Molecular materials for electronics 1441 Plastic electronics

Since the discovery of semiconducting behavior in organic materials, considerable research effort has been aimed at exploiting these properties in electronic and opto-electronic devices. Organic semiconductors can have significant advantages over their inorganic counterparts. For example, thin layers of polymers can easily be made by low-cost methods such as spin coating. High-temperature deposition from vapor reactants is generally needed for inorganic semiconductors. Synthetic organic chemistry also offers the possibility of designing new materials with different bandgaps. As noted in Section 14.2, the mobilities of the charge carriers in organic field effect transistors are low. Nevertheless, the simple fabrication techniques for polymers have attracted several companies to work on polymer transistor applications such as data storage and thin-film device arrays to address liquid crystal displays.34

Semiconducting organic films have been used similarly to inorganic semiconductors (e.g., Si, GaAs) in metal/semiconductor/metal structures. Perhaps the simplest example is that of a diode. Here, the semiconductor is sandwiched between metals of different work functions. In the ideal case, an n-type semiconductor should make an ohmic contact to a low work function metal and a rectifying Schottky barrier to a high work function metal.35 An early example is that of a phthalocyanine LB film sandwiched between aluminum and indium-tin-oxide electrodes.36

Of particular interest is the possibility of observing molecular rectification using monolayer or multilayer films. This follows the early prediction of Aviram and Ratner (noted in the Introduction) that an asymmetric organic molecule containing a donor and an acceptor group separated by a short o-bonded bridge, allowing quantum mechanical tunneling, should exhibit diode characteristics.3 Many attempts have been made to demonstrate this effect in the laboratory, particularly in LB film systems.37,38 Asymmetric current vs. voltage behavior has certainly been recorded for many LB film metal/insulator/metal structures, although some of these results are open to several interpretations as a result of the asymmetry of the electrode configuration.

Organic thin films have also been used as the semiconducting layer in field effect transistor (FET) devices.39,40 These are three-terminal structures; a voltage applied to a metallic gate affects an electric current flowing between source and drain electrodes. For transistor operation, the charge must be easily injected from the source electrode into the organic semiconductor and the carrier mobility should be high enough to allow useful quantities of source-drain current to flow. The organic semiconductor and other materials with which it is in contact must also withstand the operating conditions without thermal, electrochemical, or photochemical degradation. Two performance parameters to be optimized in organic field effect transistors are the field effect mobility and on/off ratio.40

Figure 14.14 shows a schematic diagram for the possible structure of an organic thin-film FET. In this arrangement, the organic film is deposited in the final stages of FET fabrication. It is therefore not necessary for this layer to withstand any post deposition, chemical, and thermal processing. Figure 14.15 shows how a series of transistor devices can be made on the same silicon substrate incorporating different thicknesses of LB films. The silicon serves as the gate electrode, while silicon dioxide forms the insulator. Experimental data for such a device, using an organometallic complex as the semiconductive layer, are shown in Figure 14.16.41,42 The graph shows the dependence of the saturated source-drain current vs. gate bias voltage for a device incorporating a film consisting of 59 LB layers of the iodine-doped complex on top of the interdigitated source and drain electrodes.42 The slope semiconductor Sate insulator

t insulating substrate

Figure 14.14 Schematic diagram of a field effect transistor (FET) structure.

Silicon substrate Silicon dioxide Gold

Silicon dioxide etched away to reveal silicon substrate

Gate contacts

Gate contacts

LB film stepped structure

Figure 14.15 Plan view of organic transistors on a silicon substrate. The structure is that shown in Figure 14.14. The gate is silicon. Silicon dioxide forms the gate insulator, while the semiconductor is a Langmuir-Blodgett film. A stepped structure provides different thicknesses of semiconductive film.42

LB film stepped structure

5x5 array of interdigitated electrode structures

Figure 14.15 Plan view of organic transistors on a silicon substrate. The structure is that shown in Figure 14.14. The gate is silicon. Silicon dioxide forms the gate insulator, while the semiconductor is a Langmuir-Blodgett film. A stepped structure provides different thicknesses of semiconductive film.42

Figure 14.16 (Saturated drain-source current)05 vs. gate bias voltage for a thin-film transistor incorporating 59 Langmuir-Blodgett layers of an iodine-doped charge transfer complex on top of interdigitated source and drain electrodes.41-42

Figure 14.16 (Saturated drain-source current)05 vs. gate bias voltage for a thin-film transistor incorporating 59 Langmuir-Blodgett layers of an iodine-doped charge transfer complex on top of interdigitated source and drain electrodes.41-42

of the straight line gives a carrier mobility of 0.3 cm2 V-1 sec,-1 a relatively high figure for an organic transistor device.

The operating characteristics of organic transistors have improved markedly over recent years. This has been brought about by both improvements in the material synthesis and in the thin-film processing techniques.43-48 State-of-the-art devices possess characteristics similar to those of devices prepared from hydrogenated amorphous silicon, with mobilities around 1 cm2 V-1 sec-1 and on/off ratios greater than 106. The use of ambipolar thin-film devices should lead to a significant simplification of complementary logic circuits. Thin-film transistors based on organic semiconductors are likely to form key components of plastic circuitry for use as display drivers in portable computers and pagers, and as memory elements in transaction cards and identification tags.

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