Insights into cadmium diffusion mechanisms in two-stage diffusion profiles in solar-grade Cu(In,Ga)Se₂ thin films

Cadmium diffusion experiments were performed on polished copper indium gallium diselenide (Cu(In,Ga)Se₂ or CIGS) samples with resulting cadmium diffusion profiles measured by time-of-flight secondary ion mass spectroscopy. Experiments done in the annealing temperature range between 275 °C and 425 °C reveal two-stage cadmium diffusion profiles which may be indicative of multiple diffusion mechanisms. Each stage can be described by the standard solutions of Fick's second law. The slower cadmium diffusion in the first stage can be described by the Arrhenius equation D₁ = 3 × 10⁻⁴ exp (− 1.53 eV/k_BT) cm² s⁻¹, possibly representing vacancy-meditated diffusion. The faster second-stage diffusion coefficients determined in these experiments match the previously reported cadmium diffusion Arrhenius equation of D₂ = 4.8 × 10⁻⁴ exp (−1.04 eV/k_BT) cm² s⁻¹, suggesting an interstitial-based mechanism