BiFeO₃ (BFO) single crystals were grown and the effects of Zn and Mn co-doping on the polarization and leakage current properties were investigated at 25 °C for establishing materials design based on defect chemistry. Although Zn doping or Mn doping led to a deterioration in the properties, Zn-Mn co-doping led to a large remanent polarization (36 μC/ cm²), a low coercive field (19 kV/cm), and a relatively low leakage current density (~10⁻⁸ A/cm²). It is proposed that defect dipoles composed of Zn²+ and Mn⁴+ act as effective nucleation sites for ferroelectric domains during polarization switching in BFO crystals.