OFETs ambipolar

Langmuir-Blodgett (LB) technique has been also used for the preparation of Pc-based OFET, as it allows the fine control of both the structure and the thickness of the film at the molecular level [226,227], OFET devices based on amphiphilic tris(phthalocyaninato) rare earth, triple-decker complexes have been prepared by LB technique, showing good OFET performances [228], More recently, ambipolar transport has also been realized in OFET devices through a combination of holeconducting CuPc and n-conducting Cgo fullerene, in which the asymmetry of the... 

Due to the great potential applications and the widespread interests of OFETs, several review articles with emphasis on different aspects of OFETs have been published [8-13], However, despite the extensive studies and increasing research interests in phthalocyanine as well as porphyrin semiconductors, there is still no review article that systematically generalizes the research achievements in the field of tetrapyrrole organic semiconductors for OFETs. Thus, the present contribution should interest scientists in both industrial and theoretical fields. The main contents of this chapter is as follows Firstly, some basic introduction of the structure and characteristics of OFET is provided then theoretical factors that influence the performance of OFET devices is discussed finally, the progress in phthalocyanine-based OFETs in the order of p-type, n-type, and ambipolar semiconductors is summarized. 

Fig. 3 Schematic diagram of current-voltage characteristics of (a) p-type, (b) n-type, and (c) ambipolar OFET device...

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. 

Fig. 9 Molecular structures of the reported compounds forming ambipolar OFETs with phthalo-...

Table 6 Performance of some pthalocyanine-based ambipolar OFET devices (mEn means m x 10 )... 

Contemporary OFETs are based on undoped organic semiconductors, and mobile charges in these devices must be injected from the metallic contacts. These devices can potentially operate in the electron- and the ftote-accumulation modes, depending on the polarity of the gate voltage (the so-called ambipolar operation). Often, however, the injection barrier at the contact or the held-effect threshold for either n- or... 

For several important applications in plastic optoelectronics, including the possibility of electrically pumped organic lasers, it would be very important to achieve an ambipolar operation in OFETs, with high electron and hole mobilities. Gate-controlled electroluminescence from organic small-molecule thin-film transistors... 

Interestingly, Vg-controlled electroluminescence and ambipolar characteristics have been recently observed in conjugated polymer OFETs [67,68], which indicates a balanced electron and hole injection. However, low hole and electron mobilities ( 10 cmWs), typical for polymer semiconductors, limit the channel current and therefore may present a serious problem for realizatiou of electrically pumped polymer lasers. For this reasou, ordered smaU-molecule orgauic semicouductors with higher mobilities are very promisiug for research in this direction. 

While it is simpler from a fabrication standpoint to deposit the same contact material for both the source and drain contacts (symmetric contacts), one may also consider choosing two different materials for each contact (asymmetric contacts). Based on energy band line-up considerations with the semiconductor HOMO and LUMO levels, depositing two different contact materials at either end of the transistor channel may facilitate more efficient hole and electron injection, respectively. At this point, it is unclear whether separately engineering distinct contacts for hole/elec-tron injection in ambipolar OFETs will prevail over opting for symmetric contacts. However, there will certainly be more reports on this exciting OFET subclass in the next few years. 

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. 

The first demonstration of a polymer-based ambipolar OFET was reported in 1996 by Tada ef al. [118]. In fhis early study, the workers used the polymeric... 

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