Structure What can be Learned for OFETs

As pointed out before, interfaee stmeture in OFETs is important for both charge injection (metal-semiconductor interface) and charge transport (semi-conductor-dielectric interface). The discussion of the previous section demonstrates that even for the small number of model systems considered here a large variety of sfructures is observed. Because many factors influence the interface and film sfructures, it is difficult to establish general mles. Flowever, a few important observations regarding the stmcture-forming factors can be kept hold of  

1) The presence of the metal or insulator does not only add the molecule-substrate interaction as a formative influence, but can also alter the effective intermolecular interactions. For example, whereas the crystallisation of bulk tetraeene is governed by the attractive interaction between molecules in a particular relative orientation, the surface-confined molecules (on Ag( 111)) repel each other. The modification of the effective intermolecular interaction may originate both from substrate-mediation and from the intrinsically anisotropic molecular interaction potentials. As the possibly entropy-driven ordering of tetraeene on Ag(lll) shows, the modified interactions may introduce new ordering mechanisms at the interface. 

2) As a result of the above, and of the direct competition between molecule-substrate and intermolecular interactions, the presence of the metal or insulator can induce interface polymorphs which do not exist in the bulk. Examples for this are the specific thin film phases of pentacene on insulators [16, 74, 75] or metals (e.g., Cu(llO) [16]), the a- and 3-phases of tetraeene on Ag(l 11) [69], or the square phases of PTCDA on Ag(l 11) [30] and Au(l 11) [84]. It is evident that the charge carrier mobilities of organic semiconductors will depend on the crystal phase. 

The data on 6P have been reeorded on 200 A thiek films, and the measured dispersion is thus a property of the bulk. In eontrast, the case of PTCDA re- 

As a way out of this dilemma, we have developed a two-step approach based on commensurate, highly ordered molecular layers on metal surfaces. In step 1, the powerful armoury of surface science is employed to characterise the structural and electronic properties of the metal-molecule contact. For the model molecule PTCDA, some of the results of these experiments have in fact been described in Sections 12.2 and 12.3. Once step 1 has been completed, the tip of a low-temperature STM is used to establish a covalent contact with an individual molecule of the highly ordered monolayer (step 2). Because of the excellent imaging properties of the STM, it is in fact possible to select the part of the molecule which is contacted with very high accuracy. 

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