R. Parashkov

Large Area Electronics Using Printing Methods

After the demonstration of the first organic FET in 1986, a new era in the field of electronic began: the era of organic electronics. Although the reported performance of organic transistors is still considerably lower compared to that of silicon transistors, a new market is open for organic devices, where the excellent performance of silicon technology is not required. Several commercial applications for organic electronics have been suggested: organic RFID tags, electronic papers, imagers, sensors, organic LED drivers, etc. The main advantage of organic technologies over silicon technologies is the possibility of making low-cost, large area electronics. The main processes which allow patterning with suitable resolution on a large areas are printing methods. Here we will provide an overview of methods that can be useful in the low-cost production of large area electronics.

All-organic thin-film transistors made of poly(3-butylthiophene) semiconducting and various polymeric insulating layers

We have fabricated fully patterned all-organic thin-film transistors with a variety of organic polymer insulators. Poly(3-butylthiophene) deposited by spin coating was used as the active organic layer. We have built top-gate structures with gates printed on top of the gate dielectric layer. The field enhanced current is weak with poly(4-vinyl phenol), but much stronger with polyvinyl alcohol and cyanoethylpullulan. Carrier mobilities as large as 0.04 cm2/V s were measured in the case of cyanoethylpullulan. A strong correlation is found between the solvents used for the dielectrics, dielectric constant of the insulator, and the field-effect mobility.

Vertical channel all-organic thin-film transistors

Technologically simple and cost-effective processes are essential for the fabrication of organic electronic devices. In this letter, we present a concept for making vertical channel all-organic thin-film transistors on glass substrate. This concept avoids the need for patterning processes with high lateral resolution by defining the channel length through the thickness of an insulating layer. Our devices are based on commercially available poly(ethylene dioxythiophene)/poly(styrene sulfonate) dispersion for source, drain, and gate electrodes, photoresist as the insulating layer and photosensitized poly(vinyl alcohol) as the gate insulator. Pentacene was used as the organic semiconductor. Functional devices with channel length of 2.4 μm and width of 1 mm have been realized, and we report electrical characteristics of these devices.

All-organic thin-film transistors patterned by means of selective electropolymerization

We have fabricated fully patterned all-organic thin-film transistors on polyimide substrates using selectively electropolymerized poly (3,4-ethylenedioxythiophene) doped with poly (styrene sulfonate) (PEDOT:PSS) for the source and drain contacts, PEDOT:PSS Baytron P dispersion for the gate electrodes, poly (4-vinyl phenol) or polyvinyl alcohol for the gate dielectric layers, and pentacene or poly (3-butylthiophene) for the organic active layers. We have built top-gate structures with gates printed on top of the gate dielectric layer. Carrier mobilities as large as 0.01 cm2/V s were measured. Functional all-organic transistors have been realized using a simple and potentially inexpensive technology that does not depend on photolithographical processes and that allows the preparation of feature sizes on the micrometer scale.

Organic vertical-channel transistors structured using excimer laser

Low-cost, large-area patterning of organic field-effect transistors with high-resolution is a subject of ongoing investigations. Here, we present a concept of patterning vertical-channels organic transistors using excimer laser. The channel length is controlled by the thickness of the dielectric polymer layer between the drain and source electrodes. We demonstrate that, by using this method, patterning of transistors with either metal or polymer contacts with resolutions as high as 2μm is possible. Experimental data of vertical-channel pentacene transistors with either gold or poly (3,4-ethylenedioxythiophene) as drain-source contacts are reported. Field effect mobilities of 1×10−3 and 3×10−4cm2∕Vs, respectively, have been measured in these devices.

Flexible All-Organic Field-Effect Transistors Fabricated by Electrode-Peeling Transfer

We fabricated fully patterned all-organic field-effect transistors on polymethyl methacrylate substrates by the selective electrochemical doping of electrically conducting poly(3,4-ethylenedioxythiophene) / tetraethylammoniumhexafluorophosphate into polymer films coated on an electrode surface for the source and drain contacts. Polyvinyl alcohol and pentacene were used as the insulating and active layers respectively. We built top-gate structures with gates printed on top of the gate dielectric layer. Carrier mobilities as large as 0.02 cm2/Vs were measured. Functional all- organic transistors were realized using a simple and potentially inexpensive technology that does not depend on photolithographical processes.

Enhanced mixing characteristics of GaAs/3,4,9,10-perylenetetracarboxylic dianhydride Schottky diodes

The influences on the mixing properties of GaAs Schottky diodes containing an organic 3,4,9,10-perylenetetracarboxylic dianhydride layer were investigated. The frequency conversion ability of the devices was determined by considering the I-V characteristics and high frequency reflection parameters by using a mixing technique operated in the microwave range. The results show that an organic layer with 20 nm thickness enhances the diode conversion gain for mixing applications by 3 dB and lowers the device operating bias voltage by 0.1 V. This process is related to the specific properties of the organic semiconductor and resulting organic-inorganic interface.

All-organic field effect transistors

In this work we present fully patterned organic transistors based on selective electropolymerization of conducting polymers that enables simple fabrication of micron scale features. It involves fabrication of pentacene field effect transistors in which the conducting, insulating parts as well as the substrate are all made of polymers. We have fabricated drain and source electrodes by electropolymerization of 3,4- ethylenedioxythiophene and gate by spin coating of commercially available poly( 3,4- ethylenedioxythiophene) (PEDOT:PSS) aqueous dispersion, polyvinylalcohol for the gate dielectric layer, and pentacene for the organic active layer. We have built a top-gate structure with gate dielectric layer and gate placed on the top of the pentacene layer, and in a such way obtained protection of the active layer could permit enhancement of the operating time of devices. Carrier mobility as large as 0,01 cm 2 /V s was measured. Functional all- organic transistors have been realised using a simple and potentially inexpensive technology.