Diffusion of Fe and Na in co-evaporated Cu(In,Ga)Se2 devices on steel substrates

► Impurity diffusion into Cu(In,Ga)Se2 from steel substrates is studied. ► Increased impurity content is seen when adding NaF as sodium source. ► Device performance correlates to Na and Fe contents. ► Loss in open circuit voltage for best device on steel as compared to glass substrate