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Charge Transport across Phosphonate Monolayers on Indium Tin Oxide† David M. Rampulla,‡ Christine M. Wroge,‡ Eric L. Hanson,§ and James G. Kushmerick*,‡ Surface and Microanalysis Science DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States, and Aculon Inc., San Diego, California 92121, United States ReceiVed: July 31, 2010; ReVised Manuscript ReceiVed: October 7, 2010
Transition voltage spectroscopy was used to measure the charge injection properties of monolayers of bithiophene phosphonate, quarterthiophene phosphonate, and decylphosphonate covalently bonded to an indium tin oxide surface. Hysteresis was observed for all three phosphonates, which is possibly explained by charge retention at the phosphonate-ITO interface. Unlike previous work on thiolate-based molecular junctions, there is no significant difference between the charge injection barriers of the three phosphonates, suggesting that the phosphonate moiety dominates the observed charge injection properties. Introduction The transport of charge in organic devices finds relevance in the applications of transistors, light-emitting diodes, and photovoltaics;1-4 however, the devices are often inefficient compared to their inorganic counterparts. One reason for this poor efficiency in organic devices is the barrier to charge injection between the Fermi level of the electrode and the molecular orbitals of the active organic layer. This barrier is largely dependent upon the interface between the molecule and electrode. Thus, to have a better understanding of this interface in hopes of controlling charge injection, it is beneficial to directly measure the barriers to charge injection as a function of molecular structure and composition. Previous work has demonstrated that transition voltage spectroscopy (TVS) is capable of measuring this barrier across molecular electronic junctions.5-10 Charge-carrier injection barriers on metal oxide electrodes have been successfully adjusted through the covalent attachment of phosphonate-linked self-assembled monolayers (SAMs). Modifications of indium tin oxide (ITO) by quarterthiophene phosphonate and aluminum oxide by octylphosphonate are capable of lowering the work function by 0.28 and 0.5 eV, respectively, thus leading to improved electron injection in fabricated devices.11,12 A quarterthiophene phosphonate-modified ITO surface that was p-doped with tetrafluorotetracyanoquinodimethane (F4-TCNQ) was shown to raise the ITO work function by 0.35 eV, thus lowering the hole injection barrier.11 Furthermore, a monolayer of anthracene phosphonate was used to improve pentacene-based organic thin film transistors,13 and quarterthiophene phosphonate/F4-TCNQ on ITO showed comparable device performance when compared with poly(3,4ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/ PSS) devices.11 These previous results suggest that phosphonate modification of a substrate can be tailored to induce an increase or decrease in electrode work function and potentially lead to improved organic devices. The research herein describes the current-voltage characteristics of junctions consisting of quarterthiophene phosphonic acid (4TP), bithiophene phosphonic acid (2TP), and decylphosphonic acid (DP) molecules bonded †
Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed.
[email protected]. ‡ National Institute of Standards and Technology. § Aculon Inc.
E-mail:
Figure 1. Molecular structures for decylphosphonic acid (DP), bithiophene phosphonic acid, and quarterthiophene phosphonic acid (4TP).
to ITO electrodes and contacted by a gold top electrode (Figure 1). Interestingly, the TVS for the three molecules investigated here suggests that the phosphonate-ITO interface dominates the charge injection barrier properties rather than the structure of the molecular backbone. Experimental Section Sample Preparation. Decylphosphonic acid (Alfa Aesar, 98%)14 was used as purchased. Bithiophene phosphonic acid (Sigma-Aldrich, 95%) was further purified to 99% by dissolving in hexane, followed by the addition of decolorizing carbon, filtration, and evaporation of the hexane solvent. Quarterthiophene phosphonic acid was synthesized according to a previously reported procedure.15 The ITO films (Thin Film Devices, Inc.) were 150 nm thick and patterned with a NiCr busbar for contact to external circuitry. Initially, the ITO substrates were scrubbed with a soft brush and a soapy Alconox/ deionized water solution to remove asperities that could lead to short circuits with an on-top electrode contact. Following the scrubbing, the samples were submerged in heated acetone
10.1021/jp107209m 2010 American Chemical Society Published on Web 10/20/2010
Charge Transport across Phosphonate Monolayers on ITO
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for 10 min, sonicated in acetone for 5 min, rinsed with isopropanol, and dried under a nitrogen stream. Next, the ITO samples were submerged in heated isopropanol for 10 min, sonicated in isopropanol for 5 min, rinsed with isopropanol, and dried under a nitrogen stream. Finally, the substrates were exposed to a UV ozone treatment for 10 min, rinsed with deionized water, and dried under a nitrogen stream. The “tethering by aggregation and growth” (T-BAG) method was used to grow phosphonate monolayers on the ITO films.15 Briefly, a 100 µmol solution of a chosen phosphonic acid was prepared in anhydrous tetrahydrofuran, and the ITO substrate was suspended just below the solution surface. As the solution evaporated and the meniscus lowered, the monolayer was formed. To transition from a physisorbed layer to a chemically bonded monolayer of phosphonate, the freshly coated substrate was then baked at 130 °C for 30 min under a nitrogen atmosphere. Any further impurities or unbonded phosphonic acid residues were removed by two sonication cycles in methanol for 5 min. The T-BAG process was repeated two more times to ensure a tightly packed monolayer.15 I-V Measurements. All current-voltage measurements were performed with a modified version of a crossed-wire apparatus.16-18 Briefly, a 10 µm diameter Au wire was mounted above a SAM-coated ITO surface and perpendicular to an applied magnetic field. The junction was formed by gently deflecting the Au wire with the Lorentz force generated from a small, applied dc current until it made contact with the molecular layer. The Au wires were cleaned by submerging them in 30% hydrogen peroxide for 1 h, submerging them in deionized water for 10 min, and finally submerging them in ethanol for 10 min. Results and Discussion Figure 2 shows representative current-voltage characteristics (I-V) for Au//molecule-ITO junctions formed from DP (top), 2TP (middle), and 4TP (bottom). In all cases, the molecules are bonded to the ITO surface through a phosphonate linkage, while the Au//molecule interface is merely a physical contact. The first observation from these measurements is that all three junctions exhibit some degree of hysteresis. In all of the I-V traces, the solid line corresponds to the voltage sweeping positive to negative, while the dashed line is negative to positive. To confirm that the observed hysteresis was from the junction and not an artifact of the measurement setup or ITO substrates, a junction was formed from dodecylthiol on a Au wire brought down on a clean ITO substrate. No appreciable hysteresis was observed for this junction (see Supporting Information), which demonstrates that the observed hysteresis is a result of the phosphonate-bound monolayer. The actual chemical nature of the bonding at this interface is under current research, specifically, whether the phosphonate bonds to indium or tin and whether or not it is tridentate, bidentate, or some statistical mixture of the two.19,20 Resolving the bonding question is beyond the scope of this work; however, the observed hysteresis suggests that some degree of charge trapping occurs at the phosphonate-ITO interface. In addition to the hysteresis, a small amount of rectification is observed for the 2TP and 4TP junctions. In both cases, the current at positive bias is larger than that at negative bias. Positive bias corresponds to electrons going from the ITO surface to the Au wire in our biasing scheme. Due to the asymmetric nature of the molecule and two dissimilar electrode contacts, it is not surprising that some degree of bias polarity asymmetry is observed in the I-V traces. Specifically, depending on sample preparation, ITO is found to have a work function
Figure 2. I-V characteristics for Au//molecule-ITO junctions of DP (top), 2TP (middle), and 4TP (bottom). In all three plots, the dashed (solid) line represents data taken while sweeping the bias voltage from negative to positive (positive to negative). Hysteresis in the I-V characteristics is apparent for all three junctions. Positive bias corresponds to electrons going from the ITO surface to the Au wire.
of 3.9-4.3 eV, while the work function of clean Au is 5.2 eV.21,22 The ramifications of the different work functions will be discussed in more detail below. One might be tempted to look at the overall current values to make an assessment of charge transport across these phosphonate-ITO interfaces, but because the junction area in these devices is not well-defined nor well-controlled, it is problematic to put much value on such an analysis. Fortunately, TVS is independent of junction area and thus allows us to compare the barriers to charge transport across a molecular junction without any knowledge of the number of molecules contacted. Previously, we and others have demonstrated how TVS can be used to determine the barriers to charge transport in molecular junctions.5-10 Briefly, TVS is a simple mathematical transformation of the I-V characteristics commonly obtained for molecular junctions. Plotting the acquired data on axes of ln(I/V2) versus 1/V, commonly referred to as a Fowler-Nordheim plot, lets us
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Rampulla et al. to fully map out Vtrans for negative applied biases and can thus only provide a lower limit of -1.2 V for the transition. As mentioned above, the approximately 1 eV difference in work function between the ITO surface and the Au electrode is the most likely cause for the observed rectification and the difference in positive and negative Vtrans values. Previous experiments have demonstrated how asymmetric bonding to the two electrode interfaces in molecular junctions can lead to varying degrees of rectification.16,24 This does not however explain why the DP junction is invariant to bias polarity or, more importantly, why the aromatic systems exhibit a larger transition voltage at negative bias than the alkyl system. Conclusions
Figure 3. Solid circles represent the average of 20 I-V curves for a Au//2TP-ITO junction. The vertical dashed line corresponds to the voltage at which the tunneling barrier transitions from trapezoidal to triangular (Vtrans). Also shown are representations of the barrier shape at various values of applied voltage. For clarity, the barriers are only shown for the positive biases. The negative branch appears to be approaching Vtrans, but because sufficiently high negative biases could not be applied to fully map out the transition, a definitive determination of the value is not possible. Positive bias corresponds to electrons going from the ITO surface to the Au wire.
TABLE 1: Transition Voltage transition voltage (V) junction
positive bias
negative bias
Au//DP-ITO Au//2TP-ITO Au//4TP-ITO
0.84 ( 0.09 0.94 ( 0.08 0.83 ( 0.07
0.85 ( 0.10 >1.2 >1.2
determine at what voltage the charge transport transitions from direct tunneling to field emission (Figure 3). The simplest physical picture to envision is the case of a trapezoidal barrier transitioning into a triangular barrier as the bias voltage is increased (inset to Figure 3). At low applied bias, direct tunneling is the transport mechanism, but at some elevated bias, the barrier transitions from a trapezoid to a triangular barrier, and field emission dominates. The dashed vertical line signifies this transition. While such a simple barrier picture is obviously incomplete, detailed calculations using a coherent molecular transport model justifies TVS as a spectroscopic tool.23 Table 1 lists the measured transition voltage (Vtrans) values for the three molecules studied. Surprisingly, within our statistical uncertainty, all three molecules have essentially the same value for Vtrans for positive applied bias voltage. This is in stark contrast to our earlier work of saturated and conjugated thiol molecules between two Au electrodes where the barrier to transport across saturated molecules was significantly larger than that in aromatic moieties and where a clear correlation between conjugation length and barrier height was observed.5,6 The difference between the value for the positive and negative Vtrans for the thiophene systems is also noteworthy. As mentioned above, both thiophene junctions exhibit some current rectification. The asymmetry in charge transport with respect to bias voltage polarity is even more apparent in the TVS plots. In Figure 3, we can clearly see that for a Au//2TP-ITO junction, Vtrans is 0.94 V for positive applied bias. Positive bias corresponds to electrons going from the ITO surface to the Au wire. The negative branch, however, does not exhibit a clear transition up to the maximum applied negative bias of -1.2 V. Due to junction instability at larger applied voltages, we were unable
Charge transport studies of quarterthiophene phosphonic acid, bithiophene phosphonic acid, and decylphosphonic acid molecules bonded to an ITO surface and contacted with a Au top electrode reveal hysteresis in the current-voltage characteristics. This hysteresis is only present when there is a phosphontate linkage in the junction; therefore, it is posited that charging occurs at that interface. Additionally, TVS of the three molecules demonstrates that for electron injection at the ITO electrode, there is no clear dependence on molecular structure. For hole injection at the ITO electrode, the aromatic molecules exhibit a higher barrier than the saturated molecule. Further studies incorporating these molecular layers into functioning devices are needed to better understand what the TVS is telling us about these systems. Acknowledgment. This work was supported by the National Research Council Postdoctoral Associateship Program (D.M.R.). Supporting Information Available: Current-voltage characteristics of a Au-dodecythiol//ITO junction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (2) Gelinck, G. H.; Huitema, H. E. A.; Van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; Van der Putten, J.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; Van Rens, B. J. E.; De Leeuw, D. M. Nat. Mater. 2004, 3, 106. (3) Horowitz, G. AdV. Mater. 1998, 10, 365. (4) Sheraw, C. D.; Zhou, L.; Huang, J. R.; Gundlach, D. J.; Jackson, T. N.; Kane, M. G.; Hill, I. G.; Hammond, M. S.; Campi, J.; Greening, B. K.; Francl, J.; West, J. Appl. Phys. Lett. 2002, 80, 1088. (5) Beebe, J. M.; Kim, B.-S.; Gadzuk, J. W.; Frisbie, C. D.; Kushmerick, J. G. Phys. ReV. Lett. 2006, 97, 026801. (6) Beebe, J. M.; Kim, B.-S.; Frisbie, C. D.; Kushmerick, J. G. ACS Nano 2008, 2, 827. (7) Yu, L. H.; Gergel-Hackett, N.; Zangmeister, C. D.; Hacker, C. A.; Richter, C. A.; Kushmerick, J. G. J. Phys.: Condens. Matter 2008, 20, 374114. (8) Song, H.; Kim, Y.; Jang, Y. H.; Reed, M. A.; Lee, T. Nature 2009, 462, 1039. (9) Choi, S. H.; Kim, B.; Frisbie, C. D. Science 2008, 320, 1482. (10) Lu, Q.; Liu, K.; Zhang, H.; Du, Z.; Wang, X.; Wang, F. ACS Nano 2009, 3, 3861. (11) Guo, J.; Koch, N.; Bernasek, S. L.; Schwartz, J. Chem. Phys. Lett. 2006, 426, 370. (12) Vaynzof, Y.; Dennes, T. J.; Schwartz, J.; Kahn, A. Appl. Phys. Lett. 2008, 93, 3. (13) McDowell, M.; Hill, I. G.; McDermott, J. E.; Bernasek, S. L.; Schwartz, J. Appl. Phys. Lett. 2006, 88, 3. (14) Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for this purpose.
Charge Transport across Phosphonate Monolayers on ITO (15) Hanson, E. L.; Schwartz, J.; Nickel, B.; Koch, N.; Danisman, M. F. J. Am. Chem. Soc. 2003, 125, 16074. (16) Kushmerick, J. G.; Holt, D. B.; Yang, J. C.; Naciri, J.; Moore, M. H.; Shashidhar, R. Phys. ReV. Lett. 2002, 89, 086802. (17) Kushmerick, J. G.; Holt, D. B.; Pollack, S. K.; Ratner, M. A.; Yang, J. C.; Schull, T. L.; Naciri, J.; Moore, M. H.; Shashidhar, R. J. Am. Chem. Soc. 2002, 124, 10654. (18) Seferos, D. S.; Trammell, S. A.; Bazan, G. C.; Kushmerick, J. G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8821. (19) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 7809.
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20855 (20) Li, H.; Paramonov, P.; Bredas, J. L. J. Mater. Chem. 2010, 20, 2630. (21) Chkoda, L.; Heske, C.; Sokolowski, M.; Umblach, E.; Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J. Synth. Met. 2000, 111-112, 315. (22) Michaelson, H. B. J. Appl. Phys. 1977, 48, 4729. (23) Huisman, E. H.; Guedon, C. M.; van Wees, B. J.; van der Molen, S. J. Nano Lett. 2009, 9, 3909. (24) Kushmerick, J. G.; Whitaker, C. M.; Pollack, S. K.; Schull, T. L.; Shashidhar, R. Nanotechnology 2004, 15, S489.
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