Efficient Solar Cells Using Ultra fast-Laser Micro-Textured Silicon Surfaces
Iyengar, Vikram Varadarajan, Department of Electrical Engineering, University of Virginia
Gupta, Mool C., Department of Electrical Engineering, University of Virginia
Silicon-wafer-based photovoltaics is projected to be the dominant technology until the end of this decade and beyond. Reduction in cost ($/kW.h) remains the main challenge. The solution to this obstacle is achievable by reduction in wafer thickness, process simplification and efficient device structures for reducing optical as well as electrical losses. In this dissertation, we provide alternatives for effective light management along with full scientific understanding using a laser-based approach. In this work, an ultrafast-laser texturing scheme is investigated that forms selforganized micro/nano structures on silicon surfaces. These act as excellent light trapping structures. Detailed optical, material and electrical studies are performed on ultrafast-laser micro-textured silicon surfaces in order to understand the effect of lasermatter interaction and its impact on solar cells. The ultrafast-laser micro-textured silicon surfaces demonstrate a total reflection of less than 4 % over the entire solar spectral range. This leads to higher photo-currents without the need for an antireflection coating. In order to understand the impact of laser processing, material properties of micro-textured surfaces are investigated. It is observed that most of the laser-affected region is limited to the surface. In order to remove the laser-affected near-surface areas, a combination of chemical etching and thermal annealing is introduced, after which solar cell devices are fabricated. The devices are characterized by dark and illuminated current-voltage characteristics and quantum efficiency to assess their performance. The post-texturing treatment methods are successful in recovering the original material quality and high performance laser micro-textured silicon solar cells with an efficiency of 15.3 % are fabricated for the first time. Advanced electrical characterization such as minority carrier lifetime and light-beam-currentinduced maps, are also carried out to understand the electrical loss mechanisms. Additional relevant research in the areas of simplified solar cell fabrication process, laser doping and optical studies on ultrafast-laser texturing of metal surfaces is also accomplished. Each of these aspects are studied in detail.
PHD (Doctor of Philosophy)
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