Small OFETs

Similar to the cases in other small molecules and polymers, most materials composed of phthalocyanine compounds are revealed to work as p-type semiconductors for OFET applications with only few phthalocyanine materials as n-type semiconductors and even less as ambipolar ones. Among the p-type tetrapyrrole semiconductors, monomeric phthalocyanine compounds hold all the trumps with only a few double- and triple-deckers together with some porphyrin derivatives having been reported. 

An altemative means to realize well-organized thin films of small molecules is the Langmuir-Blodgett (LB) technique, which allows a fine control of both the stmcture and thickness of the film. We note, however, that this technique is in principle restricted to amphiphilic molecules, composed of a hydrophobic chain and a hydrophobic head-group, which is not the case for most molecules used in OFETs. Nevertheless, LB-grown OFETs have been reported with mixed layers of quinquethiophene (5T) and arachidic acid, which gave well-behaved devices [56]. However, the necessary mixing of the electrically active compound with an inactive compound leads to a substantial decrease in the mobility, as compared to that of a vacuum-evaporated film. 

The performance of OFETs has continuously improved since they were first reported in 1987 [8, 9]. The rate of the progress can be visualized in Figure 14-13, where we have plotted the field-effect mobility of five prominent organic com-pounds as a lunclion of the publication date. The data include one polymer, polythiophene and its derivatives and four small molecules (three oligothiophenes, plus pentacene). Note that the highest mobility of small molecules was reported on single crystals. 

An important issue for the performance of an organic electronic device like an OFET is the injection of charge carriers, electrons or holes, from the electrode into the organic material. In case of the commonly used metal electrodes an efficient electron injection is possible only if the Fermi level of the metal and the energy of the lowest unoccupied molecular orbital (LUMO) of the organic material differs by a small amount only. A similar statement applies for hole injection, in this case the position of the highest occupied molecular orbital (HOMO) has to match with the position of the Fermi level. When noble metals, in particular Au, are being used for an electrode one may naively assume... 

Comparing the situation for PTCDA with the above discussed cases of pery-lene and pentacene which exhibit a large diffusion already at room temperature (due to their smaller sublimation enthalpy) thus indicates that the film structure of large molecules such as PTCDA prepared at room temperature may not represent the thermodynamic equilibrium structure. Finally, we note that PTCDA reveals only a rather small charge carrier mobility (for thin films values of less than 3 X 10 cmW s were reported [67]), and thus is not well suited for the fabrication of 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 ... 

To analyse the degradation of the sample it was characterised directly after the Teflon deposition and after a gap of one month. Meanwhile, the device was stored in a dark ambient atmosphere in a shielded metal box like the OFET presented in Table 18.3. Just after the deposition of the capping layer, there could be a small decline in the on-current at -40 V gate-source voltage observed, as shown in Figure 18.19. 

---