Paper-like electronic displays: large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks.

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 ( approximately 5 x 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 ( approximately 1 microm) 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 high-performance 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 ( approximately 100 degrees C) annealing to increase the on/off 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.

Gate-induced superconductivity in a solution-processed organic polymer film.

The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal-insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05-0.1 cm2 V(-1) s(-1)) at room temperature. At temperatures below approximately 2.35 K with sheet carrier densities exceeding 2.5 x 10(14) cm(-2), the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible-from insulating to superconducting.

A light-emitting field-effect transistor.

We report here on the structure and operating characteristics of an ambipolar light-emitting field-effect transistor based on single crystals of the organic semiconductor alpha-sexithiophene. Electrons and holes are injected from the source and drain electrodes, respectively. Their concentrations are controlled by the applied gate and drain-source voltages. Excitons are generated, leading to radiative recombination. Moreover, above a remarkably low threshold current, coherent light is emitted through amplified spontaneous emission. Hence, this three-terminal device is the basis of a very promising architecture for electrically driven laser action in organic semiconductors.

An organic solid state injection laser.

We report on electrically driven amplified spontaneous emission and lasing in tetracene single crystals using field-effect electrodes for efficient electron and hole injection. For laser action, feedback is provided by reflections at the cleaved edges of the crystal resulting in a Fabry-Perot resonator. Increasing the injected current density above a certain threshold value results in the decreasing of the spectral width of the emission from 120 millielectron volts to less than 1 millielectron volt because of gain narrowing and eventually laser action. High electron and hole mobilities as well as balanced charge carrier injection lead to improved exciton generation in these gate-controlled devices. Moreover, the effect of charge-induced absorption is substantially reduced in high-quality single crystals compared with amorphous organic materials.

A soluble and air-stable organic semiconductor with high electron mobility

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.

Large-scale complementary integrated circuits based on organic transistors

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.

Two-dimensional photonic crystal couplers for unidirectional light output.

We investigate the use of two-dimensional photonic crystal slabs to improve the directionality of output coupling from planar waveguides and distributed-feedback lasers. We present the theory underlying the operation of such structures and design criteria for emission in desired directions. As an example, we demonstrate a vertical coupler that is integrated with an organic distributed-feedback laser, use computer simulations to find its coupling constant and efficiency, and then discuss its feasibility.

Fabrication of photonic crystal lasers by nanomolding of solgel glasses.

We demonstrate the formation, in a single process step, of periodic arrays of features of surface relief with submicrometer lateral dimensions in hybrid organic and inorganic solgel glasses by using elastomeric molding techniques. Lasers formed with molded photonic crystal resonators that consist of triangular, square, and honeycomb lattices of cylindrical posts and holes show emission spectra and lasing thresholds that are similar to devices formed by conventional high-resolution photolithographic patterning of thick layers of thermally grown oxide.

Intrinsic Transport Properties and Performance Limits of Organic Field-Effect Transistors

The field-effect mobility in thin-film transistors based on alpha-sexithiophene (alpha-6T) and related materials displays a temperature dependence that is remarkably nonmonotonic. Above a transition temperature TT (specific to a given material) the transport is thermally activated, whereas below TT there is a very steep enhancement of the mobility. In the activated regime, the results are well described by the theoretical predictions for small polaron motion made by Holstein in 1959. An analysis of the transistor characteristics shows that the hopping transport in these devices is intrinsic. Performance limits for devices based on alpha-6T and related materials were established; these limits point to the strong possibility that better molecular materials for transistor applications may be designed from first principles.

Organic heterostructure field-effect transistors.

Organic field-effect transistors have been developed that function as either n-channel or p-channel devices, depending on the gate bias. The two active materials are alpha-hexathienylene (alpha-6T) and C(60). The characteristics of these devices depend mainly on the molecular orbital energy levels and transport properties of alpha-6T and C(60). The observed effects are not unique to the two materials chosen and can be quite universal provided certain conditions are met. The device can be used as a building block to form low-cost, low-power complementary integrated circuits.

Luminescence Enhancement by the Introduction of Disorder into Poly(p-phenylene vinylene).

A method is described for increasing luminescence in poly(p-phenylene vinylene) (PPV) light-emitting diodes. Cis linkages were engineered into the PPV chain. These linkages interrupt conjugation and interfere with the packing of the polymer chains, which results in the formation of amorphous PPV. Large-area electroluminescent devices were prepared from this polymer. Devices made of an aluminum electrode, PPV as the luminescent layer, and an electron-transporting layer have internal quantum efficiencies of 2 percent, a turn-on voltage of 20 volts, and can carry current densities of 2000 milliamperes per square centimeter. The current density is at least an order of magnitude higher than previously obtained.

Organic transistors: two-dimensional transport and improved electrical characteristics.

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.

Polymer microdisk and microring lasers.

Dye-doped polymer microlasers have been fabricated by photolithography and self-assembly. Microdisk lasers 5 to 30 microm in diameter were photolithographically patterned on thin planar polymer waveguides. We formed polymer microring lasers on thinned silica fibers by dipping the fibers in polymers and allowing the polymer droplets to cure.