Engineered Pentacenes

Pentacene routinely yields field-effect transistor (FET) devices with reliable hole mobility of 1 cm2 V-1 s 1 [6], with mobility 3 cm2 V-1 s-1 reported for thin-film devices on polymer gate dielectrics [9]. For transistors fabricated on single crystals of pentacene, the measured mobility approaches 60 cm2 V-1 s 1 [10]. 

An ingenious route to the solubilization of pentacene involves addition of substituent groups that can be removed thermally after film formation is complete. The reactive nature of the central aromatic ring in pentacene makes it a good diene for Diels-Alder type reactions, and the reversibility of this reaction makes this approach ideal for such a functionalization strategy. 

The first successful application of this method was reported by Mullen and coworkers [18]. The removal of their solubilizing group occurs thermally at temperatures as low as 180 °C, yielding pentacene and a volatile tetrahalobenzene. The tetrachloro compound 4 (Fig. 3.3) formed good-quality films from spin-cast solution, and heating the film of the soluble precursor at 200 °C for 5 s yielded the desired acene film. FET properties for devices made from solution-cast films were quite good - measured mobilities were as high as 0.2 cm2 V-1 s-1 with on/off current ratios of 106. 

Use of more complicated functionality on the solubilized pentacene (6, 7) yields materials that can be photopatterned to give devices with mobilities ranging from 0.02-0.2 cm2 V-1 s-1 [20, 21]. A very recent addition to the category of pentacene molecules functionalized with removable solubilizing groups is diketone 8 [22], which is relatively soluble and can be converted back to pentacene (and carbon monoxide) by irradiation with ultraviolet light. 

Pentacenequinones (11) are in turn conveniently prepared by a fourfold aldol 

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Saudari SR, Frail PR, Kagan CR (2009) Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts. Appl Phys Lett 95 023301... 

Functionalization of pentacene with the specific aim of improving performance in devices is a recent endeavor - the first use of a functionalized pentacene in a field-effect transistor was reported only recently (2003) [26], Functionalization of pentacene has led to the ability to engineer the solid-state arrangement, electronic, and solubility properties of this important semiconductor and to improve its stability and film-forming ability. Recent functionalized pentacene materials have yielded devices with properties comparable with those of the parent acene, have enabled the formation of devices from solution-deposited films, and have even changed the semiconductor behavior of this organic molecule from p-type to n-type. As functionalization strategies are refined, materials with all of the properties necessary for commercial device applications should soon be developed. 

L. Zhou, a-Si H and Pentacene TFT for Flexible Electronic Applications, in Electrical Engineering Department. University Park, Pennsylvania State University, 2005. 

The HPLC methods cannot be used to routinely monitor the other PAHs because the instrumentation is rather sophisticated and data interpretation and analysis requires a high level of expertise. Instead simple UV absorbance methods are used to measure the absorbance at wavelengths characteristic of certain key PAHs. These include 305 nm for coronene, 335 nm for pyrene, 345 nm for dibenzo[cdJm]perylene, and 495 nm for dinaphtho[2,l,8,7-defg 2, r8, 7 -opqr]pentacene. Tracking of each of these absorbances from process startup allows process engineers to follow the buildup of the PAHs and gain an idea of the chemistry going on within the process. 

In order to compensate the initially proposed interfacial electron traps on the Si02 dielectric, Ahles et al. [62] introduced traces of Ca to the Si02 dielectric interface. As a low work function metal, Ca was believed to act as an electron donor saturating traps in the interface near pentacene layer. The capability to accumulate mobile charge carriers at the newly engineered interface can again be examined by MIS diode experiments. The result of impedance measurements on a MIS diode... 

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