Extended Space Charge Region and Unoccupied Molecular Band Formation in Epitaxial Tetrafluorotetracyanoquinodimethane Films

Abstract

Generating well-defined molecular structures at inorganic/organic interfaces and within molecular films is fundamental for charge carrier transport and thus the performance of organic molecule-based (opto)electronic devices. Here we show by means of low-energy electron diffraction that tetrafluorotetracyanoquinodimethane (F 4 TCNQ) grows in an epitaxial fashion on the Au(111) surface, resulting in a unit cell which consists of one molecule. In this well-ordered crystalline films we found the formation of an extended space charge region and a dispersing unoccupied electronic molecular state using energy- and angle-resolved two-photon photoemission. The latter finding is a clear proof for band formation in the crystalline molecular structure. We suggest that the high electron affinity of F 4 TCNQ and a bandlike electron transport are responsible for the formation of the space charge region. Using F 4 TCNQ as a hole injection layer may open the opportunity to manipulate the hole injection barrier in a controlled way via variation of the F 4 TCNQ layer thickness.