pubs.acs.org/Langmuir © 2010 American Chemical Society
Organic Thin-Film Transistors: The Passivation of the Dielectric-Pentacene Interface by Dipolar Self-Assembled Monolayers Franziska D. Fleischli,* Stephane Suarez,† Michel Schaer, and Libero Zuppiroli EPFL STI IMX LOMM, PH D2 435, Station 31015 Lausanne, Switzerland. †Present address: EPFL STI IMX LMOM, MXG 040, Station 12, 1015 Lausanne, Switzerland Received May 21, 2010. Revised Manuscript Received August 12, 2010 In organic thin-film transistors (OTFTs), the conducting channel is located near the interface between the organic semiconductor and the oxide dielectric; this interface is crucial for transistor performance. Self-assembled monolayers (SAMs) on the interface reduce the negative influences of the oxide dielectric surface by decreasing the coupling of the carriers at the gate and the role of the active surface defects on transfer. In this paper, we show that SAMs carrying a dipole moment determine the OTFT performance by controlling the charge transfer between the oxide dielectric and the semiconductor. The charges introduced into the semiconductor by this transfer (i.e., residual carriers) lead to a threshold shift to positive values, as well as a decrease in the contact resistance and an increase in the apparent mobility. In this study, other effects of the SAMs, such as the gate potential shift in the channel or a direct reaction between semiconductor and SAM molecules, can be excluded as dominant processes.
Introduction Organic transistors built on bare SiO2 suffer from several problems linked to the nature of the oxide-organic semiconductor interface. To overcome these problems, one can introduce a selfassembled monolayer (SAM) at the interface between dielectrics and semiconductors. Although this method has been employed previously,1-8 a consensus has not yet emerged on the specific role of this layer on the transport properties in the channel. In the absence of a SAM, the conducting channel of an OFET is located near the dielectric surface and the carrier is dressed by a polarization cloud in the form of a Fr€ohlich polaron.9-12 The coupling strength is determined by the dielectric properties of the gate oxide. Moreover, the oxide surface contains defect sites which can trap charge carriers in the channel or introduce additional charges (residual carriers) into the channel. The SAM covers the defects and separates the dielectric from the semiconductor, thereby enhancing transistor performance (i.e., an increase in the apparent mobility, the on/off ratio, and a small negative threshold).1,2 The latter increases apparent mobility by decoupling the semiconductor from the dielectric surface.9 The SAM also acts as an energy barrier against charge transfer *
[email protected]. (1) Knipp, D.; Street, R. A.; Voelkel, A.; Ho, J. J. Appl. Phys. 2002, 93, 347. (2) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Appl. Phys. Lett. 2002, 81, 268. (3) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y. Nat. Mater. 2004, 3, 317. (4) Takeya, J.; Nishikawa, T.; Takenobu, T.; Kobayashi, S.; Iwasa, Y.; Mitani, T.; Goldmann, C.; Krellner, C.; Batlogg, B. Appl. Phys. Lett. 2004, 85, 2004. (5) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431. (6) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schuetz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963. (7) Koo, J. B.; Kim, S. H.; Lee, J. H.; Ku, C. H.; Lim, S. C.; Zyung, T. Synth. Met. 2006, 156, 99. (8) Von Muehlenen, A.; Castellani, M.; Schaer, M.; Zuppiroli, L. Phys. Status Solidi B 2008, 245, 1170. (9) Kirova, N.; Bussac, M. N. Phys. Rev. B 2003, 68, 235312. (10) Houili, H.; Picon, J. D.; Zuppiroli, L.; Bussac, M. N. J. Appl. Phys. 2006, 100, 023702. (11) Hulea, I. N.; Fratini, S.; Xie, H.; Mulder, C. L.; Iossad, N. N.; Rastelli, G.; Ciuchi, S.; Morpurgo, A. F. Nat. Mater. 2006, 5, 982. (12) Konezny, S. J.; Bussac, M. N.; Zuppiroli, L. Phys. Rev. B 2010, 81, 04513.
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between the dielectric and the semiconductor; this occurs by tunneling through the monolayer.13 Because of the modified dielectric surface, the semiconductor growth on the dielectric can be changed. The performance of an organic transistor containing a monolayer strongly depends on the type of SAM which is used.3,5,14 The length and the chemical nature of the SAM molecules determine the effect of the SAM. Changes in film morphology do not dominate transistor performance.5,15 One of the main effects of a self-assembled monolayer is that it hinders a charge transfer between dielectric and semiconductor by acting as an energy barrier against tunneling.13 Charge transfer through SAMs was mainly analyzed in organic light-emitting diodes (at the contacts) and in organic transistors (at the contacts and in the channel).14,16-19 Depending on the sign of the dipole moment of the SAM, tunneling is enhanced or hindered. In transistors, if the SAM is at the dielectric-pentacene interface, the dipole moment also causes a gate potential shift in the conducting channel; this has the same effect as the applied gate voltage. It was considered to cause an accumulation or depletion of charge carriers. Additional charges can also be transferred into the semiconductor by direct reaction between SAM molecules and semiconductor molecules. In general, such reactions are not likely, but they can be used in some cases to control the threshold shift.20 (13) Zuppiroli, L.; Si-Ahmed, L.; Kamaras, K.; Nuesch, F.; Bussac, M. N.; Ades, D.; Siove, A.; Moons, E.; Graetzel, M. Eur. Phys. J. B 1999, 11, 505. (14) Takeya, J.; Goldmann, C.; Haas, S.; Pernstich, K. P.; Ketterer, B.; Batlogg, B. J. Appl. Phys. 2003, 9, 5800. (15) Suarez, S.; Fleischli, F. D.; Schaer, M.; Zuppiroli, L. J. Phys. Chem. 2010, 114, 7153. (16) Nuesch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L. Chem. Phys. Lett. 1998, 288, 861. (17) Mathijssen, S. G. J.; van Hal, P. A.; van den Biggelaar, T. J. M.; Smits, E. C. P.; de Boer, B.; Kemerink, M.; Janssen, R. A. J.; de Leeuw, D. M. Adv. Mater. 2008, 20, 2703. (18) Gundlach, D. J.; Royer, J. E.; Park, S. K.; Subramanian, S.; Jurchescu, O. D.; Hamadani, B. H.; Moad, A. J.; Kline, R. J.; Teague, L. C.; Kirillov, O.; Richter, C. A.; Kushmerick, J. G.; Richter, L. J.; Parkin, S. R.; Jackson, T. N.; Anthony, J. E. Nat. Mater. 2008, 7, 216. (19) Saudair, S. R.; Frail, P. R.; Kagan, C. R. Appl. Phys. Lett. 2009, 95, 023301. (20) Possanner, S. K.; Zojer, K.; Pacher, P.; Zojer, E.; Schuerrer, F. Adv. Funct. Mater. 2009, 19, 958.
Published on Web 08/24/2010
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Figure 1. (a) OTFT architecture with a self-assembled monolayer at the dielectric-pentacene interface and (b) the molecules of the SAMs used in this work with their dipole moments.
If additional charges (residual carriers) are transferred to the semiconductor (either by tunneling or by a reaction), transistor performance is affected in several ways. First, contact resistance and on-off ratio are reduced; second, apparent mobility is increased and threshold voltage is shifted to positive values.8,21 For elevated residual carrier densities, the threshold shift depends linearly on the residual carrier density: ΔVt ¼
Pres 3 jej Cgate
ð1Þ
where Pres is the residual carrier density, e the electron charge, and Cgate the capacitance of the gate dielectric.21 In this work, pentacene-based organic thin-film transistors were modified by a coherent set of self-assembled monolayers with different dipole moments (from -2.6 to þ3.1 D). All SAM molecules have similar length, but differ in the chemical end group. We found that the dipole moment controls the charge transfer through the SAM and thus the residual carrier density in pentacene. The residual carriers, in turn, determine transistor performance such as contact resistance, apparent mobility, and threshold voltage.
Materials and Methods OTFT Architecture. The organic thin-film transistors (OTFTs) are based on pentacene as organic semiconductors and fabricated by high-vacuum evaporation in top-contact configuration (see Figure 1 a).21,22 Heavily boron-doped Si wafers (resistivity of 0.1-0.5 Ω cm) were used as substrates and served also as gate contacts. The wafer contained a thermal SiO2 layer (200 nm in thickness). An additional Al2O3 layer is necessary for SAM grafting23 and was deposited on SiO2 by radio frequency magnetron sputtering using Ar plasma with a power of 100 W, at a voltage of -290 V and a pressure of 8 10-3 mbar with a thickness of 7 nm. The dielectric is composed by the thermal SiO2 layer and the sputtered Al2O3 layer. (21) Daraktchiev, M.; Von Muehlenen, A.; Nuesch, F.; Schaer, M.; Brinkmann, M.; Bussac, M. N.; Zuppiroli, L. New J. Phys. 2005, 133, 7. (22) Pratontep, S.; Brinkmann, M.; Nuesch, F.; Zuppiroli, L. Phys. Rev. B 2004, 69, 165201. (23) Nuesch, F.; Carrara, M.; Zuppiroli, L. Langmuir 2003, 19, 4871.
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Pentacene Deposition. Pentacene is deposited by thermal evaporation (at a rate of about 1 nm/min) in a high-vacuum chamber having a pressure of 110-7 mbar. The film thickness was between 5 and 10 nm. The substrate temperature was controlled and kept at 55 °C during deposition. Pentacene was purchased at Fluka and twice purified in a furnace with a temperature gradient. Self-Assembled Monolayer Deposition. Before pentacene deposition, the self-assembled monolayers (SAMs) were deposited on the dielectric surface which was activated by an O2 plasma during a 2 min period (10 W, -50 V, 10-3 mbar). The plasma treatment is needed for grafting.24 In a recent paper,15 we have shown that in the first 10 min after oxygen plasma irradiation the main active defects are oxygen radicals, which facilitates the grafting of the carboxylate groups. The SAMs were deposited within these 10 min. The pentacene and SAM depositions were performed in the same vacuum chamber. The time between both depositions was at least 30 min to allow for the desorption of ungrafted SAM molecules. Benzoic acid derivatives are chosen to form a series of SAMs differing by the group substituted in the para-position (see Figure 1 b).23 The dipole moments of the molecules vary from -2.6 to þ3.1 D (directed from the -COOH group toward the para-substituent). Benzoic acid (-1.1 D) was used as the neutral SAM. Nitrobenzoic acid (NBA) and cyanobenzoic acid (CBA) are the positive SAMs (þ3.1 and þ3.0 D, respectively). Anisic acid (AA) and dimethylaminobenzoic acid (ABA) have negative dipole moments (-2.3 and -2.6 D, respectively). In order to have a second neutral SAM, anthracene carboxylic acid (ACA) was used (-1.1 D). The dipole moments contain the contribution of the grafting group COOH (-1.1 D).23 The surface density of benzoic acids grafted on the oxide surface was roughly estimated to be 4 1014cm-2. We used Kelvin probe measurements performed by Nuesch et al. on indium tin oxide and aluminum surfaces.23 The anthracene end group has a larger area than the benzoic group; therefore, we considered the density of the ACA monolayer to be half as large at 4 1014 cm-2. Contact Deposition. Gold contacts were deposited through a shadow mask under high vacuum (1 10-7 mbar) on the top of the pentacene thin film. The substrate was cooled to the temperature of liquid nitrogen (-185 °C) to avoid gold diffusion into pentacene. The gold contacts have a thickness of 25 nm. The channel of the OTFT has a length of 600 μm and a width of 6 mm. This geometry (long channels) ensures that our OTFTs are not contact limited. OTFT Characterization. The OTFTs were characterized by measuring output and transfer curves (Keithley 4200 SCS) under Ar atmosphere (after at least 4 h under vacuum). Drain and gate voltages were swept between þ10 and -60 V while the drain and gate currents were measured. The apparent mobility is determined by the slope of the transfer curves measured in the linear regime. The threshold voltage is the intercept between the gate voltage axis and the linear fit of the transfer curve for high negative gate voltages.25,26 The resistance of the OTFTs was measured by 2 and 4 probes (Keithley 236 source measure unit). In the 4-probes measurements, the film resistance was determined independent of the contact resistance. The contact resistance, therefore, was equal to the difference of the values measured by 2 and 4 probes. The residual carrier density in the channel at zero gate voltage is calculated by Pres ¼
σ film μapp jej
ð2Þ
(24) Carrara, M. Ph.D. Thesis 2002, No. 2564, Ecole Polytechnique Federale de Lausanne, Switzerland. (25) Di Benedetto, S.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Adv. Mater. 2009, 21, 1407. (26) Horowitz, G. J. Mater. Res. 2004, 19, 1946.
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Figure 2. (a) Output and (b) transfer characteristics of OTFT with SAM modified dielectric surfaces. Table 1. Characteristic Values of OTFTs 2
apparent mobility [cm /(V s)]
film conductivity [S 0]
threshold voltage [V]
residual carriers [cm-2]
contact resistance [Ω]
(1.2-1.5) 1013 (5.1-5.5) 1012
(2.5-5.2) 104 (1.4-2.2) 105
(1.3-1.6 1012 (0.6-1.3) 1012
(0.5-1.0) 106 (0.2-3.5) 106
(1.4-1.7) 1012 (1.2-2.7) 1012
(0.3-1.9) 106 (3.0-3.8) 107
Positive SAM NBA (þ3.1 D) CBA (þ3.0 D)
0.25-0.32 0.12-0.16
(5.0-6.6) 10-7 (1.0-1.5) 10-7
75-110 34-43
Neutral SAM BA (-1.1 D) ACA (-1.1 D)
0.11-0.17 0.07-0.12
(2.2-4.4) 10-8 (1.0-4.5) 10-8
3-6 -16 to þ20
Negative SAM AA (-2.3 D) ABA (-2.6 D)
0.12-0.19 0.03-0.05
(3.2-3.5) 10-8 (1.0-1.5) 10-8
-4 to þ12 -12 to -8
where σfilm is the film conductivity measured by 4 probes, μapp the apparent mobility, and e the electron charge.21 Film morphology is analyzed by an atomic force microscopy (AFM) in semicontact mode in the ambient atmosphere (SMENA scanner, NT-MDT).
Results Electrical Performance. The output and transfer characteristics of the OTFTs depend strongly on the SAM (see Figure 2). While OTFTs modified by a negative SAM (AA and ABA) reach saturation at drain and gate voltages of -60 V, the transistors modified by a positive SAM (NBA and CBA) do not reach saturation. In the transfer curves, the channel conductance is more than 20 times higher for a modification with NBA (þ3.1 D) than with ABA (-2.6 D). Because of the variations in OTFT performance, the apparent mobility varies from 0.03 to 0.32 cm2/(V s), the film conductivity from 0.1 to 70 10-8 S 0, the residual carrier density from 0.6 to 15 1012 cm-2 and the threshold voltage from -16 to þ110 V (for details see Table 1). Similar trends in threshold shift are observed by Kobayashi et al., Takeya et al., and Pernstich et al.3-5 For fluorinated end groups, the threshold voltage shifted to positive values. It is proposed that the effect of the SAMs depends on the dipole moment of the SAM molecules, but the exact correlation is not yet known. One reason could be that the SAM molecules used in these works have different lengths, as the transistor performance also depends on the length of the aliphatic chain in the SAM.27-29 (27) Jedaa, A.; Burkhardt, M.; Zschieschang, U.; Klauk, H.; Habich, D.; Schmid, G.; Halik, M. Org. Electron. 2009, 10, 1442. (28) Fukuda, K.; Hamamoto, T.; Yokota, T.; Sekitani, T.; Zschiechang, U.; Klauk, H.; Someya, T. Appl. Phys. Lett. 2009, 95, 203301. (29) Hill, G.; Weinert, C. M.; Kreplak, L.; van Zyl, B. P. Appl. Phys. A: Mater. Sci. Process. 2009, 95, 81.
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Figure 3. Residual carrier density vs measured threshold voltage.
For BA and ACA modified transistors, the values for the apparent mobility and threshold voltage are in the same range as the values for other short benzen and anthracene derivatives.4,5,7,8,30 Pentacene Film Morphology. The pentacene films of the present study consist of grains with a size of 3-4 μm. Pentacene is grown layer by layer directly onto the dielectric surface. After several monolayers, a three-dimensional growth starts to modify the morphology of the film. The pentacene films have a thickness of about 5 nm if they are grown on BA, CBA, and AA and about 10 nm on ACA and NBA. On ABA, the grain size is larger (about 7-8 μm in diameter) and the film thickness is about 5 nm. In general, on oxide gates, the film morphology has a much smaller effect on the transistor performance than the nature of (30) McDermott, J. E.; McDowel, M.; Hill, I. G.; Hwang, J.; Kahn, A.; Bernasek, S. L.; Schwartz, J. J. Phys. Chem. A 2007, 111, 12333.
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Figure 4. (a) Charge transfer at pentacene-dielectric interface through a SAM carrying a dipole moment and (b) SAM as energy barrier between pentacene and the defect.
dielectric-semiconductor interface.5,15 In this work, the OTFTs containing the largest grains (ABA modified interface) show the smallest values of apparent mobility and conductivity. This is an indication that the interface dominates over film morphology.
Discussion Measured Threshold Shift. The dipole moment of the SAM affects the threshold in two ways. On the one hand, it causes a gate potential shift in the channel by accumulating or depleting charge carriers at the dielectric-pentacene interface; this effect is comparable to the field effect of the gate. The threshold shift is then proportional to the dipole moment. On the other hand, the dipole moment in the SAM influences the charge transfer between dielectric and pentacene; it controls the tunneling probability through the SAM. The charges transferred into pentacene lead to a threshold shift to positive values, i.e., the threshold shift is proportional to the residual carrier density (see eq 1). If the measured threshold values are plotted versus residual carrier density, we see they are proportional (see Figure 3). The fitted slope (8.41 10-12 V 3 cm2) is comparable with the theoretical value (8.25 10-12 V 3 cm2) calculated by eq 1 (Cgate =19.5 nF/cm2). Except for vey low residual carrier densities, therefore, the threshold shift results from the residual carriers that were created. One part of the threshold shift (about -14 V) is independent of residual carrier density. In principle, the residual carriers could also be the result of a direct reaction between a pentacene and a SAM molecule. A direct reaction, however, is not likely because the energy level difference between HOMO level of unpolarized pentacene (-6.6 eV31) and LUMO level of SAM molecules (-1.5 to -3.3 eV32,33) is larger than 3.2 eV. We conclude that the residual carriers result from charge transfer between the defects on the oxide gate and the pentacene through the SAM.15 The gate potential shift due to the SAM changes the threshold and is calculated by ND ð3Þ ΔVSAM ¼ ε0 εSAM where N is the surface density of the SAM and ε0 permittivity of vacuum. (31) Gruhn, N. E.; da Silva Filho, D. A.; Bill, T. G.; Malagoli, M.; Coropceanu, V.; Kahn, A.; Bredas, J. L. J. Am. Chem. Soc. 2002, 124, 7918. (32) Iozzi, M. F.; Cossi, M. Theor. Chem. Acc. 2007, 117, 673. (33) Yoshida, M.; Samejima, M. Yakugaku Zasshi 1978, 98, 537.
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Figure 5. Energy shift due to the dipole moment of the SAM vs the charge transfer probability.
For the benzoic acids used in this work, the surface density N is 4 10-14 cm-2 and the dielectric constant εSAM is 5.3.23 Then a dipole moment of 1 D causes a potential shift of about 0.28 V. The range of potential spanned from -0.74 to þ0.88 V. These values are too small to explain the threshold voltage shifts we measured in a range of -16 V to more than 100 V (see Table 1). The gate potential shift, therefore, is not the main effect of the SAM in our transistors. Residual Carriers in the Conducting Channel. At the bare oxide-pentacene interface, charge transfer occurs due to defects on the oxide surface.21 Charge transfer probability depends on the energy level difference between HOMO level of uncharged pentacene and the energy level of the defect on the oxide surface.15 Back transfer is not likely because the HOMO level of polarized pentacene is shifted by the polarization energy of 1.5 eV.34 With the introduction of a SAM at the interface, defects and pentacene molecules are separated and the energy barrier against tunneling becomes larger,13 and the charge transfer probability is decreased. If the SAM carries a dipole moment, the defect energy level and the HOMO level of pentacene are shifted relative to one another by the internal field in the SAM. In this work, the SAMs consist of molecules of similar length therefore we estimate that the energy barriers have approximately the same height. In fact, the barrier height is determined by the (34) Bussac, M. N.; Picon, J. D.; Zuppiroli, L. Europhys. Lett. 2004, 66, 392.
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Figure 6. (a) Apparent mobility vs residual carrier density and (b) contact resistance vs residual carrier density.
LUMO level of the SAM. Because they are low (between -1.5 and -3.3 eV32,33) and the energy difference to the defect level and the HOMO level of pentacene is high, we estimate that all barriers have the same height. The energy barriers, therefore, are only varied by the dipole moment of the SAM. The internal E-field creates an energy level shift which changes the energy difference between the pentacene molecule and the defect (see Figure 4). Charge Transfer Probability. The residual carriers are introduced into pentacene by charge transfer through the SAM. The more likely it is that a charge transfer occurs, the more residual carriers are introduced; thus the charge transfer probability P is proportional to the concentration of residual carriers measured in pentacene: Pres ð4Þ Pµ Npen where Pres is the residual carrier concentration and Npen the pentacene surface density. For determining the energy level of the defect responsible for the charge transfer, we use the electron transfer theory of R. A. Marcus.35 There the charge transfer probability depends exponentially on the difference of the energy levels of the initial and final state and the reorganization energy of the system. Here, the energy levels are the defect electron accepting level on the dielectric surface and the HOMO level of the unpolarized pentacene molecule. The reorganization energy includes the effects of deforming both the pentacene molecule and the defect. Charge transfer probability is calculated by: ! - ðΔESAM þ ΔE0 - λÞ2 ð5Þ P µ exp 4λkB T where ΔESAM is the energy shift between the donor and the acceptor levels due to the dipole moment in the self-assembled monolayer, ΔE0 the difference between the energy level of the defect and the HOMO level of pentacene (Edefect - EHOMO,pen), λ the reorganization energy, kB the Boltzmann constant, and T the absolute temperature.35 The energy shift depends on the dipole moment and the molecular length: ΔESAM ¼
- 2Dβ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi -D 2 jejL 1 jejL
ð6Þ
where D is the total dipole moment of the SAM molecule, β is the transfer integral across the molecule, e is the electron charge, and (35) Marcus, R. A. Presented at the Nobel Lecture (8.12.1992), 1992.
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L is the length of the molecule (for more details, see Supporting Information). Based on Marcus, the following values are used to fit the charge transfer probability: the transfer integral β = 1 eV36 and the molecule length L = 6 A˚37 for all SAM molecules; the surface density Npen =4 1014cm-2 38 and the HOMO level EHOMO =-6.6 eV for pentacene31 and the reorganization energy λ=0.5 eV.39 The surface density of ACA monolayers is 2 1014 cm-2, and for the other SAM molecules, it is 4 1014 cm-2 (see also the Materials and Methods). By fitting the measured values, we deduce an energy level difference ΔE0 of 0.65 eV and a defect level ΔEdefect of -5.95 eV. The fitted curve and the measured values are shown in Figure 5. Earlier, we found that O-centered radical defects on Al2O3 and SiO2 indeed have energy levels between -5.5 and -7.1 eV.15 Here, we used an O2 plasma for SAM grafting so there are O-centered radicals formed on the oxide surface. Contact Resistance and Apparent Mobility. At the gold contact-pentacene interface, the Schottky barrier controls charge injection into pentacene. If there are residual carriers in pentacene in sufficient concentrations, they screen the image forces at the contacts. Thus, the contact barrier is reduced and charge injection is facilitating. As a consequence, the contact resistance decreases (see Figure 6a). The exception to this behavior are the transistors modified by ABA (-1.5 D), where the transistor performance is dominated by the contact resistance because the ratio of contact resistance to the total resistance is about 0.8. The residual carriers also screen the charges at the dielectricpentacene interface which results in a smaller trap depth. Less energy is needed to release a trapped charge; thus the apparent mobility increases (see Figure 6b). The same trend is observed for elevated residual carrier density (pres g 4 1012cm-2) by von Muehlenen et al.8 A moderate concentration of residual carriers can, therefore, enhance transistor performance.
Conclusion We fabricated and analyzed organic thin-film transistors based on pentacene in which the oxide gate dielectric is modified by six different self-assembled monolayers. The dipole moment in the SAM controls charge transfer through the monolayer between defects on the oxide surface and pentacene molecules; this resulted (36) Zuppiroli, L. Presented at the Quantum Mechanics for Engineers (Ecole polytechnique federale de Lausanne) 2008/2009, Autumn & Spring Semester. (37) Bruno, G.; Randaccio, L. Acta Crystallogr. 1980, B36, 1711. (38) Parisse, P.; Ottaviano, L.; Delley, B.; Picozzi, S. J. Phys.: Condens. Matter 2007, 19, 106209. (39) Lund, T.; Eberson, L. J. Chem. Soc., Perkin Trans. 1997, 2, 1435.
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in residual carriers (additional carriers) in the conducting channel. The presence of residual carriers shifts the threshold to positive values, increases the apparent mobility, and decreases the contact resistance. Defects with an energy level of about 6 eV are responsible for the charge transfer to pentacene. Such defects are e.g. O-centered radicals or products of the grafting reaction of the monolayer. Other effects of the SAM are too weak to explain the transistor behavior. The gate potential shift is only 0.3 V per 1 D which is too small to dominate the transistor performance. A direct reaction
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between pentacene and SAM molecules is not likely because the energy levels are far away from each other. Acknowledgment. We thank Philippe Bugnon for the purification of pentacene. This work was supported by the Swiss National Science Foundation, Switzerland, through Grant SNSF 200020-121715/1. Supporting Information Available: Text giving the calculation of the energy shift. This material is available free of charge via the Internet at http://pubs.acs.org.
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