This paper proposes a mechanism for the rectification of current by self-assembled monolayers (SAMs) of alkanethiolates with Fc head groups (SC(11)Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga2O3/EGaln) top electrodes. A systematic physical-organic study based on statistically large numbers of data (N = 300-1000) reached the conclusion that only one energetically accessible molecular orbital (the HOMO of the Fc) is necessary to obtain large rectification ratios R approximate to 1.0 x 10(2) (R = vertical bar J(-V)vertical bar/vertical bar J(V)vertical bar at +/- 1 V). Values of R are log-normally distributed, with a log-standard deviation of 3.0. The HOMO level has to be positioned spatially asymmetrically inside the junctions (in these experiments, in contact with the Ga2O3/EGaln top electrode, and separated from the Ag electrode by the SC11 moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga2O3/EGaln electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.