The interface formed between Cu 3 BiS 3 thin films and the buffer layer is a potentially limiting factor to the performance of solar cells based on Al/Cu 3 BiS 3 /buffer heterojunctions. The buffer layers of ZnS and In 2 S 3 were grown by co-evaporation, and tested as an alternative to the traditional CdS deposited by chemical bath deposition. From the Kelvin probe force microscopy measurements, we found the values of the work function of ZnS, In 2 S 3 , and CdS, layers deposited into Cu 3 BiS 3 . Additionally, different electronic activity was found for different grain boundaries (GBs), from studies under illumination, we also found the net doping concentration and the density of charged GB states for Cu 3 BiS 3 and Cu 3 BiS 3 /CdS.