Tuning of Metal Work Function with Organic Carboxylates and ItsApplication in Top-Emitting Electroluminescent Devices

Chun-Pei Cho† and Yu-Tai Tao*,†,‡

Institute of Chemistry, Academia Sinica, Taipei, 11529 Taiwan, and Department of Chemistry, NationalTsing 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 electroluminescentdevice 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 thechange in work function with the effective dipole moment along the surface normal and the currents in the hole-onlydevices was observed. These self-assembled-monolayer-modified metals were used as anodes in the fabrication oftop-emitting organic light-emitting diodes (TOLEDs). The TOLED with the Ag anode modified by the perfluoroalkanoateexhibited 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 performanceof Al-based TOLEDs could be attributed to the weaker ionic bindings of carboxylates on Al and the weaker microcavityeffect 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 becauseit allows more feasible fabrication of the displays.1 The Si thin-film transistors can be buried under the organic light-emittingdiodes (OLEDs), permitting the light to exit through the topelectrode and giving a higher aperture ratio. In a top-emittingorganic light-emitting diode (TOLED), a reflective metal as theanode is desired to direct the light output. However, due tomismatch of the metal working function and the HOMO energylevel of most organic hole-injection/transport layers, the electricaland optical characteristics of the TOLED with a metal anodewere still poor as compared with those of conventional bottom-emitting devices with ITO as the anode. Recently, it has beenreported that the charge injection from the metal anode toorganic layer could be greatly facilitated by introducing a bufferlayer at the metal surface.2-7 For example, an ultrathin plasma-polymerized CFx film, interposed between the Ag (or Au) anodeand the NPB layer, chemically tailored the metal surface andimproved the hole-injection ability of the anode.2-5 By using athin layer of Pt/Pr2O3 or C60, the hole injection of the Al anodein a TOLED could be enhanced.6,7By forming a thin Ag2O layeron the surface of the Ag anode, a TOLED showing deviceproperties competitive with those of the conventional bottom-emitting device was made.8

The use of a self-assembled monolayer (SAM) grafted on ametal surface has been shown to have great potential forsystematical tailoring of the work function of a metal.9-13 Thus,by utilizing thiol-based SAMs, the energy barrier for the holeinjection from the Au (or Cu) anode to an organic layer couldbe improved.9-13 Recently, we also reported the use of SAMsof aromatic thiolates on Ag for the fabrication of efficientTOLEDs.14Besides SAMs of various thiolates on coinage metals,n-alkanoic acids or aromatic acid derivatives have been knownto form well-ordered SAMs through ionic bonding of thecarboxylate head groups on Ag and Al surfaces.15 In this study,Ag and Al are selected and compared to be the electrode materialsdue to their highest reflectivity among metals.12The modificationof Ag and Al by SAMs of various carboxylic acids provides aninterface with well-defined structure and properties, which allowthe fine-tuning of the charge injection property from the metalto the organic layer. TOLEDs with carboxylate SAM-modifiedAg and Al as the anodes were also constructed. The hole-injectionefficiency, electroluminescence (EL) property, and deviceperformance depend profoundly on the monolayer used. Theanode modified by a monolayer with a larger positive effectivedipole moment exhibited more reduction in the energy barrierfor hole injection. The results show that the highest luminousefficiency of the Al-based TOLEDs modified by perfluorinatedalkanoates was ca. 6 cdA-1 and decayed rapidly. However, theTOLEDs with Ag anodes modified by perfluorinated alkanoatesexhibited a high luminous efficiency of ca. 12-18 cdA-1, betterthan the Ag2O-modified and other Ag-based devices.* 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 Digest2003, 974-976.(7) Lee, J. Y.Appl. Phys. Lett. 2006, 88, 073512.

(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.Langmuir1999,15, 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.

7090 Langmuir2007,23, 7090-7095

10.1021/la700648z CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 05/24/2007

Experimental Section

Figure 1 shows the TOLED structure and the acids used inelectrode modification. The acids were purchased from Aldrich orAlfa Aesar Ltd. and used as received. They were 4-CH3OPhCOOH(1a), 4-CF3PhCOOH (1b), and perfluoroalkanoic acids CF3(CF2)n-COOH (n ) 2, 2a; n ) 6, 2b; n ) 14, 2c). The anodes for thehole-only devices and TOLEDs were prepared by thermallyevaporating 150 nm of Ag or Al film on soda glasses with an areaof 6.25 mm2 that was defined by a patterned shadow mask. Thefreshly prepared metal substrates were immediately immersed in 1mM hexadecane solutions of1a, 1b, and2a-2c for 10 min to formthe SAMs. Upon withdrawal from solutions of1a and 1b, thesubstrates emerged wet and were rinsed by hexane solvent to removethe excess acid materials and then blown dry with N2 before use indevice fabrication or further characterization. On the other hand,the substrates appeared dry (oleophobic) after immersion in thesolutions of2a, 2b, and2c and were used without further rinsing.Reflection-absorption infrared spectroscopy (RAIRS) was per-formed on 2 in. metal wafers prepared in parallel to characterize themonolayer structure. The calculated thickness of a monolayer wasthe end-to-end molecular length obtained by PC Spartan Pro. Themeasured thickness of a monolayer was obtained by means of opticalellipsometry. The molecular dipole moments were estimated bythe semiempirical PM3 method with a geometry optimization(Polak-Ribiere, rms gradient of 0.05 kcal‚Å-1‚mol-1, HyperChem6.03). A photoelectron spectrometer (AC-2, RIKEN KEIKI) wasemployed to measure the work functions of the bare and SAM-modified metals. For the Ag2O-covered anode, the as-preparedbare Ag was exposed to ozone in a UV-ozone generator for 2min to generate a thin Ag2O layer on Ag. All the modified metalsubstrates were placed in a custom-designed rotating substrateholder in a vacuum chamber for the device fabrication. Theorganic layers of 4,4′,4′′-tris[(3-methylphenyl)phenylamino]-triphenylamine (m-MTDATA; 30 nm), (R-naphthylphenyl)biphe-nyldiamine (NPB; 20 nm), and tris(8-hydroxyquinoline)aluminum(Alq3; 50 nm) were deposited in sequence, followed by a triple-layered cathode of LiF (1 nm), Al (2 nm), and Ag (20 nm), toachieve effective optical transmission and electron injection. Controlexperiments were carried out showing that them-MTDATA layersdeposited on bare Ag and SAM-modified Ag were of the samethickness within experimental error. For the hole-only devices, ahole-injecting layer ofm-MTDATA (70 nm) and a hole-transportlayer of NPB (50 nm) were deposited first, followed by the sametriple-layered cathode. After the evaporation processes, the deviceswere encapsulated with transparent cover glasses by using UV-cured epoxy glue. The current density-voltage (J-V), currentdensity-brightness (J-B), and current density-efficiency (J-E)characteristics and EL spectra of the devices were measured by aPhoto Research PR650 spectroradiometer coupled with a computer-controlled Keithly 2400 source meter.

Results and Discussion

The native oxides on Ag and Al surfaces provide the basicityrequired for the deprotonation, leading to ionic bondings of thecarboxylates on the Ag and Al surfaces, which could be evidencedby the IR spectra. As revealed in Table 1 and Figure 2, all theSAMs on Ag showed a strong absorption at 1398-1406 cm-1,which is assigned as theνs(CO2

-) vibration mode. No peakassignable toνa(CO2

-) was observed. The SAMs on Al showednot only aνs(CO2

-) at 1417-1438 cm-1 but also aνa(CO2-) at

1698-1722 cm-1, as shown in Table 2 and Figure 3, demon-strating different bonding behaviors of the carboxylates on Agand Al. This suggests the carboxylate head groups bind to thesurfacesymmetrically (bridging ligand)onAgandasymmetrically(monodentate) on Al, similar to then-alkanoic acid monolayeron these surfaces.15 The aryl C-O symmetric/asymmetricstretching modes of1a on Ag and Al are located at 1261/1175and 1259/1176 cm-1, respectively, demonstrating the adsorptionof 1a as well. Compared with the IR spectra of bulk1a and1b(not shown), the relatively lower intensity ofν17bon Ag andν19a

on Ag and Al implies that1aand1bmolecules slantingly graftedonto the metal surfaces.16 The adsorbates2a, 2b, and2c, nomatter 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.

Figure 1. (a) Acids used for the formation of SAMs on electrodesurfaces. (b) Schematic architecture of a TOLED.

Figure 2. Reflection-absorption IR spectra of the SAMs on Ag.

Table 1. Assignments of Various Vibration Modes (cm-1) in theIR Spectra of SAMs on the Ag Surface

1a 1b 2a 2b 2c

νs(CO2-)a 1390 1398 1403 1401 1406

CdC stretching (ν8a)b 1607CdC stretching (ν18a)b 1021CdC stretching (ν19a)b 1512 1511aryl C-O sym stretchingb 1263aryl C-O asym stretchingb 1175aryl C-H op stretching (ν17b)b 866 872νs(CF2, A1)c 1365 1365 1378νs(CF2, E2)c 1337 1339 1322 1330ν(CC, E1)c 1291 1294 1309νa(CF2, E1)c 1251 1251 1251νa(CF2, A2)c 1225 1214 1218νs(CF2, E1)c 1125 1151 1153

a Reference 15.b Reference 16.c References 17-22.

Metal Work Function Tuning with Organic Carboxylates Langmuir, Vol. 23, No. 13, 20077091

stretching modes, as shown in Tables 1 and 2 and Figures 2 and3, confirming the adsorption of the perfluorinated SAMs.17-22

Tables 3 and 4 summarize the calculated and measuredmacroscopic properties of the carboxylate SAM-modified Agand Al, respectively. The measured thicknesses (Lm) of the SAMsare close to or smaller than the calculated length (Lc), indicatingthat the adsorbates indeed formed monolayers.9 On the basis oftheLm, the grafting angles of the adsorbates on Ag and Al couldbe approximately estimated. With the assumption that theperfluorinated carboxylates (2a-2c) grafted on Ag symmetricallywith atranszigzag chain configuration (Figure 4b), the molecularchain is expected to tilt away from the surface normal to givean effective thicknessLv. The planar sp2-hybridized carboxylatehead group could further incline (toward the back in Figure 4a)to adopt the final configuration (Figure 4a). The inclination angleθ is estimated fromLm and Lv. The θ values for1a, 1b, and2a-2con Ag are estimated to be ca. 26°, 52°, 66°, 65°, and 28°,respectively. Thus, the CF3-sbustituted benzoate tilted more thanthe CH3O-substituted benzoate did. A perfluorinated monolayer

with a shorter chain length exhibited a larger inclination. Unlikethe bidentate bondings on Ag, the carboxylates grafted onto Alvia monodentate (ester-like) bondings, as depicted in Figure 5.The measured thicknesses of2a-2con Al are very close to theirrespectiveLc. It is inferred that2a-2c stood almost perpen-dicularly on Al (θ ) 0°). To estimate the inclination angleθ of1a and1b on Al, the same head group tilt for1a and1b wasassumed according to the IR results (Figure 5). The inclinationanglesθ of 1aand1b on Al are estimated to be ca. 58° and 43°,respectively. The results revealed that the perfluorinated car-boxylates on the Al surface formed more straight-up monolayersthan those on the Ag surface, similar to the observation forn-alkanoic acids reported previously.15

The adsorbates created an interfacial dipole layer, whosemagnitude is related to different molecular dipole moments. Onthe same metal surface, it was assumed that the magnitude ofthe dipole moment has a contribution from the fragment (arylor the perfluoroalkyl moiety) attached to the carboxylate groupand the CO2- fragment at the monolayer/metal interface, whichis assumed to be the same for all molecules. Therefore, dipolemoments of radicals of the molecular fragment excluding theCO2

- moiety (µAg andµAl) were calculated, and the results aredisplayed in Tables 3 and 4. The effective dipole moments, i.e.,vector fraction ofµAg andµAl along the surface normal (µ⊥,Ag

(17) Masetti, G.; Cabassi, F.; Morelli, G.; Zerbi, G.Macromolecules1973, 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, 3855-3860.

(20) Chau, L. K.; Porter, M. D.Chem. Phys. Lett.1990, 167, 198-204.(21) Alves, C. A.; Porter, M. D.Langmuir1993, 9, 3507-3512.(22) Pawsey, S.; Reven, L.Langmuir2006, 22, 1055-1062.

Figure 3. Reflection-absorption IR spectra of the SAMs on Al.

Table 2. Assignments of Various Vibration Modes (cm-1) in theIR Spectra of SAMs on the Al Surface

1a 1b 2a 2b 2c

νs(CO2-)a 1433 1438 1417 1423 1419

νa(CO2-)a 1722 1717 1698 1702 1704

CdC stretching (V8a)b 1608CdC stretching (V19a)b 1512 1515aryl C-O sym stretchingb 1259aryl C-O asym stretchingb 1176νs(CF2, A1)c 1358 1366 1376νs(CF2, E2)c 1329 1335 1321 1328ν(CC, E1)c 1289 1296 1296νa(CF2, E1)c 1235 1251 1239νa(CF2, A2)c 1214 1217 1218νs(CF2, E1)c 1147 1152 1154

a Reference 15.b Reference 16.c References 17-22.

Table 3. Comparison of Film Thicknesses, Dipole Moments, andWork Functions of Various SAM-Modified Ag Surfaces

1a 1b 2a 2b 2c

Lca (Å) 8.08 7 5.7 10.49 20.73

Lvb (Å) 8.08 7 4.93 9.09 17.95

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 28µAg

e (D) -1.16 1.94 0.52 1.03 1.09µ⊥,Ag

f (D) -1.04 1.19 0.21 0.44 0.97ΦSAM,Ag

g

(eV)4.57 5.15 4.96 5.36 5.72

∆ΦSAM,Agh

(eV)-0.03 0.55 0.36 0.76 1.12

a Calculated thickness of a monolayer (end-to-end length of a singlemolecule) by PC Spartan Pro.b Vertical quantity ofLc along the surfacenormal.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 (excludingthe COO fragment) by HyperChem 6.03.f Effective dipole moment ofa 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 ofbare Ag (ΦAg) is 4.6 eV.

Table 4. Comparison of Film Thicknesses, Dipole Moments, andWork Functions of Various SAM-Modified Al Surfaces

1a 1b 2a 2b 2c

Lca (Å) 8.38 7.25 6.19 10.43 21.42

Lvb (Å) 8.16 7.25 6.19 10.43 21.42

Lmc (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 0µAl

e (D) -1.16 1.94 0.52 1.03 1.09µ⊥,Al

f (D) -0.61 1.43 0.52 1.03 1.09ΦSAM,Al

g

(eV)4.31 4.54 4.61 4.75 5.22

∆ΦSAM,Alh

(eV)0.03 0.26 0.33 0.47 0.94

a Calculated thickness of a monolayer (end-to-end length of a singlemolecule) by PC Spartan Pro.b Effective thicknessLv along the surfacenormal.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 (excludingthe COO fragment) by HyperChem 6.03.f Effective dipole moment ofthe 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 ofbare Al (Φ,Al) is 4.28 eV.

7092 Langmuir, Vol. 23, No. 13, 2007 Cho and Tao

andµ⊥,Al), were also obtained after correction for the tilting andinclination of the monolayers. Herein a “positive” dipole momentis defined as having a dipole direction pointing away from thesurface, and a “negative” dipole moment has a direction pointingtoward the surface. On Ag, the dipole associated with themolecular fragment increases with the perfluorinated chain length,yet the CF3-substituted phenyl group has the largest dipolemoment. The CH3O-substituted phenyl group gives an oppositedipole. On Al, the difference in dipole associated with theperfluorinated chain is smaller, due to the near-vertical orientationof the chain at the surface.

The work functions of SAM-modified metals (ΦSAM,Ag andΦSAM,Al) were measured by AC2, and the results are listed inTables 3 and 4. The work function shifts (∆ΦSAM,Ag ) ΦSAM,Ag

- ΦAg and ∆ΦSAM,Al ) ΦSAM,Al - ΦAl) are also listed. Theelectron-donating CH3O-substituted benzoate monolayer slightlylowered the work function of Ag and slightly increased that ofAl, whereas the electron-withdrawing CF3-substituted benzoatemonolayer much increased the work functions of Ag and Alsubstrates. The perfluorinated alkanoates tend to increase thework functions of both Ag and Al, and this effect is larger withincreasing chain length. It is found that both∆ΦSAM,Ag and∆ΦSAM,Al increase with an increasing and positiveµ⊥ of theSAM, but decrease with a negativeµ⊥. The correlations betweenµ⊥ and∆ΦSAM on Ag and Al are depicted in Figure 6. In thiswork,all of theperfluorinatedcarboxylatesusedhadevennumbers

of carbon atoms, so there was no so-called “odd-even” effectin the dipole moment.23 A direct proportion (near linearity) wasobserved for the series of compound2, but the aromatic acidsdo not fall in the same line (Figure 6). The discrepancy may beattributed to different dielectric constants and grafting densitiesof different types of molecules or the validity of separating thecarboxylate with rest of the molecule. Previous reports have alsocorrelated the change of the work function withµ⊥ of monolayerson ITO and the Au surface.24,25

It is interesting to note the strong dependence of the workfunction on the chain length of perfluorinated monolayers inlight of the observation that the chain length of ann-alkanethiolonly has a small effect on the work function of thiolate-modifiedAg.26 Although this can be attributed to a decreasing inclinationof the chain with increasing chain length and the fact that theterminal CF3 is the major contributor to the molecular dipole,the high sensitivity of the work function of SAM-modified Alwith respect to the chain length was still surprising. With nearperpendicular chain orientation, the dipole moment should beinsensitive to the chain length. Nevertheless, the work functionclearly increased with chain length. This may imply that not onlythe dipole moment and its direction are affecting the workfunction.27However, it is clear that the molecular dipole momentcharacteristic indeed has a major influence on the work functionshift, as also can be perceived from the hole-injection charac-teristics of the electronic devices and will be illustrated later.Thus, to choose a molecule with appropriate dipole direction andmagnitude is an efficacious way to adjust the work function ofthe 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. B2003, 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 isexpected, was reported to improve the charge injection, but the current decreasedwith increasing CFx thickness; see ref 2.

Figure 4. Film thickness estimation on Ag as a result of (a)symmetrical coordination of carboxylate to the surface and (b) chaininclination.

Figure 5. Film thickness estimation on Al as a result of asymmetricalcoordination of carboxylate to the surface.

Figure 6. Correlations betweenµ⊥ and∆ΦSAM of the SAMs on Ag(9) and Al (0).

Metal Work Function Tuning with Organic Carboxylates Langmuir, Vol. 23, No. 13, 20077093

The hole-only devices were prepared with various SAM-modified Ag as the anode. A control experiment showed that thethicknesses ofm-MTDATA deposited on different SAM surfacesare similar within experimental error. Thus, the current variationis believed to be the result of charge injection rather than thicknessvariation. TheJ-Vcurves of the hole-only devices are displayedin Figure 7. That with a Ag anode modified by a thin Ag2O layeris also included for comparison. It is apparent from the resultsthat theJ-V characteristics with different modifications aresignificantly different. The devices with Ag modified by2b and2cshowed a higher hole-injection ability than those modified byother SAMs, and the1a-modified Ag was the poorest. Thus,when the anode was modified by a carboxylate with a largerpositiveµ⊥, more work function shift and more reduction in thehole-injection barrier resulted, and as a consequence a highercurrent was obtained. However, it is noted that the effectivework function of1b-modified Ag is similar to and those of2b-and2c-modified Ag are larger than that of the HOMO of thehole-injecting layer ofm-MTDATA, which is 5.11 eV.28 AnOhmic contact of these anodes with the hole-injecting layer isexpected. The higher current injection observed for the2c-modified device than the2b-modified device may suggest theOhmic contact did not happen for all three cases, due to theadditional “interfacial” dipole formed between the electron-donatingm-MTDATA and the electron-accepting perfluorinatedsurface, which is in the opposite direction.29 The magnitude ofthis is not known but may partly offset the effective work functionincrease by the SAM adsorbed and change the injection barrier.In contrast, a merely slight work function shift was induced forthe carboxylate with a negativeµ⊥, which is unfavorable for thebarrier reduction. The1a-modified device exhibited the lowestcurrent 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 the1a-modified anode), theAg2O-based device nevertheless gave the highest current density.Presumably organic adsorbates not only modify the work functionbut also introduce a tunneling barrier which also limits the holeinjection.

Similar to the hole-only devices, the molecular dipolarcharacteristics exerted a significant influence on the EL propertyand device performance of the TOLEDs. The TOLEDs usingSAM-modified silver anodes showed basically the same trendas the hole-only devices; that is, a higher current was achievedwhen the monolayer resulting in a larger work function increaseof the anode was used. The device with1a-modified Ag as theanode gave the lowest current density. It is also interesting thatthe2c-modified device exhibited the highest luminous efficiencyof 18 cd/A and the highest brightness of 21000 cd/m2 when thecurrent density approached 120 mA/cm2, as displayed in Figure8. Under the same driving current, the luminous efficiency andbrightness of the1b-modified device were 12.6 cd/A and 15100cd/m2, respectively, slightly higher than those of the2a-modifieddevice (12.4 cd/A and 14900 cd/m2), which can also be ascribedto the difference in the effect of1b and2aon the electrode. The1a-modified device shows the lowest luminous efficiency andbrightness, 5.5 cd/A and 6500 cd/m2, presumably due to theopposite dipole leading to a higher hole-injection barrier. Eventhough the Ag2O-modified anode showed a higher hole-injectionability in the hole-only device, the Ag2O-based TOLED exhibitsa luminous efficiency of 9 cd/A and a brightness of 11000 cd/m2,merely better than the1a-modified device. This may be due tothe fact that the luminous efficiency is related to the hole/electronrecombination ratio, not solely reflected by the hole-injectioncurrent.30-32 This is also evidenced by the external quantum

(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.

(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.

Figure 7. J-V curves of the hole-only devices with Ag2O- andSAM-modified anodes.

Figure 8. (a)J-B and (b)J-E curves of TOLEDs with Ag2O- andSAM-modified anodes.

7094 Langmuir, Vol. 23, No. 13, 2007 Cho and Tao

efficiency curves of the Ag2O- and SAM-modified TOLEDsshown in Figure 9. The Ag2O-modified anode gave the highestcurrent but a lower external quantum efficiency. Within the sameseries of compound2, the trend in external quantum efficiencyparallels that of current density, which correlates with the workfunction of the modified anodes. Even though the longer chainperfluoroalkanoate is expected to impose a larger tunneling barrierfor the hole injection, the work function increase seems to

dominate the charge injection, whereas better charge balancewas obtained with the longer perfluoroalkanoate-modified device.All the Ag-based TOLEDs emitted green luminance and havethe maximum EL intensity at ca. 560 nm and an fwhm of 36 nm(not shown). This is red-shifted and much narrower relative tothe green light of the corresponding bottom-emitting device (524nm, fwhm) 100 nm). The microcavity effect may be responsiblefor the change.14

When the anode material is Al, the performance of the TOLEDsis much diminished yet has a trend similar to that of Ag-baseddevices due to the dipolar nature of the SAMs. Figure 10a showsthat the luminous efficiencies of the TOLEDs with Al anodesmodified by 1b, 2b, and2c were ca. 3-6 cd/A but decayedrapidly as the applied voltage increased. The luminous efficiencyof the TOLED with a2a-modified Al anode is approximately2 cd/A, while those of the unmodified and1a-modified deviceswere ca. 1 and 0.8 cd/A, respectively. Figure 10b shows that thebrightness of Al-based devices ranges from 500 to 3000 cd/m2,relatively lower than that of the Ag-based devices. The Al-basedTOLEDs also emitted green luminance, having the maximumEL intensity at ca. 558 nm and an fwhm of 48 nm (not shown).The broader EL peaks may be due to the lower microcavityeffect of Al. The poorer device performance and stability ofAl-based TOLEDs may also be ascribed to the weaker ionicbondings of the carboxylates on Al, the higher resistance(compared with Ag), and the lower microcavity effect resultingfrom the inferior reflectivity of the Al anode.

Conclusion

In conclusion, we have shown that well-defined SAMs ofvarious carboxylates on Ag and Al could be used to manipulatethe work function of metal surfaces. The hole-injection propertyand the device performance in top-emitting organic light-emittingdiodes can therefore be manipulated. The binding modes of theSAMs on Al and Ag are different, with adsorbates on the Alsurface forming tilted head group binding and more straight-upmonolayers for a linear chain acid. The adsorbate on Ag formedsymmetrical head group binding and a tilted chain conformation.The effective dipole moment along the surface normal, aftercorrection for molecular tilt and inclination, did not have aquantitative correlation with the work function change amongall compounds tested, but did give a direct proportional correlationin the same series of derivatives and indeed played an importantrole in affecting the work function shift. The device performanceof the TOLEDs with SAM-modified Ag and Al anodes alsorevealed a similar trend. The performances of SAM-modifiedTOLEDs were much better due to efficient hole injection anda possibly better hole-electron recombination in the devices.The Ag-based TOLEDs modified by a long-chain perfluoroal-kanoate displayed high brightness and a high luminous efficiencyof 12-18 cd/A, much superior to those of the Ag2O-modifieddevice. In contrast, the highest luminous efficiency of the Al-based TOLEDs was merely 6 cd/A and decayed rapidly with thebias.

Acknowledgment. Financial support from the Ministry ofEconomics, Taiwan, Republic of China, is gratefully appreciated.

LA700648Z

Figure 9. External quantum efficiency of the Ag2O- and SAM-modified TOLEDs.

Figure 10. (a) J-B and (b)J-E curves of TOLEDs with bare Aland SAM-modified anodes.

Metal Work Function Tuning with Organic Carboxylates Langmuir, Vol. 23, No. 13, 20077095