High Performance Organic Thin Film Transistors with Solution

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High Performance Organic Thin Film Transistors with Solution Processed TTF-TCNQ Charge Transfer Salt as Electrodes Biswanath Mukherjee*,† and Moumita Mukherjee‡ † ‡

Physics Department, Kalna College, Kalna, Burdwan, India 713409 Department of Physics, Saroj Mohan Institute of Technology, Guptipara, Hooghly, India 712512 ABSTRACT: Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the sourcedrain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as sourcedrain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of allorganics and plastic electronics devices with high throughput and low-cost processing.

1. INTRODUCTION In the past one or two decades, organic thin film transistors (OTFTs) have gained renewed research attention, and immense interest has been paid to the fabrication and investigation of highperformance OTFTs, not only because OTFTs can drive other devices, but also for the construction of organic electronic circuits.1 15 However, the conventional methods, such as photolithography and electron beam lithography, used for the deposition of passive components for interconnections in inorganic microelectronic technologies, are vulnerable to organic thin film devices, especially for short channel devices. Moreover, the contact barriers formed at the metal/semiconductor interface may greatly influence the carrier injection from electrodes into the semiconductor channel, influencing the device performances significantly. Therefore, to achieve high device performance, it is necessary to optimize carrier injection at the source/drain contacts by proper design the electrodes. The conductive tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) charge-transfer (CT) salt,16 often referred to as the “synthetic metals”, exhibits versatile electronic functionality as a result of ground-state electron transfer between the component molecules. In addition, this CT complex is a promising system for interconnect materials that can be generated from solutions of its precursors or by thermal coevaporation of the mixtures. Although the individual components are soluble in a variety of organic solvents and, in general, are nonconductive, the formation of the relatively insoluble and high-conducting charge-transfer salt occurs r 2011 American Chemical Society

spontaneously at room temperature. The high conductivity of TTF-TCNQ complex in its single crystalline form17,18 makes it attractive for applications in various electronic devices.19 21 Even the polycrystalline film of TTF-TCNQ fabricated through various methods, for example, chemical vapor deposition,22 thermal evaporation,23 or from the crystals grown from combined solutions of TTF and TCNQ,24 exhibits conductivity ∼5 10 S cm 1. Organic charge-transfer complex films fabricated by inkjet printing of individual donor and acceptor components and combining on the substrate has been demonstrated to exhibit high device performance.24 In this article, we report that poorly soluble metallic organic charge-transfer complex films can be fabricated by a simple drop casting technique in which the soluble donor and acceptor components are injected individually atop a capillary tube and combine on the substrate to form highly conducting metallic films. The presence of the capillary tube enables the deposition of highly conducting TTF-TCNQ metallic thin films on both sides of it, which is used as the source/drain contact electrodes, leaving a narrow channel at the contact line of the tube with the substrate. The spacing between the films on two sides of the capillary tube, i.e., the separation between the source and drain electrodes (channel length), is dependent on the contact area of the tube Received: May 13, 2011 Revised: July 16, 2011 Published: August 03, 2011 11246

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Langmuir with the underlying substrate. As expected, we have observed that the smaller the radius of the tube, the smaller the contact area and hence the smaller the channel length. The novelty of the present method lies in the simplicity of patterning of source/drain electrodes eliminating the need for the complicated and expensive lithographic patterning technique. Also, the lithographically patterned masks are designed for vacuum evaporated films, making them unsuitable for solution processing. The potential cost advantage associated with this solution process and the new method of patterning organic source/drain electrodes with the glass capillary tube as a “mask” is very promising for bottom contact OTFT, which may further boost the development of allorganic, cost-effective, and high-performance devices. Furthermore, the channel length of the OTFT can easily be controlled by using a capillary tube of different radius making it a facile and novel approach. Use of the TTF-TCNQ complex films as source/drain electrodes affords high-performance pentacene (p-type) and F16CuPc (n-type) thin-film transistors showing high mobility, high on/off ratio, and sharp switching at low voltages. The comparison of the device performances between TTF-TCNQ contacted OTFT and Au contacted (both top and bottom) OTFTs indicate that comparable performance or even better results are obtained in the former device, probably because of low work function and high conductivity of the organic CT complex film, which results in efficient charge injection from the electrodes into the semiconductors. Additionally, the organic organic interface between TTF-TCNQ crystals and organic thin films may have contributed to the better performance of the devices.

2. EXPERIMENTAL SECTION The OTFTs were fabricated on cleaned SiO2/Si substrates, in bottom-gate, bottom-contact source-drain electrode configuration. All the materials used in this study, viz., pentacene, F16CuPc, TTF, and TCNQ were purchased from Aldrich Chemicals and used as received. n+-Si (100) wafers with thermally oxidized SiO2 layers (300 nm) was used as the gate electrode. The oxide layer (SiO2) over the Si substrate acted as the gate dielectric, while TTF-TCNQ CT complex film was used as the bottom-contact source-drain electrodes. To deposit the source-drain electrodes over the SiO2 layer, a 3-cm-long glass capillary tube of outer wall diameter 1 mm was placed on top of the SiO2/Si substrate. Individual solutions of TCNQ and TTF (in chloroform) having concentrations of 5 mg/mL was injected separately on the capillary tube (step 1, Figure 1). The volume of each of the solutions (TTF and TCNQ) dropped on the substrate (capillary tube) was 20 μL. The solutions immediately spread along the length of the tube and also covered the sides. The CT complex formed immediately as soon as two solutions mixed on the substrate (step 2, Figure 1). The inset in step 2 (Figure 1) shows the optical microscope image of beginning of the complex film formation. The composite (mixed) solution with the capillary tube lying on the substrate was then placed on a hot plate, preset at a temperature 60 °C, and the solution was baked for 1 h under ambient conditions. With heating, the solvent evaporated completely and the CT complex film was observed to grow on both sides of the tube leaving a narrow channel (∼150 μm) at the contact line of the tube with the substrate (step 3, Figure 1). As expected, the edge profile of the electrode at the side in contact with the capillary tube is found to be slightly different from that of the other side. While the TTF-TCNQ complex film has submicroscopic gaps between the crystallites near the edge in contact with the tube, those crystallites are more compact at the noncontact edge of the electrode. Even in the surface profile scan, the slight asymmetric nature of the two edges is clearly pronounced. To

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Figure 1. Schematic of the OTFT fabrication with solution processed TTF-TCNQ CT complex film as bottom contact source-drain (S-D) electrodes. (step 1) Individual solutions are injected atop the glass capillary tube. (step 2) As soon as the solutions get mixed, CT complex film starts forming. Inset shows the OM image of complex film formation. (step 3) The film was baked at 60 °C under ambient condition to pattern the S-D electrodes. (step 4) Organic thin film was deposited on top of the TTF-TCNQ complex film electrode to complete the bottom contact OTFT fabrication. complete the OTFT fabrication, a thin film (50 nm) of pentacene (for p-type OTFT) and F16CuPc (for n-type OTFT) was thermally evaporated on top of the complex film under a high vacuum (5  10 6 Torr) at a rate of 0.3 Å/s (step 4, Figure 1). A shadow mask, used during the deposition of the organic films, defined the channel width (W) as 1000 μm. For comparison, we also have fabricated pentacene and F16CuPc based OTFTs with Au as top- and bottom-contact source-drain electrodes having the same channel dimensions. A HP4145B semiconductor parameter analyzer controlled by locally written LabView codes through a GPIB interface was used for the characterization of the devices under ambient conditions. AFM measurements of the devices were done with Nanoscope IIIa (Veeco, digital instruments).

3. RESULTS AND DISCUSSION The schematic of the device fabrication procedure is illustrated in Figure 1, the description of which has been given in the Experimental Section. Figure 2 shows the formation of TTFTCNQ CT complex on SiO2/Si substrate after injecting equally concentrated (5 mg/mL) solutions of individual components over the substrate in equal amounts, while monitoring the growth of the complex film through an optical microscope. First, TCNQ (acceptor) solution in chloroform is dropped on the substrate followed by TTF (donor) solution deposition and the mixture is dried at 60 °C for 1 h in ambient condition. The figure shows that, right after the injection of donor solution over the acceptor, CT complex starts forming (Figure 2a). It was found that the formation of the complex film started immediately within 2 s after mixing of the donor and acceptor solutions and a gradual color change takes place from yellow to green and finally to black. The figure actually shows a portion of one of the edges of the spherical droplet containing mixture of TTF and TCNQ solutions. Since the solvent evaporates faster at the edges of the sphere, the growth of the complex film takes place from the edges toward the center of the droplet. With increasing evaporation of the solvent, the TTF-TCNQ complex grows faster, and finally after complete solvent evaporation, the complex film covers the whole mixing area (Figure 2a d). After verifying the formation of charge transfer complex, we have measured the electrical conductivity of the film by evaluating 11247

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Figure 2. Growth of TTF-TCNQ complex film from edges toward the center of the spherical droplet with time monitored through an optical microscope. The images were taken after (a) 2 s, (b) 5 s, (c) 10 s, and (d) 15 s of injecting the TTF solution atop the TCNQ solution. Scale bar is 200 μm.

Figure 3. (a) I V characteristics of the TTF-TCNQ CT complex film measured laterally by two tungsten tips at different electrode separation distances. (b) I V characteristics of the individual film at electrode separation of 100 μm. Top and bottom inset shows, respectively, the OM images of drop-cast TCNQ film and TTF film.

the I V characteristics. For this, the film was deposited on SiO2/ Si substrate and annealed at 60 °C for 1 h in ambient condition. To obtain the I V characteristics of the film, two tungsten probe tips were connected on top of the film spaced 100 μm, 1 mm, and 5 mm apart. Although nonlinear behavior in I V characteristics was observed, the film exhibited high conductivity with symmetrical characteristics (Figure 3a), verifying its usefulness for conducting electrode materials. When films of individual solutions (donors or acceptors) were deposited without being mixed, the film exhibited much lower conductivity (Figure 3b). It has been found from the optical microscope images that the individual materials produced long, needle-shaped crystals which were sparsely dispersed and isolated on the substrates (inset, Figure 3b). Figure 4a shows the OM image of TTF-TCNQ electrodes, which was deposited from solution with the help of a capillary tube. High resolution image of the electrodes shows that the film consists of closely aggregated crystalline plates. Figure 4b shows the atomic force microscope (AFM) image (phase) of the TTF-TCNQ

Figure 4. (a) OM image of a typical bottom contact pentacene OTFT with solution deposited source-drain electrodes having L = 150 μm and W = 1000 μm. (b) Tapping mode AFM phase image of the TTF-TCNQ complex film depicting plate-like crystallites. (c) Height profile of the electrode as measured by a surface profilometer.

complex electrode. The characterization of the TTF-TCNQ complex film through AFM shows that the film, indeed, is composed of densely packed, elongated plate-like crystals, supporting our results obtained from the OM images. The spacing between the electrodes was measured to be ∼150 μm (from OM image) and the typical thickness of the electrode film was found from the surface profiler scan to be ∼300 nm (Figure 4c). With solution deposited TTF-TCNQ complex film as bottom-contact source-drain electrodes, organic thin-film transistors (OTFTs) were fabricated on SiO2/Si substrates. We have fabricated both p- and n-type OTFTs by depositing thin films (50 nm) of pentacene and fluorinated copper phthalocyanine (F16CuPc), respectively, to elucidate the applicability of TTFTCNQ complex film in both types of carrier injection. The output characteristics (IDS VDS) of pentacene (Figure 5a) and F16CuPc based OTFTs (Figure 5b) with TTF-TCNQ complex film as bottom contact electrodes are shown in Figure 5. The 11248

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characteristics of representative OTFT devices showed excellent gate modulation along with current saturation. For comparison, we have fabricated devices with gold (Au) electrodes having the same channel length (L) and channel width (W). Transfer characteristics (IDS VG) of p- (pentacene) and ntype (F16CuPc) OTFTs with different electrodes are shown altogether in Figure 6 in logarithmic scale. Pentacene-based devices with Au (top and bottom contact) and TTF-TCNQ CT complex film (bottom contact) as source-drain electrodes exhibited p-type features (Figure 6a), the nature of which depended significantly on the types of electrodes. Similarly, in Figure 6b, we have compared the performances of n-type OTFTs with Au top-contact and TTF-TCNQ bottom-contact sourcedrain electrodes. The mobility (μ), threshold voltage (VTh), and other parameters of p- and n-type OTFTs, fabricated with different electrodes, were calculated by using standard formula in the saturation region and are summarized in Table 1. These results have a good reproducibility with a statistical figure of 67%. It is clear from the table that TTF-TCNQ based devices attain comparable performance or even better than that with Au (top)

Figure 5. Output characteristics of (a) pentacene TFT and (b) F16CuPc TFT based on solution deposited TTF-TCNQ complex film as bottom contact source-drain electrodes.

contact devices. The TTF-TCNQ contacted p-type OTFT realizes an on/off ratio of 104 and threshold voltage of 4 V. The corresponding quantities for the n-type OTFT are 104 and 13 V, respectively. The observed mobilities, 0.16 cm2 V 1 s 1 (for pentacene) and 1.19  10 3 cm2 V 1 s 1 (for F16CuPc), are almost comparable to the reported value for the room-temperature deposition of the organic thin films.25 27 Such a low threshold voltage as observed in the transfer characteristics of TTF-TCNQ contacted OTFT device should be quite advantageous for operations at low voltages. Furthermore, it can obviously be concluded from Figures 5 and 6 that solutiondeposited thin film of TTF-TCNQ complex is suitable for both types of carrier (electron and hole) injection enabling exhibit pand n-type OTFTs. The reason TTF-TCNQ contacted OTFTs attain better (or comparable) performance than Au-contacted

Figure 6. Transistor characteristics of p- and n-type OTFTs at |VDS| = 60 V with various electrodes. Transfer characteristics of (a) pentacene (p-type) TFT with bottom-contact TTF-TCNQ complex electrode (circles), bottom-contact Au electrodes (rectangles), and top-contact Au electrodes (triangles); and (b) F16CuPc (n-type) TFT with bottom-contact TTFTCNQ complex electrodes (solid circles) and top-contact Au electrodes (open circles).

Table 1. Performance of p- and n-Type OTFTs with Different Electrodes pentacene (p-type) VTh (V)

*

on/off 4

μ (cm V 2

1

TTF-TCNQ

4V

10

0.16

Au (top) Au (bottom)

10 V 22 V

104 103

0.09 0.0013

F16CuPc (n-type) 1

s )

*

s.s. (V)

VTh (V)

on/off

μ (cm2 V

1

5

13 V

10

1.19  10

6 14

18 V -

>103 -

7.4  10 -

4

s 1) 3 4

s.s. (V) 7 12 -

s.s. = [d log(IDS)/dVG] 1. 11249

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Langmuir OTFTs is that TTF-TCNQ organic crystals would have better contact with organic semiconductors owing to the organic organic interface. Furthermore, the lower work function (highest occupied molecular energy level) of TTF-TCNQ (4.64 4.78 eV)28 compared to that of Au (5.1 eV) permitted more carrier (electron) injection into the semiconductor from the TTF-TCNQ complex film resulting in better n-type performance.

4. CONCLUSIONS In summary, we have shown that high-performance organic thin film transistors could be fabricated with solution-grown TTF-TCNQ charge transfer complex film as bottom-contact source-drain electrodes. The electrodes were patterned by a novel method, where a capillary tube was used to make a channel between the source-drain electrodes. Both p- and n-type OTFTs contacted with organic complex film have been fabricated, which showed better device performance compared to Au-contacted OTFTs. Pentacene (p-type) OTFT with TTF-TCNQ sourcedrain electrodes exhibited mobility as high as 0.16 cm2 V 1 s 1 and a threshold voltage as low as 4 V. The high conductivity, low work function of the TTF-TCNQ complex film, and better contact of TTF-TCNQ crystals with organic semiconductors owing to the organic organic interface enables higher carrier injection from electrodes into the semiconductor resulting in high device performance. The results indicate the potential of TTF-TCNQ complex as organic metal contact suitable for the fabrication of high-performance and low-cost plastic electronic devices.

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’ AUTHOR INFORMATION Corresponding Author

*Tel: +91-03454-255032; Fax: +91-03454-255861. E-mail: [email protected].

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