H

Polypyrrole r

Polythiophenes

Polyethylenedioxythiophene Poly(p-phenylene vinylene)s

Figure 14. The chemical structures of polymers for devices. Reprinted with permission from [363], http://www.conductivepolymers.com/#1. © Advanced Polymer Courses.

polymer materials have been improved quickly in recent years. For example, Ormecon Chemie, Germany, which developed polyaniline, has developed grades of polyaniline with conductivity of 300 S/cm in 1998.

8.2. Polymer Transistors and Integrated Circuits

In 1930, Lilienfeld first proposed the principle of the field-effect transistor (FET) [358]. And in 1960, Kahng and Atalla [359] fabricated the first silicon-based MOSFET. Although first descriptions of the field effect in organic semiconductors can date back to 1970, organic FETs (OFETs) have only been identified as potential elements of electronic devices since the report by Koezuka et al., in 1987, on a structure based on electrochemically polymerized polythio-phene [360]. But the first all-polymer transistor was fabricated by the team of Francis Garnier et al. after 24 years [361].

The performance of OFETs has been continuously improved since the 1980s, and some OFETs can now compete with amorphous silicon FETs. The performance of OFETs is primarily limited by the relatively low electron or hole mobility of the channel materials, either polymers or small molecules. Due to the molecular vibrations and large intermolecular distances (typically about 0.35 nm), conductivity in small molecule organic films depends on the rate of electron- or hole-hopping between the molecules. Polymers show even lower mobility than small molecules due to their less ordered structures. Figure 26 shows the highest field-effect mobilities (u) reported for OTFTs fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000 [362].

Organic FETs and integrated circuits recently reported by Philips [364], IBM, Lucent Technologies, and Pennsylvania State University [365, 366] indicate that great advances have been made.

Pentacene OTFTs have produced the highest performances with mobility as large as 1.5 cm2V-1S-1 and an on/off ratio larger that 108 in 1997 [367, 313]. However, the operating voltage requiring such performance was too high (100 V). Using much higher dielectric constant metal oxide film, barium zirconate titanate (BZT) as a gate insulator, researchers at IBM reported a similar performance of pentacene TFT fabricated on transparent polycarbonate substrates and operated at a maximum voltage of only 4 V, with the mobility values of 0.38 cm2 V-1 S-1 [362]. As most organic materials, pentacene is intolerant to exposure of various chemicals used in traditional lithography. Therefore, they are difficult to be patterned using lithography. Usually shadow masks are used during the deposition. To overcome this, researchers at Pennsylvania State University used a double-layer resist active-layer patterning technique that does not require the organic film to be exposed to process chemicals. This patterning technique also reduces the current leakage in the pentacene film outside the active device areas [365, 366]. By 2000, the maximum field-effect mobility was reported to be 2.7 cm2/VS for holes in a pentacene single crystal by Schon et al. at Bell Laboratories [369].

Researchers at the Philips Company have also made impressive progress, developing the first all-polymer ICs consisting of 326 transistors with a 2-/ m gate length and more than 300 vertical contacts in 1998 [364]. It involves reproducible fabrication of field-effect transistors in which the semiconducting, conducting, and insulating parts are all made of polymers. The interconnections between the transistors are also conducting polymers. The transistor structures are created by masking the polyaniline and irradiating it with UV radiation to reduce its conductivity by almost 10 orders of magnitude, from 103 Ohm/sq to more than 1013 Ohm/sq. Philips developed a 15-bit programmable code generator with bit rates of 30 bits/sec. The circuits remain functional when the foils (substrate) are sharply bent. Recently, Philips is also working on polymer memory chips and displays.

Lucent Bell Laboratories reported the large scales of integration up to 864 transistors per circuit and operation speeds of ~1 KHz in clocked sequential complementary circuits [370]. Bell Labs have been able to print functional circuits on flexible sheets of plastics using inexpensive screen-printing techniques in 1997 [371]. After that, Lucent Bell Laboratories and Opticom ASA have entered into an agreement to develop the plastic memory using polymer integrated circuits.

The disadvantage of using well-ordered pentacene as the active material in the TFT is that it needs the vacuum evaporator to deposit the crystalline film layer. The technologies that are believed to have the potential to produce the highest impact on polymer microelectronics are large-area stamping, ink-jet printing, screen-printing, and self-assembling using the soluble polymer semiconductors without the lithography process. Ink-jet printing deposition is a method in which the pattern can be directly printed onto the substrate, giving very high-resolution patterns and the ability to separate pixels of red, green, and blue-emitting polymers onto the substrate. Using this technology will not only lower the production cost but will also make it possible to fabricate flexible and large area devices. The University of Cambridge reported the all-polymer thin-film transistors fabricated by high-resolution ink-jet printing for the first time in 2000 [372, 373]. They realized a channel length of down to 5 / m and fabricated the TFT with a high mobility of 0.02 cm2 V-1 S-1 and an on/off ratio of more than 105 by aiding a self-aligning flow of solution.

In recent years, most studies on the solution-processable polymeric semiconductor focused on poly(3-hexylthio-phene). With the treatment of SiO2 with hexamethyldisi-lazane (HMDS) or an alkyltrichlorosilane, the OTFTs from highly regioregular poly(3-hexylthiophene) with the mobilities of 0.05 to 0.1 cm2 V-1 S-1 has been reported [374-376]. Other regioregular poly(3-alkylthiophene), such as poly(3-octylthiophene) and poly(3-dodecylthiophene) have also been used to fabricate the field-effect transistors [371].

More recently, mobilities in the range of 10-2 to 10-1 cm2 V-1 s-1 have been achieved using solution-processed, substituted oligothiophenes [377-379]. The mobility is found to strongly depend on film morphology, which can be controlled by the processing conditions. In one study, a,a'-dihexylhexathiophene (DH6T) and a,a'-dihexylquaterthiophene (DH4T) were dissolved in hot chlorobenzene or 1,2,4-trichlorobenzene, then solution-cast onto bottom-contact substrates. The solvent was evaporated in a vacuum oven at temperatures between room temperature and 100 °C. For DH6T films prepared in this way, the mobilities ranged from 4 x 10-3 to 5 x 10-2 cm2 V-1 s-1, with the highest mobilities corresponding to trichloroben-zene and 70 °C solvent evaporation. The mobilities of DH4T films ranged from 1 x 10-3 to 7 x 10-2 cm2 V-1s-1, slightly lower than those of DH6T films prepared under the same conditions [378]. In another study, additional solvents including 3-methylthiophene, anisole, and toluene were investigated. Chlorobenzene, 1,2,4-trichlorobenzene, and 3-methylthiophene all yielded smooth films and mobilities occasionally as high as 0.1 cm2 V-1 s-1 and routinely 0.03 cm2 V-1 s-1 when the DH6T concentration was less than 0.1%. From a film of a,a'-dihexylquinquethiophene (DH5T) cast from chlorobenzene onto a 50 °C substrate, the mobility of 0.03 cm2 V-1 s-1 was obtained [379]. But a subsequent study reported a mobility of 0.1 cm2 V-1 s-1 from a DH5T film solution-cast from toluene [377].

Other soluble organic oligomers have also been investigated as semiconducting materials for OFETs. Anthradithio-phene, a fused heterocycle compound similar to pentacene, is soluble in its dihexyl end-substituted form. Dihexylanthra-dithiophene (DHADT)-based transistors were fabricated by solution-casting from hot chlorobenzene, then evaporating the solvent in a vacuum oven at various temperatures. However, the electrical characteristics of the films were strongly dependent on the solvent evaporation temperature. The highest mobilities, 0.01 to 0.02 cm2V-1s-1, were obtained for a drying temperature of 100 °C [379, 380]. In comparison, vacuum-evaporated films of DHADT gave mobilities as high as 0.15 cm2 V-1 s-1 [380]. Organic FETs utilizing another thiophene-containing oligomer, trans-trans-2,5-bis-[2-{5-(2,2'-bithienyl)}ethenyl]thiophene (BTET), were fabricated by spin-coating from hot NMP. The mobility of such a device was 1.4 x 10-3 cm2 V-1 s-1 compared to 0.012 cm2 V-1s-1 for a vacuum-evaporated device [381].

Another approach to solution-processable oligomeric materials is to begin with a precursor molecule that is soluble but not semiconducting, and then convert it to its semiconducting, insoluble form. This approach has been realized for pentacene, with initial mobilities of 0.01 to 0.03 cm2 V-1 s-1 reported [383-385]. The pentacene precursor is soluble in dichloromethane and forms continuous, amorphous films when spun onto transistor substrates. The conversion to pentacene is accomplished by heating the films to a temperature of 140-220 °C in a vacuum for several minutes to 2 hours. Tetrachlorobenzene is eliminated in the conversion process, and electron microscopy reveals micro-crystallites in converted films. In a subsequent study, by treatment of the SiO2 substrate with HMDS prior to spin-coating the precursor and optimizing the conversion conditions, a mobility of 0.2 cm2 V-1 s-1 was achieved [386].

The precursor approach can also be applied to polymers. In fact, one of the first reported polymer transistors with mobilities in the range of 10-4 to 10-5 cm2 V-1 s-1used precursor-route polyacetylene as the semiconducting layer [387, 388]. Recently, another polymeric semiconductor that has been processed from a soluble precursor polymer is polythienylenevinylene (PTV). In the study, a PTV film was spun from dimethylformamide and then coated with a layer of precursor poly(p-phenylene vinylene) (PPV). Both the

PTV and PPV were converted at 100-300 °C in a N2 atmosphere, with the PPV precursor supplying protons to catalyze the PTV conversion. A mobility of 6 x 10-4 cm2 V-1 s-1 was reported for a device prepared in this way [389]. Subsequent studies have spun the PTV precursor from chloroform and dichloromethane, and have converted the PTV at 140160 °C in a N2 atmosphere with a flow of HCl gas to catalyze the reaction [364, 383, 385]. Although the mobilities remain fairly low, one advantage of this processing approach is that organic solvents can be used in subsequent device-processing steps without disturbing the converted PTV [364].

Most polymer semiconductors, including the oligomers discussed above, are p-type semiconductors. As for the n-type semiconductor, the highest mobilities have been found with C60-based TFTs: 0.08 cm2 V-1 s-1 for the neat material, and a factor of 3-4 higher for devices grown on substrates pretreated with tetrakis(dimethylamino)ethylene [382]. However, a naphthalenetetracarboxylic diimide derivative with fluorinated R groups has been reported as a solution-processable, n-type organic semiconductor [377]. This compound is soluble in hot a,a,a-trifluorotoluene. Solution-casting of this material results in morphologically nonuniform films, with some regions of the films giving mobilities greater than 0.01 cm2 V-1 s-1.

Because the field-effect mobility for the polymer TFT is relatively low and because of the large horizontal geometry of the transistors, the operating frequency of the circuits is only around 200 Hz at the working voltage of 3 V. Compared to the inorganic microelectronics, the performance and the integration level are still poor. If we can successfully fabricate the OTFTs with vertical device architecture instead of that with horizontal structure, the circuit bandwidth around several hundred MHz at the low working voltage could be expected as well as the high integration level.

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