Aluminum pastes (lead-free/low-bow) for thin wafers

Presently, a typical thickness of the silicon wafer used by the solar cell industries is about 280∼330 μm. The industry trend is to use thinner wafers to reduce the cost of solar cells. The wafer cost is about 60% of the cell fabrication cost. Going-forward industry is targeting thickness of <250 μm. As the wafer thickness decreases, the bowing of the cell due to the sintering stress generated by aluminum increases. Also, the industries are trying to make cells with metal pastes which do not contain lead due to the environment concern. Present aluminum pastes contain lead-based glass and make finished cells bow after firing. This paper deals with a development of lead-free aluminum paste for thin wafers. A typical aluminum paste contain metal powders, glasses, and additives mixed in an organic medium. The effect of paste variables such as metal powder, glass chemistry and additives on electrical performance and bowing was investigated. Also the effect of aluminum paste deposit amount on bending and the effect of aluminum paste deposit amount and firing temperature on BSF layer were studied. Based on this study new lead-free/low-bow aluminum pastes are developed. Newly developed lead-free low-bow aluminum paste (B) and (C) shows only 0.6 and 0.4 mm of bowing respectively on 240 μm thick × 5" × 5" poly crystal wafer when the wet deposition amount is ∼1.14 gram per wafer.