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Langmuir 2007, 23, 7090-7095
Tuning of Metal Work Function with Organic Carboxylates and Its Application in Top-Emitting Electroluminescent Devices Chun-Pei Cho† and Yu-Tai Tao*,†,‡ Institute of Chemistry, Academia Sinica, Taipei, 11529 Taiwan, and Department of Chemistry, National Tsing Hua UniVersity, Hsin-Chu, 30013 Taiwan, Republic of China ReceiVed March 6, 2007. In Final Form: April 9, 2007 By fine-tuning of work functions and thus the hole-injection properties of Ag and Al anodes, an electroluminescent device was achieved by using various self-assembled monolayers of organic carboxylate on the electrode surfaces. The IR spectra evidenced different binding behaviors of the carboxylates on Ag and Al. A correlation between the change in work function with the effective dipole moment along the surface normal and the currents in the hole-only devices was observed. These self-assembled-monolayer-modified metals were used as anodes in the fabrication of top-emitting organic light-emitting diodes (TOLEDs). The TOLED with the Ag anode modified by the perfluoroalkanoate exhibited a luminous efficiency as high as 18 cdA-1, superior to that of the Ag2O-based device. With Al as the anode, the highest luminous efficiency was merely 6 cdA-1 and decayed rapidly. The poorer EL property and performance of Al-based TOLEDs could be attributed to the weaker ionic bindings of carboxylates on Al and the weaker microcavity effect resulting from the inferior reflectivity of Al as compared to Ag.
Introduction In the design of active-matrix organic light-emitting diodes (AMOLEDs), the top-emitting architecture is preferred because it allows more feasible fabrication of the displays.1 The Si thinfilm transistors can be buried under the organic light-emitting diodes (OLEDs), permitting the light to exit through the top electrode and giving a higher aperture ratio. In a top-emitting organic light-emitting diode (TOLED), a reflective metal as the anode is desired to direct the light output. However, due to mismatch of the metal working function and the HOMO energy level of most organic hole-injection/transport layers, the electrical and optical characteristics of the TOLED with a metal anode were still poor as compared with those of conventional bottomemitting devices with ITO as the anode. Recently, it has been reported that the charge injection from the metal anode to organic layer could be greatly facilitated by introducing a buffer layer at the metal surface.2-7 For example, an ultrathin plasmapolymerized CFx film, interposed between the Ag (or Au) anode and the NPB layer, chemically tailored the metal surface and improved the hole-injection ability of the anode.2-5 By using a thin layer of Pt/Pr2O3 or C60, the hole injection of the Al anode in a TOLED could be enhanced.6,7 By forming a thin Ag2O layer on the surface of the Ag anode, a TOLED showing device properties competitive with those of the conventional bottomemitting device was made.8 * To whom correspondence should be addressed. E-mail: ytt@ chem.sinica.edu.tw. † National Tsing Hua University. ‡ Academia Sinica. (1) Ali, T. A.; Jones, G. W.; Howard, W. E. Soc. Inf. Disp. Symp. Digest. 2004, 35, 1012-1015. (2) Hung, L. S.; Zheng, L. R.; Mason, M. G. Appl. Phys. Lett. 2001, 78, 673-675. (3) Tang, J. X.; Li, Y. Q.; Hung, L. S.; Lee, C. S. Appl. Phys. Lett. 2004, 84, 73-75. (4) Li, Y. Q.; Tang, J. X.; Xie, Z. Y.; Hung, L. S.; Lau, S. S. Chem. Phys. Lett. 2004, 386, 128-131. (5) Peng, H. J.; Sun, J. X.; Zhu, X. L.; Yu, X. M.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2006, 88, 073517. (6) Qiu, C. F.; Peng, H. J.; Chen, H. Y.; Xie, Z. L.; Wong, M.; Kwok, H. S. SID 03 Digest 2003, 974-976. (7) Lee, J. Y. Appl. Phys. Lett. 2006, 88, 073512.
The use of a self-assembled monolayer (SAM) grafted on a metal surface has been shown to have great potential for systematical tailoring of the work function of a metal.9-13 Thus, by utilizing thiol-based SAMs, the energy barrier for the hole injection from the Au (or Cu) anode to an organic layer could be improved.9-13 Recently, we also reported the use of SAMs of aromatic thiolates on Ag for the fabrication of efficient TOLEDs.14 Besides SAMs of various thiolates on coinage metals, n-alkanoic acids or aromatic acid derivatives have been known to form well-ordered SAMs through ionic bonding of the carboxylate head groups on Ag and Al surfaces.15 In this study, Ag and Al are selected and compared to be the electrode materials due to their highest reflectivity among metals.12 The modification of Ag and Al by SAMs of various carboxylic acids provides an interface with well-defined structure and properties, which allow the fine-tuning of the charge injection property from the metal to the organic layer. TOLEDs with carboxylate SAM-modified Ag and Al as the anodes were also constructed. The hole-injection efficiency, electroluminescence (EL) property, and device performance depend profoundly on the monolayer used. The anode modified by a monolayer with a larger positive effective dipole moment exhibited more reduction in the energy barrier for hole injection. The results show that the highest luminous efficiency of the Al-based TOLEDs modified by perfluorinated alkanoates was ca. 6 cdA-1 and decayed rapidly. However, the TOLEDs with Ag anodes modified by perfluorinated alkanoates exhibited a high luminous efficiency of ca. 12-18 cdA-1, better than the Ag2O-modified and other Ag-based devices. (8) 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. (9) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121. (10) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L. Phys. ReV. B 1996, 54, R14321-R14324. (11) 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. (12) Wu, C. C.; Chen, C. W.; Lin, C. L.; Yang, C. J. J. Disp. Tech. 2005, 1, 248-266. (13) Boer, B. D.; Hadipour, A.; Mandoc, M. M.; Woudenbergh, T. V.; Blom, P. W. M. AdV. Mater. 2005, 17, 621-625. (14) Hung, M. C.; Wu, K. Y.; Tao, Y. T.; Huang, H. W. Appl. Phys. Lett. 2006, 89, 203106. (15) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358.
10.1021/la700648z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007
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Figure 1. (a) Acids used for the formation of SAMs on electrode surfaces. (b) Schematic architecture of a TOLED.
Experimental Section Figure 1 shows the TOLED structure and the acids used in electrode modification. The acids were purchased from Aldrich or Alfa Aesar Ltd. and used as received. They were 4-CH3OPhCOOH (1a), 4-CF3PhCOOH (1b), and perfluoroalkanoic acids CF3(CF2)nCOOH (n ) 2, 2a; n ) 6, 2b; n ) 14, 2c). The anodes for the hole-only devices and TOLEDs were prepared by thermally evaporating 150 nm of Ag or Al film on soda glasses with an area of 6.25 mm2 that was defined by a patterned shadow mask. The freshly prepared metal substrates were immediately immersed in 1 mM hexadecane solutions of 1a, 1b, and 2a-2c for 10 min to form the SAMs. Upon withdrawal from solutions of 1a and 1b, the substrates emerged wet and were rinsed by hexane solvent to remove the excess acid materials and then blown dry with N2 before use in device fabrication or further characterization. On the other hand, the substrates appeared dry (oleophobic) after immersion in the solutions of 2a, 2b, and 2c and were used without further rinsing. Reflection-absorption infrared spectroscopy (RAIRS) was performed on 2 in. metal wafers prepared in parallel to characterize the monolayer structure. The calculated thickness of a monolayer was the end-to-end molecular length obtained by PC Spartan Pro. The measured thickness of a monolayer was obtained by means of optical ellipsometry. The molecular dipole moments were estimated by the semiempirical PM3 method with a geometry optimization (Polak-Ribiere, rms gradient of 0.05 kcal‚Å-1‚mol-1, HyperChem 6.03). A photoelectron spectrometer (AC-2, RIKEN KEIKI) was employed to measure the work functions of the bare and SAMmodified metals. For the Ag2O-covered anode, the as-prepared bare Ag was exposed to ozone in a UV-ozone generator for 2 min to generate a thin Ag2O layer on Ag. All the modified metal substrates were placed in a custom-designed rotating substrate holder in a vacuum chamber for the device fabrication. The organic layers of 4,4′,4′′-tris[(3-methylphenyl)phenylamino]triphenylamine (m-MTDATA; 30 nm), (R-naphthylphenyl)biphenyldiamine (NPB; 20 nm), and tris(8-hydroxyquinoline)aluminum (Alq3; 50 nm) were deposited in sequence, followed by a triplelayered cathode of LiF (1 nm), Al (2 nm), and Ag (20 nm), to achieve effective optical transmission and electron injection. Control experiments were carried out showing that the m-MTDATA layers deposited on bare Ag and SAM-modified Ag were of the same thickness within experimental error. For the hole-only devices, a hole-injecting layer of m-MTDATA (70 nm) and a hole-transport layer of NPB (50 nm) were deposited first, followed by the same triple-layered cathode. After the evaporation processes, the devices were encapsulated with transparent cover glasses by using UVcured epoxy glue. The current density-voltage (J-V), current density-brightness (J-B), and current density-efficiency (J-E) characteristics and EL spectra of the devices were measured by a Photo Research PR650 spectroradiometer coupled with a computercontrolled Keithly 2400 source meter.
Figure 2. Reflection-absorption IR spectra of the SAMs on Ag. Table 1. Assignments of Various Vibration Modes (cm-1) in the IR Spectra of SAMs on the Ag Surface νs(CO2-)a CdC stretching (ν8a)b CdC stretching (ν18a)b CdC stretching (ν19a)b aryl C-O sym stretchingb aryl C-O asym stretchingb aryl C-H op stretching (ν17b)b νs(CF2, A1)c νs(CF2, E2)c ν(CC, E1)c νa(CF2, E1)c νa(CF2, A2)c νs(CF2, E1)c a
1a
1b
2a
2b
2c
1390 1607
1398
1403
1401
1406
1365 1339 1291 1251 1225 1125
1365 1322 1294 1251 1214 1151
1378 1330 1309 1251 1218 1153
1512 1263 1175 866
1021 1511 872 1337
Reference 15. b Reference 16. c References 17-22.
Results and Discussion The native oxides on Ag and Al surfaces provide the basicity required for the deprotonation, leading to ionic bondings of the carboxylates on the Ag and Al surfaces, which could be evidenced by the IR spectra. As revealed in Table 1 and Figure 2, all the SAMs on Ag showed a strong absorption at 1398-1406 cm-1, which is assigned as the νs(CO2-) vibration mode. No peak assignable to νa(CO2-) was observed. The SAMs on Al showed not only a νs(CO2-) at 1417-1438 cm-1 but also a νa(CO2-) at 1698-1722 cm-1, as shown in Table 2 and Figure 3, demonstrating different bonding behaviors of the carboxylates on Ag and Al. This suggests the carboxylate head groups bind to the surface symmetrically (bridging ligand) on Ag and asymmetrically (monodentate) on Al, similar to the n-alkanoic acid monolayer on these surfaces.15 The aryl C-O symmetric/asymmetric stretching modes of 1a on Ag and Al are located at 1261/1175 and 1259/1176 cm-1, respectively, demonstrating the adsorption of 1a as well. Compared with the IR spectra of bulk 1a and 1b (not shown), the relatively lower intensity of ν17b on Ag and ν19a on Ag and Al implies that 1a and 1b molecules slantingly grafted onto the metal surfaces.16 The adsorbates 2a, 2b, and 2c, no matter on Ag or Al, exhibited symmetric and asymmetric CF2 (16) Varsanyi, G.; Szoke, S. Vibrational Spectra of Benzene DeriVatiVes; Academic Press: New York and London, 1969.
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Cho and Tao Table 3. Comparison of Film Thicknesses, Dipole Moments, and Work Functions of Various SAM-Modified Ag Surfaces 1a
1b
2a
2b
2c
Lca (Å) 8.08 7 5.7 10.49 20.73 8.08 7 4.93 9.09 17.95 Lvb (Å) Lmc (Å) 7.25 ( 0.23 4.28 ( 0.24 2.03 ( 0.17 3.88 ( 0.25 15.9 ( 0.37 θd (deg) 26 52 66 65 28 µAge (D) -1.16 1.94 0.52 1.03 1.09 -1.04 1.19 0.21 0.44 0.97 µ⊥,Agf (D) ΦSAM,Agg 4.57 5.15 4.96 5.36 5.72 (eV) ∆ΦSAM,Agh -0.03 0.55 0.36 0.76 1.12 (eV) a
Calculated thickness of a monolayer (end-to-end length of a single molecule) by PC Spartan Pro. b Vertical quantity of Lc along the surface normal. c Measured thickness of a monolayer by optical ellipsometry. d Estimated inclined angle of a monolayer on the Ag substrate. e Calculated dipole moment of the radical of the substituent (excluding the COO fragment) by HyperChem 6.03. f Effective dipole moment of a SAM on the Ag substrate. g Work function of the SAM-modified Ag. h Work function shift of the SAM-modified Ag. The work function of bare Ag (ΦAg) is 4.6 eV.
Figure 3. Reflection-absorption IR spectra of the SAMs on Al. (cm-1)
Table 2. Assignments of Various Vibration Modes IR Spectra of SAMs on the Al Surface
Table 4. Comparison of Film Thicknesses, Dipole Moments, and Work Functions of Various SAM-Modified Al Surfaces
in the
1a
1b
2a
2b
2c
a
νs(CO2-)a νa(CO2-)a CdC stretching (V8a)b CdC stretching (V19a)b aryl C-O sym stretchingb aryl C-O asym stretchingb νs(CF2, A1)c νs(CF2, E2)c ν(CC, E1)c νa(CF2, E1)c νa(CF2, A2)c νs(CF2, E1)c a
1a
1b
2a
2b
2c
1433 1722 1608 1512 1259 1176
1438 1717
1417 1698
1423 1702
1419 1704
1358 1335 1289 1235 1214 1147
1366 1321 1296 1251 1217 1152
1376 1328 1296 1239 1218 1154
1515
1329
Reference 15. b Reference 16. c References 17-22.
stretching modes, as shown in Tables 1 and 2 and Figures 2 and 3, confirming the adsorption of the perfluorinated SAMs.17-22 Tables 3 and 4 summarize the calculated and measured macroscopic properties of the carboxylate SAM-modified Ag and Al, respectively. The measured thicknesses (Lm) of the SAMs are close to or smaller than the calculated length (Lc), indicating that the adsorbates indeed formed monolayers.9 On the basis of the Lm, the grafting angles of the adsorbates on Ag and Al could be approximately estimated. With the assumption that the perfluorinated carboxylates (2a-2c) grafted on Ag symmetrically with a trans zigzag chain configuration (Figure 4b), the molecular chain is expected to tilt away from the surface normal to give an effective thickness Lv. The planar sp2-hybridized carboxylate head group could further incline (toward the back in Figure 4a) to adopt the final configuration (Figure 4a). The inclination angle θ is estimated from Lm and Lv. The θ values for 1a, 1b, and 2a-2c on Ag are estimated to be ca. 26°, 52°, 66°, 65°, and 28°, respectively. Thus, the CF3-sbustituted benzoate tilted more than the CH3O-substituted benzoate did. A perfluorinated monolayer (17) Masetti, G.; Cabassi, F.; Morelli, G.; Zerbi, G. Macromolecules 1973, 6, 700-707. (18) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93-98. (19) Naselli, C.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1989, 90, 38553860. (20) Chau, L. K.; Porter, M. D. Chem. Phys. Lett. 1990, 167, 198-204. (21) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507-3512. (22) Pawsey, S.; Reven, L. Langmuir 2006, 22, 1055-1062.
Lc (Å) 8.38 7.25 6.19 10.43 21.42 Lvb (Å) 8.16 7.25 6.19 10.43 21.42 c Lm (Å) 4.28 ( 0.24 5.33 ( 0.33 6.06 ( 0.61 9.93 ( 0.36 21.93 ( 0.85 θd (deg) 58 43 0 0 0 -1.16 1.94 0.52 1.03 1.09 µAle (D) µ⊥,Alf (D) -0.61 1.43 0.52 1.03 1.09 ΦSAM,Alg 4.31 4.54 4.61 4.75 5.22 (eV) ∆ΦSAM,Alh 0.03 0.26 0.33 0.47 0.94 (eV) a Calculated thickness of a monolayer (end-to-end length of a single molecule) by PC Spartan Pro. b Effective thickness Lv along the surface normal. c Measured thickness of a monolayer by optical ellipsometry. d Estimated inclination angle of a monolayer on the Al substrate. e Calculated dipole moment of the radical of the substituent (excluding the COO fragment) by HyperChem 6.03. f Effective dipole moment of the SAM on the Al substrate. g Work function of the SAM-modified Al. h Work function shift of the SAM-modified Al. The work function of bare Al (Φ,Al) is 4.28 eV.
with a shorter chain length exhibited a larger inclination. Unlike the bidentate bondings on Ag, the carboxylates grafted onto Al via monodentate (ester-like) bondings, as depicted in Figure 5. The measured thicknesses of 2a-2c on Al are very close to their respective Lc. It is inferred that 2a-2c stood almost perpendicularly on Al (θ ) 0°). To estimate the inclination angle θ of 1a and 1b on Al, the same head group tilt for 1a and 1b was assumed according to the IR results (Figure 5). The inclination angles θ of 1a and 1b on Al are estimated to be ca. 58° and 43°, respectively. The results revealed that the perfluorinated carboxylates on the Al surface formed more straight-up monolayers than those on the Ag surface, similar to the observation for n-alkanoic acids reported previously.15 The adsorbates created an interfacial dipole layer, whose magnitude is related to different molecular dipole moments. On the same metal surface, it was assumed that the magnitude of the dipole moment has a contribution from the fragment (aryl or the perfluoroalkyl moiety) attached to the carboxylate group and the CO2- fragment at the monolayer/metal interface, which is assumed to be the same for all molecules. Therefore, dipole moments of radicals of the molecular fragment excluding the CO2- moiety (µAg and µAl) were calculated, and the results are displayed in Tables 3 and 4. The effective dipole moments, i.e., vector fraction of µAg and µAl along the surface normal (µ⊥,Ag
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Figure 4. Film thickness estimation on Ag as a result of (a) symmetrical coordination of carboxylate to the surface and (b) chain inclination. Figure 6. Correlations between µ⊥ and ∆ΦSAM of the SAMs on Ag (9) and Al (0).
Figure 5. Film thickness estimation on Al as a result of asymmetrical coordination of carboxylate to the surface.
and µ⊥,Al), were also obtained after correction for the tilting and inclination of the monolayers. Herein a “positive” dipole moment is defined as having a dipole direction pointing away from the surface, and a “negative” dipole moment has a direction pointing toward the surface. On Ag, the dipole associated with the molecular fragment increases with the perfluorinated chain length, yet the CF3-substituted phenyl group has the largest dipole moment. The CH3O-substituted phenyl group gives an opposite dipole. On Al, the difference in dipole associated with the perfluorinated chain is smaller, due to the near-vertical orientation of the chain at the surface. The work functions of SAM-modified metals (ΦSAM,Ag and ΦSAM,Al) were measured by AC2, and the results are listed in Tables 3 and 4. The work function shifts (∆ΦSAM,Ag ) ΦSAM,Ag - ΦAg and ∆ΦSAM,Al ) ΦSAM,Al - ΦAl) are also listed. The electron-donating CH3O-substituted benzoate monolayer slightly lowered the work function of Ag and slightly increased that of Al, whereas the electron-withdrawing CF3-substituted benzoate monolayer much increased the work functions of Ag and Al substrates. The perfluorinated alkanoates tend to increase the work functions of both Ag and Al, and this effect is larger with increasing chain length. It is found that both ∆ΦSAM,Ag and ∆ΦSAM,Al increase with an increasing and positive µ⊥ of the SAM, but decrease with a negative µ⊥. The correlations between µ⊥ and ∆ΦSAM on Ag and Al are depicted in Figure 6. In this work, all of the perfluorinated carboxylates used had even numbers
of carbon atoms, so there was no so-called “odd-even” effect in the dipole moment.23 A direct proportion (near linearity) was observed for the series of compound 2, but the aromatic acids do not fall in the same line (Figure 6). The discrepancy may be attributed to different dielectric constants and grafting densities of different types of molecules or the validity of separating the carboxylate with rest of the molecule. Previous reports have also correlated the change of the work function with µ⊥ of monolayers on ITO and the Au surface.24,25 It is interesting to note the strong dependence of the work function on the chain length of perfluorinated monolayers in light of the observation that the chain length of an n-alkanethiol only has a small effect on the work function of thiolate-modified Ag.26 Although this can be attributed to a decreasing inclination of the chain with increasing chain length and the fact that the terminal CF3 is the major contributor to the molecular dipole, the high sensitivity of the work function of SAM-modified Al with respect to the chain length was still surprising. With near perpendicular chain orientation, the dipole moment should be insensitive to the chain length. Nevertheless, the work function clearly increased with chain length. This may imply that not only the dipole moment and its direction are affecting the work function.27 However, it is clear that the molecular dipole moment characteristic indeed has a major influence on the work function shift, as also can be perceived from the hole-injection characteristics of the electronic devices and will be illustrated later. Thus, to choose a molecule with appropriate dipole direction and magnitude is an efficacious way to adjust the work function of the anode. (23) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Jr., R. C.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690-11699. (24) Khodabakhsh, S.; Poplavskyy, D.; Heutz, S.; Nelson, J.; Bradley, D. D. C.; Murata, H.; Jones, T. S. AdV. Funct. Mater. 2004, 14, 1205-1210. (25) Boer, B. D.; Hadipour, A.; Mandoc, M. M.; Woudenbergh, T. V.; Blom, P. W. M. AdV. Mater. 2005, 17, 621-625. (26) Wu, K. Y.; Huang, H. W.; Tao, Y. T. Unpublished results. (27) It is noted that fluorine is the most frequently used atom to influence the work function. A plasma-polymerized CFx, where no specific dipole direction is expected, was reported to improve the charge injection, but the current decreased with increasing CFx thickness; see ref 2.
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Figure 7. J-V curves of the hole-only devices with Ag2O- and SAM-modified anodes.
The hole-only devices were prepared with various SAMmodified Ag as the anode. A control experiment showed that the thicknesses of m-MTDATA deposited on different SAM surfaces are similar within experimental error. Thus, the current variation is believed to be the result of charge injection rather than thickness variation. The J-V curves of the hole-only devices are displayed in Figure 7. That with a Ag anode modified by a thin Ag2O layer is also included for comparison. It is apparent from the results that the J-V characteristics with different modifications are significantly different. The devices with Ag modified by 2b and 2c showed a higher hole-injection ability than those modified by other SAMs, and the 1a-modified Ag was the poorest. Thus, when the anode was modified by a carboxylate with a larger positive µ⊥, more work function shift and more reduction in the hole-injection barrier resulted, and as a consequence a higher current was obtained. However, it is noted that the effective work function of 1b-modified Ag is similar to and those of 2band 2c-modified Ag are larger than that of the HOMO of the hole-injecting layer of m-MTDATA, which is 5.11 eV.28 An Ohmic contact of these anodes with the hole-injecting layer is expected. The higher current injection observed for the 2cmodified device than the 2b-modified device may suggest the Ohmic contact did not happen for all three cases, due to the additional “interfacial” dipole formed between the electrondonating m-MTDATA and the electron-accepting perfluorinated surface, which is in the opposite direction.29 The magnitude of this is not known but may partly offset the effective work function increase by the SAM adsorbed and change the injection barrier. In contrast, a merely slight work function shift was induced for the carboxylate with a negative µ⊥, which is unfavorable for the barrier reduction. The 1a-modified device exhibited the lowest current density, representing the poorest hole-injection ability. Even though the Ag2O-modified anode has a lower work function (ca. 4.9 eV, only higher than that of the 1a-modified anode), the Ag2O-based device nevertheless gave the highest current density. Presumably organic adsorbates not only modify the work function but also introduce a tunneling barrier which also limits the hole injection. (28) Chen, S. F.; Wang, C. W. Appl. Phys. Lett. 2004, 85, 765-767. (29) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605.
Figure 8. (a) J-B and (b) J-E curves of TOLEDs with Ag2O- and SAM-modified anodes.
Similar to the hole-only devices, the molecular dipolar characteristics exerted a significant influence on the EL property and device performance of the TOLEDs. The TOLEDs using SAM-modified silver anodes showed basically the same trend as the hole-only devices; that is, a higher current was achieved when the monolayer resulting in a larger work function increase of the anode was used. The device with 1a-modified Ag as the anode gave the lowest current density. It is also interesting that the 2c-modified device exhibited the highest luminous efficiency of 18 cd/A and the highest brightness of 21000 cd/m2 when the current density approached 120 mA/cm2, as displayed in Figure 8. Under the same driving current, the luminous efficiency and brightness of the 1b-modified device were 12.6 cd/A and 15100 cd/m2, respectively, slightly higher than those of the 2a-modified device (12.4 cd/A and 14900 cd/m2), which can also be ascribed to the difference in the effect of 1b and 2a on the electrode. The 1a-modified device shows the lowest luminous efficiency and brightness, 5.5 cd/A and 6500 cd/m2, presumably due to the opposite dipole leading to a higher hole-injection barrier. Even though the Ag2O-modified anode showed a higher hole-injection ability in the hole-only device, the Ag2O-based TOLED exhibits a luminous efficiency of 9 cd/A and a brightness of 11000 cd/m2, merely better than the 1a-modified device. This may be due to the fact that the luminous efficiency is related to the hole/electron recombination ratio, not solely reflected by the hole-injection current.30-32 This is also evidenced by the external quantum (30) Kim, Y. E.; Park, H.; Kim, J. J. Appl. Phys. Lett. 1996, 69, 599-601. (31) Ganzorig, C.; Fujihira, M. Appl. Phys. Lett. 2000, 77, 4211-4213. (32) Petta, J. R.; Salinas, D. G.; Ralph, D. C. Appl. Phys. Lett. 2000, 77, 4419-4421.
Metal Work Function Tuning with Organic Carboxylates
Figure 9. External quantum efficiency of the Ag2O- and SAMmodified TOLEDs.
Langmuir, Vol. 23, No. 13, 2007 7095
dominate the charge injection, whereas better charge balance was obtained with the longer perfluoroalkanoate-modified device. All the Ag-based TOLEDs emitted green luminance and have the maximum EL intensity at ca. 560 nm and an fwhm of 36 nm (not shown). This is red-shifted and much narrower relative to the green light of the corresponding bottom-emitting device (524 nm, fwhm ) 100 nm). The microcavity effect may be responsible for the change.14 When the anode material is Al, the performance of the TOLEDs is much diminished yet has a trend similar to that of Ag-based devices due to the dipolar nature of the SAMs. Figure 10a shows that the luminous efficiencies of the TOLEDs with Al anodes modified by 1b, 2b, and 2c were ca. 3-6 cd/A but decayed rapidly as the applied voltage increased. The luminous efficiency of the TOLED with a 2a-modified Al anode is approximately 2 cd/A, while those of the unmodified and 1a-modified devices were ca. 1 and 0.8 cd/A, respectively. Figure 10b shows that the brightness of Al-based devices ranges from 500 to 3000 cd/m2, relatively lower than that of the Ag-based devices. The Al-based TOLEDs also emitted green luminance, having the maximum EL intensity at ca. 558 nm and an fwhm of 48 nm (not shown). The broader EL peaks may be due to the lower microcavity effect of Al. The poorer device performance and stability of Al-based TOLEDs may also be ascribed to the weaker ionic bondings of the carboxylates on Al, the higher resistance (compared with Ag), and the lower microcavity effect resulting from the inferior reflectivity of the Al anode.
Conclusion
Figure 10. (a) J-B and (b) J-E curves of TOLEDs with bare Al and SAM-modified anodes.
efficiency curves of the Ag2O- and SAM-modified TOLEDs shown in Figure 9. The Ag2O-modified anode gave the highest current but a lower external quantum efficiency. Within the same series of compound 2, the trend in external quantum efficiency parallels that of current density, which correlates with the work function of the modified anodes. Even though the longer chain perfluoroalkanoate is expected to impose a larger tunneling barrier for the hole injection, the work function increase seems to
In conclusion, we have shown that well-defined SAMs of various carboxylates on Ag and Al could be used to manipulate the work function of metal surfaces. The hole-injection property and the device performance in top-emitting organic light-emitting diodes can therefore be manipulated. The binding modes of the SAMs on Al and Ag are different, with adsorbates on the Al surface forming tilted head group binding and more straight-up monolayers for a linear chain acid. The adsorbate on Ag formed symmetrical head group binding and a tilted chain conformation. The effective dipole moment along the surface normal, after correction for molecular tilt and inclination, did not have a quantitative correlation with the work function change among all compounds tested, but did give a direct proportional correlation in the same series of derivatives and indeed played an important role in affecting the work function shift. The device performance of the TOLEDs with SAM-modified Ag and Al anodes also revealed a similar trend. The performances of SAM-modified TOLEDs were much better due to efficient hole injection and a possibly better hole-electron recombination in the devices. The Ag-based TOLEDs modified by a long-chain perfluoroalkanoate displayed high brightness and a high luminous efficiency of 12-18 cd/A, much superior to those of the Ag2O-modified device. In contrast, the highest luminous efficiency of the Albased TOLEDs was merely 6 cd/A and decayed rapidly with the bias. Acknowledgment. Financial support from the Ministry of Economics, Taiwan, Republic of China, is gratefully appreciated. LA700648Z
