i-ZnO layers were deposited as diffusion barriers fabricated by RF sputtering on stainless-steel substrates (SUS430, matches with AISI SUS24). It was found that the addition of ZnO layer between stainless-steel substrate and Mo back contact film deplete diffusion of metal ions from substrate and reduce recombination at CIGS layer, as identified by an SIMS depth profile, QE and C-V measurements. With such diffusion barriers, the efficiency, open-circuit voltage, short-circuit current and fill factor of CIGS solar cells all increased, compared to reference cells without diffusion barrier. For the better device performance, Na was supplied during Mo back-contact layer deposition by co-sputtering of the target, including Na-source. Efficiencies of cells were increased with increasing the quantity of Na source. Unlike barrier thickness effect, short circuit current was reduced and open circuit voltage, fill factor were increased with increasing Na-source, and achieved 12.6% efficiency without AR(anti-reflection) coating. The relationship and causality between these results and the Na-doping were analyzed using C-V measurements.