Chemical bath deposition of CdS buffer layer: prospects of increasing materials yield and reducing waste

The CdS buffer layer for CIGS-based solar cells is grown in an aqueous solution containing a cadmium salt, ammonia, and thiourea. Bottlenecks of this technique called chemical bath deposition (CBD) are the low material yield and the production of toxic CdS-containing waste. To improve yield and reduce waste, the CdS precipitate was separated from the waste after deposition by ultra-filtration, and the permeate, which contains ammonia and thiourea, was used for the next CBD process after addition of cadmium salt. The use of permeate leads to a decrease of the CdS growth rate but has no significant influence on the CdS film composition and on the Cu(In,Ga)Se-2/CdS/ZnO device performance. The prominent formation of guanidine and urea was identified and quantified by chemical analysis of the permeate. A decrease of the deposition rate is observed as a function of the number of runs, which is related to the enrichment of the permeate with reaction products and to hydroxide ion consumption. The growth rate can be maintained by adjusting the concentrations after each CBD run. (C) 2001 Elsevier Science B.V. All rights reserved.