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 (Cu1-xAgx)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 (CuZn) 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 Cuzn antisite defects of CZTSSe and improve efficiency of CZTSSe device.

In this work, the Cu2MnxZn1−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 (VOC) of 30 mV is achieved. The improvement is concerned with the enhancement of the grain size and decrease of the Cu instead of Zn (CuZn) 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.

The Cu 2 Ni x Zn 1−x Sn(S,Se) 4 (0 ≤ x ≤ 0.1) (CNZTSSe) film absorbing layer materials were successfully prepared by sol−gel means and post selenization technique. The experimental results show that the crystal quality, photoelectric properties, and device efficiency of the film can be improved by Ni doping. The grain size and crystallinity of the films significantly increase by adjusting the doping content of Ni. When x = 0.05, crystallinity and grain size reach the optimum value, and the film surface is smooth and compact. Meanwhile, Ni has successfully replaced the Zn site in the crystal lattice, which reduces the formation of harmful defects related to Zn and improves the electrical properties of the films. The hole concentration of the film is 5.03 × 10 14 cm −3 , and the maximum Hall mobility of the film is 8.95 cm 2 V −1 s −1 under the optimum Ni doping content (x = 0.05). In addition, the continuously tunable band gap (E g ) was obtained, which decreases continuously from 1.17 to 1.08 eV. Compared with the pure Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell, the open circuit voltage (V OC ) of the CZNTSSe (x = 0.05) device was increased by 42 mV, and the power conversion efficiency was boosted from 3.61 to 5.32%. KEYWORDS: thin films, Cu 2 Ni x Zn 1−x Sn(S,Se) 4 , sol−gel, photoelectric properties, solar cells

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