The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much‐needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.

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We show that doping of the transport layers can strikingly improve the properties of organic light emitting diodes (OLEDs). The electroluminescence onset voltage of diodes containing an vanadyl–phthalocyanine (VOPc) hole transport layer intentionally doped with tetrafluorotetracyano-quinodimethan (F4-TCNQ) is reduced by up to an order of magnitude compared to OLED with undoped VOPc. The improved properties of our devices can be explained by the improved conductivity and better injection for a doped transport layer.

We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation.

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