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The large open‐circuit voltage deficit (V_oc,def) is the key issue that limits kesterite (Cu₂ZnSn(S,Se)₄, [CZTSSe]) solar cell performance. Substitution of Cu⁺by larger ionic Ag⁺((Ag,Cu)₂ZnSn(S,Se)₄, [ACZTSSe]) is one strategy to reduce Cu–Zn disorder and improve kesteriteV_oc. However, the so far reported ACZTSSe solar cell has not demonstrated lowerV_oc,defthan the world record device, indicating that some intrinsic defect properties cannot be mitigated using current approaches. Here, incorporation of Ag into kesterite through a dimethyl sulfoxide (DMSO) solution that can facilitate direct phase transformation grain growth and produce a uniform and less defective kesterite absorber is reported. The same coordination chemistry of Ag⁺and Cu⁺in the DMSO solution results in the same reaction path of ACZTSSe to CZTSSe, resulting in significant suppression of Cu_Zndefects, its defect cluster [2Cu_Zn + Sn_Zn], and deep level defect Cu_Sn. A champion device with an efficiency of 12.5% (active area efficiency 13.5% without antireflection coating) and a record lowV_oc,def(64.2% Shockley–Queisser limit) is achieved from ACZTSSe with 5% Ag content.