Chemical bath deposition (CBD) techniques can offer convenient, low cost and environmentally compatible tools to address a number of manufacturing thin films for different application especially in photovoltaic. CBD has played a crucial role in thin film solar cells based on the Cu-chalcopyrite material system. At present, device efficiencies up to 19.5 % for low-gap Cu(In,Ga)Se₂ and around 12 % for high-gap CuInS₂ have been demonstrated. All these high efficiency values so far have been reported for solar cell devices with CBD-CdS buffer layers between the well-conducting n⁺-ZnO/i-ZnO window bi-layer and the Cu-chalcopyrite thin film absorber. CBD-CdS has also proven successful in the industrial pilot line and the technology of chalcopyrite solar modules with the conventional CdS buffer as hetero-junction partner is close to commercialization. However many material issues are not fully understood: a) why is the CdS buffer necessary?, b) is Cd diffusion into the absorber and n-type doping of the absorber surface creating a buried junction? c) is a Cu-deficiency phase present at the chalcopyrite surface?, d) What is the role of the band offsets at the buffer/absorber interface?, etc. In addition CdS as a heavy metal compound should be avoided in the final solar modules as well as in their production processes from an ecological as well as an economical point of view. Therefore a large number of alternative materials have been developed for the deposition by CBD. We report on the progress done for understanding the role of CBD and the recent development on alternative buffers in chalcopyrite solar cells. Particular interest will be given to the development of ZnS/Zn(S,O) bi-layer as alternative to CdS. Analysis of CuInS₂/ZnS/Zn(S,O) interfaces, the diffusion/intermixing processes affecting the absorber/buffer hetero-interface and the resulting device performances are discussed.

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1. Effects of different Zn precursors on Zn(S,O) buffer layers deposited by chemical bath for chalcopyrite based Cd-free thin film solar cells