Sodium doping of solution-processed Cu2ZnSn(S,Se)4 thin film and its effect on Cu2ZnSn(S,Se)4 based solar cells

The (NaxCu1-x)(2)ZnSn(S,Se)(4) alloy films with different Na concentrations were synthesized by using a facile and environmentally friendly sol-gel method. In this paper, we discuss the effect of Na content on the crystalline quality, and photo-electric properties of the films. The bandgap of (NaxCu1-x)(2)ZnSn(S,Se)(4) is adjusted between 0.97 and 1.02 eV with an increase of x from 0 to 0.2. The (NaxCu1-x)(2)ZnSn(S,Se)(4) film possessing the best crystal quality and optical-electrical characteristics was obtained at x = 0.10. In comparison with the Cu2ZnSn(S,Se)(4) (CZTSSe) cell, the power conversion efficiency (PCE) increased from 2.98 % to 4.96 % for the (NaxCu1-x)(2)ZnSn(S, Se)(4) (x = 0.10) cell. The enhancement in efficiency might be mainly attributed to the improvement of crystal quality for CZTSSe absorber layer and the variation of optical bandgap caused by the Na doping. The results reveal the possibility to obtain a further enhancement in the optoelectronic characteristic of the devices that could be achieved by the optimization of the (NaxCu1-x)(2)ZnSn(S,Se)(4) layer with a fine-tuning of the Na concentration.