Paul H. Wöbkenberg

High‐Mobility Low‐Voltage ZnO and Li‐Doped ZnO Transistors Based on ZrO2 High‐k Dielectric Grown by Spray Pyrolysis in Ambient Air

Sequential layers of the high-k dielectric ZrO2 and the electron transporting semiconductors ZnO and Li-doped ZnO are deposited onto conductive indium tin oxide electrodes using spray pyrolysis. With these structures, thin-film transistors are fabricated with operating voltages below 6 V and maximum electron mobilities on the order of 85 cm(2) V-1 s(-1).

Low-voltage organic transistors based on solution processed semiconductors and self-assembled monolayer gate dielectrics

Reduction in the operating voltage of organic transistors is of high importance for successful implementation in low-power electronic applications. Here we report on low-voltage n-channel transistors fabricated employing a combination of soluble organic semiconductors and a self-assembled gate dielectric. The high geometric capacitance of the nanodielectric allows transistor operation below 2V. Solution processing is enabled by analysis of the surface energy compatibility of the dielectric and semiconductor solutions. Electron mobilities in the range of 0.01–0.04cm2∕Vs and threshold voltages ⩽0.35V are demonstrated. The present work paves the way toward solution processable low-voltage/power, organic complementary circuits.

Reduced Graphene Oxide Electrodes for Large Area Organic Electronics

patterning of molecular electrode materials and carbon nanotubes. The interlayer technique involves the insertion of a layer of photoresist between the substrate and the fi lm to be patterned. The resist layer is exposed through a mask, generating a pattern that can subsequently be developed after deposition of the target material. Immersion in an appropriate developer removes the soluble parts of the resist layer together with the overlying target material, leaving a patterned fi lm of the target material over a likewise patterned fi lm of the resist. The resist and the target material are in effect patterned simultaneously in an expose‐ deposit‐develop step sequence. Furthermore, this technique is compatible with the use of standard solution processing and mechanical transfer methods for deposition of the target fi lms. Since interlayer lithography builds on the existing expertise and equipment of conventional photolithography, it is easy to implement and fully compatible with fast, cost effective sheet-to-sheet processing for large-area electronics.

Electronic properties of ZnO field-effect transistors fabricated by spray pyrolysis in ambient air

We report the application of spray pyrolysis (SP) for the deposition of high quality zinc oxide (ZnO) films and the fabrication of thin-film transistors. The chemical, structural, optical, and electronic properties of as-deposited ZnO films are studied using infrared spectroscopy, atomic force microscopy, UV-visible spectroscopic ellipsometry, and field-effect measurements. SP ZnO films are found to be uniform and polycrystalline with a band gap of 3.32 eV. ZnO transistors exhibit n-channel characteristics with electron mobility in the range 10–22 cm2/Vs. Device performance is found to depend on the work function of source/drain metal electrodes and on the device architecture employed.

Ambipolar organic transistors and near-infrared phototransistors based on a solution-processable squarilium dye†

Implementation of organic transistors in low-end, large-volume microelectronics depends, greatly, on the level of performance that can be achieved, but also on the compatibility of the technology with low-cost processing methodologies. Here we examine the suitability of a family of solution-processable zwitterionic molecules, so-called squarilium dyes, for the fabrication of organic ambipolar transistors and their application in (opto)electronic circuits. Ambipolar organic semiconductors and transistors are interesting because they could deliver performance characteristics (i.e. noise margins and signal gain) similar to that of complementary logic, but with the fabrication simplicity associated with unipolar logic (i.e. single semiconductor material and single type of metal electrodes). By designing squarilium dyes with appropriate electrochemical characteristics we demonstrate single-layer organic transistors that exhibit ambipolar charge transport with balanced electron and hole mobilities. By integrating a number of these ambipolar transistors we are also able to demonstrate complementary-like voltage inverters with wide noise margin and high signal gain. Another interesting feature of the squarilium dyes studied here is their strong absorption in the near-infrared (NIR) region of the electromagnetic spectrum. By exploring this interesting property we are able to demonstrate NIR light-sensing ambipolar organic transistors with promising operating characteristics.

TiO2 thin-film transistors fabricated by spray pyrolysis

We demonstrate electron transporting thin-film transistors based on TiO2 films deposited from solution by spray pyrolysis under ambient atmosphere. The field-effect electron mobility is found to depend strongly on the device architecture and the type of source and drain electrodes employed. For optimized transistors a maximum mobility value of 0.05 cm2/V s is obtained. Furthermore, the TiO2 transistors show air-stable operating characteristics with a shelf life time of several months. This is the only report on electron transporting transistors based on thin-films of TiO2 deposited by spray pyrolysis. Such devices could be used for the study of charge carrier transport in TiO2 and other related materials.We demonstrate electron transporting thin-film transistors based on TiO2 films deposited from solution by spray pyrolysis under ambient atmosphere. The field-effect electron mobility is found to depend strongly on the device architecture and the type of source and drain electrodes employed. For optimized transistors a maximum mobility value of 0.05 cm2/V s is obtained. Furthermore, the TiO2 transistors show air-stable operating characteristics with a shelf life time of several months. This is the only report on electron transporting transistors based on thin-films of TiO2 deposited by spray pyrolysis. Such devices could be used for the study of charge carrier transport in TiO2 and other related materials.

Ink-jet printed p-type polymer electronics based on liquid-crystalline polymer semiconductors

Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs)—currently one of the most promising class of polymer semiconductors—are known to have a low-room-temperature solubility in a broad range of common organic solvents. By judicious selection of a suitable solvent for these rigid-rod-like, liquid-crystalline materials, we ink-jet printed poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C12) into discrete thin-film field-effect transistors (FETs) that display on/off ratios of up to 107 and charge-carrier mobilities in the range of 0.05–0.1 cm2 V−1 s−1. Compared to spin-coated devices, the ink-jet printed devices typically had lower current leakage; indeed, gate-leakage currents were 1 to 2 orders of magnitude lower in ink-jet printed transistors. These overall device performance of ink-jet printed pBTTT-C12 would be sufficient for, for instance, driving simple electrophoretic displays [T. N. Ng, S. Sambandan, R. Lujan, A. C. Arias, C. R. Newman, H. Yan and A. Facchetti, Appl. Phys. Lett., 2009, 94, 233307; G. H. Gelinck, T. C. T. Geuns and D. M. de Leeuw, Appl. Phys. Lett., 2000, 77, 1487–1489; H. E. A. Huitema, G. H. Gelinck, J. van der Putten, K. E. Kuijk, C. M. Hart, E. Cantatore, P. T. Herwig, A. van Breemen and D. M. de Leeuw, Nature, 2001, 414, 599–599; H. E. A. Huitema, G. H. Gelinck, J. van der Putten, K. E. Kuijk, K. M. Hart, E. Cantatore and D. M. de Leeuw, Adv. Mater., 2002, 14, 1201–1204.]. We demonstrate here integration of such ink-jet printed pBTTT-C12 transistors into unipolar digital logic gates, which produced a signal gain of more than 16. This is one of the highest gains reported for a unipolar-based logic gate and demonstrates the potential of pBTTT polymers for high performance application employing simple ink-jet printing procedures conducted in light and ambient air without any special precautions.

Solid‐State Processing of Organic Semiconductors

By Mohammed A. Baklar , elix F Koch , Avinesh Kumar , Ester Buchaca Domingo , Mariano Campoy-Quiles , Kirill eldman , F Liyang u , Y Paul Wobkenberg , James Ball , Rory M. Wilson , Iain McCulloch , Theo Kreouzis , Martin Heeney , Thomas Anthopoulos , Paul Smith ,

Solution processed low-voltage organic transistors and complementary inverters

We demonstrate electron and hole-transporting low-voltage transistors based on self-assembling monolayer (SAM) gate dielectrics and solution processed organic small-molecule semiconductors. The studied SAMs include methyl and carboxylic acid terminated molecules. Compared to methyl terminated alkylphosphonic acids, carboxylic acid terminated SAMs are found to exhibit increased surface energy. This enables solution processing of a wide range of small molecules onto the dielectric for the fabrication of low-voltage transistors. Using these transistors we demonstrate complementary inverters operating at voltages <2 V. This work is an important step toward realizing low-voltage organic electronics.

Fluorine containing C60 derivatives for high-performance electron transporting field-effect transistors and integrated circuits

We report on electron transporting organic transistors and integrated ring oscillators based on four different solution processible fluorine containing C60 derivatives. Electron mobilities up to 0.15cm2∕Vs are obtained from as-prepared bottom-gate, bottom-contact transistors utilizing gold source-drain electrodes. Despite the high mobility, no long-range structural order could be identified with the semiconductor films exhibiting amorphouslike characteristics. The good electron transport is attributed to the structural symmetry of the fullerene derivatives and the enhanced π-π interactions between C60 units even in the case of amorphouslike films. These advantageous characteristics make fluorine containing C60 derivatives attractive for application in high-performance, large-area organic electronics.