▁metal ▁fo il ▁substr ate ▁is ▁used ▁as ▁the ▁working ▁electro de ▁in ▁industrial ▁processes . ▁An ▁in ert ▁material ▁provides ▁the ▁counter ▁electro de , ▁and ▁the ▁reference ▁electro de ▁measures ▁and ▁controls ▁the ▁potential . ▁The ▁reference ▁electro de ▁allows ▁the ▁process ▁to ▁be ▁performed ▁pot enti ost atically , ▁allowing ▁control ▁of ▁the ▁substr ate ' s ▁potential . ▁ <0x09> ▁ ▁Sim ult aneous ▁electro de position ▁must ▁overcome ▁the ▁fact ▁that ▁the ▁elements ' ▁standard ▁reduction ▁potential s ▁are ▁not ▁equal , ▁causing ▁prefer ential ▁de position ▁of ▁a ▁single ▁element . ▁This ▁problem ▁is ▁commonly ▁alle vi ated ▁by ▁adding ▁counter ing ▁ ions ▁into ▁solution ▁for ▁each ▁ion ▁to ▁be ▁depos ited ▁( C u 2 + , ▁Se 4 + , ▁In 3 + , ▁and ▁Ga 3 + ), ▁thus ▁changing ▁that ▁ion ' s ▁reduction ▁potential . ▁Further , ▁the ▁Cu - Se ▁system ▁has ▁a ▁complicated ▁behavior ▁and ▁the ▁film ' s ▁composition ▁depends ▁on ▁the ▁Se 4 + / C u 2 + ▁ion ▁flux ▁ratio ▁which ▁can ▁vary ▁over ▁the ▁film ▁surface . ▁This ▁requires ▁the ▁prec ursor ▁concentr ations ▁and ▁de position ▁potential ▁to ▁be ▁optimized . ▁Even ▁with ▁optimization , ▁reprodu ci b ility ▁is ▁low ▁over ▁large ▁areas ▁due ▁to ▁composition ▁variations ▁and ▁potential ▁dro ps ▁along ▁the ▁substr ate . ▁ <0x09> ▁The ▁resulting ▁films ▁have ▁small ▁gra ins , ▁are ▁Cu - rich , ▁and ▁generally ▁contain ▁Cu 2 − x S ex ▁phases ▁along