Improving Ag Thick Film Contacts and Al Back Surface Field Quality of PERC Silicon Solar Cells by High Speed Rapid Thermal Processing

Abstract

Rapid thermal processing (RTP) is a key technology in the screen-printed silver (Ag) and Aluminum (Al) metallization to form excellent contacts to silicon (Si) solar cell. The unique firing profiles of RTPs solve different tasks during contact co-firing, especially the ramp up and ramp down rates are very critical in achieving the precise contacts independent of the source of metallic pastes. The ramp up/down rates are controlled by the belt speed of the furnace which also determines the dwell time of the contact at elevated temperatures, which ranges from 750–800 °C. Although high belt speeds and hence high ramp up/downs, for contact co-firing are beneficial, due to the difficulties involved in reaching the peak firing temperature at high belt speeds, the commercial infrared (IR) belt furnaces are operated at belt speeds of between 180–200 inches per minute (ipm). Therefore, this paper reports on the development of high belt speeds for screen printed passivated emitter rear contacts (PERC) Si solar cell with no additional equipment. By achieving desired peak temperatures at high belt speeds (up to 375 ipm), the open circuit voltage (Voc) can be increased by ~5 mV along with 1–2 % increase in fill factor (FF) without adding any cost to production line.