Al2O3 layer deposited by atomic layer deposition was successfully used as a diffusion barrier and insulator for the stainless steel substrate of a Cu(InGa)Se-2 thin film solar cell. The 100-300 nm-thick Al2O3 coating on stainless steel improved the surface smoothness and presented reliable resistance. More importantly, the diffusion of detrimental impurities, such as Fe and Ni, was significantly reduced during CuInSe2 deposition at the substrate temperature of 550 degrees C.