Howard E. Katz

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 ∼1 kHz in clocked sequential complementary circuits.

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 (≈5 × 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 (≈1 μm) 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 (≈100°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.

Organic Transistors: Two-Dimensional Transport and Improved Electrical Characteristics

The thiophene oligomer α-hexathienylene (α-6T) has been successfully used as the active semiconducting material in thin-film transistors. Field-induced conductivity in thin-film transistors with α-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 > 106. The current densities and switching speeds are good enough to allow consideration of these devices in practical large-area electronic circuits.

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.

Electronic sensing of vapors with organic transistors

We show that organic thin-film transistors have suitable properties for use in gas sensors. Such sensors possess sensitivity and reproducibility in recognizing a range of gaseous analytes. A wealth of opportunities for chemical recognition arise from the variety of mechanisms associated with different semiconductor–analyte interactions, the ability to vary the chemical constitution of the semiconductor end/side groups, and also the nature of the thin-film morphology.

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 α-hexathienylene (α-6T) and C60. The characteristics of these devices depend mainly on the molecular orbital energy levels and transport properties of α-6T and C60. 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.

Organic smart pixels

The fabrication and characteristics of organic smart pixels are described. The smart pixel reported in this letter consists of a single organic thin-film field effect transistor (FET) monolithically integrated with an organic light-emitting diode. The FET active material is a regioregular polythiophene. The maximum optical power emitted by the smart pixel is about 300 nW/cm2 corresponding to a luminance of ∼2300 cd/m2.

Electro‐optic phase modulation and optical second‐harmonic generation in corona‐poled polymer films

Electro‐optic phase modulation was measured along with optical second‐harmonic generation in thin films of a new copolymer containing a dicyanovinyl‐terminated azo dye side chain. Orientational order was imparted to these films by poling with a corona discharge. Details of the electro‐optic measurement technique, in which the real part of the electro‐optic coefficient can be determined directly, are presented. Taking advantage of the increased orientation imparted by corona poling and the hindered motion of the nonlinear optical moiety in the side chain of the polymer leads to substantial improvements in both the magnitude and stability of nonlinear optical susceptibilities compared to guest‐host polymer systems ordered by electrode poling.

Promising thermoelectric properties of commercial PEDOT:PSS materials and their bi2Te3 powder composites.

Newly commercialized PEDOT:PSS products CLEVIOS PH1000 and FE-T, among the most conducting of polymers, show unexpectedly higher Seebeck coefficients than older CLEVIOS P products that were studied by other groups in the past, leading to promising thermoelectric (TE) power factors around 47 μW/m K(2) and 30 μW/m K(2) respectively. By incorporating both n and p type Bi(2)Te(3) ball milled powders into these PEDOT:PSS products, power factor enhancements for both p and n polymer composite materials are achieved. The contact resistance between Bi(2)Te(3) and PEDOT is identified as the limiting factor for further TE property improvement. These composites can be used for all-solution-processed TE devices on flexible substrates as a new fabrication option.

Soft, conformable electrical contacts for organic semiconductors: High-resolution plastic circuits by lamination

Soft, conformable electrical contacts provide efficient, noninvasive probes for the transport properties of chemically and mechanically fragile, ultrathin organic semiconducting films. When combined with high-resolution printing and lamination techniques, these soft contacts also form the basis of a powerful technique for fabricating flexible plastic circuits. In this approach, a thin elastomeric film on a plastic substrate supports the electrodes and interconnections; laminating this substrate against another plastic substrate that supports the gate, dielectric and semiconductor levels establishes effective electrical contacts and completes the circuits. In addition to eliminating many of the problems associated with traditional layer-by-layer fabrication strategies, this lamination scheme possesses other attractive features: the transistors and circuit elements are naturally and efficiently encapsulated, and the active organic semiconductor layer is placed near the neutral mechanical plane. We demonstrate the features of soft, laminated contacts by fabricating large arrays of high-performance thin film transistors on plastic substrates by using a wide variety of organic semiconductors.