Is there charge transfer beyond the first layer?

The adsorption of strong organic acceptors on metallic electrodes used to modify charge injection barriers in organic electronics. While for most adsorbates, the charge-transfer reaction is clearly limited to the molecules in direct contact with the substrate, for several systems experiments indicate charge-transfer to the second layer. Such long-ranged charge transfer is at variance with present level-alignment models. This raises the question if these experimental indications have been misinterpreted an a different effect, can explain the experimental findings.

In this contribution, we investigate this question exemplarily for tetracyanoethylene (TCNE) adsorbed on Cu(111). We employ the recently developed SAMPLE approach, which combines DFT with machine learning to determine the atomistic structure of the interface. These calculations show that the formation of a second layer competes with a reordering of the molecules in the first layer. This phase transition fundamentally alters the properties of the interface (including the charge state of the molecules in direct contact) and explains the measured signals better than the alleged charge transfer to the second layer.