence. In a laboratory scale Cu-Ka radiation is often used (energy of 8 keV). For such an energy, the penetration depth drastically depends upon the incident angle and the material (Z): it can vary from zero to several tens of micrometres. Moreover, X-ray diffraction averages data over an area (up to few cm2) much larger than the nano-objects under study.
Crystalline properties are often to be considered in solar cell materials. For instance, a lot of efforts concerning thin-fllm polysilicon technologies are directed towards producing a material with higher crystallographic quality than the current ones [Beaucarne and Slaoui, 2006]. Indeed it is believed that enhancing the crystallographic quality will increase the efficiency of these solar cells [Beaucarne and Slaoui, 2006].