Influence of NaF incorporation during Cu(In,Ga)Se2 growth on microstructure and photovoltaic performance

The sodium supply via thermal evaporation of NaF during different stages of a three-stage Cu(In,Ga)Se₂ (CIGS) evaporation process has been investigated. Solar cells were processed on soda lime glass with Si₃N₄ diffusion barrier and on polyimide foils at low substrate temperature of 475°C compatible with the stability of the polyimide foil. Secondary electron micrographs (SEM) of CIGS layers show inhomogeneous microstructure containing regions of small grains near the back contact when sodium is evaporated during the 1^st and the 2^nd CIGS growth stage, respectively. The CIGS layer structure is affected only to minor extent if sodium is incorporated in the 3^rd stage. In order to correlate the layer inhomogeneities with the composition profiles, the CIGS layers were investigated with depth resolved Raman scattering and sputtered neutral mass spectroscopy (SNMS). Both analyzing techniques reveal a strongly graded composition across the CIGS absorber, with an intermediate Ga-poor region and Ga-rich surface and back regions. The performance of resulting solar cells was characterized by means of current-voltage (J-V) and external quantum efficiency (EQE) measurements. It is found that the photovoltaic performance of the cells depends significantly on the NaF incorporation method. Cells developed with a low temperature growth process yielded high efficiencies of up to 16.4% without antireflection coating when NaF was supplied during the 3^rd stage of the CIGS growth process.