Down-Conversion Effect Created by SiOx Films Obtained by HFCVD and Applied over Pn-Junctions

Abstract

A top coating film of silicon oxide off-stoichiometry (SiOx) was used on silicon solar cells to improve its electrical properties by the Down-conversion effect. The SiOx films were obtained by hot filament chemical vapor deposition (HFCVD) with different hydrogen flow (HF) and different distances: the wire-source distance and the source-substrate distance, and by consequence, with different temperatures. Transmittance spectra showed 0% of transmittance in the UV region. The amount of atomic silicon % in the SiOx films was changed with the variation of the mentioned parameters, as demonstrated with the Energy-Dispersive X-ray Spectroscopy (EDS) and the Fourier transform infrared spectroscopy (FTIR) results. The SiOx films showed a wide Photoluminescence (PL) between 400 nm and 900 nm when they were excited with UV light. The SiOx films were applied as a top coating on pn junctions of silicon substrate to develop silicon solar cells. The J-V measurements and the external quantum efficiency (EQE) of the solar cells were obtained, as well as the open circuit voltage (Voc), short-circuit current density (Jsc), the efficiency (η) and the fill factor (FF). The silicon solar cells with the SiOx top coatings, with the highest PL, showed the highest efficiency. The best figures of merit were η = 5.9%, Voc = 480 mV, Jsc = 27 mA/cm2, Pmax = 5.5 mW and FF = 0.4.