The tiny diode works by means of an asymmetry caused by water molecules at the molecular junction interface. At around 60% humidity, water molecules lump together at tip side of the molecular layer and cause an imbalance between the energy levels on both sides. This strongly limits the flow of electrons.
Under dry circumstances, the molecule's symmetry doesn't break and the diode behavior disappears.

This work was carried out in the group of PD Dr. Peter Broekmann.

References:

• H. Atesci, V. Kaliginedi, J. A. Celis Gil, H. Ozawa, J. M. Thijssen, P. Broekmann, M. Haga, S. J. van der Molen;
  "Humidity-controlled rectification switching in ruthenium-complex molecular junctions"
  Nature Nanotechnology, 2018, 13(2), 117-121; doi:10.1038/s41565-017-0016-8.
• Media release of the University of Bern;
  "Humidity switches molecular diode off and on"
  04.12.2017; Details.