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Titanyl phthalocyanine ambipolar thin film transistors making use of carbon nanotube electrodes

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 485703 (http://iopscience.iop.org/0957-4484/25/48/485703) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: This content was downloaded on 13/06/2017 at 22:24 Please note that terms and conditions apply.

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Nanotechnology Nanotechnology 25 (2014) 485703 (6pp)


Titanyl phthalocyanine ambipolar thin film transistors making use of carbon nanotube electrodes Nicola Coppedè1,5, Irina Valitova2,5, Farzaneh Mahvash3, Giuseppe Tarabella1, Paolo Ranzieri1, Salvatore Iannotta1, Clara Santato3, Richard Martel4 and Fabio Cicoira2 1

IMEM CNR Institute of Materials for Electronics and Magnetism, Parco Area delle Scienze 37/A, 43124, Parma, Italy 2 Département de génie chimique, École Polytechnique de Montréal, 2500 Chemin de Polytechnique, Montreal, Québec, H3T 1J4, Canada 3 Département de génie physique, École Polytechnique de Montréal, 2500 Chemin de Polytechnique, Montréal, Québec, H3T 1J4, Canada 4 Département de chimie, Université de Montréal, Montréal, Québec H3C 3J7, Canada E-mail: [email protected], [email protected] and [email protected] Received 27 June 2014, revised 28 August 2014 Accepted for publication 2 September 2014 Published 12 November 2014 Abstract

The capability of efficiently injecting charge carriers into organic films and finely tuning their morphology and structure is crucial to improve the performance of organic thin film transistors (OTFTs). In this work, we investigate OTFTs employing carbon nanotubes (CNTs) as the source-drain electrodes and, as the organic semiconductor, thin films of titanyl phthalocyanine (TiOPc) grown by supersonic molecular beam deposition (SuMBD). While CNT electrodes have shown an unprecedented ability to improve charge injection in OTFTs, SuMBD is an effective technique to tune film morphology and structure. Varying the substrate temperature during deposition, we were able to grow both amorphous (low substrate temperature) and polycrystalline (high substrate temperature) films of TiOPc. Regardless of the film morphology and structure, CNT electrodes led to superior charge injection and transport performance with respect to benchmark Au electrodes. Vacuum annealing of polycrystalline TiOPc films with CNT electrodes yielded ambipolar OTFTs. S Online supplementary data available from stacks.iop.org/NANO/25/485703/mmedia Keywords: carbon nanotubes, organic semiconductors, charge injection, thin films, phthalocyanines, supersonic beam deposition (Some figures may appear in colour only in the online journal) 1. Introduction

injecting electrode and the energy levels of the organic semiconductor [1, 2]. The presence of injection barriers (Schottky barriers) at the interface between metal electrodes and organic semiconductors often leads to non-ohmic contacts, which strongly impact the performance of OTFTs, in turn limiting their downscaling [3]. Electrodes made of carbon nanotubes (CNTs) are emerging as effective tools to improve charge injection into OTFTs. The positive impact of CNT electrodes on OTFT performance has been demonstrated for several

The injection of charge carriers from metal electrodes into organic semiconductors is one of the key processes establishing the performance of organic thin film transistors (OTFTs). The efficiency of the charge injection process in OTFTs depends mainly on the matching between the work function of the 5

Authors contributed equally to this work.



© 2014 IOP Publishing Ltd Printed in the UK

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organic semiconductors, differently processed, such as pentacene, copper phthalocyanine, phenyl-C61-butyric acid methyl ester (PCBM), poly-3-hexylthiophene (P3HT) and rubrene [4– 10]. CNTs are electrically conductive, chemically stable, and easy to process. CNTs present a unique electrostatic behavior due to their high aspect ratio, which favors charge carrier injection into organic semiconductors [11, 12]. Furthermore, the pi-conjugated structure of CNTs is expected to create, with pi-conjugated organic semiconductors, interfaces favorable to efficient injection [13–19]. To explore the effect of film morphology/structure on charge injection from CNT electrodes to organic thin films, the behavior of OTFTs based on the same organic semiconductor, processed in different conditions, and leading to different morphology/structure needs to be investigated. Supersonic molecular beam deposition (SuMBD), where the morphology/structure of organic semiconductors can be dramatically changed as a function of the growth conditions, is a particularly suitable growth method for this type of investigation. Titanyl phthalocyanine (TiOPc) has been previously used in unipolar and ambipolar OTFTs, exhibiting mobility in the range of 10−5 – 10−3 cm2 V−1 s−1 [20, 21]. An exceptionally high mobility of 10 cm2 V−1 s−1 has been reported for films deposited at high temperature on octadecyltrichlorosilane (OTS)-modified SiO2/Si substrates [22]. In this work, we investigate the performance of OTFTs making use of CNTs as the source-drain electrodes and titanyl phthalocyanine (TiOPc) thin films as the organic semiconductor. Films were grown by SuMBD at different substrate temperatures, leading to a different morphology/ structure. A correlation between the morphology and structure of the films with the performance of the OTFTs based thereon is proposed, to extract guidelines for future applications of CNT electrodes in OTFTs.

Figure 1. SEM image (1 keV accelerating voltage and 1 μA emission

current) of single-wall CNT array electrodes (a); top view (b) and cross section (c) scheme of concentric source (S) and drain (D) electrodes.

deposition apparatus has been described in detail elsewhere [23–26]. The source was operated with a He carrier gas at a pressure of 1000 – 2000 mbar, and the kinetic energy of the TiOPc molecules in the supersonic beam was fixed at 15 eV. The deposition rate of TiOPc was ∼0.1 Å s−1. Two different substrate temperatures were investigated: 23 °C and 230 °C. The film morphology was studied with an atomic force microscope (AFM, Veeco Dimension 3100) operated in tapping mode. UV–vis spectra were collected by a Varian Cary 5000 spectrometer. Scanning electron microscope (SEM) images were acquired with a FE-SEM (Hitachi S-4700). OTFT characterization was carried out under vacuum (10−6 Torr) using a micromanipulated electrical probe station and an Agilent B1500A semiconductor parameter analyzer.

2. Experimental details CNT array electrodes (figure 1(a)), using a concentric source/ drain geometry (with interelectrode distance L and width W, such as L/W = 5/1555, 10/1540, 20/1510, and 50/1413 μm/μm), were lithographically patterned on SiO2/Si substrates with a procedure already described elsewhere [5–7]. As substrates, we used highly doped (n-type) Si (100) wafers (resistivity of 0.002 – 0.003 Ω cm) covered with either a 100 nm- or 200 nmthick thermally grown SiO2 gate dielectric (the corresponding specific capacitance Ci was 34.5 nF cm−2 and 17.3 nF cm−2), where Si acted as the common gate electrode. The electrodes consisted of a disordered array of individual (or a small bundle of) single walled CNTs with one side connected to a titanium contact pad and the other embedded into the organic film. TiOPc thin films were deposited by SuMBD during the same run on substrates patterned with CNT electrodes (to produce devices from now on indicated as CNT OTFTs) and benchmark Au electrodes with an identical geometry (the corresponding devices from now on will be indicated as Au OTFTs). TiOPc was purchased from Syntec-Sensient Imaging Technology GmbH and used as received. The SuMBD

3. Results and discussion TiOPc films are characterized by a strong polymorphism [25, 26]. Indeed, three crystalline structures are possible for TiOPc films: namely, monoclinic I (β-TiOPc), triclinic II (αTiOPc) [27], and triclinic, metastable Y [28]. Here we investigate OTFTs based on TiOPc films deposited at substrate temperatures of 23 °C (room temperature, from now on indicated as RT) and 230 °C (high temperature, HT). The morphology of the TiOPc films was found to depend on the substrate temperature, as shown by AFM phase images. At RT (figures 2(a) and (b)), films are homogeneous and completely cover the substrate surface. Films deposited at HT (figures 2(c) and (d)) are characterized by the presence of particles with different shapes, i.e., needle-like and flat squared. TiOPc thin films grown at RT and HT have different UV–vis absorption spectra, attributable to different film structures (figure 3). The spectrum of TiOPc films deposited 2

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Figure 2. AFM phase images of TiOPc films deposited by SuMBD at RT (a), (b) and HT (c), (d). Image sizes are 3.0 μm × 3.0 μm (a), 1.0 μm × 1.0 μm (b), 10.0 μm × 10.0 μm (c), and 2.4 μm × 2.4 μm (d).

at RT shows a Q-band peak located at ∼730 nm, with a well defined vibronic replica at ∼650 nm, suggesting that films are in amorphous phase [22, 25, 26, 29]. On the other hand, the spectrum of TiOPc films deposited at HT shows a Q-band peak located at ∼850 nm and a relatively high separation of the Q-band components between the shoulder at 640 nm and the peak at 850 nm. Therefore TiOPc films deposited at HT seem to be prevalently in the α-TiOPc phase. (The presence, in the TiOPc films, of limited amounts of β-TiOPc and amorphous phases cannot be excluded.) From AFM images and UV−visible spectra we can infer that TiOPc films deposited at RT and HT are prevalently in amorphous phase and α-TiOPc phase, respectively. Theoretical calculations and experimental results reported in the literature point to the good transport properties of α-TiOPc thin films, attributable to the short pi−pi intermolecular interactions present in this TiOPc phase [22, 30, 31]. After the characterization of the morphology and structure, TiOPc films grown at RT and HT were characterized for their electrical properties in a transistor configuration. From now on, OTFTs making use of CNT(Au) electrodes based on TiOPc films deposited at RT(HT) will be named CNT(Au) RT(HT) OTFTs. The output characteristics (drain-source current ID vs drain-source voltage VD for increasing values of the gate-

Figure 3. UV−visible spectra for TiOPc thin films deposited by SuMBD on quartz, at RT (23 °C, black continuous line) and HT (230 °C, red dashed line).

source voltage VG) of CNT and Au RT OTFTs show unipolar p-type behavior (figures 4(a) and (c)). In the low-voltage region (VD < 5 V, figures 4(b) and (d)), the output characteristics show a quasi linear (ohmic) and a sublinear behavior for 3

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Figure 4. Output characteristics of TiOPc transistors: CNT RT OTFTs for 0 V ⩽ VD ⩽ −60 V (a) and 0 V ⩽ VD ⩽ −5 V (b); Au RT OTFTs for 0 V ⩽ VD ⩽ −60 V (c) and 0 V ⩽ VD ⩽ −5 V (d). VD is swept by 100 mV-steps (200 nm-thick SiO2, W/L = 1540 μm/10 μm).

CNT RT OTFTs and Au RT OTFTs, respectively. For low values of VD, the ID for CNT RT OTFTs is about a factor of 40 higher than for Au counterparts (at VD = −1 V and VG = −60 V, ID ∼ −4 × 10−8 A and ∼−1.0 × 10−9 A for CNT and Au devices). From the transistor transfer characteristics at saturation (supplementary data files, SDF figure 1 and table 1), the hole mobility (μ) and the threshold voltage (VTH) were extracted using the relationship ΙD = (W/2L)μCi(VG − VTH)2. The hole mobility in the linear regime was calculated from the transconductance dID/ dVG using the relationship μ = (dID/dVG)(L/(WCiVD)). CNT electrodes lead to higher mobility with respect to Au. At VD = −50 V, average values were ∼3 × 10−4 cm2/V · s and ∼1 × 10−4 cm2/V · s for CNT and Au RT OTFTs. The average value of VTH for CNT and Au OTFTs was −12.2 and −12.9 V. In the linear regime (SDF figure 1 and table 1), the differences in the mobility between CNT and Au devices were more pronounced than at saturation. Indeed, μ was ∼3 × 10−4 and 3 × 10−5 cm2/V · s for CNT and Au RT OTFTs. A similar trend was found for OTFTs based on TiOPc films deposited at HT. The output characteristics of CNT HT OTFTs (figure 5) show at 0 V ⩽ VD ⩽ −5 V a quasi linear behavior, which contrasts with the sublinear behavior of Au counterparts. ID in the low voltage region (VD = −1 V) is two orders of magnitude higher for CNT HT OTFTs (ID ∼ −6 × 10−8 A) than for Au HT OTFTs (ID ∼ −2 × 10−10 A). From the transfer characteristics of CNT and Au HT TiOPc OTFTs in both saturation and linear regimes (figure 2 SDF), the average and best values of the μ and the average

values of VTH were extracted (table 2 SDF). In the saturation regime (VD = −50 V), the average values of μ were ∼1 × 10−3 cm2/V · s and VTH = −14.3 V for CNT HT OTFTs and μ ∼ 1 × 10−4 cm2/V · s and VTH = −32.7 for Au counterparts. In the linear regime, μ ∼ 3 × 10−4 cm2/V · s and μ ∼ 1 × 10−5 cm2/V · s were found for CNT and Au HT OTFTs. ION/IOFF extracted from the transfer curves were 104 – 105 and 103 – 104 for CNT and Au OTFTs, independently of the substrate temperature used during the growth (RT or HT). Remarkably, unipolar p-type CNT HT OTFTs were converted to ambipolar by a vacuum annealing treatment (at 110 °C for 48 h). The conversion from unipolar to ambipolar transport for OTFT with CNT electrodes has been observed for other organic semiconductors such as PCBM and copper phthalocyanine [6, 7]. The observation of the ambipolarity after thermal treatment of TiOPc films confirms the universality of the phenomenon for most organic semiconductors. Figure 6 shows the output curves of CNT HT OTFTs in both hole and electron enhancement modes after vacuum thermal annealing. The unipolar to ambipolar conversion is most likely related to the vacuum desorption of O2 and H2O molecules (the oxygen–water redox couple) from the SiO2 surface [6, 7, 35]. A qualitative scheme of the energy band diagram of TiOPc HOMO-LUMO levels and CNT and Au electrodes’ work functions (figure 7) permits us to roughly estimate the energy barrier height for the injection of holes. The Fermi level of CNTs is offset by ∼0.6 eV with respect to the HOMO 4

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Figure 5. Output characteristics of TiOPc transistors: CNT HT OTFTs for 0 V ⩽ VD ⩽ −60 V (a) and 0 V ⩽ VD ⩽ −5 V (b); Au HT OTFTs for 0 V ⩽ VD ⩽ −60 V (c) and 0 V ⩽ VD ⩽ −5 V (b). VD is swept by 100 mV-steps (W/L = 1540 μm/10 μm, 200 nm-thick SiO2).

Figure 6. Output curves of CNT HT OTFTs for n-type (a) and p-type operation (b) after 110 °C, 48 h vacuum annealing treatment. The

curves at VG = 40 V and VG = 60 V in panel (a) and VG = −40 V and VG = −60 V in panel (b) are referred to as the right y scales. VD changes by 500 mV-steps (W/L = 1540 μm/10 μm, 200 nm-thick SiO2).

level of TiOPc, whereas for Au the offset is ∼0.7 eV. Therefore, high injection barriers are expected at the interface of both CNT and Au electrodes with a TiOPc film. Such high injection barriers should result in a rather poor hole injection performance (nonlinear behavior in the output transistor curves) for both kinds of electrodes. Nonlinear output characteristics at low VD, due to the presence of a Schottky barrier, are indeed observed with OTFTs with Au electrodes. On the other hand, the quasi linear output characteristics observed at low VD for CNT based devices and the values of ID for CNT-versus Au-based devices indicate the presence of quasi electrically transparent barriers for CNT OTFTs.

4. Conclusion In conclusion, CNT electrodes have been successfully employed to improve injection performance and to achieve ambipolarity in OTFTs based on thin TiOPc films deposited by SuMBD. Efficient injection was observed for CNT OTFTs, making use of two types of thin films of TiOPc deposited at two different substrate temperature and having a prevalent amorphous or polycrystalline structure. For TiOPc deposited at RT, with amorphous structure, the injection efficiency of CNT OTFTs was 40 times higher than for Au counterparts. The increase was of two orders of magnitude for 5

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[3] Cao Y, Steigerwald M L, Nuckols C and Guo X 2010 Adv. Mater. 22 20 [4] Valitova I, Amato M, Mahvash F, Cantele G, Maffucci A, Santato C, Martel R and Cicoira F 2013 Nanoscale 5 4638–46 [5] Aguirre C M, Ternon C, Paillet M, Desjardins P and Martel R 2009 Nano Lett. 9 1457–61 [6] Cicoira F, Aguirre C M and Martel R 2011 ACS Nano 5 283–90 [7] Cicoira F, Coppede N, Iannotta S and Martel R 2011 Appl. Phys. Lett. 98 183303 [8] Xie W et al 2013 ACS Nano 13 10245–56 [9] Sarker B K and Khondaker S I 2012 ACS Nano 6 4993–9 [10] Liu B, McCarthy M A and Rinzler A G 2010 Adv. Func. Mat. 20 3440–5 [11] Avouris P and Chen J 2006 Mater. Today 9 46–54 [12] Avouris P, Chen Z and Perebeinos V 2007 Nat. Nanotech. 2 605–15 [13] Gwinner M C, Jakubka F, Gannott F, Sirringhaus H and Zaumsei J 2012 ACS Nano 6 539–48 [14] Miller A J, Hatton R A and Silva S R P 2006 Appl. Phys. Lett. 89 133117 [15] Aguirre C M, Auvray S, Pigeon S, Izquierdo R, Desjardins P and Martel R 2006 Appl. Phys. Lett. 88 183104 [16] Qi P, Javey A, Rolandi M, Wang Q, Yenilmez E and Dai H 2004 J. Am. Chem. Soc. 126 11774–5 [17] Santato C, Cicoira C and Martel R 2011 Nat. Photonics 5 392–3 [18] Tsukogoshi K, Yagi I and Aoyagi Y 2004 Appl. Phys. Lett. 85 1021–203 [19] Guo X F et al 2006 Science 103 356 [20] Tada H, Touda H, Takada M and Matsushige K 2000 Appl. Phys. Lett. 76 873 [21] Ji Z, Liu M, Shang L, Hu W, Liu G, Liu X and Wang H 2009 J. Mater. Chem. 19 5507–9 [22] Li L, Tang Q, Li H, Yang H, Hu W, Song Y, Shuai Z, Xu W, Liu Y and Zhu D 2007 Adv. Mater. 19 2613 [23] Milani P and Iannotta S 1999 Cluster Beam Synthesis of Nanostructured Materials (Berlin: Springer-Verlag) [24] Toccoli T, van Opbergen M, Boschetti A, Ciullo G, Ronchin S and Iannotta S 1999 Philos. Mag. B 79 2157 [25] Coppede N, Toccoli T, Pallaoro A, Siviero F, Walzer K, Castriota M, Cazzanelli E and Iannotta S 2007 J. Phys. Chem. 111 12550–8 [26] Coppede N, Castriota M, Cazzanelli E, Forti S, Tarabella G, Toccoli T, Walzer K and Iannotta S 2010 J. Phys. Chem. C 114 7038–44 [27] Hiller W, Strahle J, Kobel W and Hanack M 1982 Kristallogr. Z. 159 173 [28] Oka K, Okada O and Nukada K 1992 Japan. J. Appl. Phys. 31 2181–4 [29] Saito T, Sisk W, Kobayashi T, Suzuki S and Iwayanagi T 1993 J. Phys. Chem. 97 8026–31 [30] Toccoli T, Tonezzer M, Bettotti P, Coppede N, Larcheri S, Pallaoro A, Pavesi L and Iannotta S 2009 Org. Electron. 10 521–6 [31] Norton J E and Bredas J L 2008 J. Chem. Phys. 128 034701 [32] Brumbach M, Placencia D and Armstrong N R 2008 J. Phys. Chem. C 112 3142–51 [33] Suzuki S, Watanabe Y, Homma Y, Fukuba S, Heun S and Locatelli A 2004 Appl. Phys. Lett. 85 127–9 [34] Osikowicz W, de Jong M P, Braun S, Tengstedt C, Fahlman M and Salaneck W R 2006 Appl. Phys. Lett. 88 193504 [35] Aguirre C M, Levesque P L, Paillet M, Lapointe F, St-Antoine B C, Desjardins P and Martel R 2009 Adv. Mater. 21 3087–91

Figure 7. Energy band diagram of TiOPc (HOMO and LUMO,

relative to the vacuum level) [32] and work function of SWCNT (ΦCNT) [33] and Au (ΦAu) [34] electrodes. EF (Au) and EF (CNT) are the Fermi levels of Au and CNT electrodes.

TiOPc films deposited at HT with a polycrystalline structure. Therefore, CNT electrodes were able to improve OTFT injection performance regardless of the morphology and structure of the TiOPc film, thus confirming the universality of the CNT electrode approach to improve charge injection in OTFTs. The improvement in the injection characteristics is clearly more relevant for crystalline films than for amorphous films. This points to the importance of achieving a good quality of the organic semiconductor—CNT interface in terms of low charge carrier trap density and supramolecular organization. Higher mobility was obtained for OTFTs based on CNT electrodes, compared to their Au counterparts, for both amorphous and polycrystalline TiOPc films. The increase was particularly remarkable in the linear regime, where the mobility is more affected by injection. Work is in progress to deposit the TiOPc films with SuMBD on OTSmodified substrates to increase the charge carrier mobility. Our results pave the way for a novel strategy to downscale the inter-electrode distance in OTFTs.

Acknowledgments The authors acknowledge M Pola for technical support. This work is supported by NSERC Discovery grants (FC, CS, and RM). I Valitova acknowledges FQRNT for a Doctoral Scholarship and CMC Microsystems for an MNT financial assistance grant. Notes Electronic supplementary information (SDF) available: (figures 1 and 2: transfer characteristics in saturation and linear regimes for TiOPc CNT(Au) RT(HT). Tables 1 and 2: average and best values for hole mobility and threshold voltage for TiOPc CNT(Au) RT(HT)). See DOI: 10.1039/b000000x/.

References [1] Scott C 2003 J. Vac. Sci. Technol. A 21 521 [2] Braun S, Salaneck W R and Fahlman M 2009 Adv. Mater. 21 1450


Titanyl phthalocyanine ambipolar thin film transistors making use of carbon nanotube electrodes.

The capability of efficiently injecting charge carriers into organic films and finely tuning their morphology and structure is crucial to improve the ...
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