Lead free ohmic connections on large surface area Si solar cells

Abstract

Lead free ohmic connections made on industrially manufactured Si solar cells are studied. Flat copper wires coated with either Sn(96.5%)/Ag(3.5%) or Bi(58%)/Sn(42%) eutectic alloys are soldered to the bus bars of the Si cells to form environmentally friendly ohmic connections. Soldering temperatures as low as 180 0C are used for connecting the copper ribbons, coated with the Bi(58%)/Ag(42%) eutectic alloy, to the bus bars. The electrical performance of the Si solar cells with the Pb-free ohmic connections is evaluated from the analysis of the recorded I-V characteristic curves under direct or concentrated sunlight. Micro X-Ray Fluorescence measurements are performed after the completion of each individual step in the process used for making the ohmic connections. Lead is detected only on the bus bars and the quantitative analysis shows that it exceeds the limit specified by the RoHS directive. Its presence is attributed to the Pb-oxide of the glass frit in the Ag paste used during the screen printing fabrication of the Si cells.