Investigation of the diffusion behavior of sodium in Cu(In,Ga)Se₂ layers

Sodium diffusion in Cu(In,Ga)Se₂ (CIGS) layers was investigated over a temperature range from 157 °C to 400 °C. The diffusion profiles were measured by secondary ion mass spectrometry. Sodium ions diffused from a sodium fluoride (NaF) layer on the CIGS surface into the CIGS layer. From Na diffusion profiles, the diffusion along grain boundaries could be distinguished from the diffusion into the grain interior. Atom-probe tomography measurements reveal that even at a low temperature of 157 °C bulk diffusion of sodium into CIGS occurs. Based on this data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation $DNaV$ = 9.7 × 10⁻⁹ exp(−0.36 eV/k_BT) cm² s⁻¹ and the fast diffusion along the grain boundaries by $DNaGB$ = 6.5 × 10⁻⁹ exp(−0.21 eV/k_BT) cm² s⁻¹. Hence, we propose that sodium ions do not only passivate grain boundaries, but also act as dopants in the CIGS bulk.