Approaching Charge Balance in Organic Light-Emitting Diodes by

Nov 21, 2011 - The mixed SAM-modified ITO surfaces were used as the anode in the .... Erich P. Meinig , Darick J. Baker , Dana C. Olson , Reuben T. Co...
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Approaching Charge Balance in Organic Light-Emitting Diodes by Tuning Charge Injection Barriers with Mixed Monolayers Szu-Yen Yu,† Ding-Chi Huang,† Yi-Ling Chen,‡ Kun-Yang Wu,§ and Yu-Tai Tao*,†,§ †

Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan, Republic of China 300 Department of Chemistry, National Central University, Chungli, Taiwan, Republic of China 320 § Institute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China 115 ‡

ABSTRACT:

Self-assembled monolayers (SAMs) of binary mixtures of 1-butylphosphonic acid and the trifluoromethyl-terminated analogue (4,4,4-trifluoro-1-butylphosphonic acid) were formed on ITO surfaces to tune the work function of ITO over a range of 5.0 to 5.75 eV by varying the mixing ratio of the two adsorbents. The mixed SAM-modified ITO surfaces were used as the anode in the fabrication of OLED devices with a configuration of ITO/SAM/HTL/Alq3/MX/Al, where HTL was the NPB or BPAPF holetransporting layer and MX was the LiF or Cs2CO3 injection layer. It was shown that, depending on the HTL or MX used, the maximum device current and the maximum luminance efficiency occurred with anodes of different modifications because of a shift in the point of hole/electron carrier balance. This provides information on the charge balance in the device and points to the direction to improve the performance.

’ INTRODUCTION Organic light-emitting diodes (OLEDs) have become a realistic technology in fabricating flat panel displays because of the rapid progress in academia and industry research efforts.1 One major focus in OLED research is the improvement of the device luminance efficiency. According to the commonly accepted theory, the external quantum efficiency (ηext) of an OLED is described by the equation ηext ¼

ηopt  γ  ηst  q

designs for maximizing light out-coupling.5 A less well understood parameter is the electron/hole charge balance factor, although different electrode materials have been used to improve the device efficiency.6 A Schottky barrier is present at the metal/ organic interface because of the 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 energy barrier between the electrode and organic layer is in general desirable for higher charge injection. However, with the various organic semiconductors and electrode materials available nowadays, the energy alignment could be changed so much that either charge carrier (hole or electron) could become the major carrier. Improving the injection of carriers that are already in excess may improve the absolute luminance due to the greater chance of charge recombination but can have the opposite effect on the efficiency because of the deteriorated balance in charge carriers. Thus, a knowledge of the charge balance in a device is useful in choosing the strategy needed to improve the device efficiency. For a given

ð1Þ

where ηopt is the intrinsic photoluminescence quantum yield for excitons (including both fluorescence and phosphorescence), γ is the electron/hole charge balance factor (the ratio of the number of electrons to the number of holes injected from opposite electrodes), ηst is the fraction of excitons of specific multiplicity formed upon charge recombination (ηst ≈ 1/4 for singlet excitons and ηst ≈ 3/4 for triplet excitons), and q is the light out-coupling factor.2 In principle, the device efficiency can be optimized through the maximization of each parameter in the equation. Much effort has been devoted to designing new dyes with higher photoluminescence quantum yields,3 new triplet emitters and/or host materials for the full exploitation of excitons formed upon carrier recombination,4 and novel electrode surface r 2011 American Chemical Society

Received: September 16, 2011 Revised: November 13, 2011 Published: November 21, 2011 424

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Langmuir set of organic semiconductors in a device, the work function7 of the electrode greatly affects the charge injection from the electrode to the organic semiconductor, even though the surface energy can also affect the charge injection through its effect on film morphology and contact at the electrode/organic interface.8 Indium tin oxide (ITO) is the most commonly used anode in OLED fabrication because of its commercial availability, good transparency, and low resistivity. However, the relatively low work function of detergent/solvent-cleaned ITO leads to a significant barrier at the interface for hole injection into the HOMO level of most organic semiconductors. Various approaches have been used to increase the work function of ITO, including oxygen plasma treatment,9 UV ozone treatment,10 and the use of a conductive buffer layer such as poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT/ PSS).11 Self-assembled monolayers (SAMs) afford another effective way to modify an electrode surface.7,12 By choosing molecules with an appropriate dipole moment, the electrode work function and thus the Schottky barrier between the electrode and organic layer can be rationally modulated.13 SAMs of organosilanes,14 SO2Cl,15 and organophosphonic acids16 have been used to modify ITO surfaces to improve the performance of OLED devices. In contrast to the commonly used plasma-treated ITO surfaces, the organic SAM imposes about a 1 nm barrier through which charges can tunnel, with a resistivity depending on the structure of the molecule used.17 SAMs of alkylthiol or phenylthiol were shown to withstand at least a current density of ∼1 mA/cm2 passing through the layer under a bias of ∼10 MV/cm. In a typical OLED device with a total thickness of ∼100 nm for the organic layer sandwiched between the electrodes, the current density is several hundred times larger under a bias of 1 MV/cm. Although several tens times the power density is to be dissipated across the monolayer in the OLED device, the mere fact devices with certain SAM-modified ITOs as the anodes resulted in improved device durability18 compared to that using plasma-treated ITO as the anodes suggests that the monolayer used to modify the electrode will not be the cause of device failure. The improved lifetime can be due to the stable work function of ITO brought about by the covalently bound dipolar SAM in contrast to plasma-treated ITO, whose work function decays with time.18 Thus, the modulation of the work function with organic SAMs is a viable technology to be incorporated into the device fabrication. Although modification by a single-component SAM affords a specific surface property (e.g., work function or wettability), the use of a mixed monolayer allows a range of surface properties to be prepared with just two components of opposite character by adjusting the composition in the mixed monolayer. For example, the wetting characteristics have been found to be tunable by mixed monolayers containing a hydrophobic and a hydrophilic constituent.19 We20 and others21 recently reported the use of mixed monolayers from two organothiols in tuning the work function of Au or Ag surfaces. We also demonstrated that with mixed monolayers prepared from fluorinated n-decanethiol (FDT) and n-decanethiol (HDT) on Ag one can tune the work function continuously over a range of 1.7 eV. With mixed monolayer-modified Ag as the electrodes in fabricating top-emitting OLEDs, the current obtained reached a maximum with the Ag electrode modified by a 2:1 FDT/HDT mixed monolayer. Further increases in the work function using higher FDT ratios in the mixed monolayer did not increase the current further, presumably because of

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Figure 1. Scheme of OLEDs devices.

Fermi-level pinning.22 Furthermore, the luminance efficiency did not increase with improving hole injection from the modification because of the unbalanced charge injection. In such devices, enhancing the minority carrier injection (electrons) and adjusting the charge balance are essential to the improvement of the device performance.23 Electron injection continues to be an important issue because low work function metals such as Li, Ca, and Cs,24 with Fermi levels that are closer to the LUMO level of organic semiconductors are too reactive and thus difficult to use. Typically, alkaline metal salts such as LiF, CsF, and Cs2CO325 are used as an electron injection layer in conjunction with aluminum metal as the cathode. These materials are not sensitive to oxygen and proved to be effective in promoting electron injection. Among these materials, Cs2CO3 is attractive because of its lower work function and high compatibility for the selection of cathode metal. Cs2CO3 is demonstrated to be a better electron injection layer.26 In this article, we describe the use of mixed monolayers from 1-butylphosphonic acid (BPA) and its trifluoromethyl-terminated analogue 4,4,4-trifluoro-1-butylphosphoic acid (FPA) to tune the work function of the ITO surface. n-Alkanephosphonic acid is known to decrease the work function, and fluorinated phosphonic acid serves to increase the work function.15,27 A shorter chain is purposely chosen because the alkyl chain will impose an insulating tunnel barrier for charge injection. The mixtures of the two in various ratios allow for the continuous tuning of the ITO work function. A wide work function range was achieved (5.05.75 eV). These modified substrates were used as the anodes in the fabrication of organic electronic devices with variable charge injection barriers. With the use of two HTL materials with different HOMO levels and two cathodes with different electron injection abilities, we demonstrated that the charge carrier balance could be approached with particular combinations of modified ITO and the electron injection layer.

’ EXPERIMENTAL SECTION Material. BPA was purchased from Strem Chemical. FPA was synthesized and fully characterized in the laboratory. Lithium fluoride and cesium carbonate were purchased from Aldrich. α-Naphthylphenylbiphenyldiamine (NPB), (9,9-bis{4-[di(p-biphenyl)aminophenyl]}fluorene) (BPAPF), and tris(8-hydroxyquinolino) aluminum(III) (Alq3) were obtained from commercial sources. ITO-covered glass slides (30 Ω/0) were purchased from Merck. ITO Cleaning and Activation. The ITO-covered glass slides were first cleaned via a detergent/solvent cleaning procedure that consisted of sonication in detergent solution followed by sequential DI water and acetone rinses for 15 min each and then drying in a flow of nitrogen. All oxygen plasma cleaning was performed, after the cleaning protocol, in an oxygen plasma cleaner (All Real Tech. PCD150) operated at 50 W for 5 min, followed by immediate reaction with the modifier. SAM Modification of the ITO Surface. The plasma-cleaned ITO substrates were immersed in a 1 mM ethanol solution of 425

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Figure 2. XPS spectra of SAMs on ITO.

Table 1. Work Function and Contact Angle of ITO Substrates Modified by Mixed SAMs of FPA and BPAa contact angle (deg)

a

mixing ratio (FPA/BPA)

work function (eV)

H2O

hexadecane

1:0

∼5.75

∼87.0

∼35.5

2:1 1:1

∼5.62 ∼5.52

∼88.7 ∼91.4

∼31.4 ∼29.1

1:2

∼5.33

∼92.5

∼28.0

0:1

∼5.0

∼95.6

∼10.2

The work function of bare OP-ITO in this study was ∼5.65 eV.

phosphonic acid mixtures consisting of FPA and BPA in different mole ratios (1:0, 2:1, 1:1, 1:2, and 0:1) at 50 °C for 6 h. The ITO substrates were then baked at 100 °C for 12 h to achieve the completion of chemical bonding.24,28 After this annealing step, the ITO substrates were sonicated for 30 min in a 5% triethylamine/ethanol base solution to ensure that no physisorbed molecules remained. Finally, all PA-modified ITO substrates were rinsed with a copious amount of ethanol and dried in a flow of nitrogen. Characterization. XPS data were taken with an Omicron ESCA/ scanning auger system with a chamber vacuum pressure of 1  1010 Torr. The work function of the modified surface was measured with a photoelectron spectrometer (AC-2, Riken Keiki) under ambient conditions with a UV source.29 Device Fabrication. A scheme of the device is shown in Figure 1. The modified ITO substrates were placed in a custom-designed rotating sample holder in a vacuum chamber (5  106 Torr) for 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 in a cover glass with UV-cured epoxy glue. The current densityvoltage (JV) characteristics of the devices were measured with a computer-controlled Keithly 2400 source meter connected to a PR650 spectrophotometer.

Figure 3. Work function as a function of the amount of FPA in the monolayer on ITO.

a free PdO moiety. In the present case, of interest is the structure of mixed monolayers, which were prepared from solutions containing a mixture of FPA and BPA with a total concentration of 1 mM. X-ray photoelectron spectroscopy was used to characterize the mixed monolayer. The XPS spectra of F 1s, P 2p, In 4s, and Sn 3d for mixed SAMs formed on the ITO surfaces are shown in Figure 2. The intensity of the F 1s signal at 687.8 eV30 increases with increasing amounts of FPA in the monolayer. From the integration, the 1:1 ratio mixed SAM contains ∼50% of the fluorine signal of that of the single-component FPA SAM. In the P 2p spectra, the peak at 133.8 eV, assigned to PO32, did not shift for the three different PA monolayers and the integration shows that the same phosphine signal strength was obtained. It is assumed that the same binding mode and binding density on ITO were obtained for all samples. The four peaks at 452.4, 444.7, 495.3, and 486.9 eV, assigned to In 3d5/2, In 3d5/2, Sn 3d3/2, and Sn 3d5/2, respectively, serving as a reference, gave the same intensity for the three samples. On the basis of the XPS results, it is suggested that the surface composition is similar to the solution composition.28 The contact angle was measured on the modified surfaces as a function of the composition of the mixed monolayer (Table 1). All surfaces showed hydrophobicity. However, on the single-component FPA-modified surface, the water contact angle (∼87°) is lower than that on the single-component BPA modified surface (∼95.6°), presumably because hydrogen bonding between water and the fluorine atom dominates on this surface.31 The water

’ RESULTS AND DISCUSSION Characterization of ITO Substrates Modified with Phosphonic Acids. The self-assembled monolayers of phosphonic

acids on ITO have been documented.25,26 Phosphonic acids are demonstrated to create a more robust modification layer than carboxylic acids, and the PO bonds to the metal oxide lattice can be further improved by annealing.26 Mainly bidentate binding of the phosphonic acid is involved with

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Figure 4. IV characteristics and luminance efficiency as a function of the FPA/BPA ratio in the OLEDs with (a, b) device A, (c, d) device B, (e, f) device C, and (g, h) device D.

contact angle increases with increasing amounts of BPA. The hexadecane (HD) contact angle also shows a clear dependence on the composition. On a pure BPA-modified surface, the θHD is the lowest (∼10.2°) and it increases with increasing amounts of FPA in the mixed monolayer because of decreasing dispersive interactions between the hydrocarbon liquid and the fluorinated surface.32 That the θHD of the pure BPA surface is low compared to the hexadecane contact angle on the related n-alkanethiolate monolayer on Au suggests that the monolayer is not as ordered and closely packed as in SAMs formed from thiol molecules.30

The work function (j) of various modified surfaces was measured with an AC2 photoelectron spectrometer. The oxygenplasma-treated ITO substrate had a work function j of 5.65 eV. For a single-component FPA-covered ITO surface, the j value increased to about 5.75 eV, whereas for the single-component BPA-covered ITO surface, the j value decreased to about 5.0 eV. The opposite effect on the work function is suggested 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. For the mixed-monolayer-modified 427

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ITO, the measured work function was found to fall between 5.0 and 5.75 eV and vary almost linearly with the surface composition (Figure 3). The results are summarized in Table 1. Effect of Mixed SAMs on the Charge Injection between the Electrode and the Organic Layer. Because the work function of ITO can be tuned over a wide range (∼0.7 eV) using mixed monolayers, the charge injection barrier between the electrode and a contacting organic layer can be tuned accordingly. Furthermore, because all of the films are virtually a monolayer thick and have a similar effective thickness, the various mixed SAM modifications were assumed to impose similar tunneling distances for charge injection so that the energy gap between the electrode work function and the HOMO level is the major factor determining the relative charge injection. Four types of OLED devices were tested, with two different HTLs and two different cathodes. All four types of devices produced similar electroluminescence spectra, which means that the recombination area is within the same Alq3 layer. Type A devices had a configuration of ITO/SAM/NPB (50 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (80 nm), where NPB served as the HTL with a HOMO level at 5.46 eV, and Alq3 served as both the EL (emission layer) and the ETL (electron-transporting layer). The JV characteristic is shown in Figure 4a. The maximum current was obtained for an ITO electrode modified with a singlecomponent monolayer of FPA. ITO modified with a 2:1 FPA/ BPA mixed layer produced a similar current density. The use of a higher BPA content in the mixed monolayer further reduced the work function and increased the injection barrier so that the current also decreased. However, the luminance efficiency of the devices, as shown in Figure 4b, exhibits the highest value with an ITO electrode modified with a pure BPA monolayer (to yield a work function of 5.0 eV), although the efficiency decayed quickly with increasing current. The fast decay in efficiency may due to

the rather high voltage needed to reach a current comparable to that of other devices, and thus the device degraded. It is suggested that in all of these devices the hole carrier remains the dominant charge carrier. With pure BPA-modified ITO, whose work function is the lowest, the number of hole carriers injected is the least and therefore the closest to that of the electron carriers and thus the charge carriers are more balanced, although the hole carrier is still present in excess. A smaller barrier (by having increasing FPA content in the mixed monolayer) leads to even greater excesses in hole carriers, which will result in further deviations from balanced charge carriers and thus decreasing efficiency. Type B devices had the same configuration as type A devices except that 50 nm BPAPF was used as the hole-transporting layer rather than NPB. BPAPF has a higher hole mobility, yet with an even lower-lying HOMO level of BPAPF (5.62 eV),33 the hole injection barrier increased relative to that with NPB as the HTL. The device characteristics are shown in Figure 4c. Compared to type A devices, the currents are generally higher for type B devices for the same modification, even though the injection barrier is higher. This can be ascribed to the higher hole mobility for BPAPF.32 The maximum current occurred again with the ITO anode modified with a pure FPA monolayer. The incorporation of increasing amounts of BPA into the monolayer reduced the work function progressively and increased the injection barrier accordingly so that a progressively lower current was obtained. Nevertheless, the luminance efficiency of the devices was the highest with the ITO electrode modified with a 1:2 FPA/BPA monolayer (Figure 4d). Increasing the amount of FPA in the mixture led to reduced efficiency. Reducing the amount of FPA (using a pure BPA monolayer) resulted in too little current to light up the device. It is suggested that with the lower-lying HOMO of the BPAPF layer, the barrier for hole injection increases. The hole carrier is not necessarily the dominant carrier now. The presence of a luminance maximum suggests that with a 1:2 FPA/BPAmodified electrode the hole carriers from the anode and the electron carriers from the cathode appear to be more balanced. When the number of hole carriers is increased by using pure FPA-modified 2:1 FPA/BPA or 1:1 FPA/BPA-modified ITO, the charges become unbalanced, with the hole carriers dominating. With the pure BPA monolayer modification, the number of hole carriers was reduced so that electron carriers became dominant. Either way, the luminance efficiency decreased. In type C devices, a configuration of ITO/SAM/NPB(50 nm)/Alq3 (50 nm)/Cs2CO3 (1 nm)/Al (80 nm) was used, where the LiF electron injection layer was replaced by Cs2CO3, which has been shown to be a better electron injection layer.22

Figure 5. Dependence of the luminance efficiency on the amount of FPA in the monolayer on ITO in various types of OLED devices.

Figure 6. Power efficiency for (a) device A and (b) device B. 428

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Langmuir More electrons will be injected under otherwise identical conditions. The JV characteristic and luminance efficiency are shown in Figure 4e,f, respectively. The maximum device current was again obtained for the ITO electrode modified with a singlecomponent monolayer of FPA, yet the maximum efficiency of the devices occurred with ITO modified with the 1:1 FPA/BPA monolayer (with a work function of 5.33 eV). It is suggested that by comparison with type A devices, type C devices have improved electron injection and more hole carriers are needed to reach a balance with the electron carriers. This is provided by using an ITO electrode modified with a 1:1 FPA/BPA monolayer. When further increases in the number of hole carriers by using ITO modified with a monolayer of increasing FPA ratio are desired, the charge carriers will be unbalanced again and the efficiency will decrease. Fewer hole carriers provided by pure BPA or 1:2 FPA/BPA monolayer-modified ITO also lead to charge imbalance and lower efficiency. Type D devices have a configuration of ITO/SAM/BPAPF(50 nm)/Alq3(50 nm)/Cs2CO3(1 nm)/Al (80 nm). The JV characteristic and luminance efficiency are shown in Figure 4g,h, respectively. The maximum device current was again obtained for an ITO electrode modified with a single-component monolayer of FPA. A similar efficiency was obtained for devices with ITO modified with a 1:1 FPA/BPA mixed monolayer or a 1:2 FPA/BPA mixed layer. Further increases in the FPA component to the 2:1 FPA/BPA monolayer or a reduction in the FPA component to a pure BPA monolayer resulted in a lower efficiency. Either way, the efficiency decreased. Compare to device B, where the 1:2 FPA:BPA monolayer yielded a better efficiency, a better electron injection layer is used in device D so that more holes are needed to balance the electron carriers. This is achieved by using a higher FPA component in the 1:1 FPA/ BPA monolayer. Figure 5 plots the luminance efficiency as a function of the percentage of FPA in the mixed SAMs in four types of devices. Some trends can be discerned in that with NPB as the HTL and LiF/Al as the cathode the hole carriers are always present in excess no matter what kind of surface modification is used. Better balance can be reached with fewer hole carrier injections by decreasing the work function using a pure BPA monolayer to modify the ITO electrode. With BPAPF as the HTL and LiF/Al as the cathode, a turning point in the carrier balance occurrs for ITO modified by the 1:2 FPA/BPA mixed monolayer. Increasing the hole injection barrier by using more BPA in the mixed monolayer will cause the electron carriers to be present in excess whereas reducing the injection barrier by using more FPA in the mixed monolayer causes the hole carrier to be present in excess. With NPB as HTL and Cs2CO3 as the cathode, the turning point in the carrier balance occurs at ITO modified with a 1:1 FPA/BPA mixed monolayer because of better electron injection. With BPAPF as the HTL and Cs2CO3 as the cathode, the switching of charge balance occurs with ITO modified with the 1:1 FPA/BPA mixed monolayer because of more efficient electron injection and the lowering of the HOMO level at the anode/HTL interface. It is noticed that plasma-treated ITO does not always give the highest luminance efficiency compared to monolayer-modified ITO because it does not always offer hole carriers that can better balance the electron carriers. It should be noted that whereas a better charge carrier balance and a higher luminous efficiency can be obtained with a specific modification of the ITO electrode, the trend in power efficiency nevertheless deviates from the luminance efficiency in that

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devices with higher currents gave better power efficiencies (Figure 6). Presumably, higher current will increase the chance of forming excitons and thus the absolute brightness. It will also waste more carriers if the carrier is already present in excess. If the carrier balance is reached by curbing the hole carrier injection, then a much higher voltage is needed to reach a similar brightness. This would reduce the power efficiency. The result implies that an improvement in electron injection is more desirable in enhancing the luminance efficiency as well as the power efficiency.

’ CONCLUSIONS We demonstrated that the use of mixed phosphonic acid monolayers allows the continuous tuning of the work function of ITO over a wide range and thus the hole injection barrier. With mixed monolayers prepared from n-butylphosphonic acid and 4,4,4-trifluorobutyl-1-phosphonic acid in particular, the work function of ITO can be tuned over a range from 5.0 to 5.75 eV, depending on the monolayer composition. The current is always the highest with single-component FPA-modified ITO as the anode because of the lowest injection barrier between the electrode and the HTL involved. However, a higher current does not necessarily yield a higher luminance efficiency because of the deviation from the balance in charge carriers. Systematic variations in the hole carrier injection and the trend observed in the luminance efficiency may reveal information about the hole electron carrier balance in a particular combination of materials and electrodes. A decreasing luminance efficiency with a decreasing hole injection barrier can imply a device in which an excess number of hole carriers are involved and vice versa. The presence of a maximum in the luminance efficiency with a systematic change in the injection barrier may signal a switchover in the major charge carrier in the device. The work also suggests that with different combinations of materials (HTL and/or ETL) an electrode with a specific work function is needed to approach the charge carrier balance and thus optimize the luminance efficiency. The systematic fine tuning of the work function with mixed selfassembled monolayers is ideal in identifying the balancing of charge carriers with any given set of materials in a device and locating the appropriate combination of materials and electrodes for a higher luminance efficiency. In the case where the charge is not balanced, it is more desirable to increase the number of minor carriers than to increase the number of major carriers in order to improve both the luminous efficiency and the power efficiency. ’ ACKNOWLEDGMENT We thank the Ministry of Economics of Taiwan and Academia Sinica for the financial support of this work. ’ REFERENCES (1) (a) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (c) Burrows, P. E.; Gu, G.; Bulovic, V.; Shen, Z.; Forrest, S. R.; Thompson, M. E. IEEE Trans. Electron Devices 1997, 44, 1188. (2) (a) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (b) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R. Phys. Rev. B 1999, 60, 14422. (3) (a) Swanson, S. A.; Wallraff, G. M.; Chen, J. P.; Zhang, W. J.; Bozano, L. D.; Carter, K. R.; Salem, J. R.; Villa, R.; Scott, J. C. Chem. Mater. 2003, 15, 2305. (b) Bulovic, V.; Shoustikov, A.; Baldo, M. A.; 429

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dx.doi.org/10.1021/la2036423 |Langmuir 2012, 28, 424–430