Abstract Alkali‐free Cu(In,Ga)Se 2 (CIGS) absorbers grown on Mo‐coated alumina (Al 2 O 3 ) 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 V oc 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. magnified image 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 V oc and FF compared to the solar cell without K. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)