Spherical Silicon for Photovoltaic Application: Material, Modeling, and Devices
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Detailed material characterization of spherical silicon is conducted for the first time. Experimental results on crystallinity, impurities, and structural defects are presented to investigate the effect of growth mechanism and processing of the spheres. Based on the material properties, the spherical bulk is characterized from electronic point of view. A model is developed to interpret photoconductivity decay measurements and extract minority carrier lifetime, the most influential parameter in device performance. The model includes the spherical geometry as well as the radial profile of carrier lifetime and the measurement results are used to characterize the quality of the spheres and the effectiveness of the process steps. To analyze and predict the performance of spherical cells, a three dimensional opto-electric model is developed. The model separately treats the optical generation and carrier collection and is able to calculate the spectral response and the device characteristics. A simulation tool is created based on this model. The simulation results are of great importance in designing novel structures and optimizing fabrication processes. The necessary characterization methodologies are developed to measure the spectral response and I-Vcharacteristics of individual cells, as well as an array consisting of several cells. A new device design with passivated selective emitter is proposed based on simulation results. Plasma processes were developed to selectively etch back the deep emitter of the spherical cells using reactive ion etching, and then passivate the surface using plasma enhanced chemical vapor deposition of silicon nitride. The improvement in characteristics of the fabricated device is characterized using the developed measurement setups. Industrial up-scaling and manufacturability of the proposed devices and processes are also discussed.