Theoretical and experimental study of Fermi Bragg reflector as application for thin silicon solar cells

Abstract

The conception of efficient Bragg reflectors (BRs) based on dielectric layers is needed to improve the light trapping in thin film epitaxial silicon solar cells. Porous silicon multilayer structures (PSMS) used as BR show its potential to enhance optical confinement of low energy photons. In the present work, a novel design of PSMS will be studied in order to extend the reflectivity to long wavelength by adjusting BR thicknesses according to the Fermi profile law. Such new design named Fermi Bragg reflector (FBR) includes different configurations. Several FBR parameters are used in the simulation study, which is carried out by transfer matrix method. The optimal structures were realized using electrochemical etching dissolution of P+ type silicon substrate in hydrofluoric acid solution (HF) at two temperature conditions: room temperature (20 °C) and −20 °C. The measured optical response of FBR shows an enlargement in the width of high reflection range in comparing to the conventional BR. More than 80% of the photons were reflected in the spectral range 636–1155 nm for the FBR performed at ambient temperature and in the range 750–1353 nm to that carried out at −20 °C. This result proves that the FBR formed on crystalline silicon could enhance the light trapping in thin film silicon solar cells.