▁e V ▁( for ▁co pper ▁gal li um ▁s elen ide ). ▁ ▁C IG S ▁has ▁an ▁exception ally ▁high ▁absor ption ▁coefficient ▁of ▁more ▁than ▁ 1 0 5 / cm ▁for ▁ 1 . 5 ▁e V ▁and ▁higher ▁energy ▁phot ons . ▁C IG S ▁solar ▁cells ▁with ▁effic ien cies ▁around ▁ 2 0 % ▁have ▁been ▁claimed ▁by ▁the ▁National ▁Ren ew able ▁Energy ▁Labor atory ▁( N RE L ), ▁the ▁Swiss ▁Federal ▁Labor ator ies ▁for ▁Material s ▁Science ▁and ▁Technology ▁( Emp a ), ▁and ▁the ▁German ▁Zent rum ▁für ▁S onnen ener gie ▁und ▁Wasser stoff ▁Forsch ung ▁( Z SW ) ▁( trans l ated : ▁Center ▁for ▁Sol ar ▁Energy ▁and ▁H ydro gen ▁Research ), ▁which ▁is ▁the ▁record ▁to ▁date ▁for ▁any ▁thin ▁film ▁solar ▁cell . ▁ ▁Str ucture ▁▁ ▁The ▁most ▁common ▁device ▁structure ▁for ▁C IG S ▁solar ▁cells ▁is ▁shown ▁in ▁the ▁diagram ▁( see ▁Figure ▁ 1 : ▁Str ucture ▁of ▁a ▁C IG S ▁device ). ▁S oda - lime ▁glass ▁of ▁about ▁of ▁ 1 – 3   mil imet res ▁thick ness ▁is ▁commonly ▁used ▁as ▁a ▁substr ate , ▁because ▁the ▁glass ▁sheets ▁contains ▁sod ium , ▁which ▁has ▁been ▁shown ▁to ▁yield ▁a ▁substantial ▁open - circ uit ▁voltage ▁increase , ▁not ably ▁through ▁surface ▁and ▁gra in ▁boundary ▁defect s ▁pass iv ation . ▁However , ▁many ▁companies ▁are ▁also ▁looking ▁at ▁l ighter ▁and ▁more ▁flexible