Heterojunctions OFETs

In addition to its n-eleetron system, PTCDA possesses a seeond functionality, namely the carboxylic oxygen atoms in the four eomers of the molecule. Their existence creates a quadrupole moment in the molecule, allows hydrogen bonding between molecules and offers an additional interaction channel with any substrate. The interplay of all these factors makes PTCDA an interesting model molecule in the context of molecular adsorbate layers. As a consequence, PTCDA is still a fruit fly , although it is not used any more to fabricate electronic devices such as OFETs, in spite of early reports of an organic heterojunction device based on PTCDA [23,24]. 

In fact, there are still some uncertainties about the applicability of Schottky-Mott model to 0/0 heterojunctions. For example, it is not clear whether the energy-level alignment is controlled only by the constituents of the heterojunction and is independent of the substrates and the formation sequence of the junction. In this chapter, we also discuss the electronic structures of some representative 0/0 heterojunctions and investigate the substrate effects on the energy-level alignment. In Section 6.7, we address the implications of the aforementioned for the design of ambipolar OFETs and stacked OLEDs. 

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