Ammonia‐Induced Complexation for Stabilized Precursor Ink Toward Efficient Kesterite Solar Cells

Cu ₂ ZnSn(S, Se) ₄ (CZTSSe) solar cells attract extensive attention due to their excellent optoelectronic properties and earth‐abundant constituents. The solution‐processed CZTSSe absorbers based on molecular ink are a promising approach for high‐efficiency CZTSSe solar cells. However, the molecular inks suffer from poor stability because of Cu(I) oxidation, limiting device reproducibility and large‐scale production. Here, we introduce ammonia (NH ₃ ·H ₂ O) into a dimethyl sulfoxide (DMSO)‐based molecular ink to stabilize the precursor. NH ₃ preferentially complexes with Cu(I) to form stable Cu(NH ₃ ) _x ⁺ , effectively suppressing Cu(I) oxidation and extending ink stability from under 6 days to over 40 days. During precrystallization, rapid ammonia volatilization induces dense cracks in the precursor film, promoting grain growth and absorber densification upon selenization. Additionally, ammonia suppresses deep‐level defect clusters [2Cu _Zn  + Sn _Zn ]. With the optimized addition of 200 μL ammonia, the average power conversion efficiency (PCE) increases from 10.70% to 10.95%. By further employing a Zn/Sn stacking layer strategy, the optimized device achieves 11.81% PCE, accompanied by increased shunt resistance, reduced reverse saturation current density, and suppressed band‐tail states. This article provides a simple and controllable strategy for enhancing ink stability and mitigating Sn‐related deep‐level defects in solution‐processed kesterite photovoltaics.