Modeling, Optimization, and Characterization of High Concentration Photovoltaic Systems Using Multijunction Solar Cells

Recent advancements in the development of high-efficiency multijunction solar cells\ud have led to a renewed interest in the design and implementation of high concentration photovoltaic systems. With the emergence of novel materials and design structures, understanding the operation of multijunction solar cells has become a challenging task. Modeling and simulation hence play an important role in the analysis of such devices. In this dissertation, techniques for accurate optoelectrical modeling of concentrating photovoltaic systems, based on multijunction solar cells, are proposed. A 2-dimensional, distributed circuit model is proposed, parametrized to values obtained by numerical modeling of three multijunction cell designs, namely: a three-junction, lattice matched design, a three-junction lattice-mismatched, inverted metamorphic design, and a four-junction,lattice matched design. Cell performance for all the three designs is evaluated under both\ud uniform and nonuniform illumination profiles at high concentrations and efficiency enhancement by optimizing finger spacing is proposed. The effect of luminescent coupling from higher bandgap subcells is also determined.Fresnel-lens based, refractive concentrating optical systems are modeled and optimized\ud using an optical ray-tracing simulator at two different concentrations, with and without\ud a secondary optical element. The corresponding optical efficiency, acceptance angle, and the degree of nonuniformity are determined for each optical system. An integrated approach,combining optical design with electrical modeling is proposed for optimizing the multijunction solar cell in tandem with the corresponding concentrating optics. The approach is validated by on-sun, acceptance angle measurements, using a three-junction,lattice-matched cell. Also, temperature effects are modeled and are experimentally validated for a three-junction, lattice-matched design. Experimental results with a single-junction, dilute-nitride cell, targeted for four-junction operation, are presented as well. A modified distributed circuit model is used for analysis of temperature effects in a four-junction solar cell, and the results under both uniform and nonuniform temperature profiles are presented.\ud When implemented, the designs and their corresponding analyses, may result in new\ud insights into the development of CPV systems, thereby enabling enhanced efficiencies at higher concentrations