Regulating Phase Evolution and Crystallization Dynamics Enables Low Open-Circuit Voltage Deficit in Wide-Bandgap Cu2ZnSnS4 Solar Cells

Wide-bandgap kesterite Cu 2 ZnSnS 4 (CZTS) is a promising top-cell candidate for tandem photovoltaics owing to its earth-abundant composition and low-cost solution processability. However, its progress is hindered by a large open-circuit voltage ( V OC ) deficit. A key origin lies in the defects generated during sulfurization. For solution-processed CZTS absorbers, an intermediate-phase pathway involving Cu 2 SnS 3 and other intermediate phases alters the local chemical environment and crystallization behavior, leading to substantial defects and carrier recombination. Here, we develop a bilayer composition-modulated sulfurization (BCMS) strategy using a pre-stabilized Cu-rich bottom layer and a Cu-poor upper layer to regulate Cu redistribution during sulfurization. The BCMS strategy suppresses Cu 2 SnS 3 and favors the direct transformation pathway, while transforming the crystallization from surface-dominated growth to a more synchronized process across the film. The regulation of phase evolution and crystallization suppresses bulk and interface defects, reducing carrier recombination. As a result, a power conversion efficiency of 11.05% with a V OC of 747.64 mV is achieved, representing the lowest V OC deficit reported for solution-processed wide-bandgap (>1.5 eV) CZTS solar cells. The coordinated regulation of phase evolution and crystallization dynamics highlights pathway engineering as an effective strategy for defects and voltage-loss management in kesterite materials, with implications for other chalcogenide semiconductors.