  CuI has been recently rediscovered as a p-type transparent conductor with a
high figure of merit. Even though many metal iodides are hygroscopic, the
effect of moisture on the electrical properties of CuI has not been clarified.
In this work, we observe a two-fold increase in the conductivity of CuI after
exposure to ambient humidity for 5 hours, followed by slight long-term
degradation. Simultaneously, the work function of CuI decreases by almost 1 eV,
which can explain the large spread in the previously reported work function
values. The conductivity increase is partially reversible and is maximized at
intermediate humidity levels. Based on the large intra-grain mobility measured
by THz spectroscopy, we suggest that hydration of grain boundaries may be
beneficial for the overall hole mobility.
