Light Trapping in Plasmonic Solar Cells

Subwavelength nanostructures enable the manipulation and molding of light in nanoscale dimensions. By controlling and designing the complex dielectric function and nanoscale geometry we can affect the coupling of light into specific active materials and tune macroscale properties such as reflection, transmission, and absorption. Most solar cell systems face a trade-off with decreasing semiconductor thickness: reducing the semiconductor volume increases open circuit voltages, but also decreases the absorption and thus the photocurrent. Light trapping is particularly critical for thin-film amorphous Si (a-Si:H) solar cells, which must be made less than optically thick to enable complete carrier collection. By enhancing absorption in a given semiconductor volume, we can achieve high efficiency devices with less than 100 nm of active region.;In this thesis we explore the use of designed plasmonic nanostructures to couple incident sunlight into localized resonant modes and propagating waveguide modes of an ultrathin semiconductor for enhanced solar-to-electricity conversion. We begin by developing computational tools to analyze incoupling from sunlight to guided modes across the solar spectrum and a range of incident angles. We then show the potential of this method to result in absorption enhancements beyond the limits for thick film solar cells. The second part of this thesis describes the integration of plasmonic nanostructures with a-Si:H solar cells, showing that designed nanostructures can lead to enhanced photocurrent over randomly textured light trapping surfaces, and develops a computational model to accurately simulate the absorption in these structures. The final chapter discusses the fabrication of a high-efficiency (9.5%) solar cell with a less than 100 nm absorber layer and broadband, angle isotropic photocurrent enhancement. Moreover, we discuss general design rules where light trapping nanopatterns are defined by their spatial coherence spectral density.