Ultra-thin CdS layers non intentionally doped and doped with Cu were grown by chemical bath deposition and applied as buffer layers for chalcogenide CuIn1-chi Ga chi Se2 and Cu2ZnSnSe4 (CZTSe) based solar cells. We demonstrate that the use of Cu as dopant allows to reduce the CdS thickness below 30 nm while keeping the same efficiency levels as those obtained with conventional 70 nm in thickness undoped CdS. This is mainly explained by the improved V-oc values when Cu-doped CdS is employed, obtaining voltages among the highest reported values, especially for CZTSe devices. We propose the formation of a metal-insulator-semiconductor (MIS) type device for explaining the observed experimental behavior based in indirect optoelectronic characterization of the layers and devices. This opens the possibility to use ultra-thin buffer layers for high efficiency chalcogenide based solar cells, reducing the environmental impact of the CdS buffer deposition, without detrimental impact on the optoelectronic properties of the devices. (C) 2016 Elsevier B.V. All rights reserved.