Silicon Solar Cells: Structural Properties of Ag-Contacts/Si-Substrate

Abstract

The screen-printed silver (Ag) thick-film is the most widely used front side contact in industrial crystalline silicon solar cells. The front contacts have the roles of efficiently contacting with the silicon (Si) and transporting the photogenerated current without adversely affecting the cell properties and without damaging the p-n junction. Although it is rapid, has low cost and is simplicity, high quality screen-printed silver contact is not easy to make due to the complicated composition in the silver paste. Commercially available silver pastes generally consist of silver powders, lead-glass frit powders and an organic vehicle system. The organic constituents of the silver paste are burned out at temperatures below 500°C. Ag particles, which are ~70-85wt% and can be different in shape and size distribution, show good conductivity and minor corrosive characteristics. The concentration of glass frit is usually less than 5wt %; however, the glass frit in the silver paste plays a critical role for achieving good quality contacts to high-doping emitters. The optimization of the glass frit constitution can help achieve adequate photovoltaic properties. The melting characteristics of the glass frit and also of the dissolved silver have significant influence on contact resistance and fill factors (FFs). Glass frit advances sintering of the silver particles, wets and merges the antireflection coating. Moreover, glass frit forms a glass layer between Si and Ag-bulk, and can further react with Si-bulk and forms pin-holes on the Si surface upon high temperature firing. This chapter first describes the Ag-bulk/Si contact structures of the crystalline silicon solar cells. Then, the influences of the Ag-contacts/Si-substrate on performance of the resulted solar cells are investigated. The objective of this chapter was to improve the understanding of front side contact formation by analyzing the Ag-bulk/Si contact structures resulting from different degrees of firing. The observed microscopic contact structure and the resulting solar-cell performance are combined to clarify the mechanism behind the hightemperature contact formation. Samples were fired either at a optimal temperature of ~780°C or at a temperature of over-fired for silver paste to study the effect of firing temperature. The melting characteristics of the glass frit determine the firing condition suitable for low contact resistance and high fill factors. In addition, it was found the post forming gas annealing can help overfired solar cells recover their FF. The results show that after 400°C post forming gas annealing for 25min, the over-fired cells improve their FF. On the other hand, both of the optimally-fired and the under-fired cells did not show similar