Comparative Study of Different Back-Contact Designs for High-Efficiency CIGS Solar Cells on Stainless Steel Foils

In this paper, Cu(In,Ga)Se₂ (CIGS) solar cells on stainless steel foils were developed using a three-stage evaporation processes at low and high substrate temperatures. Different CIGS back-contact designs were used: a thin Ti adhesion layer together with a Mo single layer, Mo bilayer, and Mo bilayer in combination with either an Si₃N₄ or TiN impurity diffusion barrier layer. Solar cells on contacts with an Si₃N₄ and a TiN barrier coating showed no improvement in performance compared with the cells on Ti/Mo/Mo triple-layer contact, regardless of the CIGS deposition temperature. The variation of the Mo back-contact designs with no impurity diffusion barrier showed a significant decrease in solar cell performance when thin Mo contacts were used. Furthermore, capacitance to voltage measurements showed a decrease in the carrier concentration in CIGS grown on thin Mo contacts compared with the layers for solar cells on Ti/Mo/Mo triple layer structures. Best cell efficiencies of 17.3% were obtained using a Mo back-contact with a thin Ti adhesion layer, in combination with a low-temperature CIGS deposition process with no additional oxide or nitride impurity diffusion barrier layer on stainless steel foils.