Optics for High-Efficiency Full Spectrum Photovoltaics

Citation

Darbe, Sunita (2017) Optics for High-Efficiency Full Spectrum Photovoltaics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z96W9833. https://resolver.caltech.edu/CaltechTHESIS:01032017-032640614

Abstract

While the price of solar energy has dropped dramatically in the last few years, costs must be further reduced to reach wide-scale adoption. One strategy to decrease cost is to increase efficiency. Photovoltaic energy conversion is most efficient for a narrow frequency range. Lack of absorption of low energy photons and thermalization of high-energy photons leads lead to a loss of over 40% of incident solar power on a silicon cell. Current-matching and lattice-matching restrictions limit the efficiency of traditional monolithic multijunction solar cells. In order to avoid these limitations and realize ultrahigh efficiency (close to 50%), this thesis explores use of optical elements to split broadband sunlight into multiple spectral bands that can each be sent to physically separated solar cells tuned to best convert that band.

Design of a holographic diffraction grating based spectrum-splitting system resulted in a simulated module efficiency of 37%, meeting the efficiency of state-of-the-art modules. One of four holographic grating stacks is experimentally characterized. Next, a design incorporating dichroic filters, seven subcells with bandgaps spanning the solar spectrum, and concentrators with efficiency potential exceeding 45% module efficiency is presented. While prototyping this design, we also used on-going cost-modeling to ensure that our design was on-track to be a high-volume technology with low lifetime energy cost.

Finally, high-contrast gratings are used as resonant, dielectric spectrally selective mirrors in a tandem luminescent solar concentrator and as alternatives to Bragg reflectors. Gratings can have omnidirectional, high reflectivity by appropriately offsetting grating resonances in nano-patterned subwavelength thickness high-refractive index material. Subwavelength feature sizes suppress diffraction, and the high-refractive index of the grating layer leads to relatively angle-insensitive reflectance. Gratings can be fabricated by nanoimprint lithography, making them a scalable and economical option for photovoltaic applications. Simulations show hemispherically average reflectivity near 90% possible from a single subwavelength thickness layer. These properties are well suited for a variety of applications including multiple spectrum-splitting device architectures.

Thesis Files