**4. VHOE-based solar concentrators**

Since 1980, several scientific papers have been published to demonstrate that HOEs can provide improvements in solar energy collection over large incident angles and the entire solar spectrum range [2–4, 8, 10, 28, 60]. In particular, Ludman [3] described two configurations with multi-hologram lenses useful to concentrate the sunlight onto an absorber for different sun positions in the day. On the other hand, V-HOEs can be employed to obtain both a concentration and a spatial separation of the solar spectrum. This approach allowed employing different solar cell materials with optimized band gaps achieving high PV efficiency [2, 61].

Ludman et al. [5, 8] demonstrated that a well-designed holographic concentrating and spectral splitting systems can reduce the cooling requirements of the photovoltaic cells. As illustrated in **Figure 3**, PV cells were positioned at right angles with respect to the hologram orientation. This configuration allows eliminating shadow effects and facilitates cooling.

**Figure 3.** Holographic solar concentrator with spectral splitting systems [5, 8].

The holographic system was designed to direct and concentrate the red and near-infrared spectrum on one photocell and the green and blue spectrum on another one, 2hereas the farinfrared spectrum, which mainly contributes to the heating, is diffracted away from the cells. The authors compared the holographic systems with a Fresnel-based solar concentrator, demonstrating that the holographic system concentrates more total power than the Fresnel system and has a larger relative efficiency and that large heat sinks are not necessary allowing decreasing the bulk and cost.

can be produced under mild conditions, the preparation (hydrolysis and condensation) requires relatively long times to produce a consolidated material [54]. The generic approach is to dissolve the photopolymerizable species in the liquid precursors (typically modified organoalkoxysilanes) of the glassy material in a single reaction mixture, followed by hydrolysis and gelification which lead to the formation of a glassy matrix. By this approach samples of high thickness with good mechanical properties, low shrinking and high thermal and chemical stability can be obtained [55]. Some variants have been proposed to increase the refractive index change after exposure, such as the addition of titanium or zirconium alcoholates to the initial mixture [56] or using low refractive index monomers [57]. A further alternative is to increase the refractive index of the photosensitive material by introducing reactive species with high refractive index species (HRIS) [58]. This solution led to refractive index modulations up to 0,015. A further advantage of this approach seems to be that high refractive index species are dispersed in molecular form with respect to systems containing nanoparticles [59].

Since 1980, several scientific papers have been published to demonstrate that HOEs can provide improvements in solar energy collection over large incident angles and the entire solar spectrum range [2–4, 8, 10, 28, 60]. In particular, Ludman [3] described two configurations with multi-hologram lenses useful to concentrate the sunlight onto an absorber for different sun positions in the day. On the other hand, V-HOEs can be employed to obtain both a concentration and a spatial separation of the solar spectrum. This approach allowed employing different

Ludman et al. [5, 8] demonstrated that a well-designed holographic concentrating and spectral splitting systems can reduce the cooling requirements of the photovoltaic cells. As illustrated in **Figure 3**, PV cells were positioned at right angles with respect to the hologram orientation.

solar cell materials with optimized band gaps achieving high PV efficiency [2, 61].

This configuration allows eliminating shadow effects and facilitates cooling.

**Figure 3.** Holographic solar concentrator with spectral splitting systems [5, 8].

**4. VHOE-based solar concentrators**

36 Holographic Materials and Optical Systems

Muller [62] showed a variety of potential applications in architecture for utilization of solar energy, improvement of room comfort as well as design of solar light and colour effects. The possibility to record more than one transmission hologram (superimposed holograms) on the same holographic medium was detailed described by Bainier et al. [4]. The authors estimated and experimentally measured the energy efficient of a system composed of a PV cell (Siliciumor GaAs-type cell) and a holographic concentrator. In particular, they compared two types of holographic systems: the first concentrator was with a single holographic element, where the maximum of reconstruction wavelength (620 nm) was chosen to be centred in the middle of the range of the PV cells (i.e. 500–800 nm). The second concentrator system was composed of two holographic recordings, where the two maximum of the reconstruction wavelengths (514.5 and 620 nm) were chosen both to well overlap the operating spectrum of the PV cells and to avoid coupling effect. The PV cell was a GaAs with an efficiency of 23%. The authors proposed both a reflecting and transmitting version for the double hologram system. For the transmission, one of the two holograms was superimposed on the same holographic medium. Estimation and experimental evaluation of the energy efficiency of the holographic systems were of 6 and 5% for the single holographic elements and of 11 and 9% for the double holographic element, respectively.

James and Bahaj [63] published a very interesting study on the application of V-HOE-based solar concentrator for solar control of domestic conservatories and sunrooms. They demonstrated that a well-designed V-HOE applied on the glasses of a domestic conservatory can allow keeping the daytime temperatures to an acceptable level. In particular, the authors estimated the temperature reduction inside a conservatory for different configurations of the V-HOEs. Moreover, the angular selectivity of the V-HOEs allows avoiding any actuation of the incident sunlight during the winter months. So, in the winter season, the temperature reduction due to the V-HOEs is inhibited allowing to obtain a comfortable temperature.

In 2010, Hung et al. [64] proposed a superimposed structure with a doubly slanted reflecting hologram fabricated by using 488 and 632 nm laser sources. The slanted structure with an inclination of 30° assures a total internal reflection at the surfaces of the holographic medium. Thus, light emerges at the edges of the holographic plate, where appropriate PV cells can be positioned. From a theoretical point of view, the structure was analyzed as a 1D photonic band gap material. **Figure 4(a)** shows a sketch of the experimental set-up for the doubly slanted layer structures, whereas the diffraction of normally incident white light for the doubly slanted structure is illustrated in **Figures 4(b)** and **(c)**.

**Figure 4.** (a) Sketch of the experimental set-up used by Hung et al. [64] for the doubly slanted layer structure. Diffraction patterns evaluated taken from behind (b) and above (c).

Castro et al. [12] reported a detailed study on the design and characterization of a holographic grating used to address the direct sunlight on PV cell with the maximized energy efficiency possible. In particular, they analyzed the effects of incident spectra that vary hourly, daily and seasonally. To maximize the energy collection efficiency during the course of a year, the authors proposed the system based on the structure illustrated in **Figure 5(a)**.

**Figure 5.** (a) Holographic solar concentrator structure proposed by Castro et al. [12]. Dashed box: unit cell. (b) Holographic design to reduce the optical crosstalk.

The unit cell includes two cascaded holographic grating on each side of the PV cell (holograms A and B). The holograms on each side of the PV cell are conjugated (i.e. A and A' or B and B') to provide peak energy collection at different seasons. In order to reduce the optical crosstalk of the V-HOEs, the two cascaded holograms are designed to diffract light in opposite directions with the incident angles in different quadrants of the Bragg circle (**Figure 5(b)**). Moreover, the geometrical parameters of the system (such as the hologram width and the distance hologram PV cell) are optimized to assure that maximum of the diffracted rays of the sunlight within the solar responsivity spectrum of PV cell can reach the surface of the cell independently of the incident angle. An energy increase due to the concentrator averaged over a particular day of 147% can be obtained, and nearly 50% of the available energy illuminating hologram areas can be collected by photovoltaic cells without the need of tracking.

Hsieh et al. presented a solar concentrator based on a so-called 90° hologram that allows obtaining a compact and wide-angle structure [60]. The conceptual recording set-up is illustrated in **Figure 6(a)**. In particular, the reference and object beam are an edge-lit and a cylindrical converging beam, respectively. Thus, a combination of a lens and a mirror can be simultaneously recorded in the holographic medium. Then, the medium is shifted, and the recording is repeated obtaining an array of V-HOEs that assure a large angular acceptance. In fact, when the sunlight, assumed as/like a plane wave, impinges on the solar concentrator with different incident angles, the diffracted wave is guided to the edge of the recording medium where a PV cell is positioned (**Figure 6(b)**). The author demonstrated that using a 2 mm thick holographic medium, the proposed architecture increases the collection angle from 0.01° to 6°.

**Figure 6.** (a) Recording set-up and (b) configuration of volume holographic concentrator [60].

Atencia's group analyzed in detail the design and characterization of a solar PV linear concentrator based on a cylindrical holographic lens [13, 18]. In particular, a simulation tool has been developed to take into account a specific set of designed parameter, such as angular selectivity, bandwidth, optical polarization, PV cell size and so. The possibility to realize a high-efficient system that only requires one-axis tracking was demonstrated.
