Fe diffusion in polycrystalline Cu(In,Ga)Se2 layers for thin-film solar cells

Diffusion of Fe in solar-grade Cu(In,Ga)Se2 (CIGSe) layers was investigated over the temperature range from 200 to 600 °C. Either natural or radioactive iron was diffused from the front-surface of CIGSe/Mo/soda lime glass samples. Penetration profiles of stable F56e and radioactive F59e were measured by secondary ion mass spectrometry (SIMS) and ion-beam sputtering in conjunction with activity counting, respectively. The Fe diffusivity can be described by the Arrhenius equation D=1.6×10−4 exp(−0.97 eV/kBT) cm2 s−1. Taking into account Fe solubility data obtained from the SIMS profiles, we provide evidence that D is representative of diffusion along grain boundaries.