We investigated the effect of external electric fields on the relative stability of polymorphs of organic/inorganic interfaces using the example of tetracyanoethylene (TCNE) on Cu(111). TCNE is chemisorbed to the substrate and exhibits two distinctly different categories of polymorphs: "lying" and "standing". We demonstrate that the geometric and chemical differences between the two categories can be exploited to manipulate the thermodynamic properties of this system in using external electric fields.