Ambipolar organic field effect transistors and inverters

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Nov 2, 2011 - thank Günther Schwabegger, Michael Sams, and Pavel Troshin for fruitful ... 14 A. Kraus, S. Richler, A Opitz, W. Brütting, S. Haas, H. Tatsuo, ...
Ambipolar organic field effect transistors and inverters with the natural material Tyrian Purple Eric Daniel Głowacki, Lucia Leonat, Gundula Voss, Marius-Aurel Bodea, Zeynep Bozkurt et al. Citation: AIP Advances 1, 042132 (2011); doi: 10.1063/1.3660358 View online: http://dx.doi.org/10.1063/1.3660358 View Table of Contents: http://aipadvances.aip.org/resource/1/AAIDBI/v1/i4 Published by the AIP Publishing LLC.

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AIP ADVANCES 1, 042132 (2011)

Ambipolar organic field effect transistors and inverters with the natural material Tyrian Purple Eric Daniel Głowacki,1 Lucia Leonat,2 Gundula Voss,3 Marius-Aurel Bodea,4 Zeynep Bozkurt,5 Alberto Montaigne Ramil,1 Mihai Irimia-Vladu,1,6,a Siegfried Bauer,6 and Niyazi Serdar Sariciftci1 1

Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz, Altenberger Straße 69 A-4040, Linz, Austria 2 Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science, Bucharest, Romania; National Institute for Electrical Engineering, IPCE-CA, Bucharest, Romania 3 Department of Bioorganic Chemistry, University of Bayreuth, D-95440, Bayreuth, Germany 4 Institute of Applied Physics, Johannes Kepler University Linz, Altenberger Straße 69 A-4040, Linz, Austria 5 Dept. of Material Science and Engineering, Sabanci University, Istanbul, Turkey 6 Soft Matter Physics, Johannes Kepler University Linz, Altenberger Straße 69 A-4040, Linz, Austria (Received 18 August 2011; accepted 13 October 2011; published online 2 November 2011)

Ambipolar organic semiconductors enable complementary-like circuits in organic electronics. Here we show promising electron and hole transport properties in the natural pigment Tyrian Purple (6,6’-dibromoindigo). X-ray diffraction of Tyrian Purple films reveals a highly-ordered structure with a single preferential orientation, attributed to intermolecular hydrogen bonding. This material, with a band gap of ∼1.8 eV, demonstrates high hole and electron mobilities of 0.22 cm2 /V · s and 0.03 cm2 /V · s in transistors, respectively; and air-stable operation. Inverters with gains of 250 in the first and third quadrant show the large potential of Tyrian Purple for the development of integrated organic electronic circuits. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [doi:10.1063/1.3660358]

Complementary circuits (CMOS), originally developed in silicon microelectronics, are the basis technology for integrated circuits, due to the wide noise margin, durability and low power dissipation achievable.1 In organic electronics, the fabrication of CMOS-like circuits with discrete p- and n-channel devices from the same semiconductor remains challenging.2 Organic ambipolar semiconductors should have a small band gap supporting injection of electrons and holes from a single electrode material. Here we show that the natural pigment Tyrian Purple, 6,6’-dibromoindigo (figure 1(a)), is an ambipolar organic semiconductor with large electron and hole mobilities. Tyrian Purple forms highly ordered films with a single preferential orientation when prepared by evaporation. We have used such films to fabricate ambipolar organic field-effect transistors and inverters with high gain in the first and third quadrants. These devices exemplify the possibility of fabrication of high-performance biocompatible and biodegradable organic electronics.3, 4 One of the most prized dyes in antiquity, Tyrian Purple was originally produced from sea snails.5, 6 We prepared the dye synthetically via published procedures7, 8 and purified it twice by temperature gradient sublimation. Transistors were fabricated on glass substrates by first creating an AlOx dielectric layer electrochemically9 (40 nm) on a vacuum-evaporated aluminum gate electrode (100 nm). Next, a 40 nm passivation layer of the oligoethylene tetratetracontane (C44 H90 , TTC) was evaporated onto the AlOx . Easily extracted from petroleum distillation, TTC is biodegradable and

a To whom the correspondence should be addressed: E-mail: [email protected]

2158-3226/2011/1(4)/042132/6

1, 042132-1

 C Author(s) 2011

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FIG. 1. (a) Molecular structure of Tyrian Purple. (b) XRD of a 80 nm Tyrian Purple film on TTC (40 nm)|glass. A single diffraction peak at 2θ ( 5.78 is visible. TTC itself showed no diffraction peaks in the measured range. (c) Cyclic voltammogram of Tyrian Purple film (80 nm) on ITO (working electrode) with a platinum disk counter electrode, Ag(AgCl reference electrode, scan rate 20 mV/s, and 0.1M tetrabutylammonium phosphorus hexafluoride in anhydrous CH3 CN electrolyte solution. (d) UV-Vis spectrum of an 80 nm film of Tyrian Purple deposited on ITO, showing an absorption onset at ∼600 nm.

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FIG. 2. Atomic force microscopy (AFM) image of 80 nm of Tyrian Purple grown on a 40 nm film of tetratetracontane (TTC, the gate dielectric material). The film has an RMS roughness of ∼10 nm, with maximum peak-peak of ∼110 nm.

occurs abundantly in nature, being identified in large amounts (up to 10%) in medicinal plants10–13 TTC-passivated AlOx slides were annealed at 60 o C for ∼12h under nitrogen,14 giving a specific capacitance, C0d , of 43.2 nF · cm-2 . Next, 80 nm of Tyrian Purple was evaporated from a hotwall epitaxial source at a rate of 0.1 Å/s.15 Gold source and drain electrodes were evaporated through a shadow mask giving W/L of 1 mm / 80 μm. Similar to indigo, X-ray diffraction (XRD) of Tyrian Purple deposited on TTC showed only one diffraction peak (figure 1(b)) instead of a Debye ring characteristic of polycrystalline materials, suggesting a crystalline texture with a single preferential orientation.16 This high long-range order is also reflected in a large relative permittivity, εr . From impedance measurements of Al|dye|Al structures we calculated a value of εr = 6.2.17 Intermolecular hydrogen bonding between the carbonyl oxygens and amine hydrogen has been intensively characterized through X-ray diffraction18, 19 and other studies of indigoids.20, 21 Each Tyrian Purple molecule is nearly perfectly planar, with deviation from planarity of ∼ 5×10-12 m and is hydrogen bonded to four others, with an interplanar distance between two parallel molecules of 3.44 Å.19 Since the molecule alone has a low level of conjugation, two phenyl rings separated by an unconjugated bridge, we surmise that its highly-ordered growth resulting in intermolecular π -stacking is integral for charge transport. Cyclic voltammetry and UV-Vis measurements of Tyrian Purple thin films are shown in figures 1(c) and 1(d), respectively. On the basis of these we estimate a band gap of ∼1.8 eV, a HOMO level of -5.8 eV, and a LUMO level of -4.0 eV. Atomic-force microscopy (AFM) measurements of Tyrian Purple show the formation of crystalline needle-like grains with sizes in the 200-500 nm range. With the channel length used in these transistors, 80 μm, and grains of this size it is likely that overall mobility is limited by intergrain mobility. An AFM image of 80 nm of Tyrian Purple grown on TTC is shown in figure 2.

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FIG. 3. (a) Transfer characteristics of a Tyrian Purple transistor showing source-drain current (Ids ) in black type, leakage current (Igs ) in olive (b) Output characteristics in the first quadrant. (c) Output characteristics in the third quadrant. (d) Comparison of transfer characteristics during measurement in N2 atmosphere and air. Mobility for electrons drops by a factor of 2, but after this initial drop remains stable during cycling.

Transfer and output characteristics of transistor devices are shown in figure 3(a)–3(c). In the first quadrant, the output characteristics of Tyrian Purple show a superlinear increase of current at low applied gate voltages due to the injection of electrons in a channel dominated by holes. Similarly, in the third quadrant the superlinear regime occurs for decreasingly negative gate voltages, because of the injection of holes. The electron and hole field-effect mobilities are calculated in the saturation regime, according to reported methods.22 As shown in figure 3(a), the field effect mobilities are typically around 0.2 cm2 /Vs for holes and 0.03 cm2 /Vs for electrons. With a positive source-drain voltage, threshold gate voltages for holes are in the 1.5 – 1.75 V range, while for electrons, 3V – 5V. On/Off ratios were 103 – 104 for both channels. Leakage current, Igs , as shown in figure 3(a), was in the picoamp range. We found that both hole and electron mobilities show little deterioration during measurement in air, even after repeated cycles (figure 3(d)). This is due to the relatively deep LUMO value of approx. -4 eV.23 Quasi-static transfer curves and corresponding voltage gains of complementary-like inverter circuits are shown at various VDD voltages in figures 4(a)–4(d). Gains of 255 in the first and 285 in the third quadrants are among the highest reported to-date for organic ambipolar devices.2, 24 These devices showed little hysteresis upon cycling.17 In summary, we have found promising ambipolar transistor operation with the natural material Tyrian Purple. The performance of this material is compared with data on previously-reported ambipolar organic semiconductors in table I. Mobilities in Tyrian Purple are on-par with values reported in recent studies. The crystalline texture with single-preferential orientation, as evidenced by a single XRD peak, and a high dielectric constant (6.2) are interesting properties of this hydrogen-bonded semiconducting material. We fabricated devices featuring high mobilities and high voltage gains in complementary-like voltage inverters. We achieve low-voltage operation utilizing a gate dielectric of AlOx passivated with a naturally-occuring oligoethylene,

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FIG. 4. (a) Quasi steady-state transfer characteristics of complementary-like voltage inverters in the first quadrant, with (c) showing the corresponding voltage gain. (b) Quasi steady-state transfer characteristics of complementary-like voltage inverters in the third quadrant, with voltage gain shown in (d). Each measurement is performed through 1000 points with hold time, step delay and delay time of 1 second each.

TABLE I. Summary of performance of ambipolar organic semiconductors

Semiconducting Material (reference) Tyrian Purple (this work)

Reported mobility values, or range of reported values Hole Electron (cm2 /V · s) (cm2 /V · s) 0.22

0.03

Operational stability

p-channel stable, nchannel factor of 2 reversible loss PDPP-TBT (25) 0.35 0.40 Not reported Not reported PSSS-C10 (26) 0.3 4×10-3 – 0.01 P3OS (26) 2-9×10-3 2-9×10-3 Not reported Several months Nickel dithiolene 4×10-4 -1.6×10-3 2-8×10-4 (27) storage, stability during measurement not reported ∼10-4 Not reported Squarilium dye (28) ∼10-4 PDTDPP-alt-EMD (29) 0.3 0.3 Not reported PDTDPP-alt-BTZ (30) 0.1 0.09 Not reported

Inverter Performance Mobility during air W/L Gain (1st quad exposure (cm2 /V · s)(mm/μm) / 3rd quad) hole, 0.20 elec., 0.015

1 / 80

N/A N/A N/A unchanged

1 / 100 not reported 10 / 1 86 / not reported 10 / 1 Not reported 6/5 ∼6/6

N/A N/A N/A

10 / 2 1/5 2/ 50

255 / 285

10-16 / 10-16 Not reported ∼35 / 35

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tetratetracontane. This work demonstrates that high performance organic electronics can be fabricated from biomaterials. This work was partially funded by the Austrian Science Foundation “FWF” within the National Research Network NFN on Interface Controlled and Functionalized Organic Films (S09711-N08 and S09712-N08) and by the European Science Foundation. Financial support of the corresponding author from the city of Linz and the Land Ober¨osterreich is highly appreciated. The authors warmly thank G¨unther Schwabegger, Michael Sams, and Pavel Troshin for fruitful discussions. 1 R. J. Baker, CMOS Circuit design, layout, and simulation IEEE Press Series on Microelectronic Systems (Wiley Interscience,

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