Buffer layers in Cu(In,Ga)Se 2 solar cells and modules

Abstract

We report the technical progress that has occurred in the last decade in the development of Cd-free buffer layers for Cu(In,Ga)Se 2-based thin-film solar cells and modules. In different laboratories, films based on ZnS, ZnSe, ZnO, (Zn,Mg)O, In(OH) 3, In 2S 3, In 2Se 3, InZnSe x , SnO 2, and SnS 2 were deposited on differently processed absorbers and tested as an alternative to the traditional CdS buffer. The deposition methods used are: chemical bath deposition (CBD), atomic layer deposition (ALD), metal organic chemical vapour deposition (MOCVD), ion layer gas reaction (ILGAR), sputtering, thermal evaporation, and electrodeposition (ED). Several processes demonstrated efficiencies comparable with the CBD CdS standard. The highest total-area cell efficiency of 18.6% was achieved by NREL and AGU with a CBD ZnS-based buffer layer on a three-stage-process absorber. Showa Shell fabricated several 30×30 cm 2 modules with efficiencies up to 14.2% using a CBD ZnS-based buffer layer on Cu(In,Ga)(Se,S) 2 absorbers grown after the selenisation/sulphurisation of precursor metal films. ZSW fabricated 30×30 cm 2 modules with an ALD In 2S 3 buffer on in-line evaporated Cu(In,Ga)Se 2 absorbers with a maximum aperture-area efficiency of 12.9%.