Band-Gap Engineering in Cu(In,Ga)Se2 Thin Films Grown from (In,Ga)2Se3 Precursors

A three-stage process starting with the deposition of (In,Ga)2Se3 precursor films has been successful in the fabrication of graded band-gap Cu(In,Ga)Se2 thin films. In this work we examine (1) the reaction of Cu + Se with (In,Ga)2Se3, which leads to a spontaneous grading in the Ga content as a function of depth through the film, and (2) modification of the Ga content in the surface region of thefilm though a final deposition of In + Ga + Se. We show how band-gap grading can be enhanced by the formation of non-uniform precursors, how counterdiffusion limits the degree of grading possible in the surface region, and how the CuxSe secondary phase acts to homogenize the film composition.