2011, vladu. indigo - a natural pigment for high performance

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Dr. M. Irimia-Vladu, Prof. S. BauerSoft Matter PhysicsJohannes Kepler University LinzAltenberger Straße 69, 4040 Linz, Austria

E-mail: [email protected]. M. Irimia-Vladu, E. D. Głowacki, Prof. N. S. SariciftciLinz Institute for Organic Solar CellsPhysical ChemistryJohannes Kepler University LinzAltenberger Straße 69, 4040 Linz, Austria

Dr. P. A. Troshin, D. K. Susarova, O. Krystal, Prof. V. F. Razumov

Institute for Problems of Chemical Physicsof Russian Academy of Sciences

Semenov Prospect 5, Chernogolovka, Moscow region 142432,Russian Federation

L. LeonatPolitehnica University of BucharestFaculty of Applied Chemistry and Materials ScienceBucharest, Romania

G. Schwabegger, Dr. M. Ullah, Prof. H. SitterInstitute of Solid State PhysicsJohannes Kepler University LinzAltenberger Straße 69, 4040 Linz, Austria

Dr. Y. KanburDepartment of Polymer Science and TechnologyMiddle East Technical UniversityBalgat, Ankara, Turkey

Dr. M. A. BodeaInstitute of Applied PhysicsJohannes Kepler University LinzAltenberger Straße 69, 4040 Linz, Austria

DOI: 10.1002/adma.201102619

Ambipolar charge transport in organic semiconductors isessential for the development of integrated microelectronicorganic circuits and optoelectronic devices. Widespread applica-tions are currently hampered by the narrow choice of organicmaterials available. Herein we report on a “new” ambipolarorganic semiconductor: the centuries-old natural dye indigo.Indigo is an intrinsically ambipolar organic semiconductor

with a bandgap of 1.7 eV, high and well-balanced electron andhole mobilities of 1 × 10− 2 cm2 V− 1 s− 1 and good stability againstdegradation in air. Indigo-based organic field-effect transistors(OFETs) and inverters have been fabricated on natural shellacresin substrates. Inverters with high gains of 105 in the firstand 110 in the third quadrant are demonstrated. The chargetransport behavior of indigo is experimentally correlated with

strong intermolecular interactions and with reversible reduc-tion and oxidation reactions characterized by means of optical,electrical, and morphological investigations on thin indigofilms. The OFETs and complementary-like inverters are amongthe best reported organic devices to date, in terms of mobility,voltage gain, symmetry, p- and n-type channel balance, lowoperating voltage window and high ON/OFF ratio. Moreover,

these devices show that high performance electronics can befabricated entirely from non-toxic, biodegradable, and extremelycheap materials.

Technology based on complementary metal-oxide-semicon-ductor (CMOS) structures, consisting of n- and p-type transis-tors, dominates the fabrication of digital integrated circuits usedin microprocessors, microcontrollers, and random access mem-ories.[ 1 ] Low power dissipation, high signal to noise margin, andease of circuit design characterize the success of this technology.In contrast to traditional silicon-based electronic devices, organicsemiconductors can be easily processed at low temperaturesand on large substrates, enabling the fabrication of flexible solarcells, light-emitting diode displays, and integrated circuits.[ 2–4 ]

The true promise of organic electronics is the potentially ultra-low cost of mass production resulting from cheap materialsand simple processing techniques. Recent work has shown thepossibility of employing inexpensive biodegradable polymers,and natural semiconducting and insulating materials in elec-tronic devices.[ 5–7 ] Utilizing such biocompatible, biodegradableorganic semiconductors with intrinsically ambipolar chargetransporting ability would pave the way for the developmentof organic complementary CMOS-like technology. The narrowchoice of ambipolar organic materials remains the limitingfactor at present. Bilayers or mixed heterostructures of electronand hole transporting organic materials provide an alternativemeans for fabricating CMOS-like organic circuits. Some suc-cess with this approach has been demonstrated utilizing either

C60 or hexadecafluorocopper phthalocyanine (F16 CuPc) forelectron-transporting and pentacene for hole-transportingbilayers, or n- and p-type polymer blends as well as other two-material systems.[ 8–13 ] Achieving effective ambipolar perfor-mance with one semiconducting material is complicated bythe necessity of providing effective hole- and electron-injectingcontacts. For this to be possible without the use of asym-metric contacts, the bandgap of the semiconductor should besmall. Effective ambipolar devices are afforded through syn-thetic design of materials to fulfill these conditions. To date,top performance records were achieved with low-bandgapdonor–acceptor copolymers or organometallic complexes.[ 14–16 ]

Mihai Irimia-Vladu,* Eric D. Głowacki, Pavel A. Troshin, Günther Schwabegger , Lucia Leonat,Diana K. Susarova, Olga Krystal, Mujeeb Ullah, Yasin Kanbur , Marius A. Bodea,

Vladimir F. Razumov, Helmut Sitter , Siegfried Bauer , and Niyazi Serdar Sariciftci

Indigo - A Natural Pigment for High PerformanceAmbipolar Organic Field Effect Transistors and Circuits



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with respect to dilute indigo in solution. In addition, indigothin films show a broader and stronger absorption in the visiblerange than solvated indigo (Figure 1b). We found that indigoformed uniform and continuous thin films that were highlycrystalline. X-ray diffraction (XRD) measurement of purifiedindigo powders (Figure S1, Supporting Information) affordeda diffraction pattern consistent with the literature. [ 20 ] XRD of thin films yielded a single diffraction peak centered at ∼ 11.06degrees (Figure 1c), indicating a crystalline texture with a singlepreferential orientation. XRD on films of different thicknessesand on different substrate materials showed the same singlediffraction peak (Figure S2, Supporting Information). Atomicforce microscopy (AFM) investigations of the indigo films of various thicknesses show that indigo exhibits intensive islandformation with grains in the range of 250 to 300 nm (Figure S3,Supporting Information). The occurrence of strong quantummechanical coupling between nearest neighbors in crystallinefilms of indigo becomes clear from the fact that the sublimedfilms produce an absorption spectrum unlike that of indigo insolution. Moreover, as part of our experiments, we also evalu-ated the material thioindigo, a structural derivative of indigocontaining sulfurs in the place of the amine groups. Processedidentically to indigo, thioindigo showed ambipolar charge

Indigo is a dye produced from the plants Indigoferatinctoria and Isatis tinctoria , which have been cultivated for atleast 4000 years in China, India, and Egypt for coloring tex-tiles. The economic importance of indigo grew in the 19 th cen-tury, culminating in the commercialization of mass-producedsynthetic indigo in 1887.[ 17 ] Today, indigo is the leading textiledye in terms of production volume (costs of ∼ 4 USD kg− 1 ),with the dying of cotton fabric for blue jeans generating thelargest demand. Indigo has an extremely low solubility anda high melting point (390–392 ° C), explained by stabilizationfrom inter- and intramolecular hydrogen bonding. Intermo-lecular interactions of π -domains also strongly influence theelectronic and vibrational spectra of indigo in solutions anddispersions.[ 18 , 19 ] It is also likely that strong intermolecularinteractions are responsible for the good charge transport weobserved in thin indigo films.

Before characterizing indigo-based OFETs (Figure 1 ) andinverters, we studied the optical and electrochemical proper-ties of thin evaporated films of indigo. These films were pre-pared by means of evaporation, after purification of indigo bytemperature gradient sublimation. Evaporated thin films of indigo show a broad (450–730 nm) absorption band, with anabsorbance onset bathochromically shifted by about 100 nm

Figure 1. Indigo field effect transistors: a) Photograph of ambipolar indigo transistors on shellac with W /L = 1 mm/120 μ m. b) Optical absorptionspectra of a dilute indigo solution in CHCl3 and of a 40 nm evaporated indigo film, showing a bathochromic shift of absorbance onset of ∼ 100 nm inthe indigo film in comparison to the solution. c) XRD of an indigo thin film (75 nm) deposited on tetratetracontane (C44 H90 , TTC). A single diffractionis visible, indicating crystalline texture with a single preferential orientation. TTC does not show any diffraction peaks in the measured range. d) Cyclicvoltammetry scans of indigo films using ITO | indigo as the working electrode. The reversible two electron oxidation and reduction peaks suggest the

possibility of ambipolar transport. The inset shows the chemical structure of the indigo molecule.



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transport, however, with mobilities one to two orders of mag-nitude lower (Figure S4, Supporting Information). Thioindigohas no hydrogen bonding, thus further suggesting that stronghydrogen bonding-mediated intermolecular interactions playthe major part in the good performance of indigo as an ambi-polar semiconductor.

Cyclic voltammetry (CV) scans of indigo films in acetonitrileusing ITO| indigo for the working and platinum for the counterelectrode, respectively, were measured. A reversible two-electronreduction is observed, with an onset of approx. –0.9 V withrespect to Ag/AgCl (Figure 1d). This is the well-known reduc-tion to dianionic leucoindigo, utilized as the first step in textiledying. In the positive direction, a reversible two-electron oxi-dation was observed as well, with an oxidation onset at 0.8 Vand a large increase after 1 V. The HOMO and LUMO levelsapproximated from CV are shown in Table 1 . From CV we canestimate the bandgap to be 1.7 eV, agreeing with the bandgapestimated from the onset of optical absorption, 1.7 eV. Thereversible oxidation and reduction of indigo suggests ambipolarcharge transport, while the low bandgap implies the possibilityof efficient injection of both carriers from the same metal.

OFET devices were fabricated with indigo on natural resinshellac substrates. Shellac is produced by female lac beetles,and is harvested from trees in India and Thailand. [ 21 , 22 ] We havefound that drop-cast shellac resin forms smooth and uniform

substrates (0.5 nm rms roughness for a 0.5 mm thick substrate,see Figures S5 and S6, Supporting Information). Shellac is notsoluble in water or aqueous citric acid solution, recommendingit as a substrate for the anodization of aluminium to form alu-minium oxide (AlOx ) gates.[ 23 ] A thin layer of tetratetracontane(C44 H90 , TTC), a long-chain alkane, was then used to passivatethe AlOx .

[ 24 ] TTC occurs abundantly in nature, being found inplants, including medicinal plants, and is biodegradable. [ 25–27 ] XRD investigation of TTC deposited on glass, measured downto the accuracy limit of our instrument (i.e., 2θ ∼ 5 degrees)showed no evidence of crystallinity. We observed that 30 nm of TTC effectively passivated the electrochemically grown layer of AlOx ; the leakage current density at an applied gate voltageof 12 V was lower than 5 × 10− 7 A cm− 2 , thus placing TTC inthe range of high-quality passivation layers for AlOx , previouslydescribed by Klauk et al.[ 10 ] As shown in Figure S7, SupportingInformation, apart from TTC, only evaporated polyethylenefilms allowed for the observation of ambipolar transport inindigo.[ 28 ] Indigo did not show any semiconductor behavior onother investigated dielectrics, i.e. , poly(vinyl alcohol), shellac,melamine, adenine, and guanine as well as plain aluminumoxide.[ 6 , 29–31 ] Possibly, the interaction of indigo with non-polarand/or aliphatic dielectrics is vital for charge transport.

Transfer characteristics of an indigo transistor on AlOx

(45 nm) passivated with TTC (30 nm) are shown in Figure 2 a, output characteristics are displayed in Figure 2b and c. Both

Table 1. Electrochemical and optical data measured from 40 nm indigo thin films on ITO.

E ox onset vs. Ag/AgCl [V] HOMO [eV] E red onset vs. Ag/AgCl [V] LUMO [eV] E g CV [eV] E g optical [eV]

0.8 –5.5 –0.9 –3.8 1.7 1.7

Figure 2. Ambipolar OFET characteristics: a) Transfer characteristics of an ambipolar indigo OFET. The black trace shows the forward and reverseV gs scans, the dotted line shows the leakage current. Output characteris-tics for b) n-channel and c) p-channel. Channel dimensions: L = 75 μ m,W = 2 mm. Gate dielectric capacitance per area, C 0d = 87 nF cm− 2 .



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the output and transfer characteristics show ambipolar charge

transport with low hysteresis. Measured with positive source-drain voltages, the threshold voltages of indigo-based OFETsare consistently in the range of –1.5 to –3 V for holes and 4.5 to7 V for electrons. In the first quadrant, the output characteris-tics of indigo show a superlinear increase in the current at lowgate voltages due to the injection of electrons in a channel dom-inated by holes. A similar effect is observed in the third quad-rant for decreasingly negative gate voltages, as a result of theinjection of holes. Because of the low band gap of indigo and itsposition of the HOMO and LUMO energy levels (–5.5 and –3.8 eV,respectively) with respect to the work function of gold (–5.2 eV),we conclude that the superlinear increase in current is notattributable to the contact resistance between the source anddrain electrodes and the semiconductor layer. [ 32 ] The electronand hole field-effect mobilities are calculated in the saturationregime. They are typically around 1 × 10− 2 cm2 V− 1 s− 1 for elec-trons and 5 × 10− 3 to 1 × 10− 2 cm2 V− 1 s− 1 for holes. Figures S8and S9, Supporting Information, show the operational stabilityof indigo-based OFETs under various conditions, i.e., aging,dark–light exposure, cycling, and exposure to ambient condi-tions. p-channel operation occurs stably in air and n-channeloperation is stable in air only upon encapsulation. However,exposing the devices to air for several weeks does not produceirreversible degradation. The measurement in N2 environmentbefore and after extended exposure to air yielded comparableperformance: the p-channel modulation remained unaffected bythe oxygen exposure (field-effect mobility of 1 × 10− 2 cm2 V− 1 s− 1 ),

whereas the n-channel mobility decreased only by a factor of 2.5, i.e., from 1 × 10− 2 to 4.5 × 10− 3 cm2 V− 1 s− 1 (Figure S9a,Supporting Information). The slight degradation of then-channel after extensive exposure to air is a consequence of ashallow LUMO level of the indigo molecule that does not allowelectron transport to occur in an oxygenated environment. Thelatter problem was effectively mitigated by a simple encapsu-lation with polyimide tape, which allowed stable operation inair of both channels (Figure S9b, Supporting Information). Inconclusion, indigo devices are very stable with respect to chem-ical degradation. Devices stored in N2 showed no degradationeven after seven months. Table 2 shows a comparison betweenthe mobilities obtained with indigo and the mobilities reported

recently for state-of-the-art single-material ambipolar semicon-

ductors in OFETs.[ 14 – 16 , 33– 35 ] Measurements of a complementary-like voltage inverter

built with two transistors on the same shellac substrate are

Table 2. A comparison of the recent top performance reports of single material ambipolar organic transistors.

Semiconducting

Material

Reported mobility values, or range

of reported values

Operational

stability in air

Mobility during air

exposure[cm2 V− 1 s− 1 ]

W/L

[mm/μ m]

Inverter

Hole[cm2 V− 1 s− 1 ] Electron[cm2 V− 1 s− 1 ] Gain(1st quad/ 3rd quad)

Indigo (this work) 5 × 10− 3 – 1 × 10− 2 1 × 10− 2 p-channel stable, hole, 0.01 2/75 105/110 ∗

n-channel quasi- reversible loss elec., 0.0045 ∗ W / L = 1/120 [mm/μ m]

PDPP-TBT[ 14 ] 0.35 0.40 Not reported Not reported 1/100 Not reported

PSSS-C10[ 15 ] 0.3 4 × 10− 3 –0.01 Not reported Not reported 10/1 86/Not reported

P3OS[ 15 ] 2–9 × 10− 3 2–9 × 10− 3 Not reported Not reported 10/1 Not reported

Nickel dithiolene[ 16 ] 4 × 10− 4 –1.6 × 10− 3 2–8 × 10− 4 Several months storage; stability

during measurement not reported

Unchanged 6/5 ∼ 6/6

Squarilium dye[ 33 ] ∼ 10− 4 ∼ 10− 4 Not reported Not reported 10/2 10–16/10–16

PDTDPP- alt -EMD[ 34 ] 0.3 0.3 Not reported Not reported 1/5 Not reported

PDTDPP- alt -BTZ[ 35 ] 0.1 0.09 Not reported Not reported 2/50 ∼ 35/35

Figure 3. Indigo inverter circuits: a, b) Complementary inverter characteris-tics and voltage gains of ambipolar indigo for the electron and hole channels.Transistor channel dimensions: L = 120μ m, W = 1 mm; the measurementswere performed seven months after sample fabrication (with storage in aglove box under nitrogen) and showed remarkable reproducibility.



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presented in Figure 3a and b. The gain of the inverters fabri-cated, 105 in the first quadrant and 110 in the third quadrant, isremarkable for an ambipolar material with a single source anddrain contact metal. A measurement showing a complementaryinverter scanned in both directions at a frequency of 1 Hz withan external direct current (DC) voltage of ± 14 V is presented in

Figure S10a and b of the Supporting Information. The recordedgain is 90 in the first and 120 in the third quadrant.

In summary, we report OFETs and inverter circuits fullymade of natural materials performing at the state-of-the-artlevel in terms of mobilities, ambipolarity, operating voltages,stability, and gain. Indigo is an unusual organic semiconductor,featuring a highly planar but relatively small cross-conjugatedπ -electron system. We believe that strong intermolecular inter-actions resulting in high crystalline order are responsible forthe charge transporting properties, and the ability of the mol-ecule to be readily oxidized and reduced reversibly affords itstransport ambipolarity. Our work suggests that natural mate-rials may play an important role in the future development of sustainable electronic devices.

Experimental Section

Indigo was obtained from Aldrich and purified three times by vacuumgradient sublimation. In addition, two more sources of indigo wereexplored in this work for both transistor fabrication as well as materialcharacterization: an indigo batch procured from a textile companyin Moscow, Russian Federation, and another one obtained from theLaboratory for Laser Energetics in Rochester, New York, USA. All threematerials showed comparable performance in OFETs and apparentlyidentical optical, chemical, and electrical properties. Shellac wasprocured from Aldrich, Inc. and used without further purification. Shellacfilms were prepared by drop casting of a 500 mg mL− 1 shellac/ethanolsolution onto glass or metal foil substrates. Prior to drop-casting onaluminum foil wrapped glass slides, the precursor solution was stirred

at 75 ° C for 2 h and subsequently cooled and filtered through a 0.45 μ mpore filter (Whatman Inc.). The drop-cast film was dried at 75–80 ° C for∼ 10 h before being peeled off to afford a free-standing shellac substrate.A 1 mm wide, 100 nm thick aluminum gate was evaporated onto 1.5 × 1.5 cm2 shellac slides and subsequently anodized by immersing in citricacid solution and passing a step voltage (up to a maximum of 30 V) at aconstant current of 0.06 mA. The slides were rinsed with deionized waterand dried under nitrogen flush. The final thickness of aluminum oxide was∼ 45 nm. TTC (Aldrich) was deposited from a resistively heated ceramiccrucible at a pressure of 1 × 10− 6 mbar. Purified indigo was evaporatedfrom a hot-wall epitaxy source at a pressure of 1 × 10− 7 mbar.[ 36 ] ForOFETs fabrication, the typical thicknesses of TTC and indigo films were30 and 75 nm, respectively. Gold source and drain electrodes (typically100 nm) were evaporated through a shadow mask (L = 75 μ m, W = 2 mm for OFETs, and L = 120 μ m, W = 1 mm for inverters, respectively)at a pressure of 1 × 10− 6 mbar. UV-vis spectra were obtained using a

Cary UV-Vis spectrophotometer. AFM studies were performed using aDigital Instruments DIMENSION 3100 in tapping mode. XRD patternswere recorded using a Bruker AXS X-ray diffractometer (Cu K α ). Cyclicvoltammetry measurements were carried out in a N2 atmosphere with0.1 M tetrabutylammonium phosphorus hexafluoride (TBAPF6 ) in dryCH3 CN as the electrolyte solution. Ag/AgCl was utilized as a referenceelectrode, with a platinum disk counter electrode. Indium-doped tinoxide (ITO) glass with a 40 nm evaporated indigo film functioned asthe working electrode, the scan rate was 50 mV s− 1 . The ferrocene redoxcouple was used as an external standard. An Agilent E5273A instrumentwas employed for the steady state current–voltage measurements anda Novocontrol Alpha Analyzer for the frequency dependent dielectricinvestigations. Transistor characterization was carried out in a nitrogenatmosphere with < 1 ppm O2 .

Supporting Information

Supporting Information is available from the Wiley Online Library orfrom the author.

AcknowledgementsFinancial support of the principal investigator from the city of Linz andthe Land Oberösterreich is highly appreciated. This work was partiallyfunded by the Austrian Science Foundation “FWF” within the NationalResearch Network NFN on Interface Controlled and FunctionalizedOrganic Films (P20772-N20, S09712-N08, S09706-N08 and S9711-N08),by the European Science Foundation, by the Russian Foundation forBasic Research (grant 10-03-00443), by the Russian Ministry for Scienceand Education (contract 02.740.11.0749) and by the Russian PresidentFoundation (grant MK-4916.2011.3). The authors kindly thank KennethL. Marshall of the University of Rochester for indigo samples; PhilippStadler, Michael Sams, Christoph Ulbricht, and Gebhard Matt for fruitfuldiscussions and suggestions.

Received: July 8, 2011

Published online:



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2011, vladu. indigo - a natural pigment for high performance

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