# Holographic Lens Array for Solar Collector with Large Angle and Expanded Spectral Width
## 4 Discussion  
Previous research has made many attempts to improve the collection characteristics of holographic solar collectors, but has mainly focused on separately improving the collection spectrum and collection angle. In this paper, we propose a novel holographic lens array solar collector with a large acceptance angle and expanded collection spectrum. A spectrum spatial allocation method is used to expand the collection spectrum at each angle within a  
Figure 6: Normalized spectral collection efficiency and diffraction patterns at different incident angles. (**a**) Normalized spectral collection efficiency. (**b–e**) are diffraction patterns at \(-20^{\circ}\), \(-10^{\circ}\), \(20^{\circ}\), and \(10^{\circ}\) incident angles.  
large angle range; thus, the collected solar energy can be greatly improved. In addition, a lens array is used to fabricate the HLASC, so the complex structure can be fabricated with only one exposure process based on a single wavelength. The simple fabrication method is helpful for mass production. In addition, different collection parameter requirements for the HLASC can be flexibly adjusted by changing the design and fabrication parameters of the holographic lens array.  
Due to some approximations in the design process, such as approximating spherical light as many parallel lights with multiple angles and ignoring the angle changes in diffraction light during variable wavelength reconstruction, there exists the phenomenon that the spatial distribution of the wide-spectrum diffraction light on the solar cell is uneven. This can be improved by changing the lens array used for fabricating the HLASC to a micro lens array. The comparison of the two effects is shown in Figure 7, where the left and right figures, respectively, show the collection light of an HLASC fabricated by a lens array and a micro lens array. The individual lens apertures of the lens array and micro lens array are 10 mm and 2 mm, respectively. It can be seen that the collection light of the HLASC fabricated by a micro lens array has a more uniform spatial distribution in the longitudinal direction, while it also has obvious spectral separation characteristics in the transverse direction, which gives us the opportunity to use solar cells with corresponding spectral bandgaps [30; 31] to improve the photoelectric conversion efficiency. In addition, the reduction in OHL area also contributes to improving the color uniformity of the overall transmitted light of the HLASC, thereby expanding its application in see-through scenarios such as a holographic window.  
There are still some interesting studies that can be conducted in the future. On the one hand, in order to further expand the collection angle, a multi-angle parallel light incident lens array offers a promising solution for recording the multiplexed HLASC. The subsequent testing of the HLASC under real-world irradiance conditions will also expand the collection of spectra beyond the visible light range. On the other hand, based on the principle of spectrum spatial allocation in this paper, by controlling the transmittance distribution of the lens array, it is theoretically possible to control the diffraction efficiency of different wavelengths, which helps to expand the application of HOE in blue light elimination, infrared light attenuation, and other fields. Finally, due to the spatial allocation of the collection spectrum, only a partial area of the HLASC can effectively collect light from a certain spectrum, resulting in a relatively low overall broad-spectrum collection  
Figure 7: Comparison of collection light of the HLASC fabricated by a lens array and a micro lens array.  
efficiency of the HLASC. In the future, the collection efficiency can be further improved by cascading multiple layers of HOE.