Light Management and Optical Loss Mitigation for Photovoltaics: Downshifting, Downconversion, and Tandem Solar Cell Designs

Abstract

The efficiency of single-junction Si photovoltaic cells has continually increased over the past several decades, but is approaching fundamental thermodynamic limits. Holding over 95% of the solar module market share, Si modules will continue to be an integral part of the rapidly expanding photovoltaic industry, so different device technologies that increaseSi cell efficiencies beyond thermodynamic limits, or that expand the available installation sites for solar cells, are needed. In this thesis, three types of technologies are discussed that use optical design to more efficiently use the high energy solar spectrum for Si PV: downshifting, downconversion, and tandem solar cells. We first discuss the design of downshifting and concentrating devices called luminescent solar concentrators (LSCs). Tandem LSC architectures, which combine multiple luminophores to broaden the absorption spectrum, are one potential route to increase the efficiency of these devices. We first describe an analytical model to develop luminophore selection criteria for tandem LSCs. We find that luminophores with high photoluminescent quantum yield, minimal overlap between the absorption and photoluminescence spectrum, and an absorption onset closely matched to the band gap of the chosen photovoltaic cell yield the best LSC performance. We then create bilayer LSCs, which combine CdSe/CdS and Si nanocrystals in a monolithic waveguide. Through a combination of transmission measurements, position-dependent photoluminescence measurements, and ray-tracing simulations, the bilayer LSC was found to sensitize Si nanocrystal absorption and enhance the optical efficiency by 30% relative to a single layer LSC. We discuss the use of the bilayer device in agrivoltaic applications, and then explore this use further using a thin-film stack optimization methods to direct emission out one LSC side toward the plant species. The LSC extraction efficiency is increased from 13.9% to 15.1%. We next consider optical designs for downconversion, a process by which one high energy photon is converted into two lower energy photons. We consider the coupling efficiency from the downconverter to a realistic Si solar module in several different configurations, finding an optical coupling efficiency of 95.25% by placing the downconverting film directly on the Si cell. This enhances the power conversion efficiency by 2% absolute. Lastly, CdTe/Si four-terminal tandem solar cells are studied to improve the sub-band gap transparency of CdTe solar cells. We find that the surface texture of the CdTe significantly impacts light transmission into the Si bottom cell, and that the losses are dominated by the transparent conductive oxide absorption. An optical design solution is proposed that mitigates transparency loss and enhances the short circuit current density of the Si cell by 2.5 mA/cm2 , which enhances the tandem efficiency by a relative increase of 5.6%.