Investigation of Photopolymer-Based Holographic Optical Elements for Solar Applications
Document Type Theses, Ph.D
Dublin Institute of Technology, 2015.
The aim of this research was to explore the potential of photopolymer Holographic Optical Elements (HOE) for use in the collection of light from a moving source, such as the sun, and its direction into a fixed detector/convertor for application in solar concentrators. In order to increase the acceptance angle and the wavelength range of operation of the holographic device, low spatial frequency holographic recording was explored. The challenge was to record high diffraction efficiency HOEs at this spatial frequency, since it requires a material with relatively fast monomer diffusion. The acrylamide-based photopolymer developed at the Centre for Industrial and Engineering Optics has been selected, because it has previously shown such diffusion properties. In order to achieve large acceptance angle, the theoretical modelling of the angular and wavelength selectivity of the HOEs was carried out. The theoretical results confirmed that the gratings with just a few hundred lines per mili meter were of most interest in this study because the selectivity is lower.
The next challenge was to fabricate off-axis holographic spherical and cylindrical lenses with large range of operation in the photopolymer. This was achieved by stacking a number of gratings and focusing elements on top of each other. The stacked devices were characterised in two ways: (i) the regular Bragg diffraction characteristics of the stack devices were measured (ii) a set up was constructed to analyse their performance in a non-tracking system with a moving source and fixed detector/converter. The results show significant improvement for the collection of light from the higher angles. The effect of using the HOE elements with unpolarised light has also been explored. For the first time, the photopolymer was used to fabricate a combined element with symmetrically arranged off axis lens elements in order to maximise the collection area and avoid unwanted diffraction of light away from the solar cell. The combined device was tested using a solar simulator and c-Si solar cell. The maximum 60% increase was achieved.