Bottom-Contact Poly(3,3′′′-didodecylquaterthiophene) Thin-Film

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Bottom-Contact Poly(3,3′′′-didodecylquaterthiophene) Thin-Film Transistors with Gold Source-Drain Electrodes Modified by Alkanethiol Monolayers Qin Jia Cai,† Mary B. Chan-Park,*,† Zhi Song Lu,‡ Chang Ming Li,*,‡ and Beng S. Ong§ DiVision of Chemical & Biomolecular Engineering, School of Chemical & Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459, DiVision of Bioengineering, School of Chemical & Biomedical Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, and School of Materials Science and Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798 ReceiVed March 31, 2008. ReVised Manuscript ReceiVed July 15, 2008 A series of alkanethiol monolayers (CH3(CH2)n-1SH, n ) 4, 6, 8, 10, 12, 14, 16) were used to modify gold source-drain electrode surfaces for bottom-contact poly(3,3′′′-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs). The device mobilities of TFTs were significantly increased from ∼0.015 cm2 V-1 s-1 for bare electrode TFTs to a maximum of ∼0.1 cm2 V-1 s-1 for the n ) 8 monolayer devices. The mobilities of devices with alkanethiolmodified Au electrodes varied parabolically with alkyl length with values of 0.06, 0.1, and 0.04 cm2 V-1 s-1 at n ) 4, 8, and 16, respectively. Atomic force microscopy investigations reveal that alkanethiol electrode surface modifications promote polycrystalline PQT-12 morphologies at electrode/PQT-12 contacts, which probably increase the density of states of the PQT-12 semiconductor at the interfaces. The contact resistance of TFTs is strongly modulated by the surface modification and strongly varies with the alkanethiol chain length. The surface modifications of electrodes appear to significantly improve the charge injection, with consequent substantial improvement in device performance.

Introduction Contacts between metal electrodes and organic semiconductors play a crucial role in organic electronic devices.1-3 The charge injection through electrode/semiconductor contacts strongly depends on the energy alignment,4-7 dopant concentration of the organic semiconductor,8-11 and semiconductor morphology12-17 at electrode/semiconductor contacts. In organic thin* To whom correspondence should be addressed. (M.B.C.-P.) E-mail: [email protected]. Phone: +65 67906064. Fax: +65 67924762. (C.M.L.) E-mail: [email protected]. Phone: +65 67904485. Fax: +65 67911761. † Division of Chemical & Biomolecular Engineering, School of Chemical & Biomedical Engineering. ‡ Division of Bioengineering, School of Chemical & Biomedical Engineering. § School of Materials Science and Engineering. (1) Sirringhaus, H. AdV. Mater. 2005, 17, 2411. (2) Kahn, A.; Koch, N.; Gao, W. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2548. (3) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605. (4) Lyon, J. E.; Cascio, A. J.; Beerbom, M. M.; Schlaf, R.; Zhu, Y.; Jenekhe, S. A. Appl. Phys. Lett. 2006, 88, 222109. (5) Diao, L.; Frisbie, C. D.; Schroepfer, D. D.; Ruden, P. P. J. Appl. Phys. 2007, 101, No. 014510. (6) Burgi, L.; Richards, T. J.; Friend, R. H.; Sirringhaus, H. J. Appl. Phys. 2003, 94, 6129. (7) Koch, N.; Kahn, A.; Ghijsen, J.; Pireaux, J. -J.; Schwartz, J.; Johnson, R. L.; Elschner, A. Appl. Phys. Lett. 2003, 82, 70. (8) Hosseini, A. R.; Wong, M. H.; Shen, Y.; Malliaras, G. G. J. Appl. Phys. 2005, 97, No. 023705. (9) Rep, D. B. A.; Morpurgo, A. F.; Klapwijk, T. M. Org. Electron. 2003, 4, 201. (10) Cai, Q. J.; Chan-Park, M. B.; Zhang, J.; Gan, Y.; Li, C. M.; Chen, T. P.; Ong, B. S Org. Electron. 2008, 9, 14. (11) Minari, T.; Miyadera, T.; Tsukagoshi, K.; Aoyagi, Y.; Ito, H. Appl. Phys. Lett. 2007, 91, No. 053508. (12) Kymissis, I.; Dimitrakopoulos, C. D.; Purushothaman, S. IEEE Trans. Electron DeVices 2001, 48, 1060. (13) Stoliar, P.; Kshirsagar, R.; Massi, M.; Annibale, P.; Albonetti, C.; de Leeuw, D. M.; Biscarini, F. J. Am. Chem. Soc. 2007, 129, 6477. (14) Dholakia, G. R.; Meyyappan, M.; Facchetti, A.; Marks, T. J. Nano Lett. 2006, 6, 2447. (15) Bock, C.; Pham, D. V.; Kunze, U.; Kafer, D.; Witte, G.; Woll, Ch. J. Appl. Phys. 2006, 100, No. 114517. (16) Cao, Q.; Zhu, Z. T.; Lemaitre, M. G.; Xia, M. G.; Shim, M.; Rogers, J. A. Appl. Phys. Lett. 2006, 88, No. 113511.

film transistors (OTFTs), gold is usually used for source-drain (S-D) electrodes because of its strong chemical stability and high work function (∼5.1 eV), which in theory is energy compatible with the highest occupied molecular orbital (HOMO) of most p-type organic semiconductors. However, significant energy barriers for charge injection are commonly formed at Au/semiconductor contacts for OTFTs via the energy alignment.2-4,18 In addition, the reduction of the work function of Au electrodes due to environmental contamination also could affect the charge injection for solution-processable OTFTs.18,19 Increasing the dopant concentration of the organic semiconductor at electrode/semiconductor contacts could be an option to improve the charge injection.8-11 With a higher doped concentration of organic semiconductors, lower Schottky barriers at electrode/ semiconductor contacts under a bias could be achieved to improve charge injection.20 To improve the charge injection from Au electrodes into organic semiconductors, various surface modifications of Au electrodes have been used to reduce the Schottky barriers at Au/semiconductor contacts.21-24 The modulation of the organic morphology at electrode/ semiconductor contacts has also been used to affect the charge injection in OTFTs.12-17 It has been demonstrated that the charge injection from Au electrodes into pentacene can be significantly improved when the morphology of pentacene adjacent to the Au electrodes is improved by surface modification of Au electrodes (17) Koch, N.; Elschner, A.; Schwartz, J.; Kahn, A. Appl. Phys. Lett. 2003, 82, 2281. (18) Grobosch, M.; Knupfer, M. AdV. Mater. 2007, 19, 754–756. (19) Wan, A.; Hwang, J.; Amy, F.; Kahn, A. Org. Electron. 2005, 6, 47. (20) Sze, S. M.; Ng, K. K. Physics of Semiconductor DeVices, 3rd ed.; Wiley: New York, 2007; Chapter 3. (21) Zehner, R. W.; Parsons, B. F.; Hsung, R. R.; Sita, L. R. Langmuir 1999, 15, 1121. (22) Hamadani, B. H.; Corley, D. A.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Nano Lett. 2006, 6, 1303. (23) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. AdV. Mater. 2005, 17, 621. (24) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. ReV. B 1996, 54, 14321.

10.1021/la8009942 CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

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with alkanethiol self-assembled monolayers (SAMs) of alkanethiol. The treatment of Au electrodes with alkanethiol SAMs may passivate the Au electrodes and decouple or partially decouple the interactions between pentacene and Au surfaces, which results in an improved morphology of pentacene at Au/ pentacene contacts. The improved charge injection arising from the SAM modifications gives rise to the improved device performance of pentacene TFTs with a higher drain current. Recently, alkanethiol SAMs have also been used to surfacemodify Au S-D electrodes for OTFTs based on the polymer semiconductors regioregular poly(3-hexylthiophene) (P3HT) and poly[(2-methoxy-5-((2′-ethylhexyl)oxy)-1,4-phenylene)vinylene] (MEH-PPV).25 It was found that SAM modification of Au contacts suppressed the charge injection from electrodes into polymer semiconductors with respect to the untreated Au contacts. However, the polymer morphology adjacent to the Au contacts was not investigated. Since the morphologies of organic semiconductors can exert a significant influence on charge injection through electrode/semiconductor contacts, it is necessary to explore or understand the relationship between the polymer morphology and charge injection for solution-processable polymer thin film transistors (TFTs). Poly(3,3′′′-didodecylquaterthiophene) (PQT-12) is a novel, high-performance polymeric semiconductor of liquid crystalline morphology.26 It could be a good candidate for investigating the influence of polymeric morphology on charge injection at electrode/semiconductor contacts. Research using SAM modification of Au S-D electrodes for TFTs based on the PQT-12 semiconductor is limited to a single preliminary investigation,27 and that study did not consider the effect of the SAM alkyl chain length on the PQT-12 TFT performance. In this study, a series of alkanethiol (CH3(CH2)n-1SH, n ) 4, 6, 8, 10, 12, 14, 16) SAMs were employed to modify the surfaces of Au S-D electrodes for bottom-contact PQT-12 TFTs. The surface morphologies of PQT-12 semiconductor films on bare Au and alkanethiol-modified Au surfaces were investigated by atomic force microscopy (AFM). The charge injection from the electrode into the PQT-12 semiconductor was investigated using coplanar PQT-12 diodes with a short channel length (∼1 µm), and the effective contact resistance of PQT-12 TFTs was studied using the transfer line method. The AFM results suggest that the alkanethiol-modified Au surfaces result in polycrystalline PQT12 morphologies while bare Au surfaces give rise to an amorphous PQT-12 morphology. Charge injection investigations suggest that charge injection can be significantly improved via the surface modification of Au electrodes with alkanethiol SAMs. The improved charge injection could be attributable to the improved PQT-12 morphologies on alkanethiol-modified Au surfaces and results in significantly improved device performance of PQT-12 thin-film transistors.

Experimental Section Materials. 1-Butanethiol (99%), 1-hexanethiol (95%), 1-octanethiol (98.5%), 1-decanethiol (96%), 1-dodecanethiol (98%), 1-tetradecanethiol (98%), 1-hexadecanethiol (92%), octyltrichlorosilane (97%), and HPLC hexane were obtained from Sigma-Aldrich and used as received without further purification. Semiconductorgrade acetone, ethanol, and 2-propanol were received from J.T. Baker. Gold (99.999%) and titanium (99.995%) pellets for electron beam deposition were obtained from Kurt J. Lesker. Poly(3,3′′′-didode(25) Asadi, K.; Gholamrezaie, F.; Smits, E. C. P.; Blom, P. W. M.; de Boer, B. J. Mater. Chem. 2007, 17, 1947. (26) Ong, B. S.; Wu, Y.; Liu, P. J. Am. Chem. Soc. 2004, 126, 3378. (27) Cai, Q. J.; Chan-Park, M. B.; Zhou, Q.; Lu, Z. S.; Li. C. M.; Ong, B. S. Org. Electron., in press.

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Figure 1. Structural illustration of bottom-contact PQT-12 TFTs with (a) bare Au and (b) alkanethiol-modified Au source-drain electrodes.

cylquaterthiophene) solution in dichlorobenzene (0.3 wt %) was provided by the Xerox Research Centre of Canada. Patterning. Heavily n-doped silicon (Si) wafers with a 100 nm thick top oxide layer (SiO2) were patterned using photoresist AZ7220 (from Clariant). After patterning, the wafers were introduced to an electron beam evaporator (BOC Edwards A306), and thin metal films (1 nm of Ti and 50 nm of Au) were deposited on them under a 1 × 10-6 Torr vacuum. After the deposition, the patterned photoresist was cleared from the wafers by immersion in acetone assisted by sonication to finalize the electrode patterning. Device Fabrication. A series of bottom-contact PQT-12 TFTs were fabricated on heavily n-doped silicon wafers with electrode patterning. The silicon wafer and its surface SiO2 layer (100 nm) served as the gate electrode and gate dielectric, respectively. The channel lengths were 20, 40, 60, 80, and 100 µm. The channel width was 9000 µm for all devices. For TFTs with bare Au S-D electrodes, electrode-patterned wafers were briefly plasma-cleaned and treated with a 3 mM solution of octyltrichlorosilane (OTS-8) in hexane at room temperature for 15 min, rinsed with hexane and 2-propanol, and dried with nitrogen gas. For TFTs with alkanethiol-modified Au S-D electrodes, the patterned Au electrodes were additionally immersed in a 3 mM solution of alkanethiol in ethanol at room temperature for 24 h, rinsed with hexane and ethanol, and dried with nitrogen gas. Finally, PQT-12 semiconductor layers of about 60 nm thickness were deposited on the wafers by spin-coating. The semiconductor layer was then dried, annealed at 125 °C for 30 min, and cooled to room temperature overnight in a vacuum. Characterizations. The devices were characterized using an Agilent 4157B semiconductor parameter analyzer system. AFM experiments were conducted with a Nanoscope IIIa MultiMode scanning probe microscope (Digital Instruments) in tapping mode with a scan rate of 0.5 Hz. Device mobilities in the linear and saturation regimes were extracted from the following equations (neglecting contact resistance).

linear regime (Vds,VGs - VT): IDS ) VDSCiµ(VGS - VT)W/L saturation regime (Vds>VGs - VT): IDS ) Ciµ(VGS - VT)2W/2L where IDS is the drain current, Ci is the capacitance per unit area of the gate dielectric layer, and VGS and VT are gate voltage and threshold voltage.

Results and Discussion The structures of the bottom-contact devices with bare Au and alkanethiol-modified Au electrodes are illustrated in Figure 1. The output (ID-VD) characteristics of PQT-12 TFTs with bare Au S-D electrodes are shown in Figure 2a. The ID-VD curves in the low VD region exhibit obvious concave nonlinear characteristics, indicating that the charge injection process is non-Ohmic with significant suppression of charge injection from the electrodes into the semiconductor. The saturation device mobility of these devices is about 0.015 cm2 V-1 s-1, and the on/off ratio is in the range of 105-106.

Bottom-Contact PQT-12 TFTs with Au S-D Electrodes

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Figure 2. Output (VD-ID) characteristics of PQT-12 thin film transistors with (a) bare Au and (b) 1-octanethiol (C8H17SH)-modified Au electrodes. The device channel length and width are 20 µm and 9000 µm.

Figure 3. Linear (VD ) -5 V) and saturation (VD ) -55 V) device mobilities of PQT-12 thin-film transistors versus the alkanethiol chain length n.

Figure 2b shows the output characteristics of PQT-12 TFTs with Au electrodes surface-modified with 1-octanethiol (C8H17SH) SAMs. As shown in Figure 2b, the ID-VD curves in the low VD region are linear rather than concave nonlinear, suggesting the charge injection has been much improved in these devices. In addition, the saturation current of the device in Figure 2b is about 5-fold higher than that in Figure 2a. The saturation device mobility is improved to about 0.09 cm2 V-1 s-1. The on/off ratio of these devices is still high, about 106. It suggests that the device performance of PQT-12 TFTs can be significantly improved by surface modification of the electrodes with 1-octanethiol SAMs. The improved device performance of TFTs was observable at every tested SAM chain length, n, in the range from 4 to 16. Figure 3 shows the variation of the linear and saturation device mobilities with the chain length n of the alkanethiol molecules. The linear and saturation mobilities increase by factors of about 10 and 5, respectively, as n increase from n ) 0 (bare Au) to n ) 8 and then decline at larger n. This behavior is similar to that for OTFTs based on pentacene with alkanethiol-modified Au electrodes.13 All the devices with alkanethiol-modified Au electrodes gave higher linear and saturation device mobilities than those with bare Au electrodes. The results suggest that the alkanethiol SAM modifications of Au S-D electrode surfaces

probably have improved the charge injection and consequently improved the device performance of PQT-12 TFTs. The surface energy of polycrystalline Au surfaces can be reduced by chemisorption of alkanethiol SAMs.28 The reduction of the Au surface energy could result in a change in the morphologies of organic semiconductors and thereby influence the charge injection in organic TFTs.12-14 To investigate the influence of alkanethiol modifications on the PQT-12 morphology, AFM was used to characterize the PQT-12 morphologies on bare Au and alkanethiol-modified Au surfaces. Figure 4 shows AFM phase images and corresponding surface phase profiles of 20-30 nm thick PQT-12 films on a bare Au surface, a 1-butanethiol (n ) 4)-modified Au surface, a 1-octanethiol (n ) 8)-modified Au surface, and a 1-hexadecanethiol (n ) 16)-modified Au surface after postdeposition annealing under a vacuum. The postdeposition annealing temperature of about 125 °C is well below the critical temperature of about 180 °C, at which alkanethiol monolayers desorb from Au surfaces.29 As shown in Figure 4a, the AFM phase image of the PQT-12 film on the bare Au surface exhibits an amorphous morphology. In sharp contrast, AFM phase images of PQT-12 films on 1-butanethiol-, 1-octanethiol-, and 1-hexadecanethiol-modified Au surfaces show extensive nanodomains (∼35 nm in diameter) of the PQT-12 semiconductor polycrystallinity. The polycrystalline morphologies of PQT-12 were also generally observed on other alkanethiol-modified Au surfaces. The surface phase profiles of PQT-12 films on 1-butanethiol-, 1-octanethiol-, and 1-hexadecanethiol-modified Au surfaces are also much rougher than that on the bare Au surface. The formation of an amorphous PQT-12 film on the bare Au surface is probably due to interaction between the Au surface and the π-system of the PQT-12 semiconductor which prevents crystallization and self-organization of PQT-12 at the bare Au electrode. A similar interaction between oligothiophenes/pentacene and Au surfaces has been suggested to cause the formation of an amorphous semiconductor at the Au/ semiconductor contacts.12,14,30 The interaction of Au surfaces with organic semiconductors can be weakened by contaminations due to exposure to air and/or organic solvents. It has also been (28) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (29) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361. (30) Yang, Z. Y.; Zhang, H. M.; Yan, C. J.; Li, S. S.; Yan, H. J.; Song, W. G.; Wan, L. J. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 3707.

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Figure 4. Representative AFM phase images and corresponding surface phase profiles of PQT-12 films (20-30 nm in thickness) on (a) a bare Au surface, (b) a 1-butanethiol (C4H9SH)-modified Au surface, (c) a 1-octanethiol (C8H17SH)-modified Au surface, and (d) a 1-hexadecanethiol (C16H33SH)modified Au surface after postdeposition annealing.

reported that these contaminations could not effectively reduce the surface charge concentration on Au surfaces and electronically decouple the Au surface and organic semiconductors.18 In the present study, although the interaction between the bare Au surface may be weakened by Au surface contaminations and organic solvents of PQT-12, which could be removed by vacuum annealing, amorphous PQT-12 at the interface was still observable. When the Au surface was chemically passivated by alkanethiol SAMs, the surface charge concentration on the Au surface could effectively be reduced via the formation of Au-S bonds on the Au surface.28 The alkanethiol SAM treatments probably inhibited the interaction of the π-system of the PQT-12 semiconductor with the Au surface and promoted the formation of polycrystalline PQT-12. The polycrystalline PQT-12 formation probably arises from the self-organization of PQT-12 polymer chains into laminar π-π stacks similar to those promoted by SAMs of alkyl chains on octyltrichlorosilane-modified SiO2 surfaces.31 The effect of the chain length n on the PQT-12 morphology is rather subtle and cannot be easily inferred from the visual inspection or surface phase profiles of AFM phase images of PQT-12 films on different alkanethiol-modified Au surfaces. (31) Wu, Y.; Liu, P.; Ong, B. S.; Srikumar, T.; Zhao, N.; Botton, G.; Zhu, S. Appl. Phys. Lett. 2005, 86, No. 142102.

Amorphous or disordered organic semiconductors in the solid state are characteristics of low charge mobility since charge injection into and transport within amorphous organic semiconductors are dominated by hopping transport and there is scattering at every hopping step between individual molecules.32-35 In polycrystalline or highly ordered organic semiconductors, the cofacial π-π stacking structure is expected to give rise to charge carrier delocalization between molecules and thereby to facilitate charge carrier transport.32-35 The charge mobility of PQT-12 in polycrystalline states has been shown to be several orders of magnitude higher than that in amorphous states.31,36 In terms of charge injection from electrodes into organic semiconductors, it has been shown that the injected current can be significantly improved by increasing the charge mobility of organic semiconductors.37 By promoting the formation of PQT12 polycrystallinity at the Au/PQT-12 contacts and, consequently, (32) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (33) Street, R. A. Nat. Mater. 2006, 5, 171. (34) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. Chem. ReV. 2007, 107, 926. (35) Warman, J. M.; de Haas, M. P.; Dicker, G.; Grozema, F. C.; Piris, J.; Debije, M. G. Chem. Mater. 2004, 16, 4600. (36) Street, R. A.; Northrup, J. E.; Salleo, A. Phys. ReV. B 2005, 71, 165202. (37) Shen, Y.; Klein, M. W.; Jacobs, D. B.; Scott, J. C.; Malliaras, G. G. Phys. ReV. Lett. 2001, 86, 3867.

Bottom-Contact PQT-12 TFTs with Au S-D Electrodes

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Figure 5. Current versus the fourth root of the applied bias characteristics of planar PQT-12 diodes with bare Au and alkanethiol-modified Au electrodes at room temperature. PQT-12 diodes with a channel length of about 1 µm were fabricated on an octyltrichlorosilane-modified oxide Si wafer analogous to PQT-12 thin-film transistors.

higher charge carrier mobility of the PQT-12 semiconductor at the electrode/semiconductor contacts, alkanethiol SAM modifications of the Au electrodes are postulated to have a net effect of improving charge injection in PQT-12 TFTs. To further correlate the alkanethiol chain length with subtle morphological details of the PQT-12 semiconductor and charge injection at the electrode/PQT-12 contacts, charge injection in the coplanar PQT-12 diode was investigated by studying the effect of n on the effective density of states of the organic semiconductor. The effective density of states is strongly correlated to the molecular ordering of the organic semiconductor.36 According to thermionic-emission theory at room temperature, the current transport over a Schottky barrier at a metal/ semiconductor contact can be expressed as9,20

( ) ( )

J ∝ exp ∆φ )

[

qφB q∆φ exp kT kT

q3N(V + Vbi - kT/q) 8π2εs

]

(1a) 1⁄4

(1b)

where J is the current density, T is the absolute temperature, q is the electron charge, φB is the Schottky barrier height, k is Boltzmann’s constant, ∆φ is the Schottky-barrier lowering due to the combined effects of the image force and applied electric fields, N is the effective density of states, V is the applied voltage, Vbi is the built-in potential, and εs is the semiconductor permittivity. According to eqs 1a and 1b, the logarithm of the current density should scale linearly with the fourth root of the applied voltage (V1/4) neglecting the term Vbi - kT/q, with a slope ((q/kT)(q3N/8π2εs)1/4) linearly proportional to the fourth root of the effective density of states N. Figure 5 shows the current versus the fourth root of the applied voltage characteristics of planar bare Au/PQT-12 and alkanethiolmodified Au/PQT-12 diodes at room temperature. The diodes were fabricated by deposition of PQT-12 thin films between coplanar electrode contacts on octyltrichlorosilane-modified SiO2. The device structure resembles that of the PQT-12 TFTs described above, but with a much shorter channel length (∼1 µm). The short channel length was employed to study the charge transport over the electrode/PQT-12 contacts because charge transport in diodes with a short channel length is mainly interface-limited and provides direct information about the charge transport over the Schottky barrier. As shown in Figure 5a the current transport

characteristics of electrode/PQT-12 diodes are well described by eqs 1a and 1b above the lower range of the applied bias. The injection current of diodes with alkanethiol-modified Au electrodes is much higher than that with bare Au electrodes. The relative rank of the I(V) curves versus the chain length n in Figure 5a closely follows the ordering of the device mobility of TFTs versus n in Figure 3, which may be adduced as evidence in support of the above analysis that the device performance of TFTs is strongly influenced by the charge injection in these devices. Figure 5b shows the variation of the slopes of the I-V1/4 characteristics in Figure 5a with the chain length n. The slope values were derived from linearly fitting the data in Figure 5a. The slopes of the fitting lines for alkanethiol-modified Au/PQT12 contacts are significantly steeper than that of bare Au/PQT12 contacts and slightly increase to a maximum as the chain length increases from n ) 4 to n ) 12 and then decrease. The results suggest that the effective density of states of the PQT-12 semiconductor at alkanethiol-modified Au/PQT-12 contacts is higher than that at unmodified contacts. The SAM modification of electrodes leads to increased order in the semiconductor morphology (Figure 4), which should give rise to the increased delocalized charge carriers and increased effective density of states, N, in the band.36,38 With a higher PQT-12 semiconductor effective density of states at the contacts, the Schottky-barrier lowering effect could be stronger under a bias, resulting in improved charge injection at modified Au/PQT-12 contacts. The maximum slope at n ) 12 suggests maximum molecular ordering at that chain length, but this effect could also be modulated by the interfacial energetics3,20-25,39-41 and SAM thickness42-47 at (38) Salleo, A. Mater. Today 2007, 10, 38. (39) Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121. (40) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N.; Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690. (41) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. Acc. Chem. Res. 2007, 41, 721. (42) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (43) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D. J. Am. Chem. Soc. 2004, 126, 14287. (44) Nesher, G.; Vilan, A.; Cohen, H.; Cahen, D.; Amy, F.; Chan, C.; Hwang, J.; Kahn, A. J. Phys. Chem. B 2006, 110, 14363. (45) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103.

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Figure 6. Contact resistance versus the chain length n of alkanethiol molecules characteristics in PQT-12 thin-film transistors. The contact resistance of each alkanethiol modification was estimated by channel length scaling analysis using the TLM. The dashed curve is the curvefitting line for experimental data of Au electrodes surface-modified with alkanethiol SAMs of chain length n ) 8-16. The exponential curvefitting gave RC(kΩ) ) 0.53 exp(0.35n).

the interfaces. For the alkanethiol-modified Au contacts, the alkanethiol SAMs and corresponding modified Au substrates can be regarded as an integrative electrode since the charge carrier can effectively tunnel through the alkanethiol SAM layer.22-24,39-41 The effective density of states in this work should be referred to that of a PQT-12 semiconductor directly contacting alkanethiol SAMs on a Au surface. The effective contact resistance of PQT-12 TFTs was also investigated by channel length scaling analysis methods using the transmission line model (TLM).48-50 The model assumes that the device “on” resistance (Ron) approximately consists of the channel-length-dependent channel resistance (RCh ) [L/WµiCi(VGS - VT)]) and channel-length-independent contact resistance (RC) at a low drain voltage. The contact resistance can be determined by extrapolating the dependence of the device on resistance on the channel length to a zero channel length. The on resistances of PQT-12 TFTs were determined at a gate voltage of -40 V and a drain voltage of -3 V. Figure 6 shows the variation of the contact resistance with the chain length n in PQT-12 TFTs with alkanethiol-modified Au electrodes. The estimated contact resistance of bare Au electrodes is much higher than that of alkanethiol-Au electrodes. The contact resistance of PQT-12 TFTs with alkanethiol-modified Au electrodes decreases as the chain length increases from n ) 4 to n ) 8, reaching a minimum, and increases as the chain length increases from n ) 8 to n ) 16. The variation of the contact resistance with n in Figure 6 is highly correlated with that for device mobility in Figure 3, which suggests that the contact resistance in the PQT12 TFTs strongly affects the device performance. For devices with Au electrodes modified by alkanethiol SAMs of longer chain length (n > 6), the contact resistance can be described by the exponential relationship RC(kΩ) ) 0.53 exp(0.35n). The exponential relationship is probably associated with the SAM thickness and dipole moment of the alkanethiol monolayers. (46) Scaini, D.; Castronovo, M.; Casalis, L.; Scoles, G. ACS Nano 2008, 2, 507. (47) Wang, W.; Lee, T.; Reed, M. A. Phys. ReV. B 2003, 68, 035416. (48) Luan, S.; Neudeck, G. W. J. Appl. Phys. 1992, 72, 766. (49) Lefenfeld, M.; Blanchet, G.; Roger, J. A. AdV. Mater. 2003, 14, 1188. (50) Gundlach, D. J.; Zhou, L.; Nichols, J. A.; Jackson, T. N.; Necliudov, P. V.; Shur, M. S. J. Appl. Phys. 2006, 100, No. 024509.

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Charge transport through the alkanethiol monolayers is via tunneling and decreases exponentially with increasing alkanethiol chain length n.42-47 In addition, an increased chain length could increase the SAM dipole moment, which would reduce the Au work function by 10-20 meV/CH2,40 which might increase the Schottky barrier height for hole injection and in turn result in an exponential decrease of hole injection with n (eq 1a). Campbell et al.24 experimentally demonstrated that hole injection barriers were increased by 0.5 eV at the silver/MEH-PPV interface when the work function of silver was reduced by modification with alkanethiol SAMs (C10H21SH). Ab initio Hartree-Fock calculation of the alkanethiol SAM dipole moment also successfully described the surface potential change of silver due to the alkanethiol SAM treatment. Recently, the work function of Au electrodes has also been experimentally shown to be reduced by alkanethiol SAMs by about 0.8 eV.23 Hole injection was also found to be suppressed by the introduction of alkanethiol SAM treatment for Au electrodes in bottom-contact MEH-PPV TFTs.25 The exponential increase of the contact resistance with n was not observable for devices with a shorter chain length (n e 6). The decrease of the contact resistance with n for small n may be associated with the PQT-12 morphology of PQT-12/electrode contacts. As shown in Figure 5b, the slope values of alkanethiol SAMs with a short chain length (n e 6) are slightly lower than those of alkanethiol SAMs with a longer chain length (n > 8). Alkanethiol SAMs of shorter chain length probably could not fully decouple the interactions of Au surfaces and the π-system of PQT-12 and increase the PQT-12 order to the same scale as alkanethiol SAMs of longer chain length (n > 6). The observed maximum mobility and minimum contact resistance at n ) 8 for the alkanethiol SAM series in Figures 3 and 6, respectively, are probably the net effect of several competing factors, including the SAM thickness,13,42-47 the interfacial energetics associated with the dipole moments of the alkanethiol,3,21-25,39-41 and molecular orderings of the PQT-12 semiconductor at the interfaces.12-17,37 As n increases, the thickness of the alkanethiol SAMs increases to give rise to a decreased charge injection; at the same time, the dipole moment of the alkanethiol SAMs decreases the Au work function, which could also result in a decreased charge injection. However, with SAM treatment, the molecular ordering of PQT-12 could be improved to increase charge injection. Our results conclusively show the importance of morphology. More detailed investigations are needed to pinpoint the relative significance of each factor.

Conclusion We have fabricated bottom-contact PQT-12 TFTs in which the Au source-drain electrodes have been surface-modified via a series of alkanethiol SAMs. The linear and saturation device mobilities of PQT-12 TFTs were much increased by the alkanethiol SAM modification. AFM study indicates that these surface modifications are associated with, and presumably promote, the formation of PQT-12 polycrystalline morphology at the contacts. The increased molecular order of PQT-12 probably has improved the charge injection and resulted in improved device performance. The electrode surface modification reduced the contact resistance of TFTs (with optimal resistance at n ) 8 layers) and probably increased the density of states in the PQT12 semiconductor. The net effect of the microphysical consequences of the surface modifications is to improve charge injection and device mobilities of the PQT-12 TFTs. Acknowledgment. This work is financially supported by Singapore A*STAR under Grant No. 052 117 0031. LA8009942