Re-investigation of preferential orientation of Cu(In,Ga)Se_2 thin films grown by the three-stage process

摘要

The present contribution deals with the rather longstanding issue of the preferential orientation of Cu(In,Ga)Se_2 polycrystalline thin films. We investigate both the influence of the growth process parameters and that of the presence of Na on the competition between [112] and [220] orientations. The influence of the presence of Na is studied through the comparison of CIGSe layers co-evaporated on our laboratory standard Mo-coated soda lime glass (SLG/Mo) and on substrates with a sodium diffusion barrier (SLG/barrier/Mo); the process dependence of the orientation is evaluated through the comparison of films grown by the standard bithermal three-stage (400–630℃) and the derived isothermal three-stage process (620℃). For all the process/substrate combinations, the properties of the films (preferential orientation, grain size and morphology) have been determined at key steps of the growth. From these experimental results, it can be concluded that, as already suggested in the literature, the final layer orientation is strongly related to the texturation of the (In,Ga)_2Se_3 precursor; however, the amount of Na available when the film becomes Cu-rich (recrystallization at the end of the 2ndstage) can also strongly impact the film orientation. Such a phenomenon is herein interpreted by mean of the grain boundary migration model of recrystallization. In agreement with this new interpretation of the experimental data, processes have been designed in order to grow [220] textured CIGSe layers.