OPTO−ELECTRONICS REVIEW 18(2), 121–136 DOI: 10.2478/s11772−010−0008−9 Organic field-effect transistors M.J. MAŁACHOWSKI1 and J. ŻMIJA*2 1Department of Technical Education, Technical University of Radom, 29 Malczewskiego Str., 26−600 Radom, Poland 2Institute of Technical Physics, Military University of Technology, 2 Kaliskiego Str., 00−908 Warsaw, Poland The paper reviews the recent year publications concerning organic field−effect transistors (OFETs). A lot of works have been performed to help understanding the structural and electrical properties of materials used to construct OFETs. It has been established that in partially ordered systems, the charge transport mechanism is thermally activated and field−assisted hopping transport and the hopping transport between disorder−induced localized states dominate over intrinsic polaronic hopping transport seen in organic single crystals. Many research attempts have been carried out on the design of air−stable organic semiconductors with a solution process which is capable of producing OFETs with excellent properties and good stability when subjected to multiple testing cycles and under continuous electrical bias. Recent experiments have demon− strated ambipolar channel conduction and light emission in conjugated polymer FETs. These achievements are the basis for construction of OLED based displays driven by active matrix consisting of OFETs. Keywords: organic electronics, organic field−effect transistors, opto−electronic materials, information displays. 1. Introduction systems and organic field−effect transistors (OFETs) are in active development (in order to indicate the transistor struc− In the inorganic semiconductors such as Si and Ge, the at− ture, through this paper we shall use also OTFT, OFET or oms are held together with very strong covalent bonds, as OTFFET abbreviations for organic thin film transistor, high as 300 kJ/mol for Si. In these semiconductors, charge however, all transistors considered here operate as the field carriers move as highly delocalized plane waves in wide bands and indicate a high mobility of charge carriers μ >> 1 effect devices). The processing characteristics and demon− cm2/Vs. In organic semiconductors, intramolecular interac− strated performance of OFETs suggest that they can replace tions are also mainly covalent, while intermolecular interac− existing or be useful for novel thin−film−transistor applica− tions are usually induced by much weaker van der Waals tions requiring large−area coverage, low−temperature pro− and London forces resulting in energies smaller than 40 cessing, mechanical flexibility and, especially, low cost. kJ/mol. Thus, the transport bands in organic crystals are Such applications include switching devices for active−ma− much narrower than those of inorganic ones, and the band trix flat−panel displays (AMFPDs) based on either liquid structure is easily broken by disorders in such systems, crystal pixels (AMLCDs) or active matrix with organic light− leading to creation of states in the energy gap. −emitting diodes (AMOLEDs). At present, hydrogenated Almost all organic solids are insulators. However, in or− amorphous silicon (a−Si:H) is the most commonly used as ganic crystals that consist of molecules that have the p−con− active layer in TFTs backplanes of AMLCDs. The higher jugate system, electrons can move via p−electron cloud performance of polycrystalline silicon TFTs is usually re− overlaps. That is why these crystals exhibit electrical con− quired for well−performing AMOLEDs. Improvements in ductivity. Polycyclic hydrocarbons and phthalocyanide salt the efficiency of both the OLEDs and the OTFTs could crystals are examples of this type of organic semiconductor. broaden applications. OTFTs could also be used in active− Among the conjugated polymers there exist conductive and −matrix backplanes for “electronic paper” displays based on semiconductive polymers. pixels comprising either electrophoretic ink−containing Organic semiconductors, such as conjugated oligomers microcapsules or “twisting balls”. Other applications of and polymers, have become materials for a wide range of OTFTs include low−end smart cards and electronic identifi− not only electronic but also optoelectronic devices. Organic cation tags. light−emitting diodes (OLEDs) inserted into commercial Most of the investigated OFETs are not all organic as well as not flexible ones. The devices are usually con− structed on no flexible silicon substrate. In order to fabri− *e−mail: [email protected] cate the flexible AMOLED structure, rather all−organic Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski Organic field−effect transistors FETs should be used. The three−dimension bottom struc− patterned anode. Source and drain contacts can be poly− ture of this device is shown in Fig. 1(a). Figures 1(b) and meric. The active semiconducting layer can be deposited by 1(c) present top and bottom structures, respectively, cross spin coating of an appropriate solution. The gate dielectric sections of not all organic TFTs because heavily doped layer can be prepared by spin coating a solution of insulat− silicon and metal electrodes are used to fabricate them. ing polymer. The aim of numerous studies on OTFTs of Both types of construction were extensively studied and configurations shown in Fig. 1 or similar are to improve the their advantages as well as disadvantages have been transistor performance, which includes almost all elements shown. of the device as well as the charge transport mechanism The investigation results of various soluble organic se− between source and drain. a, w miconductors show that, −dihexylsexithiophene Progress in plastic electronics is achieved mainly by (DH−6T) has become the most promising candidate for rising in the mobilities of organic semiconductors, increase all−organic semiconductor integration because of its air in the capacitance of suitable gate dielectrics, and reduc− stability, good solubility, and easy synthesis. For these tions in the channel dimensions of the devices. Both poly− purposes, the polymethylmethacrylate (PMMA) is used as mer and small molecule−based organic thin films may be a dielectric layer to substitute SiO2 in order to confer the used in OFET technology. Mobile charge carriers of either transfer characteristics of OTFTs with PMMA as polarity can be injected into organic semiconductors from a dielectric layer. contacts of suitable materials. The mobilities of these All−organic TFTs on flexible polyimide substrates can charge carriers, however, are low in comparison to inor− be produced indicating electrical performance similar to that ganic semiconductors, and the device performance is often of TFTs fabricated with inorganic gate dielectrics and metal strongly lowered by carrier trapping. The transport mecha− contacts. For production the OFET devices, a technique can nism is best described as a hopping process and the charge be based on area−selective electropolymerization on a pre− carriers are polarons. For brevity, a conventional semicon− ductor language is used to refer to the negative charge carriers as electrons and to the positive charge carriers as holes. The purpose of this article is to review the recent most important papers concerning the investigations on the ef− fects governing the operation of OFETs, which have a thin film as well as crystalline form. There are known some reviews focused at different problems connected with OFETs, just to mention the papers [1–6] and others [7,8]. 2. Charge transport in organic semiconductors and OFETs As it was mentioned above, organic semiconductors have major drawbacks in relatively low mobility of charge car− riers arising from weak intermolecular interaction in solid state and in the fact that doping is problematic because it is interstitial and not substitutional as in inorganic semi− conductors. An organic semiconductor is considered with the VB or the highest occupied molecular orbital (HOMO) and the CB or the lowest unoccupied molecular orbital (LUMO) which are separated from each other by the gap which is usually rather large, EG > 2eV.Fororganic semiconductors, the manifolds of both the LUMOs and the HOMOs are characterized by random positional and energetic disorder. To model the current flow in organic semiconductors, the charge transfer complex (CT complex) idea is intro− duced. CT complex is defined as a pair of molecular groups, where one is electron−donating (donor) and the other is elec− Fig. 1. Three−dimensional view of an all−organic bottom structure tron−accepting (acceptor) and where there is a partial trans− thin film field effect transistor (a), top electrode structure of OTFT fer of charge from the acceptor to the donor in an excited (b), and bottom electrode structure of OTFT (c). molecular state. CT complexes have a transition between 122 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw the excited molecular state and a ground state and almost all 3. Principles of operation of MOSFETs CT complexes have absorption bands in the ultraviolet or and TFFETs visible region. The interaction between donor and acceptor is not only a charge transfer interaction but also electrostatic 3.1. Silicon single crystal MOSFET force. This kind of interaction is usually much weaker than interactions of the hydrogen bond and the covalent bond, We shortly discuss some well understood principal effects oc− but it is essential for constructing the crystal structures. curring in commonly used silicon single crystal (metal oxide Since the HOMO level