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Investigation of organic n-type field-effect transistor performance on the polymeric gate dielectrics
Synthetic Metals 160 (2010) 504–509
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Investigation of organic n-type field-effect transistor performance on the polymeric gate dielectrics Moumita Mukherjee, Biswanath Mukherjee, Youngill Choi, Kyoseung Sim, Junghwan Do, Seungmoon Pyo ∗ Department of Chemistry, Konkuk University, 1 Hwayang-dong, Kwangjin-gu, Seoul 143701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 October 2009 Received in revised form 26 November 2009 Accepted 30 November 2009 Available online 12 January 2010 Keywords: Organic field-effect transistors Gate dielectric Surface properties Organic semiconductor
a b s t r a c t We report a copper hexadecafluorophthalocyanine (F16 CuPc) based n-type organic field-effect transistor (OFET) with polymeric gate dielectrics with different physical/electrical properties. The gate dielectrics are four types of cross-linked poly(4-vinylphenol) and newly prepared poly(4-phenoxy methyl styrene) and those are characterized based on surface tension, leakage current and capacitance. The performance of F16 CuPc OFETs with those gate dielectrics was compared. We found that the composition of the gate dielectrics and the interfacial interaction of F16 CuPc with the gate dielectric play a decisive role in the performance of OFETs. The effect of physical/electrical properties, composition and processing condition of the gate dielectrics on the device performance was investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Considerable progress has been made in the recent years on the performance of organic field-effect transistors (OFETs) for lowcost, large-area electronic device applications, particularly those that are compatible with flexible plastic substrate [1,2]. OFETs have many unique advantages such as light weight, low-cost fabrication, mechanical flexibility, solution processability, and facile integration with electronic components [3–5]. For those organic electronic device applications, the development of important component materials such as polymeric gate dielectric and organic semiconductor are strongly required. Many polymeric gate dielectrics have been used for OFETs including poly(4-vinylphenol) [6,7], polyimide [8,9], poly(methylmethacrylate) [10], poly(vinylalcohol) [11,12], polystyrene [13,14]. Among the polymer dielectrics reported to date, the most investigated is cross-linked poly(4-vinylphenol) (PVP) due to its good film formability and electrical properties although it requires high processing temperature [15]. Organic semiconductor have already been demonstrated to possess remarkable semiconducting properties and its development is part of the main stream for progress in organic electronic devices such as solar cells, optical limiters, photoconductors and charge transporting layers in OFETs [16–20]. Compared with well-developed p-type organic semiconductors, advances in the development of
∗ Corresponding author. E-mail address: [email protected] (S. Pyo). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.11.039
n-type organic semiconductors are still in the early stage, since most of the n-type materials are unstable in air, which induces the deterioration of the electrical characteristics [21,22]. Bao et al. reported air-stable n-type OFETs based on copper hexadecafluorophthalocyanine (F16 CuPc) [23]. The fluorines introduced to metallophthalocyanines increase the electron affinity of the molecules and favor an efficient electron injection into the empty LUMO states. However, most of the OFETs with F16 CuPc are mainly fabricated with inorganic gate insulators such as SiO2 and Ta2 O5 [24–26]. Very recently, there have been reports of F16 CuPc OFETs based on chemically/mechanically stable polyimide blend gate dielectric [27] and thin cross-linked polymer gate dielectric having low operation voltage of less than 1 V and moderate charge carrier mobility [28]. In the present work, five-types of polymeric gate dielectrics was prepared for the fabrication of n-type OFETs: PVP based dielectrics prepared from PVP precursor solutions having different composition, and newly developed low-temperature processable PVP derivative, poly(4-phenoxy methyl styrene) (P4PMS). Their chemical/physical/electrical properties have been characterized by surface tension, electrical leakage current and capacitance. The performances of F16 CuPc OFETs with PVP based dielectrics that have different composition of PVP and cross-linking agent are compared each other and optimized. Additionally, we compared the performance of the optimized F16 CuPc OFETs with the PVP based dielectric with that of F16 CuPc OFETs with low-temperature processable P4PMS gate dielectric that have phenoxymethyl group (–CH2 OC6 H5 ) instead of hydroxyl group which might act as a charge
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Fig. 1. (a) Schematic of the device structure and the molecular structures of (b) copper hexadecafluorophthalocyanine (F16 CuPc), (c) poly(4-vinylphenol) (PVP) and (d) poly(4-phenoxymethyl styrene) (P4PMS).
(electron) trapping site causing device performance degradation and hysteresis. 2. Experimental Organic field-effect transistors were fabricated in top contact geometry on cleaned and patterned indium tin oxide (ITO) coated glass substrates which acted as gate electrode. All the materials including F16 CuPC and PVP were purchased from Aldrich Chemical Co. and were used as received, whereas poly(4-phenoxy methyl styrene) (P4PMS) was synthesized in our lab by the reaction of poly(4-vinylbenzylchloride) and phenol at 80 ◦ C in acetone with stirring for 48 h having K2 CO3 as catalyst. Five different gate dielectrics have been used in this study, namely, CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 and P4PMS. The four PVP based gate dielectrics (CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4) were prepared by mixing poly(4-vinylphenol) (PVP) (Mw = 20,000 g/mol) with a cross-linking agent, poly(melamine-co-formaldehyde) (Mn = 511 g/mol) in a ratio of 10:1 (CL-PVP 1), 10:5 (CL-PVP 2), 10:10 (CL-PVP 3), 10:15 (CL-PVP 4) by weight in n-butanol with overnight stirring. Toluene solution of P4PMS (10 wt%) has been used for the formation of P4PMS. All the solutions of PVP and P4PMS were spin-coated on ITO coated glass substrates. The film was soft-baked at 60 ◦ C for 10 min on a hot plate under ambient conditions and baked further at 200 ◦ C for CL-PVPs and 125 ◦ C for P4PMS in a vacuum oven (10−3 Torr) for 1 h. The thickness of the dielectrics was controlled at about 300 nm by adjusting the spin speed. On top of the dielectric, organic semiconductor (F16 CuPc) was vacuum-deposited (50 nm) at a base pressure of 5 × 10−6 Torr. The substrate temperature and the deposition rate were 25 ◦ C and 0.2 Å/s, respectively. Finally, gold (Au) was thermally evaporated on top of the film (top contact) to make the source-drain contact (40 nm) under a high vacuum (5 × 10−6 Torr) at a rate of 0.3 Å/s. A shadow mask used during the Au deposition, defined the channel length and channel width as 50 m and 1000 m, respectively. It is well-known that gold (Au) has poor capability for injecting electron carriers because of high
work-function and is not the best choice for the efficient electron injection from electrode to the F16 CuPc layers. Metal electrode such as Ca/Al (low work function metal) is more suitable for high performance n-type OFETs but it is not stable in air. For this study, since all electrical measurements were carried out under ambient conditions without any additional process for device encapsulation we used Au as a source and drain electrodes in order to exclude the effect of oxidation of low work function metal electrode on the device performance. Electrical characterization of the devices were performed at room temperature and under ambient condition using a HP semiconductor parameter analyzer (HP 4145B) controlled by the Labview program. The morphology of the F16 CuPc layers on crosslinked PVP films and P4PMS films were evaluated using tapping mode atomic force microscope (AFM; Nanoscope IIIa, Digital Instruments). The surface energies of the dielectric films were calculated from contact angle measurements (KRUSS, DSA 100) using distilled water and diiodimethane as the probe liquids. To characterize the leakage currents and capacitances of the gate dielectrics, we prepared metal–insulator–metal (MIM) capacitor type device and measured under ambient conditions. The MIM devices were fabricated as following. The bottom ITO electrode was formed by conventional lithographic method. The gate dielectric solutions were spin-coated on top of the bottom electrode and then cured at 60 ◦ C for 10 min on a hot plate under ambient conditions and baked further at 200 ◦ C for CL-PVPs and 125 ◦ C for P4PMS in a vacuum oven (10−3 Torr) for 1 h. The MIM devices were completed by evaporating the top gold electrode. The active area of MIM device was 0.3 mm2 . The capacitances were measured by an impedance/gain phase analyzer (HP 4194 A) at 1 kHz using the same MIM structure. The thickness of the gate dielectrics were measured using a surface profiler (AMBIOS XP-100). The fieldIDS versus effect mobility () was extracted from the plot of VDS in a saturation regime based on the following equation: IDS = (WCi /2L)(VGS − Vth )2 where W, L, Vth and Ci are channel width, channel length, threshold voltage and capacitance per unit area,
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Fig. 2. Plot of leakage current density vs electric field for MIM devices with P4PMS and CL-PVP gate dielectrics. The CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 were prepared by mixing poly(4-vinylphenol) (PVP) with a cross-linking agent in a ratio of 10:1 (CL-PVP 1), 10:5 (CL-PVP 2), 10:10 (CL-PVP 3), 10:15 (CL-PVP 4) by weight in n-butanol. Toluene solution of P4PMS (10 wt%) has been used for the formation of P4PMS.
respectively. Vth was determined from the plot of the square root of the IDS and VGS by extrapolating the measured data to IDS = 0. The subthreshold swing (ss), which is a measure of how sharply the device transits from the OFF to the ON state, is given by. ss =
d log(I ) −1 DS dVGS
3. Results and discussions The schematic diagram of the F16 CuPc OFET with the polymer gate dielectrics is depicted in Fig. 1(a), and the chemical structures of different materials used in this study, viz, F16 CuPc, PVP and P4PMS are shown in Fig. 1(b), (c) and (d), respectively. The CL-PVPs were prepared through a cross-linking reaction between hydroxyl group of PVP and the cross-linking agent, poly(melamineco-formaldehyde) [29]. The chemical structure of P4PMS is almost similar to PVP except for the phenoxymethyl group (–CH2 OC6 H5 ) introduced to the aromatic ring instead of hydroxyl group which might act as a charge (electron) trapping site causing device performance degradation. The electrical leakage current of gate dielectric is one of the main concerns for the fabrication of high performance OFETs. Fig. 2 shows the dependence of leakage current density on the electric field (J–E) applied to metal/insulator/metal (MIM) devices based on the gate dielectrics. It is clearly seen from the figure that leakage current density (1 × 10−5 A/cm2 at 0.5 MV/cm) of MIM based on CL-PVP 1 is relatively high. This indicates that the poly(4-vinylphenol) main chains of CL-PVP 1
were not fully cross-linked to give dense dielectric film due to the lack of cross-linking agent. It is expected that the leakage current of the MIM device would be decreased with an increase in the content of the cross-linking agent. We found that the leakage current density (1.8 × 10−8 A/cm2 at 0.5 MV/cm) of CL-PVP 2 is much lower than CL-PVP 1. However, interestingly, the leakage current density level became higher when more cross-linking agent was added. The current density of CL-PVP 3 and CL-PVP 4 was about 4.3 × 10−7 A/cm2 and 5.7 × 10−7 A/cm2 , respectively, at 0.5 MV/cm. Since the molecular weight (Mn = 511 g/mol) of the cross-linking agent is much smaller than that (Mw = 20,000 g/mol) of poly(4-vinylphenol), there is high possibility for the unreacted (residual) cross-linking molecule to be dispersed between the poly(4-vinylphenol) chains resulting in less dense film and relatively higher leakage current density. As far as the leakage current is concerned, CL-PVP 2 has most suitable electrical properties for gate dielectric for the fabrication of OFET. On the other hand, the lowtemperature processable gate dielectric, P4PMS, showed leakage current density of 7.0 × 10−8 A/cm2 at 0.5 MV/cm, slightly higher than CL-PVP 2 and lower than CL-PVP 3 and CL-PVP 4. However, it is an acceptable value for the fabrication of OFET. The capacitance of the gate dielectrics were measured from the same MIM devices and the values were listed in Table 1. The capacitance of CL-PVP 2 and P4PMS were 60 pF/mm2 and 50 pF/mm2 , respectively. In order to evaluate the surface properties of the gate dielectrics, water contact angle measurements were performed. For CL-PVP type gate dielectrics, it increased from 65◦ to 79◦ with the increase in the content of cross-linking agent. It is obvious that the increase of contact angle is due to the decreased number of hydroxyl group with the progress of cross-linking reaction. On the other hand, relatively much higher water contact angle (102◦ ) was observed from P4PMS gate dielectric. This is due to the introduction of phenoxymethyl group, much hydrophobic as compared to hydroxyl group, at 4 position of aromatic ring. The water contact angle and calculated surface energy are summarized in Table 1. The transfer characteristics (VGS –IDS ) of the F16 CuPc OFETs with CL-PVP gate dielectrics having different concentration of crosslinking agent are shown in Fig. 3. It is clearly seen from the figure that F16 CuPc OFET fabricated with CL-PVP 2 has larger on/off ratio (∼321) than other devices (7, 6, 13 for CL-PVP 1, 3, and 4, respectively). The performance parameters of the OFETs extracted from the transfer characteristics of the devices were summarized in Table 1. Comparison of the data in Table 1 indicates that as far as OFET parameters are concerned, the best performance is obtained for the device with CL-PVP 2 (5 wt% of cross-linking agent), which exhibit the best electrical characteristics as a gate dielectric (see Table 1). The results are further compared with OFET with P4PMS gate dielectric in order to investigate the effect of surface properties of gate dielectric on the device performance. The P4PMS does not have any hydroxyl groups which might act as a charge (electron) trapping site causing device performance degradation and hysteresis when it is operated under ambient conditions [30–32]. Fig. 4 shows the output characteristics (VDS –IDS ) of the F16 CuPc OFETs with P4PMS (a) and CL-PVP 2 (b). Typical
Table 1 Summary of the characteristics of the gate dielectrics and the performance parameters of F16 CuPc OFETs with different polymeric gate dielectrics. Dielectric type
Thickness (nm)
Ci (pF/mm2 )
Contact angle (◦ )
Surface energy (mJ/m2 )
Leakage current (A/cm2 )
(cm2 /V s)
On/off ratio
Vth (V)
ss (V/dec)
Nt (×1013 cm−2 eV−1 )
CL-PVP 1 CL-PVP 2 CL-PVP 3 CL-PVP 4 P4PMS
295 310 320 310 300
85 60 50 35 50
65 70 75 79 102
43.98 42.51 41.27 41.06 38.72
7.2 × 10−6 1.8 × 10−8 4.3 × 10−7 5.7 × 10−7 7.0 × 10−8
8.56 × 10−4 1.10 × 10−4 4.11 × 10−4 3.78 × 10−4 5.32 × 10−4
7 321 65 13 200
−3.5 1.68 0.37 0.46 4.60
– 20.71 28.17 29.92 12.30
– 12 16 18 6.45
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Fig. 3. Transfer characteristic curves of F16 CuPc OFETs with CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 gate dielectrics. Drain voltage was kept constant at VDS = 30 V.
n-type characteristics have been observed with a clear transition from the linear to the saturation regime. At a given VGS , IDS initially increased linearly with small positive VDS and then saturates at high VDS . A maximum saturation current of 48 nA was achieved under VGS = 40 V for F16 CuPc OFETs with P4PMS while about 25 nA was achieved with CL-PVP 2. The performance parameters of OFETs
Fig. 5. Transfer characteristic curves of F16 CuPc OFETs with (a) P4PMS (b) CL-PVP 2 gate dielectric. Drain voltage was kept constant at VDS = 30 V.
Fig. 4. Output characteristic curves of F16 CuPc OFETs with (a) P4PMS and (b) CL-PVP 2 gate dielectric.
are extracted from the transfer characteristics (VGS –IDS ) as shown in Fig. 5. The carrier mobility () of F16 CuPc OFET with P4PMS (5.32 × 10−4 cm2 /V s) is little higher than that of OFETs with CLPVP 2 (1.10 × 10−4 cm2 /V s). This difference can be attributed to the number of actual grain boundaries that mainly influences the effective charge carrier mobility. Smaller grains result in a higher density of grain boundaries which act as traps for electrons. Also, the potential barriers between the individual grains at the grain boundary is responsible for the mobility degradation as observed in OFETs with semiconductor having small grain sizes [33]. The on/off ratio of ∼3 × 102 was observed for both OFETs. The performance of OFETs can, in general, be enormously affected by the choice of gate dielectric. This in general accounts for the morphological differences in the organic semiconductor layer caused by different interfacial interactions with the gate dielectric. In order to clarify the differences in the OFET performances with CL-PVP 2 and P4PMS dielectrics, we used tapping mode atomic force microscope (AFM) to investigate the surface morphology of the F16 CuPc layers on top of the dielectric layers. Fig. 6 shows the AFM images of F16 CuPc thin films deposited on P4PMS and CL-PVP 2. It shows that grain size of F16 CuPc is larger on P4PMS (∼31.25 nm) than that on CL-PVP 2 surface (∼21.48 nm) and the deposited films over the dielectrics are very smooth and uniform having root mean square (rms) roughness less than 1 nm in both the cases. The grain size difference of the crystal of F16 CuPc may be caused by the hydrophilic/hydrophobic properties of the gate dielectric surface. As discussed in the previous section, the measured water contact angles on P4PMS and CL-PVP 2 surface are found to be 102◦ and 70◦ (see the insets of Fig. 6), respectively. The higher contact angle on P4PMS surface indicates more hydrophobic nature
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Fig. 6. Tapping mode AFM images of F16 CuPc deposited on (a) P4PMS and (b) CL-PVP 2 gate dielectric surface. The cross-sectional surface analysis of F16 CuPc films are shown together. Inset shows the images of water drops on P4PMS and CL-PVP 2 gate dielectrics.
of the surface compared to CL-PVP 2. The hydrophobic surface of P4PMS may help forming active layer of F16 CuPc with high structural organization and hence larger crystalline size [34]. This in turn, results in efficient charge transport and hence higher carrier mobility. The subthreshold slope (ss) is found to be 20.71 V/dec and 12.30 V/dec, respectively, for F16 CuPc OFETs with CL-PVP 2 and P4PMS gate dielectrics. The ss is mainly determined by the trap behavior and the quality of insulator/semiconductor interface. By assuming that densities of the interface states are independent of energy, the maximum interface trap density is given by the following relation [35] Nt =
ss log(e) KT/q
C
−1
i
q
where Nt is the maximum number of interface traps, K is the Boltzman constant, T is the absolute temperature, and q is the electronic charge. Higher value of ss leads to higher interface trap density for the OFETs with CL-PVP 2 dielectric (Nt = 12 × 1013 cm−2 eV−1 ) compared to OFETs with P4PMS dielectric (Nt = 6.45 × 1013 cm−2 eV−1 ). As the ss is directly correlated to the number of interface traps, therefore, higher value can also be attributed to the increased number of grain boundaries and consequently increased dielectric–semiconductor interface trap density. The comparative results obtained from the transfer characteristics of the devices fabricated with two different gate dielectrics were summarized in Table 1. In addition to the OFETs based on P4PMS and CL-PVP 2 dielectrics, we also fabricated F16 CuPc OFETs on SiO2 /Si substrates at room temperature and the performance was compared with OFETs with the CL-PVP 2 and P4PMS. Mobility, on/off ratio and the Vth values for the OFETs with SiO2 gate dielectric were found to be 3 × 10−4 cm2 /V s, 4 × 102 and 9.41 V, respectively. The performances of OFETs with P4PMS are better or comparable to OFET with CL-PVP 2 and SiO2 dielectric. In addition, since the P4PMS has good electrical properties and could be processed at low temperature (125 ◦ C) where flexible plastic substrates bear, the P4PMS could efficiently be used for the fabrication of high performance flexible OFETs.
4. Conclusions In conclusion, we have fabricated F16 CuPc OFETs based on different polymeric gate dielectrics. It has been demonstrated that by controlling the concentration of cross-linking agent, the electrical and surface properties of the PVP based gate dielectrics can easily be tuned and hence the device performance of F16 CuPc OFET can efficiently be optimized. Furthermore, the performance of F16 CuPc OFET based on two different dielectrics, CL-PVP 2 and P4PMS, and the effect of dielectric/semiconductor interface on the performance of OFETs was investigated. The results revealed that the presence of hydroxyl group can make a dielectric surface more hydrophilic which in turn, gives unfavorable effect on the crystal growth of F16 CuPc. The more hydrophobic, P4PMS surface on the other hand, because of its better interaction with the organic semiconductor can give larger crystal size and hence higher mobility. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy and National Research Foundation (NRF) through EPB Center (R11-2008-052-03003). This work was also supported by Seoul R&BD Program (WR090671). References [1] B. Crone, A. Dodabalapur, Y.-Y. Lin, R.W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar, H.E. Katz, W. Li, Nature 403 (2000) 521. [2] H. Klauk, M. Halik, U. Zschieschang, F. Eder, D. Rohde, G. Schmid, C. Dehm, IEEE Trans. Electron Devices 52 (2005) 618–622. [3] L. Torsi, A. Dodabalapur, L.J. Rothberg, A.W.P. Fung, H.E. Katz, Science 272 (1996) 1462. [4] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [5] M. Shtein, J. Mapel, J.B. Benziger, S.R. Forrest, Appl. Phys. Lett. 81 (2002) 268. [6] J.H. Seo, J.H. Kwon, S.I. Shin, K.S. Suh, B.K. Ju, Semicond. Sci. Technol. 22 (2007) 1039. [7] T.B. Singh, F. Meghdadi, S. Günes, N. Marjanovic, G. Horowitz, P. Lang, S. Bauer, N.S. Sariciftci, Adv. Mater. 17 (2005) 2315. [8] S. Pyo, H. Son, K.Y. Choi, M.H. Yi, S.K. Hong, Appl. Phys. Lett. 86 (2005) 133508. [9] Z. Bao, Y. Feng, A. Dodabalapur, V.R. Raju, A.J. Lovinger, Chem. Mater. 9 (1997) 1299. [10] T.S. Huang, Y.K. Su, P.C. Wang, Appl. Phys. Lett. 91 (2007) 092116.
M. Mukherjee et al. / Synthetic Metals 160 (2010) 504–509 ´ G.J. Matt, N.S. Sariciftci, R. Schwödiauer, S. Bauer, [11] T.B. Singh, N. Marjanovic, Appl. Phys. Lett. 85 (2004) 5409. [12] B. Mukherjee, M. Mukherjee, Org. Electron. 10 (2009) 1282. [13] H. Yan, Z. Chen, Y. Zheng, C. Newman, J.R. Quinn, F. Dötz, M. Kastler, A. Facchetti, Nature 457 (2009) 679. [14] Y. Lu, W.H. Lee, H.S. Lee, Y. Jang, K. Cho, Appl. Phys. Lett. 94 (2009) 113303. [15] H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, W. Weber, J. Appl. Phys. 92 (2002) 5259. [16] C.D. Dimitrakopoulos, P.R.L. Malefant, Adv. Mater. 14 (2002) 99. [17] W. Fix, A. Ullmann, J. Ficker, W. Clemens, Appl. Phys. Lett. 81 (2002) 1735. [18] H. Wang, J. Wang, H. Huang, X. Yan, D. Yan, Org. Electron. 7 (2006) 369. [19] C. Rost, D.J. Gundlach, S. Karg, W. Reiss, J. Appl. Phys. 95 (2004) 5782. [20] K.Y. Law, Chem. Rev. 93 (1993) 449. [21] S. Kobayashi, T. Takenobu, S. Mori, A. Fujiwara, Y. Iwasa, Appl. Phys. Lett. 82 (2003) 4581. [22] P.R.L. Malenfant, C.D. Dimitrakopoulos, J.D. Gelorme, L.L. Kosbar, T.O. Graham, A. Curioni, W. Andreoni, Appl. Phys. Lett. 80 (2002) 2517. [23] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207.
509
[24] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, R.W. Filas, Y.-Y. Lin, Z. Bao, J.H. O’Neill, W. Li, H.E. Katz, J. Appl. Phys. 89 (2001) 5125. [25] J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Chem. Phys. Lett. 407 (2005) 87. [26] S.H. Kim, J.H. Lee, Y.S. Yang, T. Zyung, Mol. Cryst. Liq. Cryst. 405 (2003) 137. [27] Y. Oh, S. Pyo, M.H. Yi, S.K. Kwon, Org. Electron. 7 (2006) 77. [28] M.E. Roberts, N. Queraltó, S.C.B. Mannsfeld, B.N. Reinecke, W. Knoll, Z. Bao, Chem. Mater. 21 (2009) 2292. [29] Y. Choi, H. Kim, K. Sim, K. Park, C. Im, S. Pyo, Org. Electron. 10 (2009) 1209. [30] S. Lim, S. Kim, J. Koo, J. Lee, C. Ku, Y. Yang, T. Zyung, Appl. Phys. Lett. 90 (2007) 173512. [31] S. Lee, B. Koo, J. Shin, E. Lee, H. Park, H. Kim, Appl. Phys. Lett. 88 (2006) 162109. [32] T. Jung, A. Dodabalapur, R. Wenz, S. Mohapatra, Appl. Phys. Lett. 87 (2005) 182109. [33] A.D. Carlo, F. Piacenza, A. Bolognesi, B. Stadlober, H. Maresch, Appl. Phys. Lett. 86 (2005) 263501. [34] K. Yamamoto, S. Ochiai, X. Wang, Y. Uchida, K. Kojima, A. Ohashi, T. Mizutani, Thin Solid Films 516 (2008) 2695. [35] H.W. Zan, K.H. Yen, P.K. Liu, K.H. Ku, C.H. Chen, J. Hwang, Org. Electron. 8 (2007) 450.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Investigation of organic n-type field-effect transistor performance on the polymeric gate dielectrics Moumita Mukherjee, Biswanath Mukherjee, Youngill Choi, Kyoseung Sim, Junghwan Do, Seungmoon Pyo ∗ Department of Chemistry, Konkuk University, 1 Hwayang-dong, Kwangjin-gu, Seoul 143701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 October 2009 Received in revised form 26 November 2009 Accepted 30 November 2009 Available online 12 January 2010 Keywords: Organic field-effect transistors Gate dielectric Surface properties Organic semiconductor
a b s t r a c t We report a copper hexadecafluorophthalocyanine (F16 CuPc) based n-type organic field-effect transistor (OFET) with polymeric gate dielectrics with different physical/electrical properties. The gate dielectrics are four types of cross-linked poly(4-vinylphenol) and newly prepared poly(4-phenoxy methyl styrene) and those are characterized based on surface tension, leakage current and capacitance. The performance of F16 CuPc OFETs with those gate dielectrics was compared. We found that the composition of the gate dielectrics and the interfacial interaction of F16 CuPc with the gate dielectric play a decisive role in the performance of OFETs. The effect of physical/electrical properties, composition and processing condition of the gate dielectrics on the device performance was investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Considerable progress has been made in the recent years on the performance of organic field-effect transistors (OFETs) for lowcost, large-area electronic device applications, particularly those that are compatible with flexible plastic substrate [1,2]. OFETs have many unique advantages such as light weight, low-cost fabrication, mechanical flexibility, solution processability, and facile integration with electronic components [3–5]. For those organic electronic device applications, the development of important component materials such as polymeric gate dielectric and organic semiconductor are strongly required. Many polymeric gate dielectrics have been used for OFETs including poly(4-vinylphenol) [6,7], polyimide [8,9], poly(methylmethacrylate) [10], poly(vinylalcohol) [11,12], polystyrene [13,14]. Among the polymer dielectrics reported to date, the most investigated is cross-linked poly(4-vinylphenol) (PVP) due to its good film formability and electrical properties although it requires high processing temperature [15]. Organic semiconductor have already been demonstrated to possess remarkable semiconducting properties and its development is part of the main stream for progress in organic electronic devices such as solar cells, optical limiters, photoconductors and charge transporting layers in OFETs [16–20]. Compared with well-developed p-type organic semiconductors, advances in the development of
∗ Corresponding author. E-mail address: [email protected] (S. Pyo). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.11.039
n-type organic semiconductors are still in the early stage, since most of the n-type materials are unstable in air, which induces the deterioration of the electrical characteristics [21,22]. Bao et al. reported air-stable n-type OFETs based on copper hexadecafluorophthalocyanine (F16 CuPc) [23]. The fluorines introduced to metallophthalocyanines increase the electron affinity of the molecules and favor an efficient electron injection into the empty LUMO states. However, most of the OFETs with F16 CuPc are mainly fabricated with inorganic gate insulators such as SiO2 and Ta2 O5 [24–26]. Very recently, there have been reports of F16 CuPc OFETs based on chemically/mechanically stable polyimide blend gate dielectric [27] and thin cross-linked polymer gate dielectric having low operation voltage of less than 1 V and moderate charge carrier mobility [28]. In the present work, five-types of polymeric gate dielectrics was prepared for the fabrication of n-type OFETs: PVP based dielectrics prepared from PVP precursor solutions having different composition, and newly developed low-temperature processable PVP derivative, poly(4-phenoxy methyl styrene) (P4PMS). Their chemical/physical/electrical properties have been characterized by surface tension, electrical leakage current and capacitance. The performances of F16 CuPc OFETs with PVP based dielectrics that have different composition of PVP and cross-linking agent are compared each other and optimized. Additionally, we compared the performance of the optimized F16 CuPc OFETs with the PVP based dielectric with that of F16 CuPc OFETs with low-temperature processable P4PMS gate dielectric that have phenoxymethyl group (–CH2 OC6 H5 ) instead of hydroxyl group which might act as a charge
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Fig. 1. (a) Schematic of the device structure and the molecular structures of (b) copper hexadecafluorophthalocyanine (F16 CuPc), (c) poly(4-vinylphenol) (PVP) and (d) poly(4-phenoxymethyl styrene) (P4PMS).
(electron) trapping site causing device performance degradation and hysteresis. 2. Experimental Organic field-effect transistors were fabricated in top contact geometry on cleaned and patterned indium tin oxide (ITO) coated glass substrates which acted as gate electrode. All the materials including F16 CuPC and PVP were purchased from Aldrich Chemical Co. and were used as received, whereas poly(4-phenoxy methyl styrene) (P4PMS) was synthesized in our lab by the reaction of poly(4-vinylbenzylchloride) and phenol at 80 ◦ C in acetone with stirring for 48 h having K2 CO3 as catalyst. Five different gate dielectrics have been used in this study, namely, CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 and P4PMS. The four PVP based gate dielectrics (CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4) were prepared by mixing poly(4-vinylphenol) (PVP) (Mw = 20,000 g/mol) with a cross-linking agent, poly(melamine-co-formaldehyde) (Mn = 511 g/mol) in a ratio of 10:1 (CL-PVP 1), 10:5 (CL-PVP 2), 10:10 (CL-PVP 3), 10:15 (CL-PVP 4) by weight in n-butanol with overnight stirring. Toluene solution of P4PMS (10 wt%) has been used for the formation of P4PMS. All the solutions of PVP and P4PMS were spin-coated on ITO coated glass substrates. The film was soft-baked at 60 ◦ C for 10 min on a hot plate under ambient conditions and baked further at 200 ◦ C for CL-PVPs and 125 ◦ C for P4PMS in a vacuum oven (10−3 Torr) for 1 h. The thickness of the dielectrics was controlled at about 300 nm by adjusting the spin speed. On top of the dielectric, organic semiconductor (F16 CuPc) was vacuum-deposited (50 nm) at a base pressure of 5 × 10−6 Torr. The substrate temperature and the deposition rate were 25 ◦ C and 0.2 Å/s, respectively. Finally, gold (Au) was thermally evaporated on top of the film (top contact) to make the source-drain contact (40 nm) under a high vacuum (5 × 10−6 Torr) at a rate of 0.3 Å/s. A shadow mask used during the Au deposition, defined the channel length and channel width as 50 m and 1000 m, respectively. It is well-known that gold (Au) has poor capability for injecting electron carriers because of high
work-function and is not the best choice for the efficient electron injection from electrode to the F16 CuPc layers. Metal electrode such as Ca/Al (low work function metal) is more suitable for high performance n-type OFETs but it is not stable in air. For this study, since all electrical measurements were carried out under ambient conditions without any additional process for device encapsulation we used Au as a source and drain electrodes in order to exclude the effect of oxidation of low work function metal electrode on the device performance. Electrical characterization of the devices were performed at room temperature and under ambient condition using a HP semiconductor parameter analyzer (HP 4145B) controlled by the Labview program. The morphology of the F16 CuPc layers on crosslinked PVP films and P4PMS films were evaluated using tapping mode atomic force microscope (AFM; Nanoscope IIIa, Digital Instruments). The surface energies of the dielectric films were calculated from contact angle measurements (KRUSS, DSA 100) using distilled water and diiodimethane as the probe liquids. To characterize the leakage currents and capacitances of the gate dielectrics, we prepared metal–insulator–metal (MIM) capacitor type device and measured under ambient conditions. The MIM devices were fabricated as following. The bottom ITO electrode was formed by conventional lithographic method. The gate dielectric solutions were spin-coated on top of the bottom electrode and then cured at 60 ◦ C for 10 min on a hot plate under ambient conditions and baked further at 200 ◦ C for CL-PVPs and 125 ◦ C for P4PMS in a vacuum oven (10−3 Torr) for 1 h. The MIM devices were completed by evaporating the top gold electrode. The active area of MIM device was 0.3 mm2 . The capacitances were measured by an impedance/gain phase analyzer (HP 4194 A) at 1 kHz using the same MIM structure. The thickness of the gate dielectrics were measured using a surface profiler (AMBIOS XP-100). The fieldIDS versus effect mobility () was extracted from the plot of VDS in a saturation regime based on the following equation: IDS = (WCi /2L)(VGS − Vth )2 where W, L, Vth and Ci are channel width, channel length, threshold voltage and capacitance per unit area,
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Fig. 2. Plot of leakage current density vs electric field for MIM devices with P4PMS and CL-PVP gate dielectrics. The CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 were prepared by mixing poly(4-vinylphenol) (PVP) with a cross-linking agent in a ratio of 10:1 (CL-PVP 1), 10:5 (CL-PVP 2), 10:10 (CL-PVP 3), 10:15 (CL-PVP 4) by weight in n-butanol. Toluene solution of P4PMS (10 wt%) has been used for the formation of P4PMS.
respectively. Vth was determined from the plot of the square root of the IDS and VGS by extrapolating the measured data to IDS = 0. The subthreshold swing (ss), which is a measure of how sharply the device transits from the OFF to the ON state, is given by. ss =
d log(I ) −1 DS dVGS
3. Results and discussions The schematic diagram of the F16 CuPc OFET with the polymer gate dielectrics is depicted in Fig. 1(a), and the chemical structures of different materials used in this study, viz, F16 CuPc, PVP and P4PMS are shown in Fig. 1(b), (c) and (d), respectively. The CL-PVPs were prepared through a cross-linking reaction between hydroxyl group of PVP and the cross-linking agent, poly(melamineco-formaldehyde) [29]. The chemical structure of P4PMS is almost similar to PVP except for the phenoxymethyl group (–CH2 OC6 H5 ) introduced to the aromatic ring instead of hydroxyl group which might act as a charge (electron) trapping site causing device performance degradation. The electrical leakage current of gate dielectric is one of the main concerns for the fabrication of high performance OFETs. Fig. 2 shows the dependence of leakage current density on the electric field (J–E) applied to metal/insulator/metal (MIM) devices based on the gate dielectrics. It is clearly seen from the figure that leakage current density (1 × 10−5 A/cm2 at 0.5 MV/cm) of MIM based on CL-PVP 1 is relatively high. This indicates that the poly(4-vinylphenol) main chains of CL-PVP 1
were not fully cross-linked to give dense dielectric film due to the lack of cross-linking agent. It is expected that the leakage current of the MIM device would be decreased with an increase in the content of the cross-linking agent. We found that the leakage current density (1.8 × 10−8 A/cm2 at 0.5 MV/cm) of CL-PVP 2 is much lower than CL-PVP 1. However, interestingly, the leakage current density level became higher when more cross-linking agent was added. The current density of CL-PVP 3 and CL-PVP 4 was about 4.3 × 10−7 A/cm2 and 5.7 × 10−7 A/cm2 , respectively, at 0.5 MV/cm. Since the molecular weight (Mn = 511 g/mol) of the cross-linking agent is much smaller than that (Mw = 20,000 g/mol) of poly(4-vinylphenol), there is high possibility for the unreacted (residual) cross-linking molecule to be dispersed between the poly(4-vinylphenol) chains resulting in less dense film and relatively higher leakage current density. As far as the leakage current is concerned, CL-PVP 2 has most suitable electrical properties for gate dielectric for the fabrication of OFET. On the other hand, the lowtemperature processable gate dielectric, P4PMS, showed leakage current density of 7.0 × 10−8 A/cm2 at 0.5 MV/cm, slightly higher than CL-PVP 2 and lower than CL-PVP 3 and CL-PVP 4. However, it is an acceptable value for the fabrication of OFET. The capacitance of the gate dielectrics were measured from the same MIM devices and the values were listed in Table 1. The capacitance of CL-PVP 2 and P4PMS were 60 pF/mm2 and 50 pF/mm2 , respectively. In order to evaluate the surface properties of the gate dielectrics, water contact angle measurements were performed. For CL-PVP type gate dielectrics, it increased from 65◦ to 79◦ with the increase in the content of cross-linking agent. It is obvious that the increase of contact angle is due to the decreased number of hydroxyl group with the progress of cross-linking reaction. On the other hand, relatively much higher water contact angle (102◦ ) was observed from P4PMS gate dielectric. This is due to the introduction of phenoxymethyl group, much hydrophobic as compared to hydroxyl group, at 4 position of aromatic ring. The water contact angle and calculated surface energy are summarized in Table 1. The transfer characteristics (VGS –IDS ) of the F16 CuPc OFETs with CL-PVP gate dielectrics having different concentration of crosslinking agent are shown in Fig. 3. It is clearly seen from the figure that F16 CuPc OFET fabricated with CL-PVP 2 has larger on/off ratio (∼321) than other devices (7, 6, 13 for CL-PVP 1, 3, and 4, respectively). The performance parameters of the OFETs extracted from the transfer characteristics of the devices were summarized in Table 1. Comparison of the data in Table 1 indicates that as far as OFET parameters are concerned, the best performance is obtained for the device with CL-PVP 2 (5 wt% of cross-linking agent), which exhibit the best electrical characteristics as a gate dielectric (see Table 1). The results are further compared with OFET with P4PMS gate dielectric in order to investigate the effect of surface properties of gate dielectric on the device performance. The P4PMS does not have any hydroxyl groups which might act as a charge (electron) trapping site causing device performance degradation and hysteresis when it is operated under ambient conditions [30–32]. Fig. 4 shows the output characteristics (VDS –IDS ) of the F16 CuPc OFETs with P4PMS (a) and CL-PVP 2 (b). Typical
Table 1 Summary of the characteristics of the gate dielectrics and the performance parameters of F16 CuPc OFETs with different polymeric gate dielectrics. Dielectric type
Thickness (nm)
Ci (pF/mm2 )
Contact angle (◦ )
Surface energy (mJ/m2 )
Leakage current (A/cm2 )
(cm2 /V s)
On/off ratio
Vth (V)
ss (V/dec)
Nt (×1013 cm−2 eV−1 )
CL-PVP 1 CL-PVP 2 CL-PVP 3 CL-PVP 4 P4PMS
295 310 320 310 300
85 60 50 35 50
65 70 75 79 102
43.98 42.51 41.27 41.06 38.72
7.2 × 10−6 1.8 × 10−8 4.3 × 10−7 5.7 × 10−7 7.0 × 10−8
8.56 × 10−4 1.10 × 10−4 4.11 × 10−4 3.78 × 10−4 5.32 × 10−4
7 321 65 13 200
−3.5 1.68 0.37 0.46 4.60
– 20.71 28.17 29.92 12.30
– 12 16 18 6.45
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Fig. 3. Transfer characteristic curves of F16 CuPc OFETs with CL-PVP 1, CL-PVP 2, CL-PVP 3, CL-PVP 4 gate dielectrics. Drain voltage was kept constant at VDS = 30 V.
n-type characteristics have been observed with a clear transition from the linear to the saturation regime. At a given VGS , IDS initially increased linearly with small positive VDS and then saturates at high VDS . A maximum saturation current of 48 nA was achieved under VGS = 40 V for F16 CuPc OFETs with P4PMS while about 25 nA was achieved with CL-PVP 2. The performance parameters of OFETs
Fig. 5. Transfer characteristic curves of F16 CuPc OFETs with (a) P4PMS (b) CL-PVP 2 gate dielectric. Drain voltage was kept constant at VDS = 30 V.
Fig. 4. Output characteristic curves of F16 CuPc OFETs with (a) P4PMS and (b) CL-PVP 2 gate dielectric.
are extracted from the transfer characteristics (VGS –IDS ) as shown in Fig. 5. The carrier mobility () of F16 CuPc OFET with P4PMS (5.32 × 10−4 cm2 /V s) is little higher than that of OFETs with CLPVP 2 (1.10 × 10−4 cm2 /V s). This difference can be attributed to the number of actual grain boundaries that mainly influences the effective charge carrier mobility. Smaller grains result in a higher density of grain boundaries which act as traps for electrons. Also, the potential barriers between the individual grains at the grain boundary is responsible for the mobility degradation as observed in OFETs with semiconductor having small grain sizes [33]. The on/off ratio of ∼3 × 102 was observed for both OFETs. The performance of OFETs can, in general, be enormously affected by the choice of gate dielectric. This in general accounts for the morphological differences in the organic semiconductor layer caused by different interfacial interactions with the gate dielectric. In order to clarify the differences in the OFET performances with CL-PVP 2 and P4PMS dielectrics, we used tapping mode atomic force microscope (AFM) to investigate the surface morphology of the F16 CuPc layers on top of the dielectric layers. Fig. 6 shows the AFM images of F16 CuPc thin films deposited on P4PMS and CL-PVP 2. It shows that grain size of F16 CuPc is larger on P4PMS (∼31.25 nm) than that on CL-PVP 2 surface (∼21.48 nm) and the deposited films over the dielectrics are very smooth and uniform having root mean square (rms) roughness less than 1 nm in both the cases. The grain size difference of the crystal of F16 CuPc may be caused by the hydrophilic/hydrophobic properties of the gate dielectric surface. As discussed in the previous section, the measured water contact angles on P4PMS and CL-PVP 2 surface are found to be 102◦ and 70◦ (see the insets of Fig. 6), respectively. The higher contact angle on P4PMS surface indicates more hydrophobic nature
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Fig. 6. Tapping mode AFM images of F16 CuPc deposited on (a) P4PMS and (b) CL-PVP 2 gate dielectric surface. The cross-sectional surface analysis of F16 CuPc films are shown together. Inset shows the images of water drops on P4PMS and CL-PVP 2 gate dielectrics.
of the surface compared to CL-PVP 2. The hydrophobic surface of P4PMS may help forming active layer of F16 CuPc with high structural organization and hence larger crystalline size [34]. This in turn, results in efficient charge transport and hence higher carrier mobility. The subthreshold slope (ss) is found to be 20.71 V/dec and 12.30 V/dec, respectively, for F16 CuPc OFETs with CL-PVP 2 and P4PMS gate dielectrics. The ss is mainly determined by the trap behavior and the quality of insulator/semiconductor interface. By assuming that densities of the interface states are independent of energy, the maximum interface trap density is given by the following relation [35] Nt =
ss log(e) KT/q
C
−1
i
q
where Nt is the maximum number of interface traps, K is the Boltzman constant, T is the absolute temperature, and q is the electronic charge. Higher value of ss leads to higher interface trap density for the OFETs with CL-PVP 2 dielectric (Nt = 12 × 1013 cm−2 eV−1 ) compared to OFETs with P4PMS dielectric (Nt = 6.45 × 1013 cm−2 eV−1 ). As the ss is directly correlated to the number of interface traps, therefore, higher value can also be attributed to the increased number of grain boundaries and consequently increased dielectric–semiconductor interface trap density. The comparative results obtained from the transfer characteristics of the devices fabricated with two different gate dielectrics were summarized in Table 1. In addition to the OFETs based on P4PMS and CL-PVP 2 dielectrics, we also fabricated F16 CuPc OFETs on SiO2 /Si substrates at room temperature and the performance was compared with OFETs with the CL-PVP 2 and P4PMS. Mobility, on/off ratio and the Vth values for the OFETs with SiO2 gate dielectric were found to be 3 × 10−4 cm2 /V s, 4 × 102 and 9.41 V, respectively. The performances of OFETs with P4PMS are better or comparable to OFET with CL-PVP 2 and SiO2 dielectric. In addition, since the P4PMS has good electrical properties and could be processed at low temperature (125 ◦ C) where flexible plastic substrates bear, the P4PMS could efficiently be used for the fabrication of high performance flexible OFETs.
4. Conclusions In conclusion, we have fabricated F16 CuPc OFETs based on different polymeric gate dielectrics. It has been demonstrated that by controlling the concentration of cross-linking agent, the electrical and surface properties of the PVP based gate dielectrics can easily be tuned and hence the device performance of F16 CuPc OFET can efficiently be optimized. Furthermore, the performance of F16 CuPc OFET based on two different dielectrics, CL-PVP 2 and P4PMS, and the effect of dielectric/semiconductor interface on the performance of OFETs was investigated. The results revealed that the presence of hydroxyl group can make a dielectric surface more hydrophilic which in turn, gives unfavorable effect on the crystal growth of F16 CuPc. The more hydrophobic, P4PMS surface on the other hand, because of its better interaction with the organic semiconductor can give larger crystal size and hence higher mobility. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy and National Research Foundation (NRF) through EPB Center (R11-2008-052-03003). This work was also supported by Seoul R&BD Program (WR090671). References [1] B. Crone, A. Dodabalapur, Y.-Y. Lin, R.W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar, H.E. Katz, W. Li, Nature 403 (2000) 521. [2] H. Klauk, M. Halik, U. Zschieschang, F. Eder, D. Rohde, G. Schmid, C. Dehm, IEEE Trans. Electron Devices 52 (2005) 618–622. [3] L. Torsi, A. Dodabalapur, L.J. Rothberg, A.W.P. Fung, H.E. Katz, Science 272 (1996) 1462. [4] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [5] M. Shtein, J. Mapel, J.B. Benziger, S.R. Forrest, Appl. Phys. Lett. 81 (2002) 268. [6] J.H. Seo, J.H. Kwon, S.I. Shin, K.S. Suh, B.K. Ju, Semicond. Sci. Technol. 22 (2007) 1039. [7] T.B. Singh, F. Meghdadi, S. Günes, N. Marjanovic, G. Horowitz, P. Lang, S. Bauer, N.S. Sariciftci, Adv. Mater. 17 (2005) 2315. [8] S. Pyo, H. Son, K.Y. Choi, M.H. Yi, S.K. Hong, Appl. Phys. Lett. 86 (2005) 133508. [9] Z. Bao, Y. Feng, A. Dodabalapur, V.R. Raju, A.J. Lovinger, Chem. Mater. 9 (1997) 1299. [10] T.S. Huang, Y.K. Su, P.C. Wang, Appl. Phys. Lett. 91 (2007) 092116.
M. Mukherjee et al. / Synthetic Metals 160 (2010) 504–509 ´ G.J. Matt, N.S. Sariciftci, R. Schwödiauer, S. Bauer, [11] T.B. Singh, N. Marjanovic, Appl. Phys. Lett. 85 (2004) 5409. [12] B. Mukherjee, M. Mukherjee, Org. Electron. 10 (2009) 1282. [13] H. Yan, Z. Chen, Y. Zheng, C. Newman, J.R. Quinn, F. Dötz, M. Kastler, A. Facchetti, Nature 457 (2009) 679. [14] Y. Lu, W.H. Lee, H.S. Lee, Y. Jang, K. Cho, Appl. Phys. Lett. 94 (2009) 113303. [15] H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, W. Weber, J. Appl. Phys. 92 (2002) 5259. [16] C.D. Dimitrakopoulos, P.R.L. Malefant, Adv. Mater. 14 (2002) 99. [17] W. Fix, A. Ullmann, J. Ficker, W. Clemens, Appl. Phys. Lett. 81 (2002) 1735. [18] H. Wang, J. Wang, H. Huang, X. Yan, D. Yan, Org. Electron. 7 (2006) 369. [19] C. Rost, D.J. Gundlach, S. Karg, W. Reiss, J. Appl. Phys. 95 (2004) 5782. [20] K.Y. Law, Chem. Rev. 93 (1993) 449. [21] S. Kobayashi, T. Takenobu, S. Mori, A. Fujiwara, Y. Iwasa, Appl. Phys. Lett. 82 (2003) 4581. [22] P.R.L. Malenfant, C.D. Dimitrakopoulos, J.D. Gelorme, L.L. Kosbar, T.O. Graham, A. Curioni, W. Andreoni, Appl. Phys. Lett. 80 (2002) 2517. [23] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207.
509
[24] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, R.W. Filas, Y.-Y. Lin, Z. Bao, J.H. O’Neill, W. Li, H.E. Katz, J. Appl. Phys. 89 (2001) 5125. [25] J. Wang, H. Wang, X. Yan, H. Huang, D. Yan, Chem. Phys. Lett. 407 (2005) 87. [26] S.H. Kim, J.H. Lee, Y.S. Yang, T. Zyung, Mol. Cryst. Liq. Cryst. 405 (2003) 137. [27] Y. Oh, S. Pyo, M.H. Yi, S.K. Kwon, Org. Electron. 7 (2006) 77. [28] M.E. Roberts, N. Queraltó, S.C.B. Mannsfeld, B.N. Reinecke, W. Knoll, Z. Bao, Chem. Mater. 21 (2009) 2292. [29] Y. Choi, H. Kim, K. Sim, K. Park, C. Im, S. Pyo, Org. Electron. 10 (2009) 1209. [30] S. Lim, S. Kim, J. Koo, J. Lee, C. Ku, Y. Yang, T. Zyung, Appl. Phys. Lett. 90 (2007) 173512. [31] S. Lee, B. Koo, J. Shin, E. Lee, H. Park, H. Kim, Appl. Phys. Lett. 88 (2006) 162109. [32] T. Jung, A. Dodabalapur, R. Wenz, S. Mohapatra, Appl. Phys. Lett. 87 (2005) 182109. [33] A.D. Carlo, F. Piacenza, A. Bolognesi, B. Stadlober, H. Maresch, Appl. Phys. Lett. 86 (2005) 263501. [34] K. Yamamoto, S. Ochiai, X. Wang, Y. Uchida, K. Kojima, A. Ohashi, T. Mizutani, Thin Solid Films 516 (2008) 2695. [35] H.W. Zan, K.H. Yen, P.K. Liu, K.H. Ku, C.H. Chen, J. Hwang, Org. Electron. 8 (2007) 450.