pubs.acs.org/Langmuir © 2009 American Chemical Society
Continuous Modulation of Electrode Work Function with Mixed Self-Assembled Monolayers and Its Effect in Charge Injection Kun-Yang Wu, Szu-Yen Yu, and Yu-Tai Tao* Institute of Chemistry, Academia Sinica Taipei Taiwan, Republic of China 115 Received January 6, 2009. Revised Manuscript Received March 5, 2009 Self-assembled monolayers (SAMs) of binary mixtures of n-decanethiol and the fluorinated analogue (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol) were formed on silver surface. The film structure was characterized by reflection absorption IR and XPS to be a homogeneous mixture of the two components. The mixed monolayers serve to tune the work function of silver over a wide range by varying the surface composition of the mixed monolayer from 4.1 to 5.8 eV. The mixed SAM-modified Ag surfaces were used as the anode in the fabrication of hole-only devices with the device structure Ag/SAM/HTL/Ag, where HTL represents a hole-transporting layer. It is shown that depending on the HTL used and thus the HOMO level involved, the maximum current injection into the device occurred with differently modified Ag. Top-emitting organic light-emitting diodes fabricated with differently modified silver electrodes showed that the maximum current and maximum luminance efficiency occur at anodes of different modifications due to a change in the hole-electron charge balance.
Introduction Charge injection at the interface between a metal electrode and an organic semiconductor material is involved in a variety of organic electronic devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics, etc. A Schottky barrier is present at the metal/ organic interface due to different energy level alignments of the metal work function and the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) of the organic molecule, depending on the type of charges to be injected. Reducing the barrier between the electrode and organic layer is in general desirable for efficient charge injection.1 To reduce the barrier for charge injection from the metal electrode to the organics, one can use an organic material with proper HOMO (or LUMO) level by judicious choice of the organic materials or by structural modification of the material that has been demonstrated to be promising. These approaches would nevertheless introduce other variables such as different charge mobility for the new material or change in another interface involved in a multilayered device structure. Alternatively, one can use metal electrodes of different work function. This approach would be limited by the adequacy of the particular metal in terms of conductivity, stability, or transparency/reflectivity of the metal involved. Furthermore, one can modulate the work function of a metal by surface treatment/modification.2 The size and direction of the interface dipole introduced by the modification are suggested to effect the modulation.3 Among various modifications, the use of a self-assembled monolayer (SAM) grafted on a metal surface has been shown to have great potential *Author to whom correspondence should be addressed (e-mail ytt@ chem.sinica.edu.tw). (1) (a) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 972–972. (b) Ishii, H.; Hayashi, N.; Ito, E.; Washizu, Y.; Sugi, K.; Kimura, Y.; Niwano, M.; Ouchi, Y.; Seki, K. Phys. Status Solidi a-Appl. Res. 2004, 201, 1075–1094. (2) (a) Lee, J. Y. Appl. Phys. Lett. 2006, 88, 073512. (b) Hong, K.; Lee, J. W.; Yang, S. Y.; Shin, K.; Jeon, H.; Kim, S. H.; Yang, C.; Park, C. E. Org. Electron. 2008, 9, 21–29. (c) Chen, C. W.; Hsieh, P. Y.; Chiang, H. H.; Lin, C. L.; Wu, H. M.; Wu, C. C. Appl. Phys. Lett. 2003, 83, 5127–5129. (e) Ganzorig, C.; Kwak, K. J.; Yagi, K.; Fujihira, M. Appl. Phys. Lett. 2001, 79, 272–274. (3) Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Bredas, J. L. J. Am. Chem. Soc. 2002, 124, 8131–8141.
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for systematic modulation of the work function through structural change of the molecule used. Thus, by utilizing mercaptanbased SAMs, the energy barrier for the hole injection from the Ag (or Au) anode to an organic layer could be varied.4 Besides introducing an oriented dipole, the SAM also imposes a tunneling barrier through which the charges have to pass to reach the organic layer. The current response as a function of the tunneling barrier, which can be modulated by the chain length of the SAM-forming molecules, provides insight to the charge balance situation in the device. We recently reported the use of SAMs of various organothiols on Ag for the fabrication of efficient topemitting OLEDs. The hole injection efficiency, electroluminescence (EL) property, and device performance depend profoundly on the monolayer used.5 Mixed monolayer can be prepared either by exposing a substrate to a solution containing more than one component or by ligand exchange from a preformed single-component monolayer.6 The distribution of the two components in the mixed monolayer can be phase-separated or homogeneously mixed, depending on the functional group or chain length of the molecule involved or the method of preparation.7 The mixed monolayer provides a versatile approach to alter the surface property over a wide range systematically. For example, the wetting characteristics have been shown to be tunable by mixed monolayers containing a hydrophobic and a hydrophilic constituent.6 (4) (a) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L. Phys. Rev. B 1996, 54, 14321–14324. (b) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528–3530. (c) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121–1127. (5) (a) Hung, M. C.; Wu, K. Y.; Tao, Y. T.; Huang, H. W. Appl. Phys. Lett. 2006, 89, 203106. (b) Wu, K. Y.; Tao, Y. T.; Huang, H. W. Appl. Phys. Lett. 2007, 90, 241104. (6) (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563–571. (b) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Y. Langmuir 2000, 16, 9287–9293. (7) (a) Takiue, T.; Matsuo, T.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. B 1998, 102, 5840–5844. (b) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Y. J. Phys. Chem. B 2001, 105, 2975–2980. (c) Tielens, F.; Costa, D.; Humblot, V.; Pradier, C. M. J. Phys. Chem. C 2008, 112, 182–190. (d) Lussem, B.; MullerMeskamp, L.; Karthauser, S.; Waser, R.; Homberger, M.; Simon, U. Langmuir 2006, 22, 3021–3027. (e) Stranick, S. J.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636–7646.
Published on Web 4/6/2009
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In this paper we describe the formation and structure of mixed monolayers from an n-alkanethiol (n-decanethiol, HDT) and its fluorinated analogue (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10, 10-heptadecafluoro-1-decanethiol, FDT) on silver surface. These mixed monolayers were used to tune the work function of the underlying silver metal. The n-decanethiol is known to decrease the work function, and the fluorinated thiol serves to increase the work function.4,8 The mixtures of the two allow continuous tuning of the work function simply by varying the composition in the monolayer. A wide range of work function can be achieved (4.1-5.8 eV). These modified substrates can be used as the electrode in the fabrication of organic electronic devices with variable charge injection barriers. Hole-only devices were prepared with several hole-transporting materials to show that depending on the HOMO of the organic material involved, different modifications were needed to reach the maximum current injection. The ability to tune the injection barrier for hole charges allows us to examine the hole/electron charge balance in an organic light-emitting device, which is crucial to the device efficiency.
Figure 1. Structures of thiol compounds used for the formation of mixed-SAMs.
Experimental Section Materials. HDT and FDT were purchased from Aldrich (Figure 1). Silver (99.99%) were obtained from ELECMAT. Preparation of Monolayer. The silver substrates were prepared by thermally evaporating 150 nm of silver on a 2-in. silicon wafer or microscopic slides. The silver substrates were immersed in a 1 mM ethanolic solution of thiol mixtures consisting of HDT and FDT in different mole ratios (1:0, 4:1, 2:1, 1:1, 1:2, 1:4, 0:1) for 2 h. The substrates were then removed from the solution, rinsed with absolute ethanol and acetone, and finally dried with a nitrogen flow before characterization and/or device fabrication. Characterization. Reflection-absorption IR spectra were taken with a Bio-Rad FTS-60 infrared spectrometer equipped with a DTGS detector. A custom-designed optics unit with an 86° incidence angle and p-polarized light were employed. Plasma-cleaned gold was used as reference for all spectra. XPS data were taken with an Omicron ESCA /Scanning Auger System with a chamber vacuum of 1 10-10 Torr. The work function of the modified surface was measured with a photoelectron spectrometer (AC-2, Riken Keiki) in ambient conditions with a UV source.9 Device Fabrication. The Ag electrodes were prepared by thermally evaporating 150 nm of silver layer on a soda glass, with the area (0.4 mm2) defined by a patterned shadow mask. The Ag electrodes were then immersed in 1 mM ethanolic solution of binary mixtures of thiols for 2 h. The modified Ag substrates were placed in a custom-designed rotating sample holder in a vacuum chamber (5 10-6 Torr) for the device fabrication. Various organic layers were deposited, followed by electrode deposition without breaking the vacuum. The thickness was controlled by a quartz thickness monitor. After the evaporation processes, the devices were encapsulated with a cover glass using UV-cured epoxy glue. The current density-voltage (J-V) characteristics of the devices were measured by a computer-controlled Keithly 2400 Source meter connected with a spectrophotometer PR650.
Result and Discussion Characterization of the Monolayer. The self-assembled monolayers of HDT and FDT on Au have been documented respectively before.10 Whereas trans zigzag conformation with (8) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Adv. Mater. 2005, 17, 621–625. (9) Chang, Y. M.; Wang, L.; Su, W. F. Org. Electron. 2008, 9, 968–973. (10) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (b) Tsao, M. W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317–4322.
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Figure 2. Reflection-absorption IR spectra of mixed monolayers of HDT and FDT on Ag. Table 1. Assignment of Vibration Modes in the IR Spectra of SAMs of HDT and FDT on the Ag Surface FDT (cm-1) νs(CF3) νs(CF3) νs(CF2) νa(CF2) νa(CF2) νa(CH3, ip) νs(CH3) νs(CH2) νa(CH2)
∼1333 ∼1372 ∼1153 ∼1247 ∼1218
HDT (cm-1)
∼2964 ∼2876 ∼2850 ∼2920
a chain tilt of ∼30° was reported for n-alkanethiol on Au, a nearnormal orientation of a helical conformation was reported for FDT on Au. In a mixed layer on Au, reorientation of n-alkanethiol to a near-normal direction in the presence of fluorinated alkanethiol is likely to occur.11 In contrast, n-alkanethiol on Ag adopts a chain orientation much closer to the surface normal. Change of orientation of the n-alkanethiol in mixing with fluorinated thiol is not significant, as will be shown later. In the current case, the mixed monolayer was prepared from solutions containing a mixture of HDT and FDT at a total concentration of 1 mM, a condition under which total dissolution of the FDT was observed.12. Figure 2 shows the reflection-absorption infrared spectra for the mixed monolayers formed on the silver surface. The peak frequencies and mode assignments are listed in Table 1. For a single-component HDT monolayer, the most distinct peaks are those of symmetric stretch of a methyl group at 2876 cm-1 and the in-plane asymmetric stretch of a methyl group at 2964 cm-1.10 The vibration modes at 2850 and (11) Results to be published elsewhere. (12) Scott, R. L. J. Phys. Chem. 1958, 62, 136–145.
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2920 cm-1 for symmetric and asymmetric stretching modes, respectively, for a methylene group are relatively weak, due to the near-normal orientation of n-alkanthiol on Ag.10 For a single-component FDT monolayer, the most distinct features are that for CF vibration modes appearing in the low-frequency region from ∼1000 to ∼1400 cm-1.13 For the SAMs of binary mixtures of HDT and FDT, peaks associated with CH3 and CF3 groups are present, with various intensities depending on the solution composition. It is noted that the relative intensities of various modes in the C-H stretch region are the same as that of pure HDT monolayer, with the absolute intensities shrunken in proportion to the decreasing concentration. This is an indication of a similar chain tilt in the mixed monolayer as in the pure monolayer. With the intensities of the CH3 mode at 2964 cm-1 and the CF3 mode at 1333 cm-1 as markers, the relative ratios of the HDT and FDT in the mixed monolayer were calculated on the basis of the respective integrated areas. Figure 3 shows the plot of surface composition as a function of solution composition. A nearly linear relationship was obtained. That is, the surface composition is similar to the solution composition. It is generally known that fluorocarbons and hydrocarbons are not miscible.12 How do these two components distribute on the metal surface? The IR vibration frequency is known to be sensitive to the local environment of the vibrating dipole.14 For a phase-separated monolayer, most molecules experience an environment similar to that of a single-component monolayer, except those molecules at the boundary of phase domains. For a molecularly mixed monolayer, all of the molecules experience a changing environment as the composition is varied. Figure 4 shows the spectra region between 1200 and 1400 cm-1, where the C-F stretches are observed. The frequencies of the νs (CF3) and νs (CF2) bands shift toward lower values with increasing amount of HDT in the monolayer. This is attributed to the local environment change when the nonpolar n-alkanethiol is introduced around the fluorinated chains. The peaks associated with CH3 in the 2860-2960 cm-1 region show similar shift with concentration. This suggests that the mixed monolayer is not grossly phase-separated into domains of hydrocarbons and fluorocarbons, respectively. A similar argument was invoked in the mixed monolayer of biphenylthiol and p-trifluoromethylbiphenylthiol.14 A rationale for the homogeneous distribution of the two components is the electrostatic stabilization of a dipole by opposite dipoles. X-ray photoelectron spectroscopy was also used to characterize the mixed monolayer. The XPS spectra of F 1s, C 1s, and Ag 3d for mixed SAMs are shown in Figure 5. It is noted that the intensity of the F 1s signal15 at 688.4 eV increases with increasing amount of FDT in the monolayer. In the C 1s spectra for the single-component FDT monolayer, there are three peaks at 284.4, 290.6, and 293 eV, assigned to C 1s (CH2), C 1s(CF2), and C 1s(CF3), respectively, and for the pure HDT monolayer on Ag, only C 1s (CH2) at 284.4 eV was observed. For the mixed monolayers on silver, the relative intensities of C 1s (CF2) and C 1s (CF3) to C 1s (CH2) change, as expected, with the composition of the solution: the intensities of C 1s (CF2) and C 1s (CF3) increase and that of C1s(CH2) decreases with increasing FDT concentration. It is interesting to note that the binding energies (13) Ren, Y. Z.; Iimura, K.; Ogawa, A.; Kato, T. J. Phys. Chem. B 2001, 105, 4305–4312. (14) (a) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R. Langmuir 1999, 15, 2095– 2098. (b) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662–9667. (15) (a) Shaporenko, A.; Cyganik, P.; Buck, M.; Ulman, A.; Zharnikov, A. Langmuir 2005, 21, 8204–8213. (b) Zharnikov, M.; Grunze, M. J. Vacuum Sci. Technol. B 2002, 20, 1793–1807.
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Figure 3. Plot of surface composition as a function of solution composition, based on the integrated areas under the νs (CF3) band (∼1333 cm-1) and νa (CH3) band (∼2964 cm-1), for the mixed SAMs.
Figure 4. νs (CFx) band in the range of 1400-1200 cm-1 and νs (CHx) band in the range of 2800-3000 cm-1 for mixed SAMs on Ag.
of C 1s (CF2) and C 1s (CF3) also shifted to higher binding energy with increasing amount of HDT in the mixed monolayer, whereas the binding energy of C 1s (CH2) shifts to lower values with increasing amount of FDT, presumably due to the local polarity changes. A quantitative calculation of the surface concentration based on the intensities of the F 1s signal yielded a near-linear plot similar to that obtained from IR spectra in Figure 3. This confirms that the surface composition of the mixed monolayer is similar to the solution composition. Contact angle on the monolayer-covered surface was measured using water and n-hexadecane (HD) as the wetting liquids. As shown in Figure 6a, the contact angle θ(HD) on the FDT monolayer is higher than that on HDT monolayer, suggesting the FDT monolayer surface is more “oleophobic” than the HDT monolayer surface, presumably because of the weaker dispersive interaction between the hydrocarbons and Langmuir 2009, 25(11), 6232–6238
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Figure 5. XPS spectra of various mixed SAMs on Ag.
Figure 7. Work function as a function of the amount of FDT in the monolayer on Ag. The inset shows the photocurrent trace for Ag modified with a monolayer of FDT:HDT 1:1. Table 2. Work Functions of Ag Substrates Modified by Mixed SAM of FDT and HDTa mixing ratio (FDT:HDT)
work function (eV)
1:0 ∼5.83 4:1 ∼5.53 2:1 ∼5.25 1:1 ∼4.89 1:2 ∼4.65 1:4 ∼4.40 0:1 ∼4.10 a The work function of bare Ag in this study was ∼4.67 eV.
Figure 6. Contact angle of (a) hexadecane and (b) water as a function of surface composition.
fluorinated surface than between hydrocarbons themselves.16 For a mixed monolayer, the contact angle exhibits a steady increase with increasing surface concentration of the FDT component due to a steady decrease in dispersive interaction of the surface with hexadecane. The water contact angle is also higher on fluorinated surface than on hydrocarbon surface (Figure 6b). This is in agreement with an earlier paper.17 However, the water contact angle increases with increasing amount of FDT and reaches a plateau at a surface FDT concentration of 60%. Both polar interaction and dispersive (16) Chaudhury, M. K. Mater. Sci. Eng. 1996, R16, 97–159. (17) Graupe, M.; Takenaga, M.; Koini, T.; Colorado, R.Jr.; Randall Lee, T. J. Am. Chem. Soc. 1999, 121, 3222–3223.
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interaction contribute to the work of adhesion and thus the contact angle.18 Water interacts with the surface through both dispersive interaction (with HDT) and polar interaction (with FDT). By replacing the HDT with FDT in the SAM, the dispersive component decreases and the polar component increases. The two opposing effects on contact angle may give rise to the nonmonotonous behavior in water contact angle. The work functions (j) of various mixed monolayer surfaces were measured by photoelectron spectrometer AC2. The freshly prepared bare silver substrate gave a work function j of 4.67 eV.2c For a single-component HDT-monolayer-covered Ag surface, the j value decreased to about 4.1 eV, whereas for a single-component FDT-monolayer-covered Ag, the j value increased to about 5.8 eV. The opposite effect on the work (18) Fowkes, F. M.; Riddle, F. L.Jr.; Pastore, W. E.; Weber, A. A. Colloids Surf. 1990, 43, 367–387.
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Figure 8. Chemical structures of materials used in device fabrication: (a) m-MTDATA; (b) NPB; (c) BPAPF; (d) Alq.
function is believed to be due to the opposite dipole direction of the alkyl and fluorinated alkyl moieties, mainly contributed by the terminal methyl and trifluoromethyl groups, respectively.19 Theoretical calculations also show that the surface dipole is dominated by the intrinsic dipole of the molecule, with a small contribution from sulfur-metal bond.20 For the mixed monolayer-modified Ag, the measured work functions lie between 4.1 and 5.8 eV and varied linearly with the surface compositions (Figure 7). The results are summarized in Table 2. It is noted that the square root of the counting rate as a function of photon energy shows a straight line above a photoemission threshold energy (work function) for all of the mixed monolayers (the curve for Ag modified with monolayer of 1:1 FDT:HDT is shown in the inset). This suggests a molecularly mixed monolayer because of the homogeneous environment of the substrate. The linear correlation in Figure 7 also suggests that by controlling the composition of the thiol solution, a work function anywhere between the two extreme values can be prepared. It is interesting to note that intramolecular polar bonds can be used to tune continuously the ionization energy (IE) of a thin organic film by mixing two components.21 Effect of Mixed SAM on the Charge Injection between Electrode and Organic Layer. As the work function of the metal can be tuned over a wide range (>1.7 eV) using mixed monolayers, the charge injection barrier between the electrode and an organic material can be tuned accordingly. Furthermore, because all of the films are virtually of monolayer thick and of the same effective thickness, the various mixed SAM modifications were assumed to impose a constant tunneling distance with different gaps for charge injection. We used the hole-only diodes to test the effect of work function on the charge injection. The hole-only device has a configuration of Ag/SAM/HTL (120 nm)/Ag, where the hole-transporting layers (HTLs) with different HOMO levels were used. The chemical structures of the charge-transporting materials used for device fabrication are shown in Figure 8. The first case is a bilayer HTL com(19) The work function for n-decanethiol monolayer carrying only a terminal CF3 group increases to 5.5 eV, as compared to 5.8 eV for FDT monolayer. (20) Rusu, P.; Brocks, G. J. Phys. Chem. B 2006, 110, 22628–22634. (21) Salzmann, I.; Duhm, S.; Heimel, G.; Oehzelt, M.; Kniprath, R.; Johnson, R. L.; Rabe, J. P.; Koch, N. J. Am. Chem. Soc. 2008, 130, 12870–12871.
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posed of 4,40 ,400 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA, 60 nm) and R-naphthylphenylbiphenyldiamine (NPB, 60 nm), where m-MTDATA, having a HOMO level at 5.1 eV, serves as injection promoter. The I-V characteristics are shown in Figure 9a. In contrast, the hole-only device with bare Ag electrode did not yield a reproducible I-V curve. The maximum current was obtained for Ag electrode modified with a mixed monolayer of 2:1 FDT:HDT (a work function of ∼5.25 eV). Further increase in the work function by using higher FDT content in the mixed monolayer did not increase the current further. It is suggested that the metal/organic contact reached a regime of “Fermi-level pinning”,22 with an electrode having a work function at 5.25 eV. The injection barrier did not change further with even lower Fermi level and has no advantage in terms of charge injection. When m-MTDATA was removed, so that the Ag electrode is in direct contact with NPB, which has a HOMO level at 5.46 eV, the maximum current was obtained with Ag electrode modified with pure FDT monolayer (giving a work function of 5.83 eV) (Figure 9b). Thus, the Fermilevel pinning did not yet occur with pure FDT-modified Ag electrode. When the HTL was changed to BPAPF23 (9,9-bis{4[di(p-biphenyl)aminophenyl]}fluorene), which has a HOMO level of 5.62 eV, an even more dramatic difference in the amount of injected currents was obtained. Only the device with electrode modified with pure FDT monolayer gave sizable current. Apparently the low-lying HOMO level of the BPAPF used is making the hole injection barrier much larger and the currents more sensitive to the SAM modification. The charge injection behavior between the metal and the semiconducting layer has been described either by tunneling mechanism (Fowler-Nordheim model) or by thermionic emission (Richardson-Schottky model). According to the Richardson-Schottky thermionic emission theory,24 the current density as a function of the electric field F in a diode configuration is (22) (a) Fugakawa, H.; Kera, S.; Kataoka, T.; Hosoumi, S.; Watanabe, Y.; Kudo, K.; Ueno, N. Adv. Mater. 2007, 19, 665–668. (b) Tengstedt, C.; Osikowicz, W.; Salaneck, W. R.; Parker, I. D.; Hsu, C. H.; Fahlman, M. Appl. Phys. Lett. 2006, 88, 053502. (c) Koch, N.; Vollmer, A. Appl. Phys. Lett. 2006, 89, 162107. (d) Braun, S.; Osikowicz, W.; Wang, Y.; Salaneck, W. R. Org. Electron. 2007, 8, 14–20. (23) Ko, C. W.; Tao, Y. T. Synth. Met. 2002, 126, 37–41. (24) aSze, S. M. Physical of Semiconductor Devices; Wiley: New York: 1981. (b) Wang, W.; Lee, T.; Reed, M. A. Phys. Rev. B 2003, 68, 035416.
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Figure 10. Dependence of ln(J) on ΔΦ at various driving voltages.
at interface, q is the electric charge, m* is the effective mass of carrier, h and kB are Plank’s and Boltzmann’s constants, respectively, ε0 is the vacuum permittivity, and εr is relative dielectric constant. From eq 1, a linear correlation between ln(J) and ΦB is expected if the temperature and external electric field are fixed. In contrast, in the Fowler-Nordheim tunneling model, the ln(J) is expected to correlate with (ΦB)1.5 if the tunneling distance in eq 2 is assumed to be the same with all of the mixed monolayers. pffiffiffiffiffiffiffi 3 ! 4 2mΦB 2 JFN ∼V exp 3qpF 2
Figure 9. I-V characteristics in the hole-only devices with a device configuration of Ag/SAM/HTL/Ag.
given by
JRS
pffiffiffiffi! F Φ -β B RS ¼ AT exp kB T 2
ð1Þ
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ð2Þ
Figure 10 presents the plots of ln(J) versus ΔΦ, which is the difference between the work function of mixed-SAM-modified Ag anode and HOMO level of NPB. A linear dependence of ln(J) versus ΔΦ is obtained at several driving voltages, in agreement with that expected from Richardson-Schottky thermionic emission theory. Thus, the charge injection behavior is controlled by the injection barrier heights. Top-emitting OLEDs were also fabricated to test the effect of work function on the charge injection and performance of the devices. The first device has a configuration of Ag/SAM/ m-MTDATA(20 nm)/NPB(30 nm)/Alq(50 nm)/LiF (10 nm)/Al (2 nm)/Ag (25 nm), where Alq stands for tris(8-hydroxyquinolino)aluminum(III) and m-MTDATA serves as an injection promoter as in the case of hole-only device. Very thin layer of cathode was used to allow for light output from the cathode side. The I-V characteristic is shown in Figure 11a. The maximum current was again obtained for Ag electrode modified with a mixed monolayer of 2:1 FDT:HDT. Further increase in the work function by using higher FDT content in the mixed monolayer did not increase the current further. On the other hand, the current efficiency, shown in Figure 11b and the inset, of the devices exhibits a maximum with a Ag electrode modified with a 1:1 FDT:HDT monolayer (a work function of 4.89 eV). We suggest that the hole and electron carriers are more balanced with this particular modified electrode. Larger injection barrier (by having decreasing FDT content) leads to insufficient hole carriers, and smaller barrier (by having increasing FDT content) leads to excessive hole carriers, both of which will result in lower efficiency because of deviation from a balanced charge carriers. When m-MTDATA was removed, so that the Ag electrode was in direct contact with NPB, the maximum current was obtained with a Ag electrode modified with pure FDT monolayer as in the hole-only device (Figure 12). The Ag electrode yielding maximum current in the presence of m-MTDATA gave very DOI: 10.1021/la900046b
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Figure 11. (a) I-V characteristics and (b) current efficiency plot in the TOLEDs with a device configuration of Ag (100 nm)/SAM/ m-MTDATA (30 nm)/NPB (20 nm)/Alq (50 nm)/LiF (1 nm)/Al (2 nm)/Ag (25 nm).
Figure 13. (a) I-V characteristics and (b) current efficiency plot in the TOLEDs with a device configuration of Ag (100 nm)/SAM/ BPAPF (50 nm)/Alq (50 nm)/LiF (1 nm)/Al (2 nm)/Ag (25 nm).
and luminance efficiency occurred with a Ag anode modified with pure FDT monolayer. Thus, the increased barrier between the anode and BPAPF layer results in reduced hole carrier injection. Only a FDT-modified electrode provides enough current, and the hole carriers are better matching up with the electron carriers.
Conclusion
Figure 12. (a) I-V characteristics and (b) current efficiency plot in the TOLEDs with a device configuration of Ag (100 nm)/SAM/ NPB (50 nm)/Alq (50 nm)/LiF (1 nm)/Al (2 nm)/Ag (25 nm).
low current now. Thus, with a properly modified Ag electrode, the injection layer can be eliminated. The current efficiency plot shows that the Ag electrode modified with a 2:1 FDT:HDT monolayer gave the highest current efficiency. Increasing the FDT content in the monolayer increases the current but not the current efficiency. This suggests that the hole/electron charges are more balanced with Ag modified with a 2:1 FDT:HDT monolayer. In contrast to the case with a m-MTDATA buffer layer, a higher work function is needed to have sufficient hole injection and to reach charge balance. Further increase in work function, as provided by Ag modified with a pure FDT monolayer, will increase the hole charges further to exceed the electron charges and decrease the current efficiency. The device with BPAPF as a hole-transporting layer was also prepared, and the device characteristics are shown in Figure 13. With an even lower lying HOMO level of BPAPF, both the maximum current
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DOI: 10.1021/la900046b
In conclusion, we demonstrated that the mixed monolayers formed from HDT and FDT on silver substrate are homogeneous mixtures of the two components. The opposite dipoles of the two components can be used to tune the work function of silver metal over a wide range, from 4.1 eV for HDT-modified surface to 5.8 eV for FDT-modified surface. Near-linear variation in work function was obtained by using a mixed monolayer formed from the two components. This provides easy access to all kinds of work functions that may be needed in conjunction with different HOMO levels of organic semiconductors that may used in devices. The same approach can be applied on other substrate surfaces. For example, the work function of Au can be modulated by the same mixtures here to vary from 4.4 to 5.9 eV. Hole-only devices with Ag as anodes demonstrate the effect of electrode work function on charge injection. With different HTLs used, there is a matching modification of the electrode to generate the highest current. The varied current injection can be used to tune the hole-electron charge balance in an OLED device, as demonstrated by variation of current efficiency as a function of current injection. Acknowledgment. We thank the Ministry of Economics, Republic of China, and Academia Sinica for financial support.
Langmuir 2009, 25(11), 6232–6238
