OFET materials

In the past decade, the research on organic field-effect transistors (OFETs) has experienced remarkable progress mainly because of the development of novel OFET materials, which have allowed to reach carrier mobility values good enough to compete with amorphous silicon. 

Chapter 3 explains the major classes of OFET materials which are in use, their properties, and some of the advantages and disadvantages of each system. It also discusses many of the parameters which go into material design and selection of semiconductors and doped organic conductors which can be used as electrodes. 

It is almost impossible to catalog all OFET materials new materials and forms of known materials are being developed as processing and S3mthetic techniques improve. Most materials in common use can, however, be placed into one of a few classes. A more thorough delineation of semiconducting materials for OFETs and emerging material classes can be found in several excellent review articles, some of which are listed in Appendix A. 

Fig. 3.9. Four representative n-type OFET materials, (a), (b), and (c) achieve electron deficient carbon backbones through fluorination, although other electronegative groups may also be used, (a) Hexadecafluoro copper pthalocyanine [30], (b) a per-fluorinated pentacene [31], and (c) a fluorinated thiazole-based oligomer [32]. (d) C60 is also an electron transporting material which can form OFETs [33]. (e) is a perlyene derivative, perylene 3,4,9,10 tetracarboxydiimide, which has found wide application as an n-type material in organic solar cells. A large number of n-type perylene derivatives have been developed.

Organic synthetic chemistry has yielded a tremendous variety of materials suitable for use in OFETs. Materials with improved processability, perfor-... 

Perhaps the simplest strategy is to laminate a plastic sheet on top of the transistors with a suitable adhesive [99]. It is also possible to coat OFETs with a layer of parylene, teflon, or other compatible polymer and optionally seal the devices using a metal vapor barrier layer (e.g. [100]). Heating many OFET materials for dehydration in an inert ambient is also possible, and may be important depending on the material s reactivity with water [101]. 

It would be outside of the scope of this review to address all material functionalities for every type of application. The focus is limited to FETs and transport effects in polymeric OFET. Carrier transport is a key property that links materials for all applications. The learning in OLED and OPV devices has been extremely useful in providing guidelines for OFET materials and vice versa. In OLEDs, carriers are injected from the anode and cathode and they move through the polymeric film. Ideally, the rate at which electrons and holes are supplied, i.e., their mobility, should be similar. In OPV cells, carriers are separated and transported to the respective electrodes to create a photocurrent. In OFETs, carriers... 

Benzoselenophene [111, 112] and related fused selenophenes [113-116] were previously obtained via tedious multistep reactions involving a selenocyclization reaction from barely accessible selenium-containing compounds. Such conventional synthetic methods are unsuitable for the development of selenophene-based OFET materials. Sashida s group developed a simple one-pot preparation of benzoselenophenes by an intramolecular selenocyclization reaction of acetylene compounds with a selenolate anion [117, 118]. This allowed straightforward access to sophisticated fused selenophenes, such as 52 and 53, from commercially available chemicals [119, 120]. Scheme 6.9 depicts the key selenocyclization reaction used in the synthesis of 52. This synthetic protocol was also successfully applied to the synthesis of regioisomer 54 [121]. 

This chapter has dealt with selenophene materials, which can serve as conductive, electroactive and OFET materials. In contrast to the abundant thiophene-based materials, the number of selenophene-based materials is still limited. In addition to the selenophene-containing materials mentioned here, selenophenes exhibiting liquid crystal phases [127], amorphous molecular properties [128] and biological activities [129, 130] have also been investigated. It is true that the functional properties of selenophene materials closely resemble... 

K. Takimiya, Y. Kunugi, H. Ebata and T. Otsubo, Molecular modification of 2,6-diphenylbenzo [l,2-b 4,5-fc ]dichalcogenophenes by introduction of strong electron-withdrawing groups conversion from p-to n-channel OFET materials, Chem. Lett, 35, 1200-1201 (2006). 

Figure 3.1 Chemical structures of BDT based polymers as OFET materials.

Thiophene Based Liquid Crystal Polymers for OFET Materials... 

Thiophene based polymers have been widely explored as organic electronics materials (McCulloch et al. 2009 Lu and Liu 2010). Poly(3-hexyl-thiophene) (P3HT), for example, is a benchmark material to establish the foundation of OFET research. A lot of thiophene and fused thiophene based polymers have the liquid crystalline attribute and have been demonstrated as high performance OFET materials. Some of them have been listed in Table 17.2. 

Table 17.3 Summary of a series of thiophene copolymer as p-type, ambipolar and n-type OFET materials...

Liquid Crystalline Semiconducting Polymers fin Organic Field-Effect... Table 17.4 Summary of thiazole containing polymers as OFET materials... 

---