Nanophotonic light management in thin film silicon photovoltaics

This thesis is about light-trapping in thin film silicon photovoltaic devices. Light-trapping allows more light to be absorbed inside a smaller volume of photoactive materials, therefore reducing the required active layer thickness for obtaining high optical absorption. The decrease in thickness can not only bring down the material cost of solar cells based on high-purity single-crystalline silicon, but also enable materials with poorer qualities, such as polycrystalline silicon to be used for photovoltaics, reducing both the material cost and processing cost. ❧ In this dissertation, we use three-dimensional full-vectorial electromagnetic simulation tools to explore various light-trapping schemes based on sub-wavelength nanostructures arranged in both periodic and partially-aperiodic fashion. In specific, periodic silicon nanowire and nanohole arrays were found to absorb more sunlight than an equally-thick silicon slab, if the geometry of the array is properly designed. The strong structural dependence of the optical absorption performance can be attributed to the optimal condition for exciting guided resonance modes within the nano-structured array. Furthermore, partially-aperiodic silicon nanowire/nanorod arrays show significant enhancement in light-trapping and anti-reflection performance, respectively, compared to their periodic counterparts in certain size regimes. Machine-based optimal design algorithm was utilized to maximize the optical performance enhancement. In order to verify the theoretically-predicted optical absorption enhancement effect, proof-of-concept experimental demonstration has been carried out for free-standing silicon nanomembranes patterned with both periodic and partially-aperiodic nanohole structures. Good agreement between theory and experiment was obtained, suggesting the wide applicability of electromagnetic simulations and optimal design techniques in the optical design of nano-structured thin film solar cells. Finally, the effect of plasmonic particles on the optical absorption in silicon nanowire arrays was numerically examined. It was found that due to the existence of diffractive coupling scheme afforded by the nanowire array itself, the plasmonic particles do not improve the optical absorption within the silicon nanowires.