Regulating Phase Evolution and Crystallization Dynamics Enables Low Open‐Circuit Voltage Deficit in Wide‐Bandgap Cu ₂ ZnSnS ₄ Solar Cells

ABSTRACT

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.