Influence of precursor stacking on the absorber growth in Cu(In,Ga)S2 based solar cells prepared by a rapid thermal process

Rapid thermal sulfurization of metallic precursors has proven to be a successful method for the preparation of Cu(In,Ga)S2 based solar cells. However, during the sulfurization, several problems can be encountered. Due to the difference in reaction rates between ternary sulfides, the process can result in absorbers with a layered CuInS2/CuGaS2 structure or slow and incomplete sulfurization that leads to samples where an unreacted Cu-Ga metallic phase remains at the back of the sample. The formation kinetics of single phase Cu(In,Ga)S2 is a complex process which depends on several parameters. In this work, we focus on the influence of precursor stacking and investigate the growth of Cu(In,Ga)S2 thin films using scanning electron microscopy and X-ray diffraction. It is observed that precursor alloying occurs prior to sulfurization and that the Cu(In,Ga)S2 compound is formed by the interdiffusion of the ternary CuInS2 and CuGaS2 phases. Correlation between the structural properties of the precursors/absorbers and the obtained solar cells is made.