Comparative study of different back contact designs for high efficiency CIGS solar cells on stainless steel foils

Summary form only given. The development of Cu(In, Ga)Se₂ (CIGS) thin film solar cells on flexible substrates shows high potential for reducing production costs by roll-to-roll manufacturing. However on steel foils, impurity diffusion from the steel into the CIGS absorber must be prevented to ensure high conversion efficiencies. We processed CIGS solar cells on stainless steel foils using different three-stage processes at substrate temperatures of ~450°C, ~500°C, and ~600°C. Different CIGS back contact designs were used: Mo single layers, Mo bilayers, and Mo bilayers in combination with a Si₃N₄ or TiN impurity diffusion barrier. The back contacts were deposited by an in-line magnetron sputtering process. Two different thicknesses of the Mo single layers were tested for different sputtering conditions. Solar cell experiments with Si₃N₄ and TiN based contacts showed no improvements in performance compared to the Mo bilayer contact regardless of the CIGS deposition temperature. The variation of the Mo back contact designs without an impurity diffusion barrier showed a significant change in solar cell performance when thin contacts at certain sputtering conditions were used. Corresponding secondary ion mass spectroscopy measurements showed an increase in Fe diffusion from the steel into the absorber, whereas no influence on the Cr diffusion was found. The Fe diffusion intensity was in good correlation with the solar cell performance, which was measured by current density to voltage technique. Best cell efficiency of 17.7% (certified by Fraunhofer ISE) was obtained using a Mo bilayer back contact with appropriate sputtering conditions in combination with a low temperature CIGS process without any additional impurity diffusion barrier layer on stainless steel foils.