Structural and Optical Properties of Annealed Porous Silicon Bragg Reflector for Thin-Film Crystalline Silicon Solar Cells

Implementing an annealed porous silicon Bragg reflector between a crystalline silicon thin-film solar cell and its cheap silicon substrate improves greatly its performance. Such a multilayer features indeed a high reflectivity, allowing the optical confinement which is essential for these cells. Annealing the porous silicon above 900 °C under H2 modifies significantly the structure of the porous silicon. This phenomenon lowers its resistivity to avoid high cell series resistance and allows the subsequent epitaxial growth of the silicon cell. The different behaviors of porous silicon structure alteration are studied and rationalized. The pores shapes and sizes, the porosities and the thicknesses of layers are affected. These changes depend on the initial porosities and thicknesses and the annealing parameters. Pores can erupt at the surface when they are neighboring, thus causing a roughness of characteristic length according to the porosity of the layer. The pores sizes increase in all cases. Different multilayer reflectivities are presented, the highest ones being up to 98% over a wide wavelength range. It allows a better understanding of this annealing process and the optimization of the reflectivity of the Bragg reflector in order to apply it on solar cells.