▁substr ates ▁such ▁as ▁poly im ide ▁or ▁metal ▁fo ils . ▁A ▁mol y b den um ▁( Mo ) ▁metal ▁layer ▁is ▁depos ited ▁( comm only ▁by ▁s put ter ing ) ▁which ▁serves ▁as ▁the ▁back ▁contact ▁and ▁reflect s ▁most ▁un abs or bed ▁light ▁back ▁into ▁the ▁C IG S ▁absor ber . ▁Following ▁mol y b den um ▁de position ▁a ▁p - type ▁C IG S ▁absor ber ▁layer ▁is ▁grown ▁by ▁one ▁of ▁several ▁unique ▁methods . ▁A ▁thin ▁n - type ▁buffer ▁layer ▁is ▁added ▁on ▁top ▁of ▁the ▁absor ber . ▁The ▁buffer ▁is ▁typically ▁cad m ium ▁sul f ide ▁( C d S ) ▁depos ited ▁via ▁chemical ▁bath ▁de position . ▁The ▁buffer ▁is ▁over la id ▁with ▁a ▁thin , ▁intr insic ▁z inc ▁ox ide ▁layer ▁( i - Z n O ) ▁which ▁is ▁c apped ▁by ▁a ▁th icker , ▁al umin um ▁( Al ) ▁do ped ▁Z n O ▁layer . ▁The ▁i - Z n O ▁layer ▁is ▁used ▁to ▁protect ▁the ▁C d S ▁and ▁the ▁absor ber ▁layer ▁from ▁s put ter ing ▁damage ▁while ▁depos iting ▁the ▁Z n O : Al ▁window ▁layer , ▁since ▁the ▁latter ▁is ▁usually ▁depos ited ▁by ▁DC ▁s put ter ing , ▁known ▁as ▁a ▁dam aging ▁process . ▁The ▁Al ▁do ped ▁Z n O ▁serves ▁as ▁a ▁transparent ▁conduct ing ▁ox ide ▁to ▁collect ▁and ▁move ▁electrons ▁out ▁of ▁the