SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

Abstract

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si3N4) deposited over the n+-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si3N4 layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n+-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n+-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρc (in Ω-cm2) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρc = 2.53 × 10−5 Ω-cm2 is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρc is two orders of magnitude lower than the typical value of ρc = 3 × 10−3 Ω-cm2 reported previously for the Ag contacts of the solar cell. Such low value of ρc for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρc versus impurity doping level (Ns) shows that measured value of the ρc follows a linear relationship with the Ns as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n+-Si surface of the silicon solar cell.