Cation substitution is a promising approach to reduce the antisite defects and further improve the efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) cells. In this paper, silver (Ag) has been introduced into Cu₂ZnSn(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.

Cu₂Mg_xZn_1−xSnS₄ (0 ≤ x ≤0.6) thin films were prepared by a simple, low-temperature (300 °C) and low-cost sol–gel spin coating method followed by post-annealing at optimum conditions. We optimized the annealing conditions and investigated the effect of Mg content on the crystalline quality, electrical and optical performances of the Cu₂Mg_xZn_1−xSnS₄ thin films. It was found that the Cu₂Mg_xZn_1−xSnS₄ film annealed at 580 °C for 60 min contained large grain, less grain boundaries and high carrier concentration. Pure phase kesterite Cu₂Mg_xZn_1−xSnS₄ (0 ≤ x ≤ 0.6) thin films were obtained by using optimal annealing conditions; notably, the smaller Zn²⁺ ions in the Cu₂ZnSnS₄ lattice were replaced by larger Mg²⁺ ions. With an increase in x from 0 to 0.6, the band gap energy of the films decreased from 1.43 to 1.29 eV. When the ratio of Mg/Mg + Zn is 0.2 (x = 0.2), the grain size of Cu₂Mg_xZn_1−xSnS₄ reaches a maximum value of 1.5 μm and the surface morphology is smooth and dense. Simultaneously, the electrical performance of Cu₂Mg_xZn_1−xSnS₄ thin film is optimized at x = 0.2, the carrier concentration reaches a maximum value of 3.29 × 10¹⁸ cm⁻³.

In this work, the Cu₂Mn_xZn_1−xSn(S,Se)₄ (0 ≤ x ≤ 1) (CMZTSSe) alloy films were fabricated by a sol-gel method. Meanwhile, the effects of Mn substitution on the structural, morphological, electrical, optical, and device performance were studied systematically. The clear phase transformation from Cu₂ZnSn(S,Se)₄ (CZTSSe) with kesterite structure to Cu₂MnSn(S,Se)₄ (CMTSSe) with stannite structure was observed as x = 0.4. The scanning electron microscope (SEM) results show that the Mn can facilitate the grain growth of CMZTSSe alloy films. Since the x was 0.1, the uniform, compact, and smooth film was obtained. The results show that the band gap of the CMZTSSe film with a kesterite structure was incessantly increased in a scope of 1.024–1.054 eV with the increase of x from 0 to 0.3, and the band gap of the CMZTSSe film with stannite structure was incessantly decreased in a scope of 1.047–1.013 eV with the increase of x from 0.4 to 1. Meanwhile, compared to the power conversion efficiency (PCE) of pure CZTSSe device, the PCE of CMZTSSe (x = 0.1) device is improved from 3.61% to 4.90%, and about a maximum enhanced the open-circuit voltage (V_OC) of 30 mV is achieved. The improvement is concerned with the enhancement of the grain size and decrease of the Cu instead of Zn (Cu_Zn) anti-site defects. Therefore, it is believed that the adjunction of a small amount of Mn may be an appropriate approach to improve the PCE of CZTSSe solar cells.