Ultrasonically sprayed Zinc sulfide buffer layers for Cu(In,Ga)(S,Se)2 solar cells

Zinc sulfide (ZnS) buffer layer deposited by an ultrasonic spray pyrolysis (USP) method is a feasible alternative to the chemical bath deposited cadmium sulfide (CdS) buffer layer. In the present work we report the results of a low-cost, non-vacuum and in-line compatible method to grow ZnS thin films. We investigated the properties of USP-ZnS films grown at different substrate temperatures and spray solution precursors. Rutherford backscattering spectrometry measurements were done for quantitative chemical composition information, revealing chlorine impurities depending on the deposition temperature as well as on the chemical precursors. By optimizing the spray parameters of USP-ZnS buffer layers on Cu(In,Ga)(S,Se)2 absorbers, a maximum solar cell efficiency of 10.8% after air-annealing was achieved, whilst the CdS reference fabricated on a similar absorber reached 11.4%. A significant increase of the short circuit current is observed as compared to the CdS reference due to a gain in the blue wavelength region.