Spray Pyrolysis Technique Enables Uniform Precursor Layer for Large‐Area Flexible CZTSSe Solar Cells

Kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) is a promising candidate for sustainable photovoltaics, realizing high efficiency via low‐cost fabrication methods remains a challenge due to the difficulty of defect passivation. Herein, we report a feasible approach for fabricating high‐quality CZTSSe absorber using the spray pyrolysis technique. Through optimizing the effects of spray flow rate and substrate temperature, we establish a critical processing window that ensures the formation of a dense, void‐free precursor layer. The optimized absorber exhibits superior crystallinity with significantly reduced surface roughness, facilitating a high‐quality interface with the CdS buffer layer. Deep‐level defect analysis reveals that this optimized crystallization path can effectively suppress Cu/Zn cation disorder and minimize band‐tail states, yielding a low Urbach energy of 33.98 meV. Consequently, the non‐radiative recombination is substantially mitigated, accompanied by a reduced interface trap density and lower defect activation energy. The optimized CZTSSe solar cell achieves a PCE of 9.53% (active area: 0.21 cm 2 ), accompanied by an enhanced open‐circuit voltage. Furthermore, a large‐area flexible CZTSSe solar cell with an active area of 1.22 cm 2 achieves a PCE of 7.65% and retains 91.2% of its initial efficiency after 4 000 bending cycles. This work presents a straightforward and scalable approach for developing large‐area kesterite solar cells.