Effects of Bi Incorporation on Cu(In_1-x,Ga_x)Se₂ Thin Films and Solar Cells

The effects of bismuth (Bi) incorporation into Cu(In_1-x,Ga_x)Se₂ (CIGS) thin films and solar cells have been investigated. 10–50-nm-thick Bi thin layers were deposited onto Mo-coated soda-lime glass (SLG) and SiO_x-coated SLG substrates by vacuum evaporation. CIGS thin films were then deposited by a three-stage process at substrate temperatures of 450–550 °C. The grain growth of CIGS thin films was enhanced, and the open-circuit voltage and hence the conversion efficiency was improved by the Bi incorporation when the SLG substrates were used. However, little effect was observed when the alkali barrier SiO_x layer was deposited on SLG substrates. As a result, we found that the Bi incorporation is beneficial for improving the cell performance when sodium exists simultaneously in CIGS layers.