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In this work, the Cu₂Mn<i>_x</i>Zn_1−<i>x</i>Sn(S,Se)₄ (0 ≤ <i>x</i> ≤ 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 <i>x</i> = 0.4. The scanning electron microscope (SEM) results show that the Mn can facilitate the grain growth of CMZTSSe alloy films. Since the <i>x</i> 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 <i>x</i> 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 <i>x</i> from 0.4 to 1. Meanwhile, compared to the power conversion efficiency (PCE) of pure CZTSSe device, the PCE of CMZTSSe (<i>x</i> = 0.1) device is improved from 3.61% to 4.90%, and about a maximum enhanced the open-circuit voltage (<i>V_OC</i>) 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.