▁in ▁the ▁film ▁at ▁the ▁interface ▁between ▁the ▁α ▁phase ▁and ▁the ▁O DC . ▁The ▁recom bin ation ▁velocity ▁at ▁the ▁C IG S / C d S ▁interface ▁is ▁decre ased ▁by ▁the ▁hom oj unction ' s ▁presence . ▁The ▁drop ▁in ▁interface ▁recom bin ation ▁att rib utable ▁to ▁O DC ▁formation ▁is ▁demonstrated ▁by ▁experiments ▁which ▁have ▁shown ▁that ▁recom bin ation ▁in ▁the ▁bulk ▁of ▁the ▁film ▁is ▁the ▁main ▁loss ▁mechanism ▁in ▁Cu ▁def ic ient ▁films , ▁while ▁in ▁Cu ▁rich ▁films ▁the ▁main ▁loss ▁is ▁at ▁the ▁C IG S / C d S ▁interface . ▁▁ <0x09> ▁ ▁S od ium ▁incorpor ation ▁is ▁necessary ▁for ▁optimal ▁performance . ▁Ide al ▁Na ▁concentration ▁is ▁considered ▁to ▁be ▁approximately ▁ 0 . 1 %. ▁Na ▁is ▁commonly ▁supplied ▁by ▁the ▁s oda - lime ▁glass ▁substr ate , ▁but ▁in ▁processes ▁that ▁do ▁not ▁use ▁this ▁substr ate ▁the ▁Na ▁must ▁be ▁deliber ately ▁added . ▁Na ' s ▁benef icial ▁effects ▁include ▁increases ▁in ▁p - type ▁conduct ivity , ▁texture , ▁and ▁average ▁gra in ▁size . ▁Furthermore , ▁Na ▁incorpor ation ▁allows ▁for ▁performance ▁to ▁be ▁maintained ▁over ▁larger ▁sto ichi ometric ▁devi ations . ▁Sim ulations ▁have ▁predicted ▁that ▁Na ▁on ▁an ▁In ▁site ▁creates ▁a ▁shall ow ▁accept or ▁level ▁and ▁that ▁Na ▁serves ▁to ▁remove ▁In ▁on ▁Cu ▁defect s ▁( don ors ), ▁but ▁reasons ▁for ▁these ▁benefits ▁are ▁controvers ial . ▁Na ▁is ▁also ▁cred ited