Laser-induced spatial composition gradients in SiGe core fiber

Abstract

An important contributor to global warming is greenhouse gas emission caused by burning fuels such as oil and coal. To preserve the environment for the whole world and to simultaneously meet growing global energy demand, environmentally friendly renewable energy attracts wide interests. Among the options, solar energy is a promising prospect due to the large amount of energy available. Tremendous effort has been devoted into this research area, on every aspect from materials science to device development, and commercial crystalline silicon solar cells were developed. But silicon solar cells still have many disadvantages such as high energy cost and low conversion efficiency.

This thesis is centered on materials studies needed for improvements to the most common silicon solar cells. Thin solar wires can be used to reduce material costs and semiconductor purity requirements in solar cells. This structure enables low-quality silicon materials to collect photo-generated electrical carriers more effectively. We add small amounts of Ge to the silicon to assist with material processing and to increase energy absorption. The Ge absorbs longer wavelengths than the silicon and can improve solar cell efficiency, if physically located where needed. We developed laser processing methods for SiGe-core fiber that allow writing of spatial composition structures in the SiGe. Finally, the graded composition SiGe fibers were assembled into a vertically aligned array suitable for fabrication into solar prototype cells. In addition, SiGe fibers with radial Ge concentration gradients were explored for their potential application as waveguides for longer wavelengths, suitable for energy and information transfer.