Influence of sodium in fabrication process of high efficiency Cu(In,Ga)Se₂ thin film solar cells

Abstract

Influence of Na on Cu(In,Ga)Se₂ thin film solar cells were studied by using NaF precursor in the three-stage deposition process. The X-ray diffraction spectra showed the increase of (112) preferred-orientation with the thickness of NaF precursor leading to the increase of sharp grains seen on surface morphology. The CIGS thin film solar cells using NaF precursor showed a significant increase of the open-circuit voltage, the fill factor and the conversion efficiency. The optimum Na concentration in the CIGS film was approximately 0.25 at.% to achieve the best cell efficiency of 16% in this work. A simplified Na-assist growth model in the three-stage process was introduced to explain growth mechanism and doping mechanism. It was proposed that Na located at the bottom, during the first stage, affected the preferred orientation of γ-(In,Ga)2Se3 film by enhancing (105) orientation. Then, Na formed Na-related compounds, e.g. Na(In,Ga)Se₂ and Na₂Se₂ in the mixture of CIGS and Cu₂₋xSe compounds in the film during the second stage. They reduced (220)(204) orientations of CIGS films and enhanced roughness with slightly sharper grain. Finally, the Na compounds decomposed and contributed to free Na atoms which were then driven to the surface and thus leaving the vacancies around the grain boundaries. The increase of doping was related to the formation of Na compounds and the Na vacancy defects in the CIGS films which could be observed from the increase of the open-circuit voltage.