Building OFETs

OFETs constructed on a silicon wafer do not lake advantage of one of the main interest of organic materials, namely the possibility of building electronic devices on plastic substrates. A second important drawback of the silicon-based structure is the difficulty to individually address the gale of transistors built on the same wafer, which would prevent the achievement of integrated circuits. 

Based on these results, we build up OFETs with a ferroelectric gate insulation, consisting of P(VDF-TrFE). 

The oligothiophene films were prepared by solution cast methods and by vacuum deposition. Depending on the cast procedure, different layer thicknesses could be achieved. Preparation from solution either by drop casting or spincoating resulted in films ranging from 50 nm to 200 nm in thickness. Vacuum sublimation yielded very thin layers in the range of 10 nm to 15 nm, which nevertheless allowed to build-up and operate an OFET structure of molecular thickness as demonstrated in this chapter. 

Novel oligomers based on P-substituted thiophene derivatives were synthesised with the aim to build-up a small molecule organic field-effect transistor (OFET). The developed material, a,o)-dicyano-P,P -dibutylquaterthiophene (DCNDBQT), exhibits exeellent thermal, optical and electrochemical stability. 

Modeling and device characterization is explained in chapter 6. The legacy strategy usually encountered in the literature, the IEEE standard method, and several emerging strategies for device characterization are presented. This section also discusses a number of non-ideal effects and builds a small signal device model which can be used for characterization and modeling purposes. Chapter 7 summarizes several application areas in which OFETs have been applied and shows the structure of the circuits used in many of these applications. 

Ong et al. have synthesized solution-processable alkyl-substituted poly(ter- and quarterthiophenes) 203-205 [369-371] and realized OFETs with hole mobilities up to0.14cm V s and on/off ratio over 10 in the case of203 [369]. Similar results have been reported by others [372]. Another approach consists in limiting rr-electron delocalization by incorporation of building block, such as thieno[2,3-b]thiophene that interrupts the conjugated pathway in the chain. The solution-processable polymers 206 leads to OFETs with hole mobility of 0.12-0.15 cm V s [373]. 

FIGURE 7.9 Experimental and transmission line simulation results (colored and dashed lines, respectively) of OFET charging current for different gate voltages (Vd= —8 [V]). All of the simulations are with the same parameters V7- = —3.6 [V], /r = 1.4 x 10 " [cm /V s], and I aise = 9 x 10 [Aj. (From Roichman, Y. and Tessler, N., Tum-on and Charge Build-Up Dynamics in Polymer Field Effect Transistors, San Francisco MRS, 2005. With permission.)... 

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