Alkali‐free Cu(In,Ga)Se2(CIGS) absorbers grown on Mo‐coated alumina (Al2O3) substrates were doped with potassium (K) after CIGS growth by a potassium fluoride (KF) post‐deposition treatment (PDT). The addition of K to the absorber leads to a strong increase in cell efficiency from 10.0% for the K‐free cell to 14.2% for the K‐doped cell, mainly driven by an increase in the open‐circuit voltage Voc and the fill factor FF, and to an increase in the net charge carrier density. Hence K doping by KF‐PDT is comparable to doping with Na. J –V characteristics of a CIGS solar cell with KF‐PDT (red solid line) and of a K‐free reference cell without treatment (black dashed line). The solar cell treated with K shows a strong increase in Voc and FF compared to the solar cell without K. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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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/kBT) cm2 s−1 and the fast diffusion along the grain boundaries by DNaGB = 6.5 × 10−9 exp(−0.21 eV/kBT) cm2 s−1. Hence, we propose that sodium ions do not only passivate grain boundaries, but also act as dopants in the CIGS bulk.