## 1 Introduction  
Solar energy conversion processes provide clean, sustainable and renewable energy, thus there is a growing interest in research and development in the study of conversion systems and their cost effectiveness. An improvement in this sense needs simultaneous consideration of three issues: initial cost, durability-reliability and performance [1]. Considering the current technology, photovoltaic (PV) cells with triple-junction InGaAs are the most efficient (37%); nevertheless, their high cost makes them unattractive, even if their use for domestic applications, as well as in the aerospace industry, is desirable. This problem can be solved by using optical concentrators that allow focussing the sun's rays onto the active solar cell area, thus letting to reduce a significant amount of the expensive PV material [2]. Consequently, in the last 15 years, solar concentration technologies were explored to direct all the available light towards small solar cells. Currently, there are two principal types of concentrators involved in conversion technology: conventional solar concentrators (e.g., lenses or mirrors) and holographic solar concentrators. Holographic optical elements (HOEs) could in part overcome the limitations of the conventional solar concentrator, such as complex designs, thermal management due to the excessively heated of the solar cell when illuminated with concentrated solar radiation and active solar tracking. Additionally, holography is much more versatile and cheaper with respect to other concentrating optical systems. In particular, volume holographic optical elements (V-HOEs) have been proposed for use as solar concentrators [3; 4] thanks to the following features: (i) potential to achieve nearly 100% efficiency for certain wavelengths and directions; (ii) very rapid and low-cost effective manufacturing and easy customizability of the recorded devices; (iii) narrowness and lightness; (iv) potential to fabricate elements with multiple optical responses (multiplexing); and (v) potential to be very inexpensive in mass production. V-HOEs applications range from spectral splitting applications to increase the conversion efficiency of PV cells [3; 5; 6] to simultaneous concentration and spectral splitting applications [7]. Despite all these advantages offered by V-HOEs, currently only a few holographic concentrators, patented by Prism Solar Technologies [8], are on market. They work by total internal reflection by means of multiplexed gratings [9], have low cost (around 1 \(\$\)/W) and are easy to be integrated into buildings [10]. Nevertheless, Prism Solar Modules are still under test and maybe this is the main reason for which this technology is not yet so diffused. However, with the aim of obtaining high performance of V-HOEs as solar concentrators, it is necessary to keep in mind that volume holograms have high efficiency only when the incident rays vary in a given portion of the plane (angular selectivity) and their efficiency depends on the wavelength: it is high for a bandwidth centred on a wavelength determined by both the refractive index modulation obtained in the recording material and the angle of incidence (chromatic selectivity) [4]. Thus, V-HOEs should be designed to have a high efficiency for the spectrum of the sunlight inside the PV conversion range (for multijunction PV cells, \(350\div 1750\) nm [11]). It is worth noting that, due to the chromatic selectivity, the V-HOE diffraction efficiency depends on the wavelength [10]. Exploiting this feature, the heating of the cell, due to the solar spectrum region associated with wavelengths above 1200 nm, can be managed. Thus, one of the main problems of conventional solar concentrators can be overcome allowing a higher conversion efficiency and so lower cost/watt [12; 13]. V-HOEs can be used as solar concentrators in both earth and space (satellites) applications.