Ute Zschieschang

The University of Tokyo, Tokyo, Tokyo-to, Japan

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Publications (125)590.96 Total impact

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    ABSTRACT: Organic thin-film transistors (TFTs) have potential as pixel drivers in flexible active-matrix organic light-emitting diode displays [1]. Hence it is essential to analyze the charge-carrier injection and extraction dynamics of organic TFTs to gain a better understanding of the trapping and detrapping at the TFT interfaces. From the current-voltage characteristics of the TFTs, many important parameters can be extracted, such as carrier mobility, threshold voltage, on/off ratio, subthreshold slope and transconductance. But to quantitatively evaluate the trapping and detrapping dynamics, displacement current measurements on two-terminal long-channel capacitors (LCCs) are far more useful [2, 3]. The cross-section and the layout of an LCC are schematically shown in Fig. 1. Unlike a TFT, an LCC has only one contact, so that carriers are injected into and extracted from the semiconductor through the same contact. To increase the signal-to-noise ratio, a very large channel length (up to 6 cm) is employed. While Liang et al. have performed displacement current measurements on pentacene-based LCCs [2,3], we report here on displacement current measurements on LCCs based on four different organic semiconductors: pentacene, dinaptho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), 2,9-didecyl-DNTT (C10-DNTT) and diphenyl-DNTT (DPh-DNTT). In TFTs, these semiconductors show hole mobilities ranging from 1 to 7 cm2/Vs. The goal of the displacement current measurements reported here is to study how the choice of the semiconductor affects the trapping and detrapping dynamics in organic TFTs.
    2014 72nd Annual Device Research Conference (DRC); 06/2014
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    ABSTRACT: This paper presents analysis of the charge storage behavior in organic thin-film transistors (OTFTs) by means of admittance characterization, compact modeling, and 2-D device simulation. The measurements are performed for frequencies ranging from 100 Hz to 1 MHz and bias potentials from zero to -3 V on top-contact OTFTs that employ air-stable and high-mobility dinaphtho-thieno-thiophene as the organic semiconductor. It is demonstrated that the dependence of the intrinsic OTFT gate-source and gate-drain capacitances on the applied voltages agrees very well with Meyer's capacitance model. Furthermore, the impact of parasitic elements, including fringe current and contact impedance, is investigated. The parameters used for the simulation and modeling of all the dynamic characteristics correspond closely to those extracted from static measurements. Finally, the implications of the admittance measurements are also discussed relating to the OTFTs dynamic performance, particularly the cutoff frequency and the charge response time.
    IEEE Transactions on Electron Devices 01/2014; 61(1):98-104. · 2.06 Impact Factor
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    ABSTRACT: A process for the fabrication of bottom-gate, top-contact (inverted staggered) organic thin-film transistors (TFTs) with channel lengths as short as 1 μm on flexible plastic substrates has been developed. The TFTs employ vacuum-deposited small-molecule semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 3 V. The p-channel TFTs have an effective field-effect mobility of about 1 cm2/Vs, an on/off ratio of 107, and a signal propagation delay (measured in 11-stage ring oscillators) of 300 ns per stage. For the n-channel TFTs, an effective field-effect mobility of about 0.06 cm2/Vs, an on/off ratio of 106, and a signal propagation delay of 17 μs per stage have been obtained.
    Design Automation and Test in Europe; 01/2014
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    ABSTRACT: The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b:2′,3′-f]-thieno-[3,2-b]-thiophene (C10-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C10-DNTT is 8.5 cm2/V s for a nominal semiconductor thickness of 10 nm and a substrate temperature during the semiconductor deposition of 80 °C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4 nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C10-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100 μm, a relatively small contact resistance of 0.33 kΩ cm was determined. TFTs with the C10-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8 ± 0.3) cm2/V s, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3 ± 0.1) cm2/V s.
    Organic Electronics 12/2013; 14(12):3213–3221. · 3.84 Impact Factor
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    ABSTRACT: In the late 1960s, a new concept was proposed for an infrared absorbing device called a “rectenna” that, combining an antenna and a nanoscale metal-insulator-metal diode rectifier, collects electromagnetic radiation in the terahertz regime, with applications as detectors and energy harvesters. Previous theories hold that the diode rectifies the induced terahertz currents. Our results, however, demonstrate that the Seebeck thermal effect is the actual dominant rectifying mechanism. This new realization that the underlying mechanism is thermal-based, rather than tunneling-based, can open the way to important new developments in the field, since the fabrication process of rectennas based on the Seebeck effect is far simpler than existing processes that require delicate tunnel junctions. We demonstrate for the first time the fabrication of a rectenna array using an efficient parallel transfer printing process featuring nearly one million elements.
    IEEE Transactions on Nanotechnology 11/2013; 12(6):1144-1150. · 1.80 Impact Factor
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    ABSTRACT: Organic thin-film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9-bis(perfluoroalkyl)-substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core-brominated TAPPs should exhibit n-channel semiconducting properties. Current-voltage characteristics of the TFTs established electron mobilities of up to μn = 0.032 cm2 V−1 s−1 for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN).
    Advanced Functional Materials 08/2013; 23(31). · 10.44 Impact Factor
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    ABSTRACT: Air-stable bottom-gate, top-contact n-channel organic transistors based on a naphthalene diimide exhibiting electron mobilities up to 0.8 cm2/Vs at low voltages were fabricated. Transistors with channel lengths of 1 μm show a transconductance of 60 mS/m, but are significantly limited by the contact resistance. Transmission line measurements in combination with contact resistance models were applied to investigate this influence. Both contact resistance and contact resistivity are proportional to the inverse gate overdrive voltage. Organic complementary ring oscillators were fabricated on a flexible plastic substrate showing record signal delays down to 17 μs at a supply voltage of 2.6 V.
    Applied Physics Letters 06/2013; 6(6). · 3.52 Impact Factor
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    ABSTRACT: Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1 μm have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT). The transistors have an effective field-effect mobility of 1.2 cm2/V s, an on/off ratio of 107, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 ns per stage at a supply voltage of 3 V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V.
    Organic Electronics 06/2013; 14(6):1516–1520. · 3.84 Impact Factor
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    ABSTRACT: This paper presents S-parameter characterization and a corresponding physics-based small-signal equivalent circuit for organic thin-film transistors (OTFTs). Furthermore, the impact of misalignment between the source/drain contacts and the patterned gate on the dynamic TFT performance is explored and a simple method to estimate the misalignment from the measured S-parameters is proposed. An excellent fit between theoretical and experimental S-parameters is demonstrated. For this study, OTFTs based on the air-stable organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) having a channel length of 1 μm and a gate-to-contact overlap of 5 or 20 μm and being operated at a supply voltage of 3 V are utilized. The intentional asymmetry between gate-to-source and gate-to-drain overlaps is precisely controlled by the use of high-resolution silicon stencil masks.
    Organic Electronics 05/2013; 14(5):1318–1322. · 3.84 Impact Factor
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    ABSTRACT: Low-operating-voltage flexible organic thin-film transistors with high thermal stability using DPh-DNTT and SAM gate dielectrics are reported. The mobility of the transistors are decreased by 23% after heating to 250 °C for 30 min. Furthermore, flexible organic pseudo-CMOS inverter circuits, which are functional after heating to 200 °C.
    Advanced Materials 04/2013; · 14.83 Impact Factor
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    ABSTRACT: This letter presents the first comprehensive experimental studies on the frequency response of staggered low-voltage organic thin-film transistors (OTFTs) using S-parameter measurements. The transistors utilize air-stable dinaphtho-thieno-thiophene as the organic semiconductor with various channel lengths and gate overlaps. A peak cutoff frequency of 3.7 MHz for a channel length of 0.6 $mu{rm m}$, gate overlap of 5 $mu{rm m}$, and a supply voltage of 3 V is achieved. In view of the low supply voltage and air-stability, this result marks a record achievement in OTFT technology. The channel length dependence of the cutoff frequency is described in a compact model and a close correspondence to the measured data of OTFTs with variable device dimensions is shown. Moreover, the cutoff frequencies at different gate biases are found to be proportional to the dc transconductance.
    IEEE Electron Device Letters 04/2013; 34(4):520-522. · 2.79 Impact Factor
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    ABSTRACT: We measure the gap density of states and the Fermi level position in thin-film transistors based on pentacene and dinaphtho[2,3-b:2^{'},3^{'}-f]thieno[3,2-b]thiophene (DNTT) films grown on various surfaces using Kelvin probe force microscopy. It is found that the density of states in the gap of pentacene is extremely sensitive to the underlying interface and governs the Fermi level energy in the gap. The density of gap states in pentacene films grown on bare silicon dioxide (SiO_{2}) was found to be larger by 1 order of magnitude compared to that in pentacene grown on SiO_{2} treated with hexamethyldisilazane and larger by 2 orders of magnitude compared to that of pentacene grown on aluminum oxide (AlO_{x}) treated with a self-assembled monolayer (SAM) of n-tetradecylphosphonic acid (HC_{14}-PA). When DNTT was grown on HC_{14}-PA-SAM-treated AlO_{x}, the gap density of states was even smaller, so that the Fermi level pinning was significantly reduced. The correlation between the measured gap density of states and the transistor performance is demonstrated and discussed.
    Physical Review Letters 01/2013; 110(3):036803. · 7.73 Impact Factor
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    ABSTRACT: Here we report hexathienocoronenes (HTCs), fully thiophene-annelated coronenes in which six double bonds in the periphery are thieno-fused. The derivatives tetrasubstituted with hexyl and dodecyl chains show a phase formation that strongly depends on the chain length. HTCs are remarkably stronger donors than the known thiophene-annelated coronenes but do not readily assemble into well-ordered films when deposited from the vapor phase. Thus, thin-film transistors fabricated by vacuum deposition have only modest field-effect mobilities of 0.002 cm(2) V(-1) s(-1).
    Journal of the American Chemical Society 10/2012; · 10.68 Impact Factor
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    ABSTRACT: We demonstrate the fabrication of ultra-thin titanium oxide films by plasma-induced surface oxidation. Ellipsometry measurements indicate an oxide thickness of about 2 nm. Electrical characterization was performed on microscale and nanoscale metal-insulator-metal tunneling diodes. Electrical fields up to 22 MV/cm were applied without destroying the titanium oxide films. The current-voltage-characteristic of the diodes are found to be asymmetric with respect to zero bias when employing electrodes with different work functions. The permittivity of the ultra-thin titanium oxide was determined to be less than 6, which is the smallest permittivity that has been reported for titanium oxide.
    Applied Physics Letters 08/2012; 101(8). · 3.52 Impact Factor
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    ABSTRACT: A series of new tetraazapyrene (TAPy) derivatives has been synthesized by reducing 1,4,5,8-tetranitronaphthalene to its corresponding tin salt (I) and reacting it with perfluorinated alkyl or aryl anhydrides. The resulting 2,7-disubstituted TAPy molecules and the known parent compound 1,3,6,8-tetraazapyrene (II) have been further derivatized by core chlorination and bromination. The brominated compounds served as starting materials for Suzuki cross-coupling reactions with electron-poor arylboronic acids. Single-crystal X-ray analyses established polymorphism for some TAPy compounds. The ground-state geometries of all new TAPy derivatives were modeled with DFT methods [B3PW91/6-31 g(d,p) and B3PW91/6-311+g(d,p)], especially focusing on the energies of the lowest unoccupied molecular orbital (LUMO) and the electron affinities (EA) of the molecules. The results of the calculations were confirmed experimentally by cyclic voltammetry to evaluate the substitution effects at the 2 and 7 position and the core positions, respectively, and gave LUMO energy levels that range from -3.57 to -4.14 eV. Fabrication of organic field-effect transistors (OFETs) with several of these tetraazapyrenes established their potential as organic n-type semiconductors.
    The Journal of Organic Chemistry 06/2012; 77(14):6107-16. · 4.56 Impact Factor
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    ABSTRACT: Nanoscale metal-insulator-metal (MIM) diodes represent important devices in the fields of electronic circuits, detectors, communication, and energy, as their cutoff frequencies may extend into the "gap" between the electronic microwave range and the optical long-wave infrared regime. In this paper, we present a nanotransfer printing method, which allows the efficient and simultaneous fabrication of large-scale arrays of MIM nanodiode stacks, thus offering the possibility of low-cost mass production. In previous work, we have demonstrated the successful transfer and electrical characterization of macroscopic structures. Here, we demonstrate for the first time the fabrication of several millions of nanoscale diodes with a single transfer-printing step using a temperature-enhanced process. The electrical characterization of individual MIM nanodiodes was performed using a conductive atomic force microscope (AFM) setup. Our analysis shows that the tunneling current is the dominant conduction mechanism, and the electrical measurement data agree well with experimental data on previously fabricated microscale diodes and numerical simulations.
    ACS Nano 03/2012; 6(3):2853-9. · 12.03 Impact Factor
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    ABSTRACT: A range of 2,9-perfluoroalkyl-substituted tetraazaperopyrene (TAPP) derivatives (1-5) was synthesised by reacting 4,9-diamino-3,10-perylenequinone diimine (DPDI) with the corresponding carboxylic acid chloride or anhydride in the presence of a base. The reaction of compounds 1-4 with dichloroisocyanuric acid (DIC) in concentrated sulphuric acid resulted in the fourfold substitution of the tetraazaperopyrene core, yielding the 2,9-bisperfluoroalkyl-4,7,11,14-tetrachloro-1,3,8,10-tetraazaperopyrenes 6-9, respectively. The optical and electrochemical data demonstrate the drastic influence of the core substitution on the properties. All compounds are highly luminescent (fluorescence quantum yields of up to Φ=0.8). The LUMO energies of the tetrachlorinated TAPP derivatives (determined by cyclic voltammetry and computed by DFT calulations) were found to be below -4 eV. In the course of this work the performance of TAPP derivatives in organic thin-film transistors (TFTs) was investigated, and their n-channel characteristics with field-effect mobilities of up to 0.14 cm(2)  V(-1)  s(-1) and an on/off current ratio of >10(6) were confirmed. Long-term stabilities of 3-4 months under ambient conditions of the devices were established. Complementary inverters and ring oscillators with n-channel TFTs based on compound 8 and p-channel TFTs based on dinaphtho-[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) were fabricated on a glass substrate.
    Chemistry - A European Journal 02/2012; 18(12):3498-509. · 5.93 Impact Factor
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    ABSTRACT: Using the recently developed organic semiconductor, 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT), we have fabricated organic thin-film transistors and ring oscillators on flexible polymeric substrates. By utilizing a gate dielectric with a small thickness (5.3 nm) and a large capacitance (800 nF cm−2), the transistors can be operated with relatively low voltages of about 2 to 3 V. To improve the charge exchange between the organic semiconductor and the gold source and drain contacts, a thin layer of a non-alkylated organic semiconductor (DNTT) sandwiched between two thin layers of a strong organic dopant (NDP-9) was inserted between the C10-DNTT and the gold contacts. The optimized transistors have a field-effect mobility of 4.3 cm2 V−1 s−1, an on/off current ratio of 108, and a subthreshold swing of 68 mV per decade. The ring oscillators have a signal propagation delay of 5 μs per stage at a supply voltage of 3 V, making these the fastest organic circuits at supply voltages below 7 V reported to date.
    Journal of Materials Chemistry 02/2012; 22(10):4273-4277. · 6.63 Impact Factor
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    ABSTRACT: Nano diodes show great potential for applications in detectors, communications and energy harvesting. In this work, we focus on nano transfer printing (nTP) to fabricate nm-scale diodes over extensive areas. Using a temperature-enhanced process, several millions of diodes were transfer-printed in one single step. We show the reliable transfer of functioning MIM diodes, which were electrically characterized by conductive Atomic Force Microscopy (c-AFM) measurements. Quantum-mechanical tunneling was determined to be the main conduction mechanism across the metal-oxide-metal junction.
    Silicon Nanoelectronics Workshop (SNW), 2012 IEEE; 01/2012
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    ABSTRACT: Nano diodes show great potential for applications in detectors, communications and energy harvesting. However, to make them suitable for low-cost mass production, these nano devices have to be fabricated reliably over large areas while minimizing process time and costs. Printing techniques are promising candidates to overcome these economical drawbacks of conventional nanolithography without a significant loss in structure quality. In this work, we focus on nano transfer printing (nTP) to fabricate nm-scale diodes over extensive areas. Using a temperature-enhanced process, several millions of diodes were transfer-printed in one single step. We show the reliable transfer of functioning Schottky and MIM diodes of different sizes, which demonstrates the versatility and usability of our approach (nTP), paving the way to numerous applications in the fields of e.g. infrared detection or energy harvesting. The nano devices are characterized electrically by conductive Atomic Force Microscopy (c-AFM) measurements. For these MIM structures, quantum-mechanical tunneling was determined to be the main conduction mechanism across the metal-oxide-metal junction.
    Nanotechnology (IEEE-NANO), 2012 12th IEEE Conference on; 01/2012

Publication Stats

3k Citations
590.96 Total Impact Points

Institutions

  • 2008–2013
    • The University of Tokyo
      • • Department of Applied Physics
      • • Center for Quantum-Phase Electronics
      • • Department of Electrical Engineering and Information Systems
      Tokyo, Tokyo-to, Japan
  • 2006–2013
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 2012
    • Universität Heidelberg
      • Institute of Inorganic Chemistry
      Heidelberg, Baden-Wuerttemberg, Germany
  • 2011–2012
    • Technische Universität München
      • Institute of Nanoelectronics
      München, Bavaria, Germany
    • Leibniz Institute for Solid State and Materials Research Dresden
      • Institute for Integrative Nanosciences
      Dresden, Saxony, Germany
  • 2010
    • McMaster University
      • Department of Electrical and Computer Engineering
      Hamilton, Ontario, Canada
  • 2002–2005
    • Infineon Technologies
      München, Bavaria, Germany