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Solution-Processed Naphthalene Diimide Derivatives as n-Type Semiconductor Materials Ya-Lien Lee, Hui-Lin Hsu, Szu-Ying Chen, and Tri-Rung Yew* Department of Materials Science and Engineering, National Tsing-Hua UniVersity, 101, Sec. 2, Kuang-Fu Rd., Hsinchu, Taiwan 300 ReceiVed: August 5, 2007; In Final Form: NoVember 2, 2007

Two soluble 1,4,5,8-naphthalenetetracarboxylic diimide (NTCDI) derivatives with phenylmethyl and (trifluoromethyl)benzyl groups (NTCDI-P and NTCDI-F, respectively) were synthesized and used as semiconductor layers in organic thin-film transistors (OTFTs) by a spin-coating process in air. These two synthesized materials were characterized by 1H NMR and UV-vis spectra as well as mass analysis. The morphology and crystallinity of spin-coated NTCDI-P and NTCDI-F films have been inspected using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. The channel mobilities of these two NTCDI derivatives were calculated to be about 1.2 × 10-3 cm2 V-1 s-1 in air, which degraded slightly while stabilized for NTCDI-F OTFTs after exposure in air for 1 month.

Introduction Organic electronics such as TFTs, displays, and sensors have drawn a lot of attention because of the advantages of low cost for large area fabrication and flexible substrates.1-3 For complementary metal oxide semiconductor (CMOS) circuits, both p-channel and n-channel semiconductor materials are required. The p-type organic semiconductor materials have been widely studied. Relatively fewer n-type materials have also been reported, including C60 and its derivatives,4,5 oliothiophene and its derivatives,6 copper hexadecafluorophthalocyanine,7 naphthalene diimides, and perylene diimides.8-13 However, some of organic n-channel semiconductor materials cannot be operated in air due to electron trapping by oxygen and carrier injection issues.14 The approaches to achieve n-channel materials with air stability and high mobility have been reported by the incorporation with strong electron-withdrawing groups, such as -CN and -F.15-17 Currently, the highest mobility of n-type organic semiconductor is obtained from a thiazole oligomer derivative (1.83 cm2 V-1 s-1) measured in a vacuum environment18 or from a perylene diimide derivative (PDI-FCN2) with a mobility of 0.64 cm2 V-1 s-1 measured in air. However, both materials are deposited by vacuum thermal evaporation. Low-cost organic thin-film transistor (OTFT) applications require a solution process in place of vacuum thermal deposition. Compared to evaporation-deposited n-type semiconductor materials, relatively fewer studies of solution-processed semiconductors on n-type oligomers or polymers19-21 via dropcasting or spin-coating have been conducted due to their poor solubility in solvents and environment-sensitive properties. For solution-process technologies such as spin-coating or inject printing, process conditions affect the molecule’s crystal structure and thin-film morphology significantly. Besides, most of the solution-processed semiconductor materials exhibit an amorphous structure with presumingly lower mobility than thermally deposited ones.22,23 Moreover, the poor solubility of organic semiconductors in a given solvent increases further the * Corresponding author. Phone: 886-936347230. Fax: 886-3-5722366. E-mail: [email protected].

challenges of their solution process. Until now, the highest mobilities of solution-processed n-type semiconductors were achieved from quaterthiophene24 (0.2 cm2 V-1 s-1) measured in a vacuum and from naphthalene diimide (NTCDI) with a long-chain fluorinated alkyl group (0.01 cm2 V-1 s-1) solutionprocessed in air by Katz et al.8 However, this solution-processed n-type NTCDI thin film was deposited via drop-casting, which might impact film uniformity,8 and its stability after exposure in air for a period of time has not been reported yet. Here we report the synthesis of new n-type NTCDI derivatives with different side groups. Different from previous works,9 we focus on their solution process via spin-coating for OTFT applications. The air-stability test of OTFT devices is also conducted. According to previous research, the substituted NTCDI semiconductors show high-mobility and air-stability characteristics, while most of them are deposited by vacuum thermal deposition.8-9,25,26 In this work, two new n-channel NTCDI derivatives that could be solution-processed in air were investigated, including N-phenylmethyl (NTCDI-P) and N(trifluoromethyl)benzyl (NTCDI-F) groups shown in the insets of Figure 1a,b, respectively. Experimental Methods Material and Sample Preparation. 1,4,5,8-Naphthalenetetracarboxylic dianhyride, excess amine, and zinc acetate were heated in quinoline for several hours under N2 and then cooled to room temperature and poured into stirred methanol. The solution was filtered, which was followed by the wash with hot NaHCO3, water, and methanol sequentially. It was dried and further purified by column chromatography on silica gel (CHCl3) and then characterized by 1H NMR and mass analyses with the results shown as following. N,N′-Bis(phenylmethyl)naphthalene-1,4,5,8-tetracarboxylic diimide (NTCDI-P): white powder (yield, 75%); 1HNMR (CDCl3) δ (ppm) 8.74 (4H, s), 7.5 (4H, d), 7.3 (4H, t), 7.25 (2H), 3.7 (4H, s); [M+] calcd m/e 446.1267, found m/e 446.1263. N,N′-Bis(3-fluoro-5-(trifluoromethyl)benzyl)-1,4,5,8-naphthalene diimide (NTCDI-F): white powder (yield, 63%); 1HNMR

10.1021/jp076278w CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008

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Figure 1. UV-vis absorption and emission spectra of (a) NTCDI-P and (b) NTCDI-F in toluene solvent. Each condition was repeated with four different samples (n ) 4), which show similar results.

TABLE 1: Melting Points, Decomposition Temperatures, Optical Absorption Maxima (λabs), and Fluorescence Emission Maxima (λf) for Various NTCDI Derivatives λabs (nm)

λf (nm)

calcd band gap

compd

mp (°C)

dec temp (°C)

solna

filmb

Ega (eV)

Egb (eV)

solna

filmb

NTCDI-P NTCDI-F

275-277 296-298

288 310

363, 383 367, 386

385 389

3.23 3.21

3.22 3.18

486 487

423 430

a 10-5 M NTCDI derivatives in toluene. b 2 mg/mL NTCDI/toluene drop-cast on glass substrate. All spectra were taken at room temperature with similar results repeated from four different samples for each data point (n ) 4).

(CDCl3) δ (ppm) 8.79 (4H, s), 7.59 (2H, s), 7.44 (2H, d), 7.21 (2H, d), 5.39 (4H, s); [M+] calcd m/e 618.0826, found m/e 618.0679. Optical and Fluorescence Spectra Measurement. All NTCDI derivatives were dissolved in toluene and then cast on a glass substrate. Optical absorption spectra were measured by Hitachi U-2010/3010 UV-vis spectrometers. Emission spectra were obtained by using Hitachi F-4500 fluorescence spectrometers at room temperature. Thin-Film Characterization. Atomic force microscopy (AFM) images of NTCDI derivative thin films were measured by using a Veeco DI-3100 AFM in tapping mode. The XRD measurement was carried out using an XRD 6000 Shimaru with a beam wavelength of 1.5406 Å and device operated at 30 keV and 20 mA. The glancing-angle XRD (GAXRD) was carried out using a Japan MAC Science MXP18 with a beam wavelength of 1.5406 Å with the device operated at 40 keV and 150 mA. Transistor Fabrication. The doped n+-Si wafer (resistivity: 5-25 Ω cm) was used as a back-gate electrode and substrate. The thermally grown SiO2 (200 nm, capacitance/unit area Ci ) 17.2 nF/cm2) was used as a gate dielectric layer for OTFT devices. For hexamethydisilazane (HMDS) pretreatment, the chemical obtained from Arcos Chemical Co. was spin-coated at 4000 rpm for 30 s on SiO2 or PVP followed by baking at 85 °C for 30 min. NTCDI derivatives dissolved in toluene (concentration: 5-10 mg/mL) were spin-coated (1000 rpm for 30 s) at different solution and substrate temperatures and then baked at 80-85 °C for 20 min. Gold pads as source and drain electrodes with a channel length (L) and width (W) of 100 µm and 1000 µm, respectively, were deposited through shadow mask. Electrical characteristics of OTFT devices were measured in air under the accumulation mode using an Agilent 4155 semiconductor parameter analyzer. Results and Discussion Optical and Fluorescence Properties. Ultraviolet-visible (UV-vis) optical absorption and photoluminescence (PL) emission spectra of all NTCDI derivatives were examined in solution and as thin films with the results shown in Figure 1

and Table 1 (similar results repeated from four different samples for each data point, n ) 4). The UV-vis/PL peaks are redshifted from NTCDI-P to NTCDI-F. The highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps of NTCDI-P and NTCDI-F films calculated from the end-absorption maxima in UV-vis spectra are 3.22 and 3.18 eV, respectively, which are all smaller than that of unsubstituted NTCDA (3.3 eV).27 The largest red-shift for NTCDI-F reflects that the highly electronegative F atom influences the naphthalene core in NTCDI and depresses the electronic energy level more than others, which is consistent with those reported earlier.11,28 Morphological Studies. The solution and substrate temperatures, surface pretreatment, and baking temperature were found to be the key parameters to determine the film uniformity, surface morphology, and electrical properties of NTCDI derivatives. The NTCDI derivatives are hard to be spin-coated onto the SiO2 surface without HMDS pretreatment. It is believed that HMDS can modify the SiO2 from hydrophilic to hydrophobic so as to enhance the adhesion of NTCDI derivatives onto SiO2 for nucleation attributed to the bonding between OH- on SiO2 and -NH in HMDS.29 As no homogeneous film could be coated at a baking temperature lower than 50 °C, the coated film was subjected to a baking process at 80-85 °C for 20 min. In addition, the solution and substrate temperatures were found to play an important role in determining film quality. Figure 2a-f shows AFM images of NTCDI-derivative films processed using solution temperatures at 40, 80, and 110 °C (similar results repeated from three different samples for each temperature, n ) 3), with the SiO2/Si substrate kept at the same temperature as that of solution during spin-coating. The NTCDI derivatives coated at a solution and substrate temperature (Ts) of 40 °C show dendrite and smaller grain structures. Increasing Ts to 80 °C can enlarge their grain sizes. However, they exhibit rougher or less continuous surface morphologies at the Ts of 110 °C. Possible reasons to explain films with smaller grain size or degradation in uniformity at the Ts of 40 and 110 °C are as following. To form homogeneous thin films with lager grains, it requires a sufficient energy that provides solute mobility for grain growth. Besides, it needs adequate time for the solute to

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Figure 2. AFM topographies of various NTCDI derivatives spin-coated onto HMDS-treated SiO2/Si at different solution/substrate temperatures: (a) NTCDI-P, 40 °C; (b) NTCDI-P, 80 °C; (c) NTCDI-P, 110 °C; (d) NTCDI-F, 40 °C; (e) NTCDI-F, 80 °C; (f) NTCDI-F, 110 °C. Each condition was repeated with three different samples (n ) 3), which show similar results.

Figure 3. XRD patterns (bottom) and GAXRD diffraction patterns (top) for the (a) powder (bottom) and thin-film (top) of NTCDI-P and (b) powder (bottom) and thin-film (top) of NTCDI-F. Each measurement was repeated with four different samples (n ) 4), which show similar results.

move so as to increase grain size. Increasing Ts (such as 110 °C) can provide sufficient energy and enhance the grain growth, while it may also speed up the solvent evaporation (boiling temperature of toluene solvent: 110 °C) and hence reduce the time for solute to move.30,31 Consequently, a proper Ts (such as 80 °C in this work) may provide sufficient energy and time, because of proper evaporation rate, for grain growth. In addition to the process temperatures, the types of side groups in NTCDI derivatives also can determine the surface morphologies as shown in Figure 2. The NTCDI-P films with benzyl groups exhibit ball-like grains (size: 500-650 nm), and NTCDI-F with fluorinated groups reveals a small needlelike structure (300-400 nm in length). It has been reported that NTCDI-P and NTCDI-F with phenyl-substituted groups exhibit high intramolecules steric hindrance. In addition, the NTCDI-F with strong electronegative F atoms at the end-groups shows more electrostatic repulsion between molecules.32 Above may explain why it is difficult to form large-grain thin films for NTCDI-P and NTCDI-F at a Ts lower than 110 °C. Microstructure Studies. X-ray diffraction (XRD) patterns of NTCDI-derivative powders and their thin films coated on HMDS-treated SiO2/Si at an optimized Ts (80 °C) are shown in Figure 3, with similar results repeated from four different samples for each plot (n ) 4). From the diffraction patterns of NTCDI-P and NTCDI-F powders in Figure 3a,b, it can be

observed that both NTCDI derivatives exhibit triclinic/monoclinic packing characteristics. This is consistent with the singlecrystal structure of similar NTCDI derivatives published earlier with π-stacking in a triclinic or monoclinic lattice.33,34 Besides, the glancing-angle XRD (GAXRD) patterns of NTCDI-P and NTCDI-F thin films (∼100 nm) in Figure 3a,b, respectively, all exhibit polycrystalline structures. For NTCDI-P, the packinglattice spacing (∼1.1 nm) from XRD diffractions is very close to their respective lengths calculated along the molecular long axis (∼1.4 nm). The shorter packing length indicates that the molecular packing axis is slightly tilted with respect to the substrate surface.35 From Figure 3b, it can be observed that the lattice spacing of the fluorinated compound (NTCDI-F) is about 4.8 Å, which is similar to the short axis of the calculated molecular length (∼4.9 Å), indicating that the fluorinated molecules are laid flat onto the substrate. In a comparison of these two substitutions, the NTCDI-P molecules show an edgeon arrangement and the NTCDI-F molecules exhibit a face-on arrangement. It seems that the branched substitutions of NTCDI-F change the planarity of the aromatic ring and lead to side-by-side molecular stacking.36,37 Transistor Characteristics. The NTCDI derivatives as semiconductor layers for OTFT fabrication were also investigated, with a channel width and length of 1000 and 100 µm, respectively. The spin-coated NTCDI-derivative films used for OTFT fabrications were about 100 nm in thickness, confirmed by an AFM height-scan measurement. The OTFT output characteristics (drain-current versus drain-to-source voltage, IdVd) and transfer characteristics (drain-current versus gate-tosource voltage, Id-Vg) of NTCDI derivatives measured in air with the source grounded are shown in Figure 4 (similar results repeated from four different samples, with four devices for each sample), with the insets showing their corresponding OTFT devices in cross section. The OTFTs with fluorinated compound (NTCDI-F) as a semiconductor layer show comparable mobility and Ion/Ioff ratio (1.2 × 10-3 cm2 V-1 s-1 and 102, respectively) with a threshold voltage (Vt) of 29 V relative to those of NTCDI-P with phenyl substitution (1.2 × 10-3 cm2 V-1 s-1 and 50-60, respectively) with a Vt of 31 V. Though there might be a contact effect, however, the trace flattening of the NTCDI-F device is mostly from traditional channel saturation behavior

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Figure 4. Electrical characteristics for OTFTs: (a) Id-Vd, (b) Id-Vg, and (c) (Id)1/2-Vg for as-coated NTCDI-F; (d) Id-Vd, (e) Id-Vg, and (f) (Id)1/2-Vg of as-coated NTCDI-P. NTCDI-F or NTCDI-P was used as a semiconductor layer coated at a solution and substrate temperature of 80 °C and channel width (1000 µm)/length (100 µm) ) 10. Each measurement was repeated from four different samples, with four different devices for each sample, which show similar results.

as Id-Vg curves at Vg > Vt do not deviate seriously from linearity,38 which is shown in Figure 4b,e for NTCDI-F and NTCDI-P, respectively. The mobilities were calculated on the basis of (Idsat)1/2 ) [Wµ/2(C/L)]1/2(Vg - Vt), where Idsat, C, Vg, and Vt represent the saturation current (Idsat ) ∼10 × 10-9 A at Vg ) 40 V and Vd ) 20 V), gate dielectric capacitance (17.2 nF/cm2), gate-to-source voltage () 40 V), and extrapolated threshold voltage (Vt), which can be calculated from a plot of (Idsat)1/2 vs Vg as shown in Figure 4c,f. The NTCDI-F semiconductors were also spin-coated onto an HMDS-treated poly(vinylphenol) (PVP) gate-dielectric layer in air for OTFT fabrication with patterned Au as a bottom-gate. Transistor characteristics were also measured in air, showing a mobility of about 4 × 10-4 cm2 V-1 s-1, Ion/Ioff ratio of 102, and Vt of 15 V using the same equation as above and Vg ) 20 V and Vd ) 20 V. However, the OTFT characteristics of the above devices show higher leakage current and drain offset which are

most likely contributed from nonpatterned bottom-gate or nonpatterned NTCDI-derivative semiconducting layers.39 A device with the smaller channel length or patterned semiconductor layer needs to be fabricated to further reduce the leakage current. The air stability of NTCDI-P and NTCDI-F was also tested by exposing their OTFT devices to air. The OTFTs with NTCDI-F semiconductor layers show a slight degradation on mobility while stabilizing after 7 days and a further 1 month storage in air as shown in Figure 5, while those with NTCDI-P show significant degradation in mobility. This indicates that NTCDI-F exhibits a certain degree of air stability, mostly contributed by the electron withdrawing of fluorinated substitution, which makes it less susceptible to oxidation as reported.28 Though the device performances of NTCDI derivatives developed are not as good as those reported by Katz et al.8,9 via dropcasting using R,R,R-trifluorotoluene as a solvent, we have

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Figure 5. (a) Id-Vd characteristics of OTFTs after 7 days of storage in air using NTCDI-F as a semiconductor layer coated at a solution and substrate temperature of 80 °C and channel width (1000 µm)/length (100 µm) ) 10. (b) Air-stability test of NTCDI-F OTFTs on mobility change. Each measurement was repeated from four different samples, with four different devices for each sample, which show similar results.

demonstrated the feasibility of using a new solution-processed NTCDI-F as an n-type semiconductor layer by a spin-coating process to fabricate OTFT devices in air with a certain degree of air stability. Conclusion In summary, the feasibility of using two new solutionprocessed NTCDI derivatives, NTCDI-P and NTCDI-F, as semiconductor materials via a spin-coating process in air for their application on OTFTs with SiO2 or PVP gate dielectrics has been investigated. The band gaps were measured to be 3.22 and 3.18 eV for NTCDI-P and NTCDI-F, respectively, from the UV-vis optical absorption spectra. The AFM morphologies show that the HMDS surface pretreatment, a proper baking temperature (80-85 °C), and an appropriate solution and substrate temperature (80 °C) are required to produce homogeneous thin films of NTCDI derivatives on SiO2 and PVP using toluene as a solvent. The NTCDI-P films show ball-like grains (size: 500-650 nm) and the NTCDI-F films reveal needlelike grains (size: 300-400 nm). The layer-to-layer spacing was estimated by using XRD analyses, showing that NTCDI-P packed along the molecular long axis and NTCDI-F laid onto the substrate along a molecular short axis. Transistor characteristics of OTFTs using NTCDI derivatives as semiconductor layers have been achieved, with a mobility of about 1.2 × 10-3 cm2 V-1 s-1 measured in air for NTCDI-F and NTCDI-P OTFTs. The air-stability test of NTCDI-F OTFTs shows slight mobility degradation while stabilizing after exposure to air for 1 month. However, the performance of OTFT devices needs further improvement via the process optimization of spin-coating and device integration including the fabrication of patterned or small devices for future applications in flexible integrated circuits (ICs). Acknowledgment. We acknowledge financial support from the ITRI-EOL under Project Nos. 95A023J4 and 96A0233J4. We also thank Prof. Jeng-Hua Wei for very valuable discussion on devices and Yu-Jung Peng for PVP thin-film coating information. References and Notes (1) Forrest, S. R. Nature 2004, 428, 911. (2) Dimitrakopoulos, C. D.; Malenfant, P. AdV. Mater. 2002, 14, 99. (3) Reese, C.; Roberts, M.; Ling, M.-M.; Bao, Z. Mater. Today 2004, Sep., 20.

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