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Ambipolar organic field-effect transistors based on CuPc and F16CuPc: Impact of the fine microstructure at organic–organic interface
Synthetic Metals 161 (2011) 1915–1920
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Ambipolar organic field-effect transistors based on CuPc and F16 CuPc: Impact of the fine microstructure at organic–organic interface Sébastien Nénon a , Daiki Kanehira b , Noriyuki Yoshimoto b , Frédéric Fages a , Christine Videlot-Ackermann a,∗ a b
Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), UPR CNRS 3118, Aix Marseille Université, Campus Luminy, Case 913, 13288 Marseille Cedex 09, France Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
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Article history: Received 18 April 2011 Received in revised form 9 June 2011 Accepted 21 June 2011 Available online 22 July 2011 Keywords: Phthalocyanines Ambipolar OTFTs Thin film Morphology XRD
a b s t r a c t Ambipolar organic thin film transistors were fabricated by using hexadecahydrogen copper phthalocyanine (CuPc) and hexadecafluoro copper phthalocyanine (F16 CuPc) as p-and n-type semiconductors, respectively. Ambipolar transport was observed in thin films based on either heterojunction (CuPc/F16 CuPc or F16 CuPc/CuPc with CuPc and F16 CuPc as first deposited layer, respectively) or blend (CuPc:F16 CuPc) architectures. Structure and morphology of thin films have been studied by atomic force microscopy (AFM) and X-ray diffraction (XRD). A careful study of the fine microstructure formed at the interface between CuPc and F16 CuPc highlights the presence of three intermediate phase layers ensuring a continuously grown between highly ordered CuPc and F16 CuPc polycrystalline thin films. Due to a better distribution between the phases (CuPc, F16 CuPc and the intermediate phase layers), CuPc/F16 CuPc heterojunctions give rise to an optimized ambipolar transport. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Performances of organic thin film transistors (OTFTs) have been continually improving up to the point of now being viable in integrated circuit applications, with potential use in future products such as low-cost flexible radiofrequency identification (RFID) tags, display drivers, smart cards and sensing devices [1–6]. As already used in organic light emitting diodes (OLEDs), several organic layers of different materials allows the effective transport of electrons and holes towards and away the organic–organic interface. The use of more complex structures than just one organic thin film deposited on a substrate needs the joint use of p-channel transistors combined with n-channel transistors to open new perspectives in the field of organic electronics. Such ambipolar OTFTs have been realized by three approaches: (i) using two stacked layers of discrete hole- and electron-transporting semiconducting materials to form a p/n stacked heterojunction, (ii) using a two-component layer comprising a blend of unipolar electron- and hole-transporting organic semiconductors listed as a p:n heterojunction, and (iii) using a single-component layer with symmetric or asymmetric electrodes. Phthalocyanines (Pcs) are today clearly regarded as optical materials, thin applies to organic dye layers in general. The elec-
∗ Corresponding author. Tel.: +33 6 1724 8193; fax: +33 4 9182 9580. E-mail address: [email protected] (C. Videlot-Ackermann). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.035
trical and optical properties of Pcs are determined by a central metal ion and side groups [7]. Copper phthalocyanines with copper (Cu) as the central metal ion, have excellent chemical and thermal stability in environmental condition. The copper phthalocyanine with hexadecahydrogen as a side group (H16 CuPc or CuPc) shows p-type semiconducting nature, while copper phthalocyanine with hexadecafluorine as a side group (F16 CuPc) shows n-type semiconducting nature [8–10]. These commercially available materials have the great advantage to be easily sublimed and resulting in high purity thin films without decomposition. It has been known that phthalocyanines with high thermal and chemical stability represent one of the most promoting candidates for modern opto-electronic devices such as optical recording, organic light-emitting diodes, gas sensors, solar cells and thin film transistors [8,9,11–17]. A relative high mobility in OTFTs can be achieved by employing elevated substrate temperatures during deposition (Tsub ), which affect directly the morphology of films. Typically, a hole mobility of 0.01–0.02 cm2 /V s is obtained for CuPc-based OTFTs at Tsub = 120–175 ◦ C [8,18]. Despite a wide range of recent air-stable n-type organic semiconductors during the last years as cyano-substituted perylene tetracarboxylic diimide core (PDI-8CN2) and an anthracenedicarboximide core (ADI8-CN2) [19,20], F16 CuPc remains one of the most well-known n-type semiconductors. By far F16 CuPc was among the most widely used n-channel materials due to its relatively high mobility (i.e., 0.01 cm2 /V s at Tsub = 120 ◦ C and 0.03 cm2 /V s at Tsub = 125 ◦ C) [10,18] and commercially low cost availability. Because CuPc and
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S. Nénon et al. / Synthetic Metals 161 (2011) 1915–1920
Fig. 1. Molecular structure of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16 CuPc). Energy diagram of heterojunction between CuPc and F16 CuPc together with gold (Au).
F16 CuPc possess similar molecular shape and crystal packing structure as well as comparable performances, it is convenient to obtain ambipolar transport by using these two compounds. Ye et al. adopted laminated structure to fabricated ambipolar transistors [21,22]. They argued that CuPc and F16 CuPc could be grown through an intermediate phase layer to form homostructure. Wang et al. claimed that, in the CuPc/F16 CuPc heterojunction (with CuPc as the first deposited organic layer), there might be charge transfer from CuPc to F16 CuPc, which resulted in a dipolar layer in the interface of CuPc/F16 CuPc [23,24]. This result was confirmed by an independent work on unipolar OTFTs with thick CuPc and F16 CuPc layers stacked in a p/n heterojunction [25]. As shown by the schematic energy diagram in Fig. 1, a low energy level difference occurs between the highest occupied molecular orbital (HOMO) of CuPc and the lowest unoccupied molecular orbital (LUMO) of F16 CuPc. This situation favors a charge transfer between CuPc and F16 CuPc, which results in the accumulation of charge carriers, electrons in F16 CuPc and holes in CuPc, at both sides of the heterojunction interface, and, in turn, in a build-in electrical field. The induced dipolar layer could act as a new conductive channel, which could improve field-effect mobility and shift the threshold voltage. The use of more complex structures as heterojunctions containing two semiconducting materials, in two stacked or blend layers, instead of a thin film based on just one organic semiconductor will involve specific properties. Especially, the microscopic structure of organic heterojunctions plays a key role in current and future electronic devices built from organic semiconducting molecules. Very little is known about how such organic–organic interfaces evolve during growth and how the emerging morphology and structure affects the performances of organic devices, such as ambipolar OTFTs. Previously, we have reported on highly ordered polycrystalline thin films based on either CuPc or F16 CuPc deposited on Si/SiO2 substrates under similar optimized growth conditions [18].
Fig. 2. Schematic representation of organic field-effect transistors (OTFTs) in bilayer (a and b) and blend (c) architectures.
Such layers were implemented as active layer to realize unipolar p- or n-channel OTFTs in both top-contact (TC) and bottom-contact (BC) configurations. In this study, we discuss the impact of the interface between CuPc and F16 CuPc in heterojunctions, based on two stacked layers (CuPc/F16 CuPc and F16 CuPc/CuPc) or on a blend layer (CuPc:F16 CuPc), on the electrical properties of ambipolar OTFTs. Structure and morphology of CuPc and F16 CuPc heterostructure thin films were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD). 2. Experimental The source samples of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16 CuPc) were purchased from Aldrich (France) with a sublimation grade (>99%) and 80% of purity, respectively. The molecular structures of CuPc and F16 CuPc are shown in Fig. 1. CuPc and F16 CuPc were involved in OTFT devices as bilayers and blend layers. The schematic representation of OTFT architectures used in the present study is shown in Fig. 2. Highly ndoped silicon wafers (gate), covered with thermally grown silicon ˚ gate dielectric), were purchased from Vegatec dioxide SiO2 (3000 A, (France) and used as device substrates. The capacitance per unit area of SiO2 dielectric layers was 1.2 × 10−8 F/cm2 . Organic active layers were deposited on substrate by vapor deposition, using a Edwards Auto 306 apparatus, at a rate of 4–7 nm/min under a pressure of 1–2 × 10−6 mbar. In bilayer OTFT architectures, CuPc and F16 CuPc thin films were successively vacuum-deposited from two independent deposition sources. In blend OTFT architectures, CuPc and F16 CuPc thin films were vacuum-deposited simultaneously by heating both deposition sources. The nominal thickness of CuPc and F16 CuPc were controlled by two independent in situ quartz crystal monitors. Substrate temperature (Tsub ) during deposition was con-
S. Nénon et al. / Synthetic Metals 161 (2011) 1915–1920
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trolled to be 30 or 120 ◦ C, by heating in the last case the block on which the substrates are mounted. The Au source and drain electrodes (channel length L = 50 m, channel width W = 1 mm) were evaporated on top of the organic thin films through a shadow mask in the top-contact (TC) configurations or directly on SiO2 layer prior the organic depositions in the bottom-contact (BC) configurations (Fig. 2). Current–voltage characteristics were obtained at room temperature under ambient conditions (air, temperature and light) with a Hewlett-Packard 4140B pico-amperemeter-DC voltage source. The source-drain current in the saturation regime (I)sat is governed by the equation: (I)sat =
W C (Vg − Vt )2 2L i
(1)
where Ci is the capacitance per unit area of the gate insulator layer, Vg is the gate voltage, Vt is the threshold voltage, and is the fieldeffect mobility. All the data in Table 1 were obtained by randomly measuring 2–4 individual OTFTs. Atomic force microscopy (AFM) measurements were done on thin films in air with a SII NanoTechnology Inc., S-image operating in tapping mode. The used cantilever was made of silicon, the resonance frequency was 100 kHz. Thin films analyzed by X-ray film diffractometry (XRD) were fabricated by vacuum deposition in a pressure of 5 × 10−5 Pa using K-cell type crucible. Si wafer (covered by SiO2 layer 300 nm thick) was used as substrates. The substrate temperature (Tsub ) was controlled to be 30 or 120 ◦ C. The nominal thickness of CuPc and F16 CuPc were controlled by two independent in situ quartz crystal monitors. The deposition rates ˚ were 0.1–0.2 A/s. The as-deposited thin films were characterized using X-ray diffraction in air using an X-ray diffractometer (Regaku Co., ATX-G) which was specially designed for characterization of thin films. The used wavelength of X-ray in the experiments was 0.1542 nm. 3. Results Top-contact (TC) and bottom-contact (BC) OTFT devices were fabricated by vacuum evaporation process using two stacked CuPc and F16 CuPc thin films on Si/SiO2 substrates at Tsub = 120 ◦ C. An elevated substrate temperature during deposition (Tsub ) was employed to achieve highest mobility values as observed for unipolar OTFTs based on either CuPc or F16 CuPc with mobility values as high as ∼0.01 to 0.02 cm2 /V s in TC-OTFTs at Tsub = 120 ◦ C for 10 nmthick films [18]. P/n heterojunctions realized in the present study are CuPc/F16 CuPc and F16 CuPc/CuPc with CuPc and F16 CuPc as the first deposited layer, respectively. OTFT devices exhibited the typical electrical characteristics of ambipolar transistors, as shown in Fig. 3. At positive gate voltages |Vg | < 20 V, the current (I) does not saturate and rapidly increases for |Vd | − |Vg | > 30 V, which can be explained by the contribution of drain-induced holes. Similarly, at negative gate voltages, p-channel operational characteristics are not identical to those of OTFTs with only a CuPc layer. For |Vg | < 40 V, Table 1 OTFT data of ambipolar organic field-effect transistors based on CuPc and F16 CuPc as active layers deposited on Si/SiO2 substrates at Tsub = 120 ◦ C in bilayer architectures with CuPc(10 nm)/F16 CuPc(10 nm) and F16 CuPc(10 nm)/CuPc(10 nm). Bottom contact (BC) and top contact (TC) configurations were used for source-drain electrodes. Active layer
Contact TC
CuPc/F16 CuPc BC TC F16 CuPc/CuPc BC
Channel
(cm2 /V s)
Vt (V)
p n p n p n p n
1.3 × 10−3 1.3 × 10−2 9.4 × 10−5 3.2 × 10−3 5.2 × 10−4 1.1 × 10−2 2.4 × 10−4 3.5 × 10−3
44 −49 −50 −41 −12 −29 −43 −57
Fig. 3. Output characteristic of an ambipolar OTFT based on CuPc and F16 CuPc as active layers deposited on Si/SiO2 substrates at Tsub = 120 ◦ C in the specific CuPc(10 nm)/F16 CuPc(10 nm) architecture.
an increase in non-saturated current could also be observed, which originates from drain-induced electrons. The hole and electron mobilities were calculated from the respective saturation regime of the transfer curves. OTFT performances, field-effect mobility () and threshold voltage (Vt ), are summarized in Table 1. As unipolar OTFTs based on either CuPc or F16 CuPc, it was found that the ambipolar devices have highest mobilities in TC configuration. The highest hole and electron mobilities of 0.0013 and 0.013 cm2 /V s, respectively, were found for a CuPc(10 nm)/F16 CuPc(10 nm) heterojunction containing devices. Similar values were obtained in the literature for such bilayer containing devices with however a higher electron mobility in our case [22]. For devices with a blend CuPc:F16 CuPc as active layer, OTFT data were poorly reproducible with lower performances than stacked layers. The highest hole and electron mobilities observed for CuPc(10 nm):F16 CuPc(10 nm) in TC configuration at Tsub = 120 ◦ C were 10−5 and 10−6 cm2 /V s, respectively. This reduction in charge carrier mobility compared to stacked bilayer systems was also observed by A. Opitz et al. in CuPc:F16 CuPc interpenetrating networks for applications in solar cells [26,27]. We found that the dependence of the mobility values on the contact configuration (TC or BC) and on the heterojunctions, i.e. stacked or blend layers, is related to the morphology of organic thin films, as shown in Fig. 4. From the AFM phase images of thin films, it can be seen that both CuPc/F16 CuPc and F16 CuPc/CuPc heterojunctions have different morphologies than single layers with top layers largely influenced by the previously deposited organic layer. In the case of CuPc/F16 CuPc, elongated bent strips of CuPc and interstices are entirely covered by F16 CuPc based small grains. On the contrary, such CuPc elongated grains cannot be formed on the F16 CuPc surface in F16 CuPc/CuPc bilayers revealing only round grains. In CuPc:F16 CuPc blend system, an interconnected network of grains as small bundled fibers are formed during the simultaneous evaporation of both organic molecules. We speculate that the charge transport in ambipolar semiconductor thin films is related to the intimate contact between CuPc and F16 CuPc molecules. In order to better understand the influence of morphology on performances, X-ray diffraction patterns were performed on each system, stacked and blend layers, for substrate temperatures of Tsub = 30 and 120 ◦ C. Fig. 5 shows as an example the X-ray diffraction patterns for both bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and the blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. The more relevant observation is a single diffraction peak at 2
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Fig. 4. AFM pictures of single layers (CuPc and F16 CuPc), bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
Table 2 Data obtained from /2 mode of X-ray diffraction patterns and corresponding deconvolution peaks for CuPc(10 nm)/F16 CuPc(10 nm) and F16 CuPc(10 nm)/CuPc(10 nm) bilayers together with CuPc(10 nm):F16 CuPc(10 nm) blend layers deposited on Si/SiO2 substrates at Tsub = 30 and 120 ◦ C. Phase
2 (◦ )
d (Å)
CuPc/F16 CuPc (%)
F16 CuPc/CuPc (%)
F16 CuPc 1 2 3 CuPc
5.56 5.86 6.19 6.56 6.78
1.588 1.507 1.427 1.346 1.302
7.3 17.9 19.8 16.2 38.8
0.08 2.2 16.5 13.5 67.7
F16 CuPc 3 CuPc
6.13 6.45 6.88
1.444 1.373 1.287
29.5 28.9 41.6
23.3 2.7 74
a
No deconvolution possible as the pattern showed a single peak with no participation of CuPc and F16 CuPc.
CuPc:F16 CuPc (%) 29.4 0 0 68.4 2.2 0a ∼100a 0a
Tsub
120 ◦ C
30 ◦ C
S. Nénon et al. / Synthetic Metals 161 (2011) 1915–1920
Fig. 5. /2 mode of X-ray diffraction patterns for both bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and the blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
between 5◦ and 8◦ for both substrate temperatures. In thin films based on either CuPc or F16 CuPc, these observations imply that CuPc and F16 CuPc molecules of herringbone-like ␣-phase were well ordered along the (2 0 0) direction corresponding the –* orbital staking [18]. Furthermore, the intensity of this diffraction peak for both stacked and blend layers was found to increase with the substrate temperature during deposition, which indicates a consistent increase in ordering and crystallinity in p/n heterojunctions. However, the peaks do not appear as single symmetrical peaks but show evidence of substructures. Fig. 6 shows the detailed X-ray diffraction patterns and the corresponding deconvolution peaks between 5◦ and 8◦ for 10 nm-thick CuPc and F16 CuPc layers, CuPc(10 nm)/F16 CuPc(10 nm) and CuPc(10 nm)/F16 CuPc(10 nm) bilayers, as well as F16 CuPc(10 nm):CuPc(10 nm) blend layer deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. From Fig. 6a, interplanar d spacing values of 1.302 and 1.588 nm for CuPc and F16 CuPc
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deposited on Si/SiO2 at Tsub = 120 ◦ C can be extracted, respectively, and represented as phase CuPc and phase F16 CuPc in Fig. 7. Such values are consistent with previous reported values for thin films vacuum deposited in the same conditions [18]. For more complex heterojunctions as two stacked or blend layers containing CuPc and F16 CuPc, the deconvolution reveals the presence of new peaks in addition to those of CuPc and F16 CuPc with interlayer spacing gradually changing from CuPc to F16 CuPc. From the detailed XRD analysis, the ratios of the integrated intensity (%) for these XRD peaks in the various heterojunctions for both Tsub = 30 or 120 ◦ C are shown in Table 2. Whatever the interface configuration, the main peak can be decomposed into several components with always a participation of CuPc and its counterpart F16 CuPc, with however an exception for the blend architecture at Tsub = 30 ◦ C where a single peak is observed with no visible participation of CuPc and F16 CuPc. Such particularity is consistent with the literature where only one peak located between those of CuPc and F16 CuPc is observed for a mixed layer deposited at low substrate temperature [26,27]. By increasing the substrate temperature during the simultaneous deposition of CuPc and F16 CuPc, the same observation was also reported in the literature [26,27]. The molecules are intimately mixed at the molecular level in a co-evaporation to give a homogeneous mixture throughout the active layer as schematically represented in Fig. 7 for the phase 3 voluntarily drawn ordered for a better visualization but not necessarily the case in the layer. However, it is significant to note that in our case the results of deconvolution at Tsub = 120 ◦ C highlight the presence of not one but three peaks corresponding to the mixed phthalocyanine phase together with a specific involvement of both CuPc and F16 CuPc as a phase separation effect with higher substrate temperature (Fig. 6d). To such mixed phthalocyanine phases, phase 3 at Tsub = 120 ◦ C and phase 3 at Tsub = 30 ◦ C, correspond interplanar ˚ respectively. These interplanar disdistance of 1.346 and 1.373 A, tances, always intermediate between those of CuPc and F16 CuPc, follow the same substrate temperature dependence, i.e. a slight increase with the substrate temperature due to a variation in the tilted angle of such CuPc and F16 CuPc-based homogeneous mixed
Fig. 6. /2 mode of X-ray diffraction patterns and corresponding deconvolution peaks: (a) CuPc(10 nm) and F16 CuPc(10 nm) layers, (b) CuPc(10 nm)/F16 CuPc(10 nm) bilayer, (c) F16 CuPc(10 nm)/CuPc(10 nm) bilayer and (d) CuPc(10 nm):F16 CuPc(10 nm) blend layer deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
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Fig. 7. (a) Structural formulae of CuPc and F16 CuPc. (b) Schematic molecular arrangement for neat (phases CuPc and F16 CuPc) and mixed (phases 1–3) phthalocyanine films deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. For easier visualization an ordered structure of the blend is assumed.
phase. Additionally as a single angle variation of such phase was observed for thin films realized by co-evaporation at Tsub = 120 ◦ C, phases 1 and 2 observed for both stacked layers at Tsub = 120 ◦ C will find their origin in an effect directly linked to the growth process as it will be explained below. Nevertheless, the dominant presence of phase 3 does not lead to an efficient charge transport, electrons and holes, in blend thin films at Tsub = 120 ◦ C. For both stacked layers at Tsub = 120 ◦ C, i.e. CuPc/F16 CuPc and F16 CuPc/CuPc, three peaks (listed from 1 to 3 in Fig. 6b and c) appeared for both p/n architectures where phases 1 and 2 were intermediate phases between F16 CuPc single layer and the homogeneous mixture (Fig. 7). As such phases were not present in crystalline domains obtained for both stacked layers at Tsub = 30 ◦ C or in any thin films realized by co-evaporation, the hypothesis of new phases with different intermixing ratio between CuPc and F16 CuPc molecules was favored. As represented in Fig. 7, phases 1 and 2 contain mostly F16 CuPc molecules with some CuPc ones integrated into F16 CuPc-based crystalline domains where the molecules of the second layer gradually fill the interstices of the first surface layer deposited. Under the same conditions, Mori et al. described the existence of a single intermediate phase layer [22]. In the present study, a more detailed analysis of diffraction spectra highlights the fine microstructure of the interface between CuPc and F16 CuPc by the presence of three intermediate phase layers. CuPc and F16 CuPc polycrystalline thin films could be continuously grown via intermediate phase layers in the heterostructure. Theses intermediate phase layers originate from the interactions or relaxations between CuPc and F16 CuPc at the interface. By the compositions at the molecular level of phases 1 and 2, a better percolation pathway is offered to electrons throughout the active layer until the F16 CuPc layer in the case of F16 CuPc is progressively deposited on CuPc in CuPc/F16 CuPc heterojunction. Moreover, this CuPc/F16 CuPc heterojunction gives a better distribution of the different phases (F16 CuPc, 1, 2, 3 and CuPc) than F16 CuPc/CuPc and could explain the optimized ambipolar transport in this CuPc/F16 CuPc heterojunction. 4. Conclusion Ambipolar OTFTs were fabricated by using CuPc and F16 CuPc semiconductors in heterojunctions as two stacked (CuPc/F16 CuPc and F16 CuPc/CuPc) or blend (CuPc:F16 CuPc) layers. The initial inter-
ests associated to similar molecular shape and crystal packing structure as well as comparable performances turn out not to be factors of security for an efficient ambipolar transport. The highest ambipolar mobilities are observed for CuPc/F16 CuPc heterostructure on Si/SiO2 substrates at Tsub = 120 ◦ C with hole and electron mobilities of 0.0013 and 0.013 cm2 /V s, respectively. Due to a better distribution between the phases ensuring a continuously grown between polycrystalline CuPc and F16 CuPc-based thin films at the organic–organic interface, CuPc/F16 CuPc heterojunctions give rise to an optimized ambipolar transport. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
J. Zaumseil, H. Sirringhaus, Chem. Rev. 107 (2007) 1296. C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. A. Facchetti, Mater. Today 10 (2007) 28. A.R. Murphy, J.M.J. Fréchet, Chem. Rev. 107 (2007) 1066. L. Zhou, A. Wanga, S.-C. Wu, J. Sun, S. Park, T.N. Jackson, Appl. Phys. Lett. 88 (2006) 083502. L. Torsi, A. Dodabalapur, Anal. Chem. 77 (2005) 380. W. Brütting, Physics of Organic Semiconductors, Wiley-VCH, Weinheim, 2005, p. 23. Z. Bao, A.J. Lovinger, A. Dodabalapur, Appl. Phys. Lett. 69 (1996) 3066. Z. Bao, A.J. Lovinger, A. Dodabalapur, Adv. Mater. 9 (1997) 42. Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207. G. Guillaud, J. Simon, J.P. Germain, Coord. Chem. Rev. 178–180 (1998) 1433. M. Ottmar, D. Hohnholz, A. Wedel, M. Hanack, Synth. Met. 105 (1999) 145. M.I. Newton, T.K.H. Starke, G. McHale, M.R. Willis, Thin Solid Films 360 (2000) 10. H. Deng, Z. Lu, Y. Shen, H. Mao, H. Xu, Chem. Phys. 231 (1998) 95. J. Zhang, J. Wang, H. Wang, D. Yan, Appl. Phys. Lett. 84 (2004) 142. K. Xiao, Y. Liu, G. Yu, D. Zhu, Appl. Phys. A: Mater. Sci. Process. 77 (2003) 367. R. Ye, M. Baba, K. Suzuki, K. Mori, Thin Solid Films 517 (2009) 3001. S. Nénon, D. Kanehira, N. Yoshimoto, F. Fages, C. Videlot-Ackermann, Thin Solid Films 518 (2010) 5593. B.A. Jones, A. Facchetti, M.R. Wasielewski, T.J. Marks, J. Am. Chem. Soc. 129 (2007) 15259. Z. Wang, C. Kim, A. Facchetti, T.J. Marks, J. Am. Chem. Soc. 129 (2007) 13362. R. Ye, M. Baba, Y. Oishi, K. Mori, K. Suzuki, Appl. Phys. Lett. 86 (2005) 253505. R. Ye, M. Baba, K. Suzuki, K. Mori, Appl. Surf. Sci. 254 (2008) 7885. J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Appl. Phys. Lett. 87 (2005) 093507. J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Chem. Phys. Lett. 407 (2007) 253510. C. Videlot-Ackermann, J. Ackermann, F. Fages, Synth. Met. 157 (2007) 551. A. Opitz, B. Ecker, J. Wagner, A. Hinderhofer, F. Schreiber, J. Manara, J. Pflaum, W. Brütting, Org. Electron. 10 (2009) 1259. A. Opitz, J. Wagner, B. Ecker, U. Hörmann, M. Kraus, M. Bronner, W. Brütting, A. Hinderhofer, F. Schreiber, Mater. Res. Soc. Symp. Proc. 1154 (2009), 1154-B0912.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Ambipolar organic field-effect transistors based on CuPc and F16 CuPc: Impact of the fine microstructure at organic–organic interface Sébastien Nénon a , Daiki Kanehira b , Noriyuki Yoshimoto b , Frédéric Fages a , Christine Videlot-Ackermann a,∗ a b
Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), UPR CNRS 3118, Aix Marseille Université, Campus Luminy, Case 913, 13288 Marseille Cedex 09, France Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 9 June 2011 Accepted 21 June 2011 Available online 22 July 2011 Keywords: Phthalocyanines Ambipolar OTFTs Thin film Morphology XRD
a b s t r a c t Ambipolar organic thin film transistors were fabricated by using hexadecahydrogen copper phthalocyanine (CuPc) and hexadecafluoro copper phthalocyanine (F16 CuPc) as p-and n-type semiconductors, respectively. Ambipolar transport was observed in thin films based on either heterojunction (CuPc/F16 CuPc or F16 CuPc/CuPc with CuPc and F16 CuPc as first deposited layer, respectively) or blend (CuPc:F16 CuPc) architectures. Structure and morphology of thin films have been studied by atomic force microscopy (AFM) and X-ray diffraction (XRD). A careful study of the fine microstructure formed at the interface between CuPc and F16 CuPc highlights the presence of three intermediate phase layers ensuring a continuously grown between highly ordered CuPc and F16 CuPc polycrystalline thin films. Due to a better distribution between the phases (CuPc, F16 CuPc and the intermediate phase layers), CuPc/F16 CuPc heterojunctions give rise to an optimized ambipolar transport. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Performances of organic thin film transistors (OTFTs) have been continually improving up to the point of now being viable in integrated circuit applications, with potential use in future products such as low-cost flexible radiofrequency identification (RFID) tags, display drivers, smart cards and sensing devices [1–6]. As already used in organic light emitting diodes (OLEDs), several organic layers of different materials allows the effective transport of electrons and holes towards and away the organic–organic interface. The use of more complex structures than just one organic thin film deposited on a substrate needs the joint use of p-channel transistors combined with n-channel transistors to open new perspectives in the field of organic electronics. Such ambipolar OTFTs have been realized by three approaches: (i) using two stacked layers of discrete hole- and electron-transporting semiconducting materials to form a p/n stacked heterojunction, (ii) using a two-component layer comprising a blend of unipolar electron- and hole-transporting organic semiconductors listed as a p:n heterojunction, and (iii) using a single-component layer with symmetric or asymmetric electrodes. Phthalocyanines (Pcs) are today clearly regarded as optical materials, thin applies to organic dye layers in general. The elec-
∗ Corresponding author. Tel.: +33 6 1724 8193; fax: +33 4 9182 9580. E-mail address: [email protected] (C. Videlot-Ackermann). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.035
trical and optical properties of Pcs are determined by a central metal ion and side groups [7]. Copper phthalocyanines with copper (Cu) as the central metal ion, have excellent chemical and thermal stability in environmental condition. The copper phthalocyanine with hexadecahydrogen as a side group (H16 CuPc or CuPc) shows p-type semiconducting nature, while copper phthalocyanine with hexadecafluorine as a side group (F16 CuPc) shows n-type semiconducting nature [8–10]. These commercially available materials have the great advantage to be easily sublimed and resulting in high purity thin films without decomposition. It has been known that phthalocyanines with high thermal and chemical stability represent one of the most promoting candidates for modern opto-electronic devices such as optical recording, organic light-emitting diodes, gas sensors, solar cells and thin film transistors [8,9,11–17]. A relative high mobility in OTFTs can be achieved by employing elevated substrate temperatures during deposition (Tsub ), which affect directly the morphology of films. Typically, a hole mobility of 0.01–0.02 cm2 /V s is obtained for CuPc-based OTFTs at Tsub = 120–175 ◦ C [8,18]. Despite a wide range of recent air-stable n-type organic semiconductors during the last years as cyano-substituted perylene tetracarboxylic diimide core (PDI-8CN2) and an anthracenedicarboximide core (ADI8-CN2) [19,20], F16 CuPc remains one of the most well-known n-type semiconductors. By far F16 CuPc was among the most widely used n-channel materials due to its relatively high mobility (i.e., 0.01 cm2 /V s at Tsub = 120 ◦ C and 0.03 cm2 /V s at Tsub = 125 ◦ C) [10,18] and commercially low cost availability. Because CuPc and
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Fig. 1. Molecular structure of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16 CuPc). Energy diagram of heterojunction between CuPc and F16 CuPc together with gold (Au).
F16 CuPc possess similar molecular shape and crystal packing structure as well as comparable performances, it is convenient to obtain ambipolar transport by using these two compounds. Ye et al. adopted laminated structure to fabricated ambipolar transistors [21,22]. They argued that CuPc and F16 CuPc could be grown through an intermediate phase layer to form homostructure. Wang et al. claimed that, in the CuPc/F16 CuPc heterojunction (with CuPc as the first deposited organic layer), there might be charge transfer from CuPc to F16 CuPc, which resulted in a dipolar layer in the interface of CuPc/F16 CuPc [23,24]. This result was confirmed by an independent work on unipolar OTFTs with thick CuPc and F16 CuPc layers stacked in a p/n heterojunction [25]. As shown by the schematic energy diagram in Fig. 1, a low energy level difference occurs between the highest occupied molecular orbital (HOMO) of CuPc and the lowest unoccupied molecular orbital (LUMO) of F16 CuPc. This situation favors a charge transfer between CuPc and F16 CuPc, which results in the accumulation of charge carriers, electrons in F16 CuPc and holes in CuPc, at both sides of the heterojunction interface, and, in turn, in a build-in electrical field. The induced dipolar layer could act as a new conductive channel, which could improve field-effect mobility and shift the threshold voltage. The use of more complex structures as heterojunctions containing two semiconducting materials, in two stacked or blend layers, instead of a thin film based on just one organic semiconductor will involve specific properties. Especially, the microscopic structure of organic heterojunctions plays a key role in current and future electronic devices built from organic semiconducting molecules. Very little is known about how such organic–organic interfaces evolve during growth and how the emerging morphology and structure affects the performances of organic devices, such as ambipolar OTFTs. Previously, we have reported on highly ordered polycrystalline thin films based on either CuPc or F16 CuPc deposited on Si/SiO2 substrates under similar optimized growth conditions [18].
Fig. 2. Schematic representation of organic field-effect transistors (OTFTs) in bilayer (a and b) and blend (c) architectures.
Such layers were implemented as active layer to realize unipolar p- or n-channel OTFTs in both top-contact (TC) and bottom-contact (BC) configurations. In this study, we discuss the impact of the interface between CuPc and F16 CuPc in heterojunctions, based on two stacked layers (CuPc/F16 CuPc and F16 CuPc/CuPc) or on a blend layer (CuPc:F16 CuPc), on the electrical properties of ambipolar OTFTs. Structure and morphology of CuPc and F16 CuPc heterostructure thin films were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD). 2. Experimental The source samples of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16 CuPc) were purchased from Aldrich (France) with a sublimation grade (>99%) and 80% of purity, respectively. The molecular structures of CuPc and F16 CuPc are shown in Fig. 1. CuPc and F16 CuPc were involved in OTFT devices as bilayers and blend layers. The schematic representation of OTFT architectures used in the present study is shown in Fig. 2. Highly ndoped silicon wafers (gate), covered with thermally grown silicon ˚ gate dielectric), were purchased from Vegatec dioxide SiO2 (3000 A, (France) and used as device substrates. The capacitance per unit area of SiO2 dielectric layers was 1.2 × 10−8 F/cm2 . Organic active layers were deposited on substrate by vapor deposition, using a Edwards Auto 306 apparatus, at a rate of 4–7 nm/min under a pressure of 1–2 × 10−6 mbar. In bilayer OTFT architectures, CuPc and F16 CuPc thin films were successively vacuum-deposited from two independent deposition sources. In blend OTFT architectures, CuPc and F16 CuPc thin films were vacuum-deposited simultaneously by heating both deposition sources. The nominal thickness of CuPc and F16 CuPc were controlled by two independent in situ quartz crystal monitors. Substrate temperature (Tsub ) during deposition was con-
S. Nénon et al. / Synthetic Metals 161 (2011) 1915–1920
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trolled to be 30 or 120 ◦ C, by heating in the last case the block on which the substrates are mounted. The Au source and drain electrodes (channel length L = 50 m, channel width W = 1 mm) were evaporated on top of the organic thin films through a shadow mask in the top-contact (TC) configurations or directly on SiO2 layer prior the organic depositions in the bottom-contact (BC) configurations (Fig. 2). Current–voltage characteristics were obtained at room temperature under ambient conditions (air, temperature and light) with a Hewlett-Packard 4140B pico-amperemeter-DC voltage source. The source-drain current in the saturation regime (I)sat is governed by the equation: (I)sat =
W C (Vg − Vt )2 2L i
(1)
where Ci is the capacitance per unit area of the gate insulator layer, Vg is the gate voltage, Vt is the threshold voltage, and is the fieldeffect mobility. All the data in Table 1 were obtained by randomly measuring 2–4 individual OTFTs. Atomic force microscopy (AFM) measurements were done on thin films in air with a SII NanoTechnology Inc., S-image operating in tapping mode. The used cantilever was made of silicon, the resonance frequency was 100 kHz. Thin films analyzed by X-ray film diffractometry (XRD) were fabricated by vacuum deposition in a pressure of 5 × 10−5 Pa using K-cell type crucible. Si wafer (covered by SiO2 layer 300 nm thick) was used as substrates. The substrate temperature (Tsub ) was controlled to be 30 or 120 ◦ C. The nominal thickness of CuPc and F16 CuPc were controlled by two independent in situ quartz crystal monitors. The deposition rates ˚ were 0.1–0.2 A/s. The as-deposited thin films were characterized using X-ray diffraction in air using an X-ray diffractometer (Regaku Co., ATX-G) which was specially designed for characterization of thin films. The used wavelength of X-ray in the experiments was 0.1542 nm. 3. Results Top-contact (TC) and bottom-contact (BC) OTFT devices were fabricated by vacuum evaporation process using two stacked CuPc and F16 CuPc thin films on Si/SiO2 substrates at Tsub = 120 ◦ C. An elevated substrate temperature during deposition (Tsub ) was employed to achieve highest mobility values as observed for unipolar OTFTs based on either CuPc or F16 CuPc with mobility values as high as ∼0.01 to 0.02 cm2 /V s in TC-OTFTs at Tsub = 120 ◦ C for 10 nmthick films [18]. P/n heterojunctions realized in the present study are CuPc/F16 CuPc and F16 CuPc/CuPc with CuPc and F16 CuPc as the first deposited layer, respectively. OTFT devices exhibited the typical electrical characteristics of ambipolar transistors, as shown in Fig. 3. At positive gate voltages |Vg | < 20 V, the current (I) does not saturate and rapidly increases for |Vd | − |Vg | > 30 V, which can be explained by the contribution of drain-induced holes. Similarly, at negative gate voltages, p-channel operational characteristics are not identical to those of OTFTs with only a CuPc layer. For |Vg | < 40 V, Table 1 OTFT data of ambipolar organic field-effect transistors based on CuPc and F16 CuPc as active layers deposited on Si/SiO2 substrates at Tsub = 120 ◦ C in bilayer architectures with CuPc(10 nm)/F16 CuPc(10 nm) and F16 CuPc(10 nm)/CuPc(10 nm). Bottom contact (BC) and top contact (TC) configurations were used for source-drain electrodes. Active layer
Contact TC
CuPc/F16 CuPc BC TC F16 CuPc/CuPc BC
Channel
(cm2 /V s)
Vt (V)
p n p n p n p n
1.3 × 10−3 1.3 × 10−2 9.4 × 10−5 3.2 × 10−3 5.2 × 10−4 1.1 × 10−2 2.4 × 10−4 3.5 × 10−3
44 −49 −50 −41 −12 −29 −43 −57
Fig. 3. Output characteristic of an ambipolar OTFT based on CuPc and F16 CuPc as active layers deposited on Si/SiO2 substrates at Tsub = 120 ◦ C in the specific CuPc(10 nm)/F16 CuPc(10 nm) architecture.
an increase in non-saturated current could also be observed, which originates from drain-induced electrons. The hole and electron mobilities were calculated from the respective saturation regime of the transfer curves. OTFT performances, field-effect mobility () and threshold voltage (Vt ), are summarized in Table 1. As unipolar OTFTs based on either CuPc or F16 CuPc, it was found that the ambipolar devices have highest mobilities in TC configuration. The highest hole and electron mobilities of 0.0013 and 0.013 cm2 /V s, respectively, were found for a CuPc(10 nm)/F16 CuPc(10 nm) heterojunction containing devices. Similar values were obtained in the literature for such bilayer containing devices with however a higher electron mobility in our case [22]. For devices with a blend CuPc:F16 CuPc as active layer, OTFT data were poorly reproducible with lower performances than stacked layers. The highest hole and electron mobilities observed for CuPc(10 nm):F16 CuPc(10 nm) in TC configuration at Tsub = 120 ◦ C were 10−5 and 10−6 cm2 /V s, respectively. This reduction in charge carrier mobility compared to stacked bilayer systems was also observed by A. Opitz et al. in CuPc:F16 CuPc interpenetrating networks for applications in solar cells [26,27]. We found that the dependence of the mobility values on the contact configuration (TC or BC) and on the heterojunctions, i.e. stacked or blend layers, is related to the morphology of organic thin films, as shown in Fig. 4. From the AFM phase images of thin films, it can be seen that both CuPc/F16 CuPc and F16 CuPc/CuPc heterojunctions have different morphologies than single layers with top layers largely influenced by the previously deposited organic layer. In the case of CuPc/F16 CuPc, elongated bent strips of CuPc and interstices are entirely covered by F16 CuPc based small grains. On the contrary, such CuPc elongated grains cannot be formed on the F16 CuPc surface in F16 CuPc/CuPc bilayers revealing only round grains. In CuPc:F16 CuPc blend system, an interconnected network of grains as small bundled fibers are formed during the simultaneous evaporation of both organic molecules. We speculate that the charge transport in ambipolar semiconductor thin films is related to the intimate contact between CuPc and F16 CuPc molecules. In order to better understand the influence of morphology on performances, X-ray diffraction patterns were performed on each system, stacked and blend layers, for substrate temperatures of Tsub = 30 and 120 ◦ C. Fig. 5 shows as an example the X-ray diffraction patterns for both bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and the blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. The more relevant observation is a single diffraction peak at 2
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Fig. 4. AFM pictures of single layers (CuPc and F16 CuPc), bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
Table 2 Data obtained from /2 mode of X-ray diffraction patterns and corresponding deconvolution peaks for CuPc(10 nm)/F16 CuPc(10 nm) and F16 CuPc(10 nm)/CuPc(10 nm) bilayers together with CuPc(10 nm):F16 CuPc(10 nm) blend layers deposited on Si/SiO2 substrates at Tsub = 30 and 120 ◦ C. Phase
2 (◦ )
d (Å)
CuPc/F16 CuPc (%)
F16 CuPc/CuPc (%)
F16 CuPc 1 2 3 CuPc
5.56 5.86 6.19 6.56 6.78
1.588 1.507 1.427 1.346 1.302
7.3 17.9 19.8 16.2 38.8
0.08 2.2 16.5 13.5 67.7
F16 CuPc 3 CuPc
6.13 6.45 6.88
1.444 1.373 1.287
29.5 28.9 41.6
23.3 2.7 74
a
No deconvolution possible as the pattern showed a single peak with no participation of CuPc and F16 CuPc.
CuPc:F16 CuPc (%) 29.4 0 0 68.4 2.2 0a ∼100a 0a
Tsub
120 ◦ C
30 ◦ C
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Fig. 5. /2 mode of X-ray diffraction patterns for both bilayers (CuPc/F16 CuPc and F16 CuPc/CuPc) and the blend layer (CuPc:F16 CuPc) deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
between 5◦ and 8◦ for both substrate temperatures. In thin films based on either CuPc or F16 CuPc, these observations imply that CuPc and F16 CuPc molecules of herringbone-like ␣-phase were well ordered along the (2 0 0) direction corresponding the –* orbital staking [18]. Furthermore, the intensity of this diffraction peak for both stacked and blend layers was found to increase with the substrate temperature during deposition, which indicates a consistent increase in ordering and crystallinity in p/n heterojunctions. However, the peaks do not appear as single symmetrical peaks but show evidence of substructures. Fig. 6 shows the detailed X-ray diffraction patterns and the corresponding deconvolution peaks between 5◦ and 8◦ for 10 nm-thick CuPc and F16 CuPc layers, CuPc(10 nm)/F16 CuPc(10 nm) and CuPc(10 nm)/F16 CuPc(10 nm) bilayers, as well as F16 CuPc(10 nm):CuPc(10 nm) blend layer deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. From Fig. 6a, interplanar d spacing values of 1.302 and 1.588 nm for CuPc and F16 CuPc
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deposited on Si/SiO2 at Tsub = 120 ◦ C can be extracted, respectively, and represented as phase CuPc and phase F16 CuPc in Fig. 7. Such values are consistent with previous reported values for thin films vacuum deposited in the same conditions [18]. For more complex heterojunctions as two stacked or blend layers containing CuPc and F16 CuPc, the deconvolution reveals the presence of new peaks in addition to those of CuPc and F16 CuPc with interlayer spacing gradually changing from CuPc to F16 CuPc. From the detailed XRD analysis, the ratios of the integrated intensity (%) for these XRD peaks in the various heterojunctions for both Tsub = 30 or 120 ◦ C are shown in Table 2. Whatever the interface configuration, the main peak can be decomposed into several components with always a participation of CuPc and its counterpart F16 CuPc, with however an exception for the blend architecture at Tsub = 30 ◦ C where a single peak is observed with no visible participation of CuPc and F16 CuPc. Such particularity is consistent with the literature where only one peak located between those of CuPc and F16 CuPc is observed for a mixed layer deposited at low substrate temperature [26,27]. By increasing the substrate temperature during the simultaneous deposition of CuPc and F16 CuPc, the same observation was also reported in the literature [26,27]. The molecules are intimately mixed at the molecular level in a co-evaporation to give a homogeneous mixture throughout the active layer as schematically represented in Fig. 7 for the phase 3 voluntarily drawn ordered for a better visualization but not necessarily the case in the layer. However, it is significant to note that in our case the results of deconvolution at Tsub = 120 ◦ C highlight the presence of not one but three peaks corresponding to the mixed phthalocyanine phase together with a specific involvement of both CuPc and F16 CuPc as a phase separation effect with higher substrate temperature (Fig. 6d). To such mixed phthalocyanine phases, phase 3 at Tsub = 120 ◦ C and phase 3 at Tsub = 30 ◦ C, correspond interplanar ˚ respectively. These interplanar disdistance of 1.346 and 1.373 A, tances, always intermediate between those of CuPc and F16 CuPc, follow the same substrate temperature dependence, i.e. a slight increase with the substrate temperature due to a variation in the tilted angle of such CuPc and F16 CuPc-based homogeneous mixed
Fig. 6. /2 mode of X-ray diffraction patterns and corresponding deconvolution peaks: (a) CuPc(10 nm) and F16 CuPc(10 nm) layers, (b) CuPc(10 nm)/F16 CuPc(10 nm) bilayer, (c) F16 CuPc(10 nm)/CuPc(10 nm) bilayer and (d) CuPc(10 nm):F16 CuPc(10 nm) blend layer deposited on Si/SiO2 substrates at Tsub = 120 ◦ C.
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Fig. 7. (a) Structural formulae of CuPc and F16 CuPc. (b) Schematic molecular arrangement for neat (phases CuPc and F16 CuPc) and mixed (phases 1–3) phthalocyanine films deposited on Si/SiO2 substrates at Tsub = 120 ◦ C. For easier visualization an ordered structure of the blend is assumed.
phase. Additionally as a single angle variation of such phase was observed for thin films realized by co-evaporation at Tsub = 120 ◦ C, phases 1 and 2 observed for both stacked layers at Tsub = 120 ◦ C will find their origin in an effect directly linked to the growth process as it will be explained below. Nevertheless, the dominant presence of phase 3 does not lead to an efficient charge transport, electrons and holes, in blend thin films at Tsub = 120 ◦ C. For both stacked layers at Tsub = 120 ◦ C, i.e. CuPc/F16 CuPc and F16 CuPc/CuPc, three peaks (listed from 1 to 3 in Fig. 6b and c) appeared for both p/n architectures where phases 1 and 2 were intermediate phases between F16 CuPc single layer and the homogeneous mixture (Fig. 7). As such phases were not present in crystalline domains obtained for both stacked layers at Tsub = 30 ◦ C or in any thin films realized by co-evaporation, the hypothesis of new phases with different intermixing ratio between CuPc and F16 CuPc molecules was favored. As represented in Fig. 7, phases 1 and 2 contain mostly F16 CuPc molecules with some CuPc ones integrated into F16 CuPc-based crystalline domains where the molecules of the second layer gradually fill the interstices of the first surface layer deposited. Under the same conditions, Mori et al. described the existence of a single intermediate phase layer [22]. In the present study, a more detailed analysis of diffraction spectra highlights the fine microstructure of the interface between CuPc and F16 CuPc by the presence of three intermediate phase layers. CuPc and F16 CuPc polycrystalline thin films could be continuously grown via intermediate phase layers in the heterostructure. Theses intermediate phase layers originate from the interactions or relaxations between CuPc and F16 CuPc at the interface. By the compositions at the molecular level of phases 1 and 2, a better percolation pathway is offered to electrons throughout the active layer until the F16 CuPc layer in the case of F16 CuPc is progressively deposited on CuPc in CuPc/F16 CuPc heterojunction. Moreover, this CuPc/F16 CuPc heterojunction gives a better distribution of the different phases (F16 CuPc, 1, 2, 3 and CuPc) than F16 CuPc/CuPc and could explain the optimized ambipolar transport in this CuPc/F16 CuPc heterojunction. 4. Conclusion Ambipolar OTFTs were fabricated by using CuPc and F16 CuPc semiconductors in heterojunctions as two stacked (CuPc/F16 CuPc and F16 CuPc/CuPc) or blend (CuPc:F16 CuPc) layers. The initial inter-
ests associated to similar molecular shape and crystal packing structure as well as comparable performances turn out not to be factors of security for an efficient ambipolar transport. The highest ambipolar mobilities are observed for CuPc/F16 CuPc heterostructure on Si/SiO2 substrates at Tsub = 120 ◦ C with hole and electron mobilities of 0.0013 and 0.013 cm2 /V s, respectively. Due to a better distribution between the phases ensuring a continuously grown between polycrystalline CuPc and F16 CuPc-based thin films at the organic–organic interface, CuPc/F16 CuPc heterojunctions give rise to an optimized ambipolar transport. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
J. Zaumseil, H. Sirringhaus, Chem. Rev. 107 (2007) 1296. C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. A. Facchetti, Mater. Today 10 (2007) 28. A.R. Murphy, J.M.J. Fréchet, Chem. Rev. 107 (2007) 1066. L. Zhou, A. Wanga, S.-C. Wu, J. Sun, S. Park, T.N. Jackson, Appl. Phys. Lett. 88 (2006) 083502. L. Torsi, A. Dodabalapur, Anal. Chem. 77 (2005) 380. W. Brütting, Physics of Organic Semiconductors, Wiley-VCH, Weinheim, 2005, p. 23. Z. Bao, A.J. Lovinger, A. Dodabalapur, Appl. Phys. Lett. 69 (1996) 3066. Z. Bao, A.J. Lovinger, A. Dodabalapur, Adv. Mater. 9 (1997) 42. Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207. G. Guillaud, J. Simon, J.P. Germain, Coord. Chem. Rev. 178–180 (1998) 1433. M. Ottmar, D. Hohnholz, A. Wedel, M. Hanack, Synth. Met. 105 (1999) 145. M.I. Newton, T.K.H. Starke, G. McHale, M.R. Willis, Thin Solid Films 360 (2000) 10. H. Deng, Z. Lu, Y. Shen, H. Mao, H. Xu, Chem. Phys. 231 (1998) 95. J. Zhang, J. Wang, H. Wang, D. Yan, Appl. Phys. Lett. 84 (2004) 142. K. Xiao, Y. Liu, G. Yu, D. Zhu, Appl. Phys. A: Mater. Sci. Process. 77 (2003) 367. R. Ye, M. Baba, K. Suzuki, K. Mori, Thin Solid Films 517 (2009) 3001. S. Nénon, D. Kanehira, N. Yoshimoto, F. Fages, C. Videlot-Ackermann, Thin Solid Films 518 (2010) 5593. B.A. Jones, A. Facchetti, M.R. Wasielewski, T.J. Marks, J. Am. Chem. Soc. 129 (2007) 15259. Z. Wang, C. Kim, A. Facchetti, T.J. Marks, J. Am. Chem. Soc. 129 (2007) 13362. R. Ye, M. Baba, Y. Oishi, K. Mori, K. Suzuki, Appl. Phys. Lett. 86 (2005) 253505. R. Ye, M. Baba, K. Suzuki, K. Mori, Appl. Surf. Sci. 254 (2008) 7885. J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Appl. Phys. Lett. 87 (2005) 093507. J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Chem. Phys. Lett. 407 (2007) 253510. C. Videlot-Ackermann, J. Ackermann, F. Fages, Synth. Met. 157 (2007) 551. A. Opitz, B. Ecker, J. Wagner, A. Hinderhofer, F. Schreiber, J. Manara, J. Pflaum, W. Brütting, Org. Electron. 10 (2009) 1259. A. Opitz, J. Wagner, B. Ecker, U. Hörmann, M. Kraus, M. Bronner, W. Brütting, A. Hinderhofer, F. Schreiber, Mater. Res. Soc. Symp. Proc. 1154 (2009), 1154-B0912.