| Solar radiation can be converted directly into electricity by using the photovoltaic effect, which represents the principle of operation of solar cells. Currently, most solar cells are made of crystalline silicon and have a conversion efficiency of about 20% or less. Multi-junction solar cells, made of III-V compound semiconductors, can have efficiencies in excess of 40%. The main factor that prohibits such high-efficiency technologies from wider acceptance is the cost. An alternative approach to using large-area expensive solar cells is to employ lower cost optics and concentrate the solar radiation to smaller cell area, which is the basic principle of solar concentrators. In this thesis, we consider a solar concentrator module that consists of a combination of a lens array and a slab waveguide with etched conical holes on one side of the waveguide, which are aligned with the lenslets. Sunlight coming through each of these lenslets is focused on the backside of the waveguide, where a coupling structure (an etched cone) is fabricated. This coupler changes the propagation direction of the incident light in such a way that light is guided through total internal reflection (TIR) within the glass slab and eventually reaches a solar cell, which is properly mounted on the side of the slab. The concept of this concentrated photovoltaic (CPV) system is based on a planar light guide solar concentrator module, proposed earlier by another group. This project builds on the original idea by including the following substantial modifications. The lens array is to be made of solid glass by a mold technology and provided to us by our industrial partner, Libbey, Inc., as opposed to silicone on glass technology, in which the lenses are made out of silicone and sit on a glass substrate. The coupling structures are cone-shaped holes etched directly into the solid glass waveguide, as opposed to coupling structures that are formed by addition of polymeric layer and consequent patterning. The fabrication of the etched holes in the glass is proposed to be based on a self-aligned process using a laser-induced backside etching (LIBWE) method, which is discussed in this project and its feasibility is examined. The role of different parameters to the concentration level and the optical efficiency of the CPV system are studied by simulations in ZEMAX (which is a leading optical analysis/design software) using non-sequential ray tracing. The optical efficiency of this design under different light concentration level is studied and discussed. The main contributions of this research consist of a new design of a waveguide-based CPV system which can be made entirely of glass by a low-cost glass fabrication method, and a feasibility study in terms of critical fabrication steps and optical performance. |
