Minority-carrier lifetime degradation in silicon co-doped with iron and gallium [solar cells]

The effect of Fe-Ga pair defects on minority-carrier lifetime τ in high-purity, dislocation-free float-zoned Si solar cells is presented, and their properties are documented by deep-level transient spectroscopy and annealing studies. The typical τ of Fe-Ga co-doped crystals (0.4 μs) was dramatically lower than τ of crystals doped with similar amounts of either Fe alone (12 μs) or Ga alone (1,400 μs), contrary to the τ behavior at low injection of Fe-B pair defects. Thermal dissociation of Fe-Ga pairs increased τ, confirming a stronger carrier-recombination activity at Fe-Ga pairs than at interstitial Fe. Defect energies (relative to the valance band) equal to 0.10 and 0.21 eV were observed for Fe-Ga co-doped samples, and the corresponding hole-capture cross sections (at T=∞) were 3×10 ^-16 and 6×10^-15 cm². An estimation of the binding energies resulted in much lower values than expected (0.09 eV and 0.15 eV, respectively)