Fabrication, Electrical Characterization and Modeling of SiO/SiO2 Passivation Coatings on MOS Solar Cells for Energy Harvesting

This article shows the fabrication, electrical characterization and modeling of SiO/SiO2 layers for passivation of Indoor MOS solar cells operating in inversion mode. After the electrical characterization of Al/SiO/SiO2/p-Si structures as MOS (Metal-Oxide-Semiconductor) capacitors, the effective positive charge in the SiO layer was obtained from the Capacitance x Voltage (CV) characteristics. This positive charge will promote the formation of the depletion mode at the exposed regions SiO/SiO2/p-Si around the metal grid that is the gate of Al/SiO2/p-Si solar cells. The tunneling process through the Al/SiO2/p-Si structure was modeled as a lumped parallel conductance Gp and substrate and contact resistance as a lumped series resistance RS. On the other hand, the effective positive charge of the SiO layer, ~180-195 nm thick, was obtained from the flat band voltage (VFB) shift since the Al/SiO/SiO2/p-Si structure is not influenced by Gp and RS due the high thickness of the SiO/SiO2 dielectrics (~180-195 nm + 1.8 nm or 4.0 nm). As a result, the effective positive charge density from the Al/SiO/SiO2/p-Si structure was 1.24x1012/cm2 and the accumulation regime was modelled by an approximately constant parallel conductance Gp of ~20mS in the Al/SiO2/p-Si structure. Furthermore, an experimental improvement of the conversion yield of light to electricity was observed for SiO/SiO2 passivation layers on indoor MOS solar cells after annealing at 300 oC during 30 min in N2 + H2(10%). Current density x voltage curves (JxV), using solar simulator of 100 mW/cm2 (1 sun) or indoor light illumination of 5.0 mW/cm2 (LED) or 11.7 mW/cm2 (halogen lamp) at 25oC on the sample surface, were extracted for ~180-195 nm SiO thick and the main electrical parameters were obtained for the MOS photovoltaic cells such as the short circuit current (Jsc), the open circuit voltage (Voc) and energy conversion efficiency (η). As a result, a significant increase of the in-door energy conversion efficiency was obtained for the MOS solar cell recovered with SiO layer illuminated with 5.0 mW/cm2 (LED) or 11.7 mW/cm2 (halogen lamp), this is to say, η ≈ 1.0% and η ≈ 5.5%, respectively, which was explained by an increase of the effective positive charges in the SiO layers after annealing.