Research On The Pentancene Organic Thin Film Transistors

In 1986, Tsumura et al. reported the first organic thin film field-effecttransisitor (OTFT), which used ploythiophenes as the active layer andwhose mobility was only 10-5cm2/Vs. Since then, more and more attentionhas been paid to OTFT by academe at home and abroad. With lucubratingconstantly, people found that the active layer used by small moleculeconjugated organic polymers has remarkable predominance, thus, pentacenehas been a alternative material. A lot of researches showed that theproperties of OTFTs prepared by using a pentacene film as the active layerare comparable with that of a-Si:H devices, even superior to a-Si:H devicesin some aspects. In addition, the maximal dominance of OTFTs includeslow temperature processing, capable of fabricating on a flexible substrateand overall low cost. Therefore, they are very attractive and promising.In this paper, we first introduced the research progress, the key structureand process of OTFTs, also gave the potential perspective about the devices.Then, we set forth the basic properties, structure characteristics oforganic semiconductor, gave the theory models of carrier injection andtransport. The researches on mechanics are necessary to optimum design oforganic/polymer devices, and are significant to reduce the threshold voltageof the devices and increase the field-effect mobility. Because there aren'tlong-range order in organic semiconductors, molecular interaction force isVan der Waals'force and chemical bond is very feeble, molecular orbitaloverlap and molecular charge exchange are also very weak, classicalinorganic semiconductor theory is not suitable for organic semiconductorany more. When we deal with the problems in organic/polymer devicessuch as charge transport, we often need some theory models to apply. In thispaper, we chiefly introduced Tunneling model, Thermal emission model,Space charge limited injection, Trap-limited transport model andInterface-limited injection. Sometimes, only a kind of model doesn'tdescribe the current-voltage(I-V) characteristics in the whole voltage rangevery well for there may be extrinsic trap states, impurities and inducedbreakage during the film deposition in organic materials, several kinds ofmechanism are involved in a device. For different material system anddifferent structure devices, accordant models are always different. In thispaper, we explained the phenomena observed in our experiment reasonablyby these models. Afterwards, we made experimental researches on the OTFTs using thepentacene film as the active layer, optimized the devices from fabricationprocess, structure and the gate insulator selection respectively. Our purposeis to increase the field-effect mobility of the devices and reduce the loadvoltage. By groping and optimizing the experiment condition constantly, weefficiently improved the electrical characteristics. In the fabrication aspect, we took advantage of the full-evaporationmethod. By this method, our devices were completed one-off withoutvacuum breaks, which simplified the device processing and reduce thestained probability of the devices during fabrication. At the same time, thefilm thickness can be controlled exactly. The thin insulator film was souniform and compact that the threshold voltage of devices can be reduced. In the structure design aspect, we used the bottom source and drainstructure, the source and drain electrodes were formed on a cleanedindium-tin-oxide(ITO) coated glass substrate by the standardphotolithography technique. Thereafter, the pentacene active layer film, thegate insulator and the gate electrode were deposited in turn by thefull-evaporation method. Active layer, the gate insulator and the gateelectrode were patterned by a mask, which is available to reduce the devicesize and improve the integrated level. In our structure, the pentacene activelayer is encapsulated efficiently, the contamination of H2O, O2 and otherimpurities, which significantly improve the stability of the devices. In performance optimization aspect, we found that the characteristics canbe improved by changing the thickness of the gate insulator and using theappropriate insulating material. Firstly, we used the Teflon film as theinsulator. In our experiment, we observed that the threshold voltage can beremarkably reduced and the field-effect mobility can be improved bydecrease the thickness of the insulator in the devices using the Teflon filmas the insulator. And then, we used Teflon (dielectric constant is 2.0) andpoly(methylmethacrylate)(PMMA)( dielectric constant is 3.0) as theinsulator, respectively. By testing and comparing the devices using the twomaterials as the insulator, we found that the field-effect mobility of thedevices using PMMA as the insulator is 0.167 cm2/Vs, which is six times aslarge as that of the devices using Teflon as the insulator and their thresholdvoltage approaches 0V, which are chiefly due to the interface improvementof PEN/PMMA relative to PEN/Teflon, and the trap reduction at theinterface, consequently, the field-effect mobility is improved remarkably,the threshold voltage was reduced and on-state current increased markedly,...