The efficiency of Cu₂ZnSnS₄ (CZTS) solar cells is limited due to interfacial band misalignment and severe non-radiative recombination. ZnSnO (ZTO) is a promising Cd-free buffer layer, offering a potential for favorable band alignment with CZTS absorber. Here, we demonstrate that optimizing the temperature-dependent deposition during reactive magnetron sputtering significantly promotes elemental interdiffusion. For the proposed CZTS/ZTO interface, a favorable “spike-like” band alignment is achieved, effectively enhancing the carrier transport efficiency and reducing the interfacial defect density. Furthermore, Zn diffusion mitigates Cu_Zn (that is, copper atoms sit at sites normally occupied by zinc atoms) antisite defects, reducing the non-radiative recombination and improving the absorber quality. Finally, the champion device achieved the highest power conversion efficiency (PCE) of 10.90% by sputtering ZTO as buffer layer in CZTS solar cell so far, with a high open circuit voltage (V_OC) of 740 mV and a fill factor (FF) of 61.79%. This strategy highlights the potential of sputtered ZTO as a scalable and eco-friendly buffer layer for Cd-free CZTS solar cells.