Light management in nanostructures: nanowire solar cells and optical epitaxial growth

2015-08-20This dissertation work studies the use of nanostructures to control the flow of light in two application areas: photovoltaics and material self-assembly, which are discussed in I and Part II, respectively. ❧ The work in photovoltaics focuses on designing high-efficiency nanowire array solar cells. Nanowire arrays are a promising candidate for the next generation of low-cost, high efficiency, flexible photovoltaic cells. Nanowire structures relax the lattice-matching constraints and allow the usage of materials with different lattice constants for multijunction cells. This opens up a much wider range of materials choices than for traditional, planar cells. The design of a high-efficiency cell involves two factors: optical absorption and carrier collection. In this dissertation, I first use the full-wave electromagnetic simulation to investigate the absorption properties of periodic nanowire arrays and provide the optimal designs in a single junction and a tandem wire-on-substrate cell configurations. I then study strategies for optimal carrier collection by finite-element method electronic device simulations. I optimize the p-n junction geometry, doping parameters, and surface passivation scheme. This work not only establishes the fundamental limits of nanowire solar cells’ designs but also provides practical guidelines and solutions for high performance nanowire solar cell devices. ❧ The work of material self-assembly is based on the light-assisted, templated self-assembly (LATS) technique developed in our group. In this method, we shine light through a photonic crystal (or template) to create an array of optical traps. The traps drive the self-assembly of nanoparticles into regular patterns. In this dissertation work, I discover the crucial effect of inter-particle interactions on the pattern formation of metallic particles in the LATS system. I envision the analogy between the optical assembly of nanoparticles and atomic level epitaxial growth and suggest that the system can be viewed as “optical eptaxial growth”. I develop the modeling and simulation technique to explain and predict experimental results. Such model leads to the first successful demonstration of the optical assembly of a 2D periodic gold nanoparticle array