Tailoring selenization dynamics: How heating rate manipulates nucleation and growth boosts efficiency in kesterite solar cells.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has emerged as a promising photovoltaic material due to its low cost and high stability. The CZTSSe film for high-performance solar cells can be obtained by annealing the deposited CZTS precursor films with selenium (a process known as selenization). The design of the selenization process significantly affects the quality of the absorber layer. In this work, we systematically investigate the impact of heating rate on the selenization kinetics and the microstructural characteristics of the films using a two-step selenization method. The results indicate that a slow heating rate promotes surface crystallization, resulting in a thick and dense layer of large grains at the film surface that impedes the diffusion of Se vapor. Conversely, a rapid heating rate enhances the diffusion of Se into the interior of the film, synthesizing more low-melting-point intermediate compounds that facilitate grain growth and reduce the thickness of fine grains at the film bottom. Ultimately, a CZTSSe solar cell with an efficiency of 10.17% was fabricated at a heating rate of 200 °C/min. This research deepens the understanding of thin film growth mechanisms and advances the development of high-performance solar cells.