▁cell ▁while ▁absor bing ▁as ▁little ▁light ▁as ▁possible . ▁ ▁The ▁Cu In Se 2 - based ▁materials ▁that ▁are ▁of ▁interest ▁for ▁phot ov olta ic ▁applications ▁include ▁several ▁elements ▁from ▁groups ▁I , ▁III ▁and ▁VI ▁in ▁the ▁periodic ▁table . ▁These ▁sem icon duct ors ▁are ▁especially ▁attract ive ▁for ▁solar ▁applications ▁because ▁of ▁their ▁high ▁optical ▁absor ption ▁coefficients ▁and ▁vers atile ▁optical ▁and ▁elect rical ▁characteristics , ▁which ▁can ▁in ▁principle ▁be ▁manip ulated ▁and ▁tun ed ▁for ▁a ▁specific ▁need ▁in ▁a ▁given ▁device . ▁ ▁Con version ▁efficiency ▁ ▁C IG S ▁is ▁mainly ▁used ▁in ▁the ▁form ▁of ▁pol yc ry st all ine ▁thin ▁films . ▁The ▁best ▁efficiency ▁achieved ▁as ▁of ▁September ▁ 2 0 1 4 ▁was ▁ 2 1 . 7 %. ▁A ▁team ▁at ▁the ▁National ▁Ren ew able ▁Energy ▁Labor atory ▁achieved ▁ 1 9 . 9 %, ▁a ▁record ▁at ▁the ▁time , ▁by ▁modifying ▁the ▁C IG S ▁surface ▁and ▁making ▁it ▁look ▁like ▁C IS . ▁These ▁examples ▁were ▁depos ited ▁on ▁glass , ▁which ▁meant ▁the ▁products ▁were ▁not ▁mechan ically ▁flexible . ▁In ▁ 2 0 1 3 , ▁scient ists ▁at ▁the ▁Swiss ▁Federal ▁Labor ator ies ▁for ▁Material s ▁Science ▁and ▁Technology ▁developed ▁C IG S ▁cells ▁on ▁flexible ▁polym er ▁fo ils ▁with ▁a ▁new ▁record ▁efficiency ▁of ▁ 2 0 . 4 %. ▁These ▁display ▁both ▁the ▁highest ▁efficiency ▁and ▁greatest ▁flex ibility . ▁ ▁The ▁U . S