Electronic devices based on organic semiconductors offer an attractive alternative to conventional inorganic devices due to potentially lower costs, simpler packaging and compatibility with flexible substrates. As is the case for silicon-based microelectronics, the use of complementary logic elements-requiring n- and p-type semiconductors whose majority charge carriers are electrons and holes, respectively-is expected to be crucial to achieving low-power, high-speed performance. Similarly, the electron-segregating domains of photovoltaic assemblies require both n- and p-type semiconductors. Stable organic p-type semiconductors are known, but practically useful n-type semiconductor materials have proved difficult to develop, reflecting the unfavourable electrochemical properties of known, electron-demanding polymers. Although high electron mobilities have been obtained for organic materials, these values are usually obtained for single crystals at low temperatures, whereas practically useful field-effect transistors (FETs) will have to be made of polycrystalline films that remain functional at room temperature. A few organic n-type semiconductors that can be used in FETs are known, but these suffer from low electron mobility, poor stability in air and/or demanding processing conditions. Here we report a crystallographically engineered naphthalenetetracarboxylic diimide derivative that allows us to fabricate solution-cast n-channel FETs with promising performance at ambient conditions. By integrating our n-channel FETs with solution-deposited p-channel FETs, we are able to produce a complementary inverter circuit whose active layers are deposited entirely from the liquid phase. We expect that other complementary circuit designs can be realized by this approach as well.

Thin-film transistors based on molecular and polymeric organic materials have been proposed for a number of applications, such as displays and radio-frequency identification tags. The main factors motivating investigations of organic transistors are their lower cost and simpler packaging, relative to conventional inorganic electronics, and their compatibility with flexible substrates. In most digital circuitry, minimal power dissipation and stability of performance against transistor parameter variations are crucial. In silicon-based microelectronics, these are achieved through the use of complementary logic-which incorporates both p- and n-type transistors-and it is therefore reasonable to suppose that adoption of such an approach with organic semiconductors will similarly result in reduced power dissipation, improved noise margins and greater operational stability. Complementary inverters and ring oscillators have already been reported. Here we show that such an approach can realize much larger scales of integration (in the present case, up to 864 transistors per circuit) and operation speeds of approximately 1 kHz in clocked sequential complementary circuits.

Electronic systems that use rugged lightweight plastics potentially offer attractive characteristics (low-cost processing, mechanical flexibility, large area coverage, etc.) that are not easily achieved with established silicon technologies. This paper summarizes work that demonstrates many of these characteristics in a realistic system: organic active matrix backplane circuits (256 transistors) for large (Ϸ5 ؋ 5-inch) mechanically flexible sheets of electronic paper, an emerging type of display. The success of this effort relies on new or improved processing techniques and materials for plastic electronics, including methods for (i) rubber stamping (microcontact printing) high-resolution (Ϸ1 m) circuits with low levels of defects and good registration over large areas, (ii) achieving low leakage with thin dielectrics deposited onto surfaces with relief, (iii) constructing highperformance organic transistors with bottom contact geometries, (iv) encapsulating these transistors, (v) depositing, in a repeatable way, organic semiconductors with uniform electrical characteristics over large areas, and (vi) low-temperature (Ϸ100°C) annealing to increase the on͞off ratios of the transistors and to improve the uniformity of their characteristics. The sophistication and flexibility of the patterning procedures, high level of integration on plastic substrates, large area coverage, and good performance of the transistors are all important features of this work. We successfully integrate these circuits with microencapsulated electrophoretic ''inks'' to form sheets of electronic paper.T he backplane circuit consists of a square array of 256 suitably interconnected p-channel transistors. Fig. 1 shows the circuit layout. Fig. 2 presents a cross-sectional illustration of a transistor and a top view of a unit cell. The completed display (total thickness Ϸ1 mm) comprises a transparent frontplane electrode of indium tin oxide (ITO) and a thin unpatterned layer of flexible electronic ''ink'' mounted against a sheet that supports square pixel electrode pads and pinouts; these pixel pads attach, via a conductive adhesive, to the back planes. Each transistor functions as a switch that locally controls the color of the ink, which consists of a layer of polymeric microcapsules filled with a suspension of charged pigments in a colored fluid (1, 2). In each of the four quadrants of the display, transistors in a given column have connected gates, and those in a given row have connected source electrodes. Applying a voltage to a column (gate) and a row (source) electrode turns on the transistor located at the cell where these electrodes intersect. Activating the transistor generates an electric field between the frontplane ITO and the corresponding pixel electrode. This field causes movement of a pigment within the microcapsules, which changes the color of the pixel, as observed through the ITO: when the pigments flow to the ITO side of the capsules, the color of the pigment (white in this case) determines the color of the pixel; when they ...

The thiophene oligomer alpha-hexathienylene (alpha-6T) has been successfully used as the active semiconducting material in thin-film transistors. Field-induced conductivity in thin-film transistors with alpha-6T active layers occurs only near the interfacial plane, whereas the residual conductivity caused by unintentional doping scales with the thickness of the layer. The two-dimensional nature of the field-induced conductivity is due not to any anisotropy in transport with respect to any molecular axis but to interface effects. Optimized methods of device fabrication have resulted in high field-effect mobilities and on/off current ratios of > 10(6). The current densities and switching speeds are good enough to allow consideration of these devices in practical large-area electronic circuits.

An essential aspect of the development of organic-based electronics is the synthetic chemistry devised for the preparation of the semiconductor materials responsible for the activity of organic field-effect transistors. Access to organic semiconductors in sufficient purity and variety has led to breakthroughs in solid-state physics and circuit realization. In this Account, we review the synthetic methods that have been most useful for preparing a range of semiconductors, including thiophene-based oligomers, several kinds of fused rings, and polymers. The tradeoff between process efficiency and target purity is emphasized.