Self-designed selenium thermal-cracking system is used to produce selenium vapor with high activity for sufficient selenization of CZTSSe absorber. CZTS precursor film prepared by magnetron sputtering of CZTS quaternary target. By adjusting the heating temperature of selenium source to 600 °C, macromolecular Se n can be cracked to small molecule Se n with high reaction activity, which is helpful to form densely packed large grains in the CZTSSe absorber due to the reduction of selenium vacancies and the enhancement of surface migration. The crystallization quality of CZTSSe absorber is further improved by the optimization of selenium source heating rate. A peak performance of solar cells achieved exhibiting a power conversion efficiency of 7.9%, with V oc of 410 mV, J sc of 35.25 mA/cm2 and FF of 54.98%. J-V characterization, electrochemical impedance spectra and capacitance-voltage tests are also employed to analysis the device performance enhancement mechanism. • Selenium vapor with high activity for sufficient selenization. • Macromolecular Se n (n ≥ 5) can be cracked to small molecule Se n with high reaction activity. • Higher suppression ability for carrier recombination. • Inherent performance enhancement mechanism. [ABSTRACT FROM AUTHOR]