The electrical and microstructural properties of electroplated screen-printed Ag metal contacts in crystalline silicon solar cells

Abstract

In the present study, electrical and microstructural properties of electroplated screen-printed Ag metal contacts in crystalline silicon solar cells have been investigated. The metal contacts were formed by a fire-through process at different firing temperatures (700–850 °C) in ambient air. This is followed by a low temperature annealing step at 450 °C in ambient nitrogen. Subsequently, an electroless Ag deposition step using AgNO3 solution under light illumination was performed to improve the electrical properties of the Ag contacts. The specific contact resistance, ρc (Ω-cm2) of the Ag contacts was measured by applying the three-point probe method. It shows that a ρc = 2.16 × 10−5 Ω cm2 was measured for the fire-through Ag contacts. It is found that the specific contact resistance of the fire-through Ag contacts was improved to about 2.35 × 10−6 Ω cm2 after applying an electroless Ag deposition step on the Ag contacts. The cross-sectional view of the Ag metal/doped Si contact region by SEM measurement shows that the Ag metal firmly coalesces with the doped Si region. However, the microstructure of the Ag metal has found a large number of micropores and voids. These may be formed due to the burn off of the volatile organic constituents of the Ag metal paste during the sintering process. The deposition of the pure silver (Ag) metal during the electroless Ag deposition step fills up the micropores and voids in the Ag contacts. As a result, finger conductivity and the electrical properties of the fire-through processed Ag contacts are improved.