## 5 V-HOE Based Solar Concentrators
### Multiplexed Hologram  
To improve the angular selectivity, multiplexed solutions have been considered. [PERSON] et al. [7] reported a detailed study on superimposed transmission holograms recorded on the same holographic medium based on DCG. In particular, the authors compared a system consisting of a single holographic element as a concentrator with the maximum of reconstruction wavelength (620 nm) centred in the middle of the range of the PV cell (i.e., 500-800 nm) and a system composed of two holographic recordings with the two maximum reconstruction wavelengths (514.5 and 620 nm) designed both to overlap the operating spectrum of the PV cell (GaAs with an efficiency of 23%) and to avoid the coupling effect. Both the reflecting and transmitting version for the double hologram system were characterized, with one of the two holograms superimposed on the same holographic medium in the transmission configuration. Theoretical and experimental evaluation of the energy efficiency of the holographic systems were carried out. Values of 6 and 5% for the single holographic elements and of 11 and 9% for the double holographic element were obtained, respectively.  
A newsworthy structure with a double superimposed slanted reflecting hologram tilted of 30\({}^{\circ}\) and recorded on DCG by using 488 and 632 nm laser sources, was proposed in 2010 [52]. This  
Figure 2: Holographic solar concentrator with spectral splitting systems [50; 5].  
geometry gives rise to a total internal reflection at the surfaces of the holographic medium. The PV cells were placed at the edges of the holographic plate where the collected light emerges. A sketch of the experimental set-up used for the doubly slanted layer structures is reported in Figure 3a, while the diffraction pattern of normally incident white light for the doubly slanted hologram is showed in Figure 3b,c.  
[PERSON]'s group [53] realized an angular multiplexed HOE recorded on a photopolymer. They optimized a suitable recording method for an angular multiplexed HOE solar concentrator and find that by using the iterative recording method, the low efficiency of multiplexed holograms due to overexposure is compensated. Moreover, the authors evaluated the performance of the HOE as a solar concentrator by introducing a new efficiency calculation method, named concentrated diffraction efficiency (CDE), that is considered as the percentage ratio between the effective concentration rate (ECR) of the HOE and the ECR of the convex lens used in the recording process. The fabricated HOE that uses the modified iterative recording method shows CDEs of 26.73%, 35.31% and 22.78% from incidence angles \(-10^{\circ}\), \(0^{\circ}\) and \(+10^{\circ}\), respectively. These values are considerably higher than those obtained considering multiplexed holograms recorded with equal exposure time without considering the saturation time.  
[PERSON] et al. recorded a focusing HOE as well as multiplexed gratings on an acrylamide-based photopolymer. High diffraction efficiency HOE consisting of a single spherical lens using thin layers and lower spatial frequency was recorded and a larger acceptance angle was obtained respect to the optical component. Regarding the multiplexed geometry, the authors calibrated the intensity of the beam and then the exposure energy to obtain high efficiency. A diffraction efficiency of three multiplexed gratings at 51.9 \(\pm\) 3.5% was demonstrated [54].