The interface formation in CuIn1-xGaxSe2 (CIGS) based solar cells with In2S3 buffer layer deposited by ultrasonic spray pyrolysis (USP) is presented. The photovoltaic properties of solar cells with USP-In2S3 buffer depend strongly on the [Ga]/([Ga] +/- [In]) ratio in the bulk of the absorber and on the surface composition. It was found that the band alignment between the CIGS and In2S3 changes from spike to cliff at a [Ga]/ ([Ga] +/- [In]) ratio between 0.22 and 0.23. A significant increase in V-OC and FF is observed for CIGS solar cells with USP-In2S3 buffer layer after device annealing at 200 degrees C in air, which can be correlated to an increase in activation energy caused by a band gap widening of the surface region of the absorber due to copper depletion. (C) 2011 Published by Elsevier B.V.