The fire-through processed screen-printed Ag thick film metal contacts formed on an electrochemically etched porous silicon antireflection coating of silicon solar cells

Abstract

The fabrication of reproducible low resistance ohmic contacts on electrochemically etched porous silicon is an existing challenge due to the irregular and nanoscale sized Si crystallites and the voids in the structure of the porous silicon. In this study, the effect of the peak firing treatment by a fire-through process at different temperatures (700–825 °C) and electroless Ag deposition steps on the electrical properties of the Ag electrodes have been investigated. The cross-sectional view of the Ag metal/doped Si region by SEM shows that Ag metal firmly coalesces with the doped Si surface. The sintering of the metal contact at a higher peak firing temperature enables the firing-through of the Ag metal paste onto the high resistivity porous silicon that leads to the interaction with the doped Si region. As a result, a good ohmic contact between the Ag metal and doped Si emitter is formed. The specific contact resistance, ρ_c (Ω.cm²) measurement of the Ag electrode by the three-point probe method shows that the ohmic properties of the just sintered Ag electrodes are very good, with ρ_c ≅ 1.025 × 10⁻⁴ Ω.cm². An electroless Ag deposition step using AgNO₃ solution under constant light illumination was applied to improve the electrical properties of the sintered Ag contacts. It is found that the ρ_c of the Ag electrodes was improved to about 3.25 × 10⁻⁵ Ω.cm² after applying the electroless Ag deposition on the sintered Ag contacts. The deposition of the pure silver metal on the Ag contacts fills up the micropores and voids and improves the electrical properties of the contact structure.