Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se 2 thin films

Abstract

Cu(In,Ga)Se 2 (CIGS) layers have been deposited using the three-stage process on Mo coated soda-lime glass substrates with an alkali diffusion barrier. Sodium has been incorporated in such layers by NaF coevaporation, by deposition of NaF precursor layers and by diffusion of Na from a glass substrate without barrier. Scanning electron microscope pictures showed a reduction in grain size of the CIGS films when Na was available during growth. This effect depends on the Na concentration of the finished film, but not on the incorporation method. Furthermore, the [Ga]/[In] concentration ratio is decreased in Na containing CIGS layers at depths at around one-quarter of the absorber thickness. X-ray diffraction patterns indicate a preferred orientation change only in the case of NaF precursors, which is attributed to CIGS growth on a modified surface and/or to the high Na availability during the initial stages of film growth. Solar cells with an efficiency exceeding 15% have been processed from such absorber layers containing Na.