## 4 Holographic Materials for Solar Concentrators
### Photopolymers  
Photopolymers are the most studied holographic materials since the 1970s; they are based on polymerization and cross-linking reactions induced by absorption of light and they offer several advantages respect to silver halide and DCG. Indeed, photopolymerizable materials show high diffraction efficiencies, allow real-time monitoring of the recording process, do not require development processes but only a bleaching process, can be produced from raw materials at low cost and give the possibility to modulate the properties through chemical synthesis [4]. Additional, photopolymers can be mass produced allowing to reducing production costs [30; 31]. Typically, a photopolymer is made from a photoinitiator system, one or more polyfunctional monomers or oligomers and a polymeric binder. When it is exposed to light, the polymerization occurs in the areas of constructive interference (high-light intensity areas) leading to increased consumption of the monomers, while the polymerization is limited or absent in the areas of destructive interference (low-light intensity areas). The difference of monomer consumption rate leads to a concentration gradient that drives monomer diffusion from dark to illuminated areas [32; 33; 34]. Thus, the polymer concentration distribution will take over the sinusoidal pattern of the light intensity, resulting in a permanent modulation of the refractive index, that is, a volume-phase hologram. Some examples of the most commonly used photopolymers as recording material for holography are: (i) a photopolymer developed within Bayer MaterialScience based upon an orthogonal two chemistry formulation; it is capable of achieving transmission above 90% in the film samples [35]; (ii) photopolymer materials developed by DuPont for recording volume phase holograms, in particular new materials with panchromatic sensitivity, designed for multicolour holographic recording have been developed [36]; (iii) acrylamide-based photopolymers have also been extensively used for fabrication of HOEs in solar energy applications [37]. Regarding the resistance of these materials for holographic concentrators in environmental conditions, there are durability tests on some of the most widely used photopolymers in conventional Fresnel-type solar concentrators, such as acrylic polymers [38]. The fabrication of lenses for concentrators and protective layers for photovoltaic cells is often made by using polymethyl methacrylate (PMMA) [39] since it allows a good resistance to UV radiation and high transmittance (>92%). The UV sensitivity can be further reduced by introducing protective layers or by adding radical scavengers or antioxidants to the formulation of the material. Also polydimethylsiloxane (PDMS) shows very high stability against UV radiation and making it suitable for use in the space environment [40]. Additional, PDMS features greater optical transmittance compared to PMMA. Considering all the advantages offered by photopolymeric materials, to obtain solar compliant holographic materials that should be less sensitive to thermal and photochemical degradation phenomena, the study of new photopolymers based on inorganics or hybrid organic/inorganic components instead of organic material is still in progress. Some examples in this sense are photopolymers containing nanoparticles of inorganic species such as SiO\({}_{2}\), ZrO\({}_{2}\) and TiO\({}_{2}\)[41], that show a lower shrinkage due to the polymerization and a higher refractive index modulation [42; 43] and photopolymer containing zeolite nanocrystals as inorganic dopant, that allows to improve compatibility between inorganic particles and polymer and reduce the optical losses due to scattering [44]. Finally, the sol-gel chemistry versatility allows obtaining a high level of interpenetration between the organic and inorganic networks, as reported in Refs. [4; 45; 46; 47]. Photopolymer with a very low outgassing level and a high resistance to strong thermal excursions, that can be useful for some applications, was obtained by sol-gel techniques [4].