Energy-Level Engineering in Self-Contact Organic Transistors

Article pubs.acs.org/JPCC

Energy-Level Engineering in Self-Contact Organic Transistors Prepared by Inkjet Printing Tomofumi Kadoya,† Sumika Tamura,† and Takehiko Mori*,†,‡ †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama 2-12-1, Tokyo, 152-8552, Japan ACT-C, JST, Honcho, Kawaguchi, 332-0012, Japan

‡

ABSTRACT: Organic field-effect transistors with chemically doped source/drain electrodes are fabricated by selectively transforming the active layer to the conducting material, organic charge-transfer (CT) complex, using inkjet printing. The formation of the CT complex is investigated by X-ray diffraction and Raman spectroscopy, and the resulting CT complex is conducting enough to realize satisfactory transistor performance. The solvent of the ink is a key factor to achieve the optimal chemical doping to form a well-ordered CT complex. In such a manner, we can obtain semiconductor-specific electrodes that significantly reduce the Schottky barrier at the electrode/semiconductor interface, because ideal energy-level engineering is automatically attained between the same type of molecules.

1. INTRODUCTION Chemical doping is a conventional tool to construct a conducting part in silicon technology but usually not used in organic devices. In general, p-channel and n-channel organic semiconductors are regarded as an electron donor and an electron acceptor, respectively, and these molecules potentially constitute conducting CT complexes with various counterions. However, organic semiconductors are basically used as an intrinsic form, namely, as a single-component material, and this is one of the most important characteristics of organic electronic devices. This is partly because representative organic semiconductors do not form stable and highly conducting charge-transfer (CT) complexes. In organic transistors, active layers are also composed of single-component organic semiconductors, whereas the performance is not solely governed by the inherent properties of the organic semiconductors, but largely influenced by the contact effect between the metal electrode and the organic active layer. In order to reduce the contact resistance, there are several remedies such as insertion of a buffer layer1−5 and thiol treatment of the Au electrode.6,7 In this connection, conducting organic CT complexes have been used as the electrode material and proved to significantly reduce the contact resistance.8−12 Recently, solution-processable silver and gold nanoparticles have been used widely, and the channel length as small as 4.5 μm has been achieved by the use of laser ablation and sintering.13−16 Analogously, carbon electrodes fabricated by laser sintering have achieved low contact resistance together with high resolution and transparency.17 Since laser sintering converts many organic compounds to conducting carbon, we have constructed organic transistors by selectively transforming thin films of organic semiconductors to conducting carbon by laser irradiation.18 Such devices are called “self-contact” organic © 2014 American Chemical Society

transistors, because the contact part originates in the organic semiconductor. This is an ultimately easy way to fabricate organic transistors because the active layer and the source/drain (S/D) contacts are both made from a single organic film. However, this method is not applicable to a flexible organic substrate because the laser process damages the substrate. We have recently developed another type of self-contact organic transistors by selectively transforming tetramethyltetrathiafulvalene (TMTTF) to a conducting CT complex by evaporating 7,7,8,8-tetracyano-p-quinodimethane (TCNQ).19 Here TMTTF works as an excellent p-channel organic semiconductor, whereas (TMTTF)(TCNQ) is a highly conducting organic metal. By using (TMTTF)(TCNQ) as the electrode, smooth carrier injection is realized from the electrode to the active layer TMTTF. In the present paper, self-contact organic transistors are fabricated using a solution process, namely, inkjet printing a solution of counterions. Inkjet printing has attracted considerable attention as a low-cost, large area, and highly selective patterning tool for organic electronics.20−26 In addition to our previous p-channel transistors, we have achieved n-channel transistors. Figure 1 illustrates component molecules and fabrication of self-contact organic transistors using inkjet printing. Here, dimethyldicyanoquinonediimine (DMDCNQI) is used as a n-channel material27,28 and hexamethylenetetrathiafulvalene (HMTTF) as a p-channel organic semiconductor.29,30,32 First, an organic semiconductor film of DMDCNQI or HMTTF is vacuum evaporated on the substrate. Then a CuI or TCNQ solution is selectively printed on the semiconductor Received: July 16, 2014 Revised: August 27, 2014 Published: September 10, 2014 23139

dx.doi.org/10.1021/jp5070819 | J. Phys. Chem. C 2014, 118, 23139−23146

The Journal of Physical Chemistry C

Article

Figure 1. (a) Structures of organic semiconductors and organic charge-transfer complexes. (b) Illustration of self-contact organic transistors fabricated by inkjet printing.

Figure 2. (a) CuI solution dots ejected from the inkjet head. (b) Optical image of a printed Cu(DMDCNQI)2 electrodes (black regions) on a yellow DMDCNQI film. (c) Expanded view of the device region.

Figure 3. (a) MAXRD pattern and (b) AFM image of the DMDCNQI film. (c) MAXRD pattern and (d) AFM image of the Cu(DMDCNQI)2 region after printing a 1 wt % CuI solution of 1:1 AN/DMSO three times. The insets of (a) and (c) show the optical images of the sample, where the bright spots are X-ray irradiated regions.

film. DMDCNQI is reduced by a CuI solution to the highly conducting complex, Cu(DMDCNQI)2, according to the following reaction.33

Advantages of these self-contact transistors are discussed from the viewpoint of the energy-level engineering.

2. EXPERIMENTAL SECTION 2.1. DMDCNQI/Cu(DMDCNQI)2 Transistors. Transistors were fabricated onto an n-doped Si substrate with a thermally grown SiO2 dielectric layer (300 nm, C = 13 nF cm−2). DMDCNQI was purified by sublimation.27 After octadecyltrimethoxysilane treatment,35 DMDCNQI was vacuum evapo-

2DMDCNQI + CuI → Cu(DMDCNQI)2 + 1/2I 2

HMTTF is also converted to the TCNQ complex.32,34 After the conducting CT complexes are formed, these parts are used as the S/D electrodes in an organic transistor (Figure 2). 23140



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rated under the vacuum of 10−3 Pa.28 The film thickness of DMDCNQI was about 500−600 nm. A 1 wt % CuI solution of dehydrated acetonitrile (AN)/dimethyl sulfoxide (DMSO; volume ratio = 1:1 to 1:2) was ejected on the DMDCNQI film with a speed of ∼4 m s−1 (Figure 2a) using a piezoelectric inkjet printer head (Microjet JHB-1000). In the typical pattering method, the dots were successively deposited with an interval of 100 μm. After 15 min, the CuI printing was repeated in the reverse direction. The electrode pattern had a width of about 150 μm. The channel width W = 950−1000 μm and length L = 100−200 μm were measured by an optical microscope (Figure 2c). Channel length was defined at the minimum; this method does not overestimate the mobility. The remaining solvent and iodine were removed by evacuation in a desiccator (≅1 Pa) over a period of 12 h. The transistor characteristics were measured at first in vacuum (10−3 Pa), then in the ambient conditions by using a Keithley 4200 semiconductor parameter analyzer. The mobility was evaluated in the saturated region. Sheet resistance of the resulting selfcontact region is measured by the two-probe method. Micro area X-ray diffraction (MAXRD) measurements of the thin films were carried out by Bruker AXS D8 DISCOVER μHR. Atomic force microscopy (AFM) images were obtained with a Seiko Instruments SPA-300 and SPI3800 probe system by using an Si3N4 cantilever. Electrode thickness profiles were measured by KOSAKA ET-200. Scannig electron microscopy (SEM) images were taken with a HITACHI FE-SEM S-4700. 2.2. HMTTF/(HMTTF)(TCNQ) Transistors. HMTTF transistors were prepared similarly to the DMDCNQI transistors. As for the surface treatment, polystylene (ε = 2.5 and 100 nm) in toluene was spin-coated, where the resulting total capacitance of the gate dielectrics was 9.0 nF cm−2.19 HMTTF was purified by sublimation and vacuum evaporated up to the thickness of about 200 nm under 10−3 Pa. A 0.5 wt % TCNQ solution in 1:1 AN and DMSO was ejected repeatedly as following. The initial dots were deposited with an interval of 100 μm, and then these dots are interconnected by the second deposition (W = 700−800 μm and L = 100−200 μm), then annealed at 60 °C for 12 h in vacuum (10−3 Pa). As another way, a 0.5 wt % TCNQ solution in 1:2 chlorobenzene (CB) and DMSO was used. Then, the substrate was dried in vacuum (≅1 Pa) over the period of 12 h. Raman spectra were recorded on a JASCO NRS-2100 spectrometer. For comparison, ordinary Au-top-contact transistors were fabricated. The resulting transistor characteristics were measured similarly to the DMDCNQI transistors.

The solvent of the ink is important for patterning the CT complex. It has been known that the high boiling point and large surface energy are essential for the ink solvent.38 In the case of the DMDCNQI transistors, when CuI is dissolved in pure AN, the ejected droplets spread widely on the substrate, and the patterning is not successful. On the other hand, when the ratio of DMSO is too large, the reaction is very sluggish, and conductivity of the electrode is low. The optimal ratio is AN/DMSO = 1:1 to 1:2. DMDCNQI readily reacts with CuI to form the complex, and we can use the device after drying the solvent. At the same time, the formed iodine is removed completely. The rapid reaction is partly because DMDCNQI dissolves in the inkjet solvent, AN. (HMTTF)(TCNQ) does not show clear XRD peaks, but the formation of the CT complex is confirmed by Raman spectroscopy.31 Figure 4 shows Raman spectra of HMTTF

Figure 4. Raman spectra of an HMTTF self-contact transistor. The red curve corresponds to the neutral HMTTF film and the blue curve is from the (HMTTF)(TCNQ) region. The insets show the optical images of the HMTTF and (HMTTF)(TCNQ) regions, where the bright spot of the former is the laser irradiated region.

and (HMTTF)(TCNQ) films. The neutral HMTTF film shows a ν3 CC stretching mode at 1541 cm−1.39 After inkjet printing, this peak shifts to a lower wavenumber owing to the complex formation. As a result, new peaks appear at 1459 and 1551 cm−1, corresponding to the ν3 and ν2 modes of the CC stretching of HMTTF, as well as at 1413 and 1606 cm−1 according to the ν4 and ν3 modes of the CC stretching of TCNQ in (HMTTF)(TCNQ).39 Figure 5a demonstrates that the sheet resistance Rs decreases with increasing the printing times N. Height profiles are shown in Figure 5b,c. After repeated printing, the plateaus of the electrode parts become gradually clear. The initial printing leads to unlinked regions, but additional printing improves the connectivity within the electrode. The reaction of HMTTF does not proceed smoothly owing to the low solubility in the inkjet solvent, to which the relatively high Rs and no XRD peaks are attributed. In this case, annealing is necessary to attain the sufficient complex formation. 3.2. Transistor Characteristics. Transistor characteristics are shown in Figure 6 and Table 1. The self-contact transistors show as high performance as the ordinary Au-top-contact transistors. In particular, the printed HMTTF self-contact transistors realize excellent mobilities μ around 1 cm2 V−1 s−1 together with the small threshold voltage, Vth, and the small hysteresis. It is noteworthy that these transistors exhibit

3. RESULTS AND DISCUSSION 3.1. Thin Film Properties. Micro area X-ray diffraction (MAXRD) patterns are shown in Figure 3a. The observed dvalue of 8.1 Å corresponds to half the crystallographic a axis of the DMDCNQI single crystal.36 The DMDCNQI molecular planes are standing perpendicular to the substrate and forming a two-dimensional brickwork structure. The atomic force microscopy (AFM) images in Figure 3b show block-like grains of the DMDCNQI film. In the electrode region (Figure 3c), additional peaks are observed in the MAXRD; the d-value of 10.8 Å corresponds to the crystallographic a axis of the singlecrystal Cu(DMDCNQI)2.37 In Figure 3d, the surface of Cu(DMDCNQI)2 is not so smooth, and needle-like crystals are observed. From these results, formation of Cu(DMDCNQI)2 on the DMDCNQI film is confirmed. 23141



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Figure 5. (a) Sheet resistance of electrodes as a function of printing times. The red circles for Cu(DMDCNQI)2, blue circles for (HMTTF)(TCNQ) from AN/DMSO, blue squares for (HMTTF)(TCNQ) from CB/DMSO, and the orange circle for an Au film. Height profiles of (b) Cu(DMDCNQI)2 and (c) (HMTTF)(TCNQ) films after the repeated inkjet printing.

Figure 6. Output and transfer characteristics of self-contact organic transistors measured in vacuum: (a) DMDCNQI and (b) HMTTF.

excellent performance even in air similarly to in vacuum (Table 1). Figure 7 shows contact resistance and scanning electron microscopy (SEM) images at the electrode/semiconductor interface. Although the electrode edges are not perfectly straight, the L values are sufficiently determined to carry out the transfer-line method. For DMDCNQI, the obtained contact resistance, 235 kΩ cm, is larger than 130 kΩ cm of similarly prepared Au-top-contact transistors. A small gap is potentially formed between the CT complex and the active layer. The (HMTTF)(TCNQ)/HMTTF interface is more smoothly

connected, and the resulting contact resistance 6 kΩ cm is in the same order as 3 kΩ cm in the Au top-contact transistors (Table 1). In the case of HMTTF, the chemical reaction is confined in the upper surface of the semiconductor film owing to the comparatively sluggish complex formation, and the real top-contact geometry is achieved. Then the carrier injection occurs in the longitudinal direction from the electrode to the semiconductor, and the current flows in the laterally connected organic semiconductor. In contrast, the rapid reaction of DMDCNQI with CuI results in the complex formation penetrating down to the deep region and leads to the relatively 23142



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Table 1. Transistor Characteristics of DMDCNQI and HMTTF Transistors active layer

solventa

contact

DMDCNQI

Au TC Cu(DMDCNQI)2 Au TC

AN/DMSO (N = 2)

HMTTF

(HMTTF)(TCNQ)

AN/DMSO (N = 3) AN/DMSO (N = 4) CB/DMSO (N = 2)

a

condition vacuum air vacuum vacuum air vacuum air vacuum vacuum air

μ (cm2 V−1 s−1) 0.045−0.1 0.043−0.1 0.048−0.078 0.75−0.94 0.82−1.76 0.46−1.0 0.6−1.6 0.8−1.1 0.8−0.9 1.2−1.9

RCWa,b (kΩ cm)

Vth (V)

on/off

16−28 12−41 4−25 −7−0 −4−3 −11−0 −9−8 −14 − −12 −6−3 −2−8

89−400 × 10 7−200 × 103 40−350 × 103 4−9.4 × 103 0.2−1.6 × 103 0.7−1.4 × 103 0.1−0.5 × 103 0.4−0.5 × 103 0.3−1 × 103 0.2−0.3 × 103 3

130 (60 V) 235 (60 V) 3 (−80 V) 6 (−80 V) 7 (−80 V) 28 (−80 V, N = 4)

N: Printing times. bVG in the parentheses.

Figure 7. (a) Estimation of the contact resistance by the transfer-line method, and (b) a SEM image of the electrode/semiconductor interface region in a DMDCNQI self-contact transistor. (c) Transfer-line method and a SEM image for HMTTF self-contact transistors from (d) CB/DMSO and (e) AN/DMSO. The insets of (a) and (c) show the optical images of the device regions.

poor lateral connectivity across the semiconductor/electrode interface regions (Figure 7b). The reaction rate is also seriously influenced by the solubility of the organic semiconductor in the inkjet solvent. The rapid reaction of DMDCNQI is associated with the good solubility of DMDCNQI in AN, whereas the slow reaction of HMTTF is related to the poor solubility in AN and DMSO. HMTTF shows better solubility in CB than AN,

so that the combination of CB/DMSO shows larger contact resistance than AN/DMSO (Table 1). This is related to the better morphological connection between the electrode and the semiconductor (Figure 7e). The dopant (TCNQ), however, has to show a good solubility in the inkjet solvent. An appropriately slow reaction rate is preferable for realizing the well-defined top-contact geometry. 23143



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Figure 8. Energy-level engineering in self-contact (a) DMDCNQI and (b) HMTTF transistors.

3.3. Energy-Level Engineering. Energy-level matching between the inorganic-metal electrode and the organic semiconductor is not simple because the interfacial potential usually makes the Schottky barrier larger than the naive energy level difference.40 However, the present method guarantees that the Fermi level of the electrode appears close to the energy level of the active layer, because the electrode and the active layer are composed of the same molecule. In addition, the interfacial polarization is unimportant in the organic/organic interface.12,42 After chemical doping, the CT complex has a partially filled energy band. Cu(DMDCNQI)2 is an anionradical salt, where the electron transport is maintained by the organic acceptor. Since Cu is Cu4/3+ and DMDCNQI has 2/3− charge,41,42 the LUMO level of DMDCNQI is one-third filled. When the band center of the Cu(DMDCNQI)2 band is located at the same position as the original LUMO level of DMDCNQI (4.65 eV), after the band formation with the bandwidth of 0.8 eV,41,42 the Fermi level is pushed down by approximately 0.1 eV from the original LUMO level. A similar interpretation is applicable to the cation-radical salts. On the other hand, (HMTTF)(TCNQ) is a donor−acceptor complex in which the energy bands come from both the donor and the acceptor molecules. The bandwidths of HMTTF and TCNQ are 0.3 and 0.4 eV, respectively.43 Upon band formation, the Fermi level is expected to be located in between the HOMO (4.60 eV) of HMTTF and LUMO (4.58 eV) of TCNQ.44−47 As a consequence, the Fermi level is located very close to the energy level of the active layer (Figure 8). In contrast, the energy level of the standard electrode material, Au, appears around 5.1 eV.48 Electrical conductivity of the bulk CT complex is considerably lower than that of Au and this is responsible for the comparatively high sheet resistance (Figure 5a). Nonetheless, the contact resistance and the mobility are nearly the same in the self-contact transistors. These results indicate that efficient carrier injection is achieved due to approximately the same energy levels. The present “self-contact” technique is a method that automatically guarantees a close energy-level alignment at the electrode/semiconductor interface, and provides an ideal semiconductor-specific electrode. Among the chemically doped polymers, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) PEDOT/PSS has been used widely, but unfortunately, PEDOT does not work as a good organic semiconductor. When a single organic molecule forms the active layer of an organic device and a highly conducting and stable CT complex working as the electrode parts at the same time, we can construct self-contact transistors. As far as these requirements are fulfilled, the present selfcontact technique is widely applicable in other materials, even in solution-processed polymers. This is a promising strategy to

use the method of conventional silicon technology in organic electronics.

4. CONCLUSION In conclusion, a pattern of a conducting CT complex is formed on an organic semiconductor film by selective chemical doping, using inkjet printing. In addition to the HMTTF-based pchannel transistors, we have realized n-channel self-contact transistors by using DMDCNQI. So fabricated n- and pchannel self-contact transistors maintain comparable performance to the ordinary metal-electrode transistors. In the printing process, choice of the inkjet solvent is important. The solvent should be a good solvent of the dopant but a poor solvent of the active layer, because once the active layer dissolves, the rapid reaction results in a disordered electrode/semiconductor interface. By the chemical doping, we can obtain the semiconductor-specific electrode, whose Fermi level is inherently located very close to the energy level of the active layer. Such energy-level engineering realizes smooth charge carrier injection and high-performance organic transistors without using inorganic metal electrodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Tokyo Institute of Technology, Center for Advanced Materials Analysis for the XRD, SEM, and Raman measurements. This work was partly supported by a Grant-in Aid for Scientific Research (B) (No. 23350061) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. T.K. is grateful to JSPS for a research fellowship.

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Energy-Level Engineering in Self-Contact Organic Transistors

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