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Planar micro-optic solar concentration


Solar radiation can be converted directly into electricity with materials exhibiting a photovoltaic response. Most photovoltaic arrays use crystalline silicon cells assembled in large modules which convert <20% of incident light into electricity. More recently, multijunction solar cells, comprised of multiple semiconducting layers, have exceeded 41% conversion. The drawback to these devices is the high cost associated with materials and fabrication, making them impractical as rigid panels. The field of concentrator photovoltaics pairs these costly devices with inexpensive collection optics which reduce the amount of active cell area. Most commercial systems rely upon simple lenses or mirrors focusing through secondary optics, yet these approaches lead to hundreds of individual components which must be assembled, aligned and interconnected. In this dissertation, I present an alternative concentration approach which replaces discrete optics with a segmented lens array and common slab waveguide. Sunlight collected by each small lens aperture focuses onto mirrors placed on the waveguide surface which reflect rays at angles that guide by total internal reflection. This configuration directs light from thousands of arrayed lenses into the same waveguide which connects to a single photovoltaic cell. We refer to this approach as planar micro-optic concentration because the waveguide remains uniform in cross-section and is compatible with large-scale microfabrication techniques such as roll-to-roll processing. In the following chapters, I discuss the concept and tradeoffs associated with waveguide coupling and propagation. I present optimized systems which demonstrated >80% optical efficiency at 300x geometric concentration. In addition, I develop a self-aligned fabrication process to assemble several small-scale prototypes using commercially-available components. These systems were experimentally measured at 52.3% optical efficiency. Lastly, I show how the waveguide geometry can be exploited to increase performance and add functionality within concentrator photovoltaic systems

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