Formation of Alloyed\nCu<sub>2</sub>Co<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>Sn(S,Se)<sub>4</sub> Absorption Layer and Its Application\nin Solar Cells

Partial substitution of cations is crucial for suppressing\nharmful\ndefects in Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> thin-film solar cells.\nIn this study, based on the mixed <i>n</i>-butylammonium\nand butyrate solution system, the alloyed Cu<sub>2</sub>Co<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>Sn(S,Se)<sub>4</sub> phase can be prepared by substituting Zn<sup>2+</sup> with\nCo<sup>2+</sup>, which can suppress harmful defects and optimize the\ncrystallinity of the Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> absorption\nlayer, and improve the photoelectric conversion efficiency (PCE) of\ndevices. By systematic investigation of the impact of Co content on\nthe performance of devices, the optimal substitution amount of Zn<sup>2+</sup> with Co<sup>2+</sup> is 0.05. At this time, PCE, the open-circuit\nvoltage (<i>V</i><sub>OC</sub>), current density (<i>J</i><sub>SC</sub>), and fill factor (FF) of the devices can\nreach 9.0%, 416 mV, 33.87 mA/cm<sup>2</sup>, and 64%, respectively.\nIt is the first time that the replacement of Zn<sup>2+</sup> with\nCo<sup>2+</sup> is applied to optimize PCE of CZTSSe solar cells.\nThe excellent results also demonstrate that the substitution of Zn<sup>2+</sup> with Co<sup>2+</sup> can become a new approach for further\nperformance optimization of Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> solar\ncells.