S ▁solar ▁cells ▁offer ▁total ▁area ▁effic ien cies ▁of ▁ 1 5 . 0 % ▁and ▁ 9 . 5 %, ▁respectively . ▁In ▁ 2 0 1 5 , ▁the ▁gap ▁with ▁the ▁other ▁thin ▁film ▁techn ologies ▁has ▁been ▁closed , ▁with ▁record ▁cell ▁effic ien cies ▁in ▁labor ator ies ▁of ▁ 2 1 . 5 % ▁for ▁C d Te ▁( First S olar ) ▁and ▁ 2 1 . 7 % ▁for ▁C IG S ▁( Z SW ). ▁( See ▁also ▁N RE L ▁best ▁research ▁cell ▁efficiency ▁chart .) ▁ ▁Properties ▁▁ ▁All ▁high ▁performance ▁C IG S ▁absor bers ▁in ▁solar ▁cells ▁have ▁similar ities ▁independent ▁of ▁production ▁technique . ▁First , ▁they ▁are ▁pol yc ry st all ine ▁α - phase ▁which ▁has ▁the ▁ch al copy rite ▁cry stal ▁structure ▁shown ▁in ▁Figure ▁ 3 . ▁The ▁second ▁property ▁is ▁an ▁overall ▁Cu ▁def ic iency . ▁Cu ▁def ic iency ▁increases ▁the ▁majority ▁car rier ▁( hole ) ▁concentration ▁by ▁increasing ▁the ▁number ▁of ▁( elect ron - accept ing ) ▁Cu ▁vac an cies . ▁When ▁C IG S ▁films ▁are ▁In ▁rich ▁( C u ▁def ic ient ) ▁the ▁film ' s ▁surface ▁layer ▁forms ▁an ▁ordered ▁defect ▁comp ound ▁( O DC ) ▁with ▁a ▁sto ichi ometry ▁of ▁Cu ( In , G a ) 3 Se 5 . ▁The ▁O DC ▁is ▁n - type , ▁forming ▁a ▁p - n ▁hom oj unction