Thin OFETs

In this chapter we investigate and discuss the thermal, optical, electrical properties of the oligothiophene derivatives by means of differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and UV-Vis spectroscopy. The thin films of these compounds produced by solution cast and vacuum deposition methods are characterised by AFM measurements in contact and non-contact mode, and by X-ray diffraction. Finally, an ultra-thin OFET is built, and the transistor characteristics are determined. 

As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

The most representative element belonging to such interfaces is the OFET. A simplified scheme of an OFET based on an organic thin film is given in Fig. 4.31. 

For an OFET, the dependence of the drain current / on the source-drain voltage Vd and the source-gate voltage Vb is described hy the classical equations derived from inorganic-hased thin him transistors (Horowitz, 1998) ... 

Jones BA, Facchetti A, Wasielewski MR, Marks TJ (2008) Effects of arylene diimide thin film growth on n-channel OFET performance. Adv Funct Mater 18 1329-1339... 

CuPc thin films, and the enhanced physical connection between source-drain electrodes and semiconductor channel associated with the PMMA polymer layer, the OFET performance of this bottom-contact device was significantly improved with leakage current being reduced by roughly one order of magnitude and on-state current enhanced by almost one order of magnitude. The hole mobility of this bottom-contact OFET device reached 0.01 cm2 V-1 s 1, which is comparable with that of top-contact device but much higher than that of normal bottom-contact device without polymer layer [45],... 

By introducing Lewis-acid V2O5 thin film between semiconductor and insulator layers, Minagawa et al. fabricated CuPc-based OFETs that can be turned on by positive gate voltage, which showed improved hole mobility in comparison... 

M = Tb, Lu) into organic thin-film transistors by LB technique and reported their field effect mobility, which represented the first report for p-type OFETs based on bis(phthalocyaninato) rare earth complexes prepared via LB method [88], Due to the highly ordered molecular arrangement of M(Pc)[Pc(OC8Hi7)g] (M = Tb, Lu) in LB films and the appropriate HOMO energy level of these double-deckers relative to the Au source-drain electrodes, the OFETs reported in that work exhibited higher hole transfer mobility of 1.7 x 10-3 cm2 V-1 s-1 in comparison with those fabricated from monomeric phthalocyanine LB films. 

As expected, n-type OFET devices fabricated from single crystals display higher performance than those made from thin solid films because of the more... 

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. 

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