The identification of performance‐limiting factors is a crucial step in the development of solar cell technologies. Cu2ZnSn(S,Se)4‐based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin‐film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance‐limiting role of deep‐trap‐level‐inducing 2CuZn+SnZn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu«sub>2</sub>ZnSnS4 and Cu2CdSnS4. It is shown that these deleterious defect clusters can be suppressed by substituting Zn with Cd in a Cu‐poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the CuZn+ZnCu and CuCd+CdCu antisites. Detailed investigation of the Cu2CdSnS4 series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu‐poor composition, presumably via the presence of VCu, in improving the optoelectronic properties of the cation‐substituted absorber. Finally, a 7.96% efficient Cu2CdSnS4 solar cell is demonstrated, which shows the highest efficiency among fully cation‐substituted absorbers based on Cu2ZnSnS4.