Holographic Optical Elements for Spectrum-Splitting Photovoltaic Systems

Abstract

Spectrum-splitting is a technique for increasing the conversion efficiency of a photovoltaic system. In a spectrum-splitting system, an optical element such as a dichroic filter or a diffraction grating is used to divide the solar spectrum between a set of laterally separated photovoltaic cells with different energy bandgaps. In the past, one of the main challenges for spectrum splitting systems was a lack of inexpensive photovoltaic cells at a variety of energy bandgaps. However, the recent development of perovskite solar cells opens up new opportunities for spectrum splitting systems. Perovskite solar cells are efficient and inexpensive and have been developed at a variety of bandgap energies ranging from 1.25eV to 2.3eV. In literature, perovskite solar cells are arranged in a vertical stack and achieve spectral separation through absorptive filtering. However, this approach is limited since each perovskite cell parasitically absorbs 10% to 20% of the incident light before transmitting to the underlying cells. One of the main questions considered in this dissertation is what the optimal cell arrangement is for spectrum splitting systems with two, three, and four energy bandgaps. This question is approached by designing and comparing a variety of spectrum splitting systems. First, a particular approach for lateral spectral separation using volume holographic lens arrays is selected and developed in depth for a two-bandgap system. Next, the approach is extended for a three-bandgap system by designing and simulating a cascaded volume holographic lens array. Lastly, a hybrid cell arrangement is proposed which combines both vertically stacked and laterally separated cell arrangements. Three- and four- bandgap systems are designed in the hybrid cell arrangement and are shown to have greater conversion efficiency than either cell arrangement individually. A variety of issues related to the design and fabrication of volume holographic lens arrays are also addressed. First, the environmental stability of Covestro Bayfol HX is shown to be insufficient for solar applications due to yellowing of the film after only several weeks of exposure. However, the other holographic material candidate, dichromated gelatin (DCG), is difficult to work with and is well known for yielding different results when processed in different atmospheric conditions. A reproducibility study is conducted and it is found that the variation in the measured spectral diffraction efficiency is reduced by a factor of six when the humidity is regulated at 65% during the drying stage of the film preparation process. Lastly, a replication system for volume holographic lens arrays is proposed that is suitable for mass manufacturing. In the replication method, the object beam, reference beam, and aperture of the copy hologram are all recorded in a composite master hologram and replayed by illuminating with a single laser beam aligned at normal incidence. This replication system is used to fabricate a 9.6cm by 6.0cm volume holographic lens array with 36 elements that each have greater than 95% diffraction efficiency.