The Firing Process (Sintering) in Silicon Solar Cell Manufacturing

The firing process, also referred to as sintering, is one of the key steps with which the front-metal contact is formed in a silicon solar cell. In this process, the thick film paste is dried at about 150oC to remove much of the solvents. The presence of solvents can cause excessive out gassing which can lead to cracks and voids. The dried substrates are then fired inside a firing furnace. The firing process consists of four primaries. The first step is the initial temperature ramp up where the paste solvents are volatilized. The second step is the burn out. The objective of the burn out phase is to remove all of the organic binder that was used in paste formation. The burn out phase is carried out at 300-4000C. The third step is the sintering, or the firing process, which is done between the ranges of 700-800C. During this process, the Ag metal forms a bond with the underlying silicon substrate to form metal contact. The final step in the firing process is the wafer cool down phase. The mechanism behind contact formation in a fire through contact is very complex and not fully understood [4]. According to Mohammed Et al, the process starts by evaporating the solvent between 100-200C and then burning out the polymer between 200 and 400C. Later, from 400-600C, the glass frit melts and the sintering of the Ag particles take place. Furthermore, from 600-800C, molten glass with some amount of dissolved Ag etches the silicon nitride anti reflection coating and then reaches the Si surface. Here, it reacts and etches a very thin layer of Si. Ag in the glass will then precipitate onto the Si surface in the form of crystallites. The quality of the contact influences the shunt resistance, series resistance and junction leakage current, which have a significant impact on the efficiency of the solar cell. Hence, it is very important to understand various aspects of the firing process to be able to achieve higher efficiencies. Some of them include; peak temperature, dwell time, and the temperature at which Si –Ag alloy formation happens.  Over the next few blogs I will discuss these!  Make sure to follow us on twitter  www.twitter.com/torreyhillstech