Buffer layers in Cu(In,Ga)Se2 solar cells and modules

We report the technical progress that has occurred in the last decade in the development of Cd-free buffer layers for Cu(In,Ga)Se2-based thin-film solar cells and modules. In different laboratories, films based on ZnS, ZnSe, ZnO, (Zn,Mg)O, In(OH)3, In2S3, In2Se3, InZnSex, SnO2, and SnS2 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 cm2 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 cm2 modules with an ALD In2S3 buffer on in-line evaporated Cu(In,Ga)Se2 absorbers with a maximum aperture-area efficiency of 12.9%.