Substitution of Ag for Cu in Cu ₂ZnSn(S,Se) ₄: Toward Wide Band Gap Absorbers with Low Antisite Defects for Thin Film Solar Cells

Abstract

Cation substitution is a promising approach to reduce the antisite defects and further improve the efficiency of Cu ₂ZnSn(S,Se) ₄ (CZTSSe) cells. In this paper, silver (Ag) has been introduced into Cu ₂ZnSn(S,Se) ₄ (CZTSSe) thin film to replace Cu partially and form (Cu _1-xAg _x) ₂ZnSn(S,Se) ₄ (0 ≤ x ≤ 1) (CAZTSSe) alloy films by combination of solution method and a rapid annealing technique. The fundamental properties of the mixed Ag-Cu kesterite compound are systematically reported as a function of the Ag/(Ag+Cu) ratio. The results show that band gap of kesterite CAZTSSe is incessantly increased by adjusting the Ag doping content, indicating that the CAZTSSe alloy film is a potentially applicable bandgap grading absorption layers material to obtain higher CZTSSe device. Furthermore, CAZTSSe alloy films with better electrical performance were also obtained by adjusting the Ag content during film fabrication. Finally, we also observed an increment in open circuit voltage (Voc) by 160 mV and an accompanying rise in device efficiency from 4.24 to 5.95%. The improvement is correlated to the improved grain size and decreased antisite defects of Cu instead of Zn site (Cu _Zn) in the lattice. The Voc enhancement evidences that the solution method is facile and viable to achieve proper cation substitution toward higher efficiency kesterite solar cells. In addition, the CAZTSSe cell also displays better charge collection performance because of the higher fill factor (FF) and power conversion efficiency (PCE). Therefore, it can be concluded that the doping of Ag is a potentially appropriate method to reduce the Cu _zn antisite defects of CZTSSe and improve efficiency of CZTSSe device.