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Triarylamine Substituted Arylene Bisimides as Solution Processable Organic Semiconductors for Field Effect Transistors. Effect of Substituent Position on Their Spectroscopic, Electrochemical, Structural, and Electrical Transport Properties Adam Pron,*,† Renji R. Reghu,‡ Renata Rybakiewicz,§ Hubert Cybulski,|| David Djurado,† Juozas V. Grazulevicius,*,‡ Malgorzata Zagorska,*,§ Irena Kulszewicz-Bajer,§ and Jean-Marie Verilhac^ †
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INAC/SPrAM (UMR 5819, CEA-CNRS-Univ. J. Fourier-Grenoble 1) Laboratoire d’Electronique Moleculaire Organique et Hybride, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble, France ‡ Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania § Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00664 Warszawa, Poland Department of Physical Chemistry and Center for Research in Biological Chemistry and Molecular Materials, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain ^ CEA/LITEN/LCI, 38054 Grenoble, France
bS Supporting Information ABSTRACT: New, solution processable organic semiconductors, namely arylene bisimides core- or N-functionalized with triarylamines have been synthesized in view of their potential application in organic electronics. In N-functionalized compounds the electrochemically determined oxidation potentials are 0.46 and 0.48 V vs Fc/Fc+ for perylene (P1) and naphthalene (N1) bisimides, respectively. For the core functionalized perylene bisimide (P2) this potential is shifted to higher values (0.55 V vs Fc/Fc+). A reversed trend is observed for the first reduction potential since P2 is being reduced at the lowest potential (1.09 V) whereas N1 and P1 at 1.05 and 1.02 V, respectively. These shifts observed in the case of P2 are induced by donoracceptor interactions between the substituent and the core. The results of DFT calculations performed for N-substituted bisimides indicate a clear separation in space of the HOMO and LUMO orbitals, the former being located on the triarylamine substituent whereas the latter is on the bisimide core. To a first approximation, their UVvisNIR spectra can be considered as a superposition of the triarylamine spectrum and those of bisimides containing nonchromophore (alkyl) substituents. This indicates a very weak or essentially nonexistent conjugation between the aromatic core and the N-substituents. In powders, the N-functionalized bisimides show liquid crystalline-type structural organization whereas in thin, spin-coated films they are amorphous. The coresubstituted bisimide shows different properties. In this case a new band of a charge transfer (CT) character appears in its UVvis spectrum. In accordance with the spectroscopic data, the DFT calculations indicate that in the core-substituted compound the HOMO electron density spreads from the triarylamine substituent to the bisimide core. Powders of the core-substituted bisimide crystallize in a 3D structure whereas thin spin-coated films show liquid crystalline-like structural organization. Taking into account their electrochemical properties, all three bisimides studied seem to be good candidates for the fabrication of air operating ambipolar transistors. However, N-functionalized bisimides show only p-type behavior with the hole mobility approaching 104 cm2 V1 s1 in the all organic (CYTOP dielectric) field effect transistor configuration. The coresubstituted bisimide shows, however, the expected ambipolar behavior with the hole and electron mobilities of 1.5 103 and 3.5 104 cm2 V1 s1, respectively.
1. INTRODUCTION Specific electronic and optoelectronic properties of low and high molecular weight organic semiconductors have been drawing enormous research interest of the scientific community for more than two decades.13 This is caused by the fact that these materials can be used in the fabrication of different types of electronic devices like light emitting diodes (LEDs),4 photodiodes (PDs),5 r 2011 American Chemical Society
photovoltaic cells (PCs),6 or field effect transistors (FETs).7 The role of synthetic and polymer chemistry in the development of this field of research cannot be overestimated. The application of Received: March 17, 2011 Revised: June 3, 2011 Published: June 22, 2011 15008
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The Journal of Physical Chemistry C so-called “building block approach”, together with the use of a variety of cross-coupling reactions (including those for which Suzuki, Negishi, and Heck were granted Nobel Prize in chemistry in 2010), led to several dozens of new processable organic semiconductors whose redox, spectroscopic, and electrical transport properties could be precisely tuned and adapted to a given application. Low molecular weight compounds and polymers suitable for the fabrication of FETs constitute an important class of organic semiconductors. Large majority of them are used in p-channel FETs. The search for semiconductors which could be applied in n-channel transistors intensified in recent years and, as a result, several new solution processable compounds, yielding air operating devices, have been developed.8 The design and preparation of semiconductors for air operating, one-component ambipolar transistors is the most delicate matter. First, they should exhibit a low lying lowest unoccupied molecular orbital (LUMO) level (below 3.9 eV with respect to the vacuum level) which facilitates the injection of electron charge carriers and assures their air stability in the operating conditions. For similar reasons their highest occupied molecular orbital (HOMO) level should be slightly lower than 5.0 eV. This position makes the injection of holes effective and renders the material resistant toward oxidative degradation in air operating conditions. It is therefore not unexpected that the number of air stable organic semiconductors showing ambipolar behavior is low.911 Moreover, some of them are hard to solution process and for this reason difficult to be used as semiconductor materials in large area printed electronics. Several types of conjugated molecules were tested so far as components of ambipolar transistors. The first type embraces low and high molecular weight thiophene derivatives, among them linear oligothiophenes differently end-capped with electron accepting cyano groups.12,13 Donoracceptor copolymers containing donor oligothiophene segments alternating with different acceptor ones like pyrrolopyrrole dione-14 or naphthalenebiscarboximide-type15 also show ambipolar behavior. Oligothiophenes functionalized with strongly electron withdrawing pyrazine constitute an electron accepting unit in alternating copolymers with fluorene donor unit, yielding ambipolar molecular semiconductors.16 Poly(dialkylterthiophene)s are well-known semiconductors for p-channel FETs.1719 However, their selenium analogues, i.e., poly(dialkylterselenophene)s, exhibit a lower band gap (1.8 eV) and ambipolar charge transport properties.20 Functionalized acenes like for example anthracene containing cyano functions and linked to thienyl terminal groups via vinylene spacers constitute another group of molecular semiconductors showing ambipolar electrical transport properties.21 Other ambipolar molecules of this group embrace tetrafluorotetraceno[2,3b]thiophenes functionalized with a trialkylsilylethynyl anchor group.22 Ambipolar behavior can also be found in some azaacenes.23 Arylene bisimides with small aromatic core like naphthalene and perylene bisimides usually show n-type behavior. However, their analogues with larger aromatic cores like, for example, terrylene bisimides in some circumstances may exhibit ambipolar properties.24 In this paper we report on the synthesis and detailed characterization of new solution processable semiconductors. We have selected N- and core-substituted arylene bisimides as objects of our investigations since several studies, including theoretical ones,25 clearly show that their electronic properties can be precisely tuned by these types of substitutions. The new
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compounds consist of an arylene bisimide central part and triarylamine substituents attached either to the imide nitrogen or directly to the aromatic core. Taking into account that arylene bisimides are known semiconductors for n-channel FETs whereas triarylamines and their high molecular weight analogues can be used in the fabrication of p-channel FETs, we were tempted to verify whether their assembly in one molecule would result in organic semiconductors suitable for the fabrication of ambipolar transistors. In particular, we demonstrate that the position of the substituent not only affects the spectroscopic and redox properties of the studied semiconductors but also changes their behavior with respect to the application in FETs. Bisimides with triaryl amine N-substituents show field effect only in the p-channel configuration while the core substituted ones yield ambipolar transistors.
2. EXPERIMENTAL SECTION The details on the synthesis of all semiconducting compounds studied can be found in the Supporting Information which in addition contains all spectroscopic (1H NMR, 13C NMR, IR, and MS) and elemental analysis data. Solution UVvis spectra were measured in chloroform (2 105 M) using a Varian Cary 5000 spectrophotometer. Cyclic voltammograms were registered in a dry argon atmosphere on an Autolab potentiostat (Eco Chimie). The electrolytic medium consisted of the semiconducting compound studied (c = 1 103 M) dissolved in 0.1 M Bu4NBF4/CH2Cl2 (DCM) electrolyte. The three electrode electrochemical cell consisted of a platinum working electrode of the surface area of 3 mm2, a platinum wire counter electrode and an Ag/0.1 M AgNO3/ CH3CN reference electrode. One should note that the use of a methylene chloride - based electrolyte (imposed by better solubility of the synthesized compounds in this solvent) together with an acetonitrile-based electrolyte of the reference electrode may generate an additional junction potential. Therefore, the potential of the reference electrode with respect to the ferrocene redox couple was always measured after each experiment. Two types of X-ray experiments were performed. Powder diffractograms of N1, P1, and P2 were recorded on a X-Pert Pro MPD Philips diffractometer (Co KR1 radiation; λ = 1.789 Å) using BraggBrentano (θ/2θ) reflection geometry. The detector was moved by 2θ steps of 0.04 and the counting time was at least 15 s per step. The divergence slit was automatically adjusted giving a constant 10 mm irradiated length. Thin layers (ca. 100 nm thick) of the investigated molecules have been deposited on Si wafers by spin-coating. BraggBrentano X-ray measurements of these layers have been carried out using the same model of diffractometer and the same cobalt radiation. In these measurements, the divergence slit was fixed at 0.16 while Soller slits were mounted both in the incident beam path (0.04 rad) and in the diffracted beam path (0.01 rad). The 2θ steps were still fixed at 0.04 and the counting time at 15 s per step. The fabrication of the test transistors can briefly be described as follows. In the first step 30 nm thick gold source and drain electrodes were vacuum deposited on a poly(ethylene naphthalate) (PEN) substrate and then patterned by photolitography giving a channel width W of 0.9 mm and a channel length L of 40 μm. The surface of the deposited electrode was then modified by oxygen plasma (1 min) and by treatment with either 4-methoxythiophenol or pentafluorobenzenethiol as recommended.26,27 In the subsequent step thin layers of the investigated organic 15009
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The Journal of Physical Chemistry C
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Scheme 1. Synthetic Route for the Preparation of N1 and P1a
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Reagents and conditions: (i) Zn(OAc)2 2H2O, NMP, 180, 3 h.
semiconductors were spin coated from a chloroform solution (concentration 7 mg/mL, spin speed of 600 rpm during 20 s) and then annealed (10 min at 100 C, under N2 flow) with the goal to remove the remaining traces of the solvent. Typical thickness of the deposited semiconducting layer as determined by Dektak profilometer was ∼100 nm. Commercially available fluorinated polymer, CYTOP, (Asahi glass, Japan) served as the dielectric layer. It was deposited on the top of the semiconducting layer by spin-coating and then dried at 100 C. The deposited, 1 μm thick dielectric layer showed the capacitance of 1.8 nF cm2. In the last step 500 nm thick gate electrode was deposited by inkjet using a Dimatix. Commercial suspension of Ag nanoparticles in alcohol (Cabot) was used as a source of silver. It should be noted that a thin layer (
