Abstract
<title>Abstract</title> <p> The morphology of precursor thin films exerts a significant influence on the fabrication of CZTSSe thin films. In this work, CZTSSe thin films were fabricated using 2-methoxyethanol (MOE) as the solvent. The morphology of the precursor films was regulated by varying the thiourea-to-metal (Tu/M) ratio in the precursor solution, and the crystallization evolution of the precursor films with distinct morphologies as well as the resulting performance of the CZTSSe solar cells were systematically investigated. Experimental results reveal that an optimized Tu/M ratio in the precursor solution effectively tailors the film porosity, which accelerates the inward transport of Se into the film interior during selenization, thereby significantly improving the vertical crystallization uniformity of the absorber. Additionally, the tailored film porosity promotes the formation of the Cu <sub>2−x</sub> Se phase, enhancing the initial crystallization quality of the films. The enhanced crystallization of the CZTSSe films leads to optimized defect properties and improved device interface characteristics, which suppresses internal carrier recombination and reduces the contact resistance at the back contact. Consequently, the power conversion efficiency (PCE) of the solar cell is substantially boosted from 8.1% to 12.1% (without cation doping and anti-reflection coating). </p>