Performance Enhancement of Polymer Light-Emitting Diodes by Using

Apr 9, 2009 - State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate S...
0 downloads 16 Views 665KB Size
Download PDF
7898

J. Phys. Chem. C 2009, 113, 7898–7903

Performance Enhancement of Polymer Light-Emitting Diodes by Using Ultrathin Fluorinated Polyimide Modifying the Surface of Poly(3,4-ethylene dioxythiophene):Poly(styrenesulfonate) Baohua Zhang,†,‡ Wenmu Li,†,‡ Junwei Yang,† Yingying Fu,†,‡ Zhiyuan Xie,*,† Suobo Zhang,† and Lixiang Wang† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: December 9, 2008; ReVised Manuscript ReceiVed: March 4, 2009

Herein, an insulating fluorinated polyimide (F-PI) is utilized as an ultrathin buffer layer of poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in polymer light-emitting diodes to enhance the device performance. The selective solubility of F-PI in common solvents avoids typical intermixing interfacial problems during the sequential multilayer spin-coating process. Compared to the control device, the F-PI modification causes the luminous and power efficiencies of the devices to be increased by a factor of 1.1 and 4.7, respectively, along with almost 3-fold device lifetime enhancement. Photovoltaic measurement, singlehole devices, and X-ray photoelectron spectroscopy are utilized to investigate the underlying mechanisms, and it is found that the hole injection barrier is lowered owing to the interactions between the PEDOT:PSS and F-PI. The F-PI modified PEDOT:PSS layer demonstrates step-up ionization potential profiles from the intrinsic bulk PEDOT:PSS side toward the F-PI-modified PEDOT:PSS surface, which facilitate the hole injection. Moreover, the insulating F-PI layer at the PEDOT:PSS surface is also favorable for the hole injection by blocking the electrons and strengthening the local electric field at the interface. I. Introduction Polymer light-emitting diodes (PLEDs) are considered to be a promising candidate for next-generation flat panel display because of their low-cost solution process and high-quality device performance including low power consumption, wide viewing angle, good contrast, and video rate operation. Poly(3,4ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely employed in PLEDs due to its numerous advantages including good transparency in visible region, variable conductivity and work functions, flattening effects, and excellent air-stability.1-13 As a result, both the efficiencies and lifetime of PLEDs have been remarkably improved, which are considered as the most critical development for the commercialization of this low-cost display techniques. However, some problems relating to the PEDOT:PSS still exist. First, the hole injection barrier (HIB) at the PEDOT:PSS/emissive polymer layer (EML) interface is still a limiting factor for enhancing the light-emitting efficiency, especially in the case that the overlying emissive polymer has a much higher highest-occupied molecular orbital (HOMO) level. For example, the HOMO level of the blue fluorescent poly(9,9-dioctylfluorene) is ca. 5.8 eV,14 resulting in the HIB of 0.6 eV with respect to the PEDOT:PSS (5.2 eV); Second, the PEDOT:PSS film may undergo a series of intrinsic5,10,15-17 and extrinsic4,5 degradations under long-term device operations. Researchers from Cambridge Display Technologies, Ltd. at several conferences suggested that the superfluous electrons can traverse through the EML into the PEDOT: * To whom correspondence should be addressed. E-mail: xiezy_n@ ciac.jl.cn. † State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry. ‡ Graduate School of Chinese Academy of Sciences.

PSS layer and attack the PEDOT:PSS to form the residues or byproduct (e.g., sulfate released from PSS), which diffuse into the light-emitting polymer layer and degrade the device lifetime.18 The interface engineering between the PEDOT:PSS surface and the overlying EML is one of the most promising solutions to tune the HIB and to prevent the adverse effects of the PEDOT:PSS on the device stability. Adjusting the work function of the spin-coated PEDOT:PSS film has been widely pursued and confirmed to be an effective approach to realize better hole injection and enhance the device lifetime.2,12,13 Lee et al. demonstrated remarkably efficient PLEDs by doping the PEDOT:PSS with a perfluorinated ionomer to form a gradient work function profiles through the PEDOT:PSS layer by selforganization. Other methods including electrochemical treatment, postdeposition treatment, layer-by-layer electrostatic selfassembly process, and controlling the surface composition have also been utilized to enhance the work function of PEDOT: PSS surface.2,6,13,19 It is worth noting that the high content of insulating species at the surface of PEDOT:PSS film, such as PSS in PEDOT:PSS and the insulating additives,12,13 is always favorable for enhancing the hole injection and the device stability. In this work, we deliberately introduce an ultrathin insulating layer of a kind of regioirregular fluorinated polyimides (F-PI) via spin-coating between the PEDOT:PSS and EML. As a prerequisite, the distinctly selective solubility of F-PI,20 that is, diffluent in tetrahydrofuran (THF) but insoluble in toluene (used as the solvent of the emissive polymer in this work) is critical to avoiding frequently happened uncontrollable interfacial mixing or confusion21 at the interfaces during sequential spin-coating. The device performance including the lightemitting efficiency and the device stability is largely enhanced

10.1021/jp810824m CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

Enhancement of Polymer Light-Emitting Diodes

Figure 1. Chemical structures of F-PI and GPFO polymers, respectively.

compared to the control device. The underlying mechanisms of the ultrathin insulating F-PI modification are investigated. II. Experimental Section Materials. The polymer F-PI (Tg >300 °C, dielectric constant 2.7, Mn 5-8 × 104) employed in this work was synthesized in our laboratory, and the detailed synthesis was published previously.20 Poly(9,9-dioctylfluorene-2,7-diyl-co-2,5di(phenyl-4′-yl)-2,1,3-benzothiadiazole) (hereafter designated as GPFO; Mn, 19400; Mw, 46400) was used as the emissive polymer, which was synthesized according to the literature.22 The chemical structures of F-PI and GPFO are shown in Figure 1. Both of these materials were used as received without any further purification. Absorption Spectroscopy Characterization. The UV-vis absorption measurement was conducted using Perkin-Elmer 35 UV-visible spectrophotometer on quartz substrates. To characterize the toluene resistivity of F-PI film, the absorption of the F-PI film spin-coated from THF solution was measured before and after spin washing three times with toluene. Additionally, the thicknesses of the ultrathin F-PI films (see Supporting Information for details) were indirectly obtained by comparing their UV-vis absorption spectra with the thick F-PI films whose thicknesses were measured by Dektak 6 M Stylus Profiler.21,23 Surface Topography. The evolutions of surface morphology of PEDOT:PSS film without or with covering an ultrathin F-PI layer were studied by SPI3800N atomic force microscopy (AFM) instrument (Seiko Instrument Inc.) in the tapping mode with a 2 N m-1 probe and at a scan rate of 1 Hz under ambient conditions. The sample preparation for this scanning is just the same as the situations for the device fabrication. X-ray Photoelectron Spectroscopy (XPS) Characterization. XPS experiments were performed in Thermo ESCALAB 250 using monochromatized Al KR at hυ ) 1486.6 eV. The binding energies were calibrated to the C1s peak at 284.6 eV. Photovoltaic Measurements of PLEDs. As for the details concerning the photovoltaic measurements of PLEDs, a white light illumination with intensity of 100 mW/cm2 was exposed onto the PLEDs from the indium tin oxide (ITO) glass substrate side. The photocurrent-voltage characteristic was recorded using a computer-controlled Keithley 236 source meter. Opencircuit voltages (VOC) of devices were defined as the voltage at which the corresponding photocurrent is equal to zero. Photoluminescence (PL) Spectra Measurements. PL spectra of the EML were obtained in the structure of glass substrate/ PEDOT:PSS/without or with F-PI/EML on a Perkin-Elmer LS 50B luminescence spectrometer with xenon discharge lamp excitation. The samples were excited with 370 nm monochromic light from the EML side. Fabrication and Characterization of PLEDs. The PLEDs have a structure of ITO/PEDOT:PSS/F-PI/GPFO/Ca/Al. The

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7899 ca. 50-nm-thick PEDOT:PSS (Baytron P AI 4083) is used to serve as the hole injection layer that is spin-coated from a water dispersion onto the precleaned ITO substrate and then annealed at 120 °C for 45 min in air. The F-PI layer, if it is included, is deposited via spin-coating from THF solution on top of the PEDOT:PSS layer and dried at 75 °C for 15 min in a vacuum oven. The GPFO is spin-coated onto the F-PI layer or PEDOT: PSS layer from toluene solution and is baked at 75 °C for 30 min in the vacuum oven to obtain a 120-nm-thick EML. Finally, the cathode of Ca (10 nm)/Al (100 nm) is thermally evaporated in vacuum at a base pressure of 4 × 10-4 Pa. The active area of the PLEDs is 0.12 cm2. The current density (J)-voltage (V)-luminance (L) characteristics as well as electroluminescent (EL) spectra were measured with a Keithley 2400 source meter and a coupled PR650 Spectroscan photometer. All the measurements were carried out at room temperature under ambient conditions. III. Results and Discussion The traditional polyimide with regular and symmetrical backbone structure is merely soluble in the very high polar solvents, while the herein used fluorinated regioirregular and asymmetrical F-PI possesses much better solubility, which is soluble in relatively lower polar solvents, such as DMSO, DMF, THF, and CHCl3, but still insoluble in toluene that is usually used as the solvent to dissolve the emissive polymer.20 We further confirmed its toluene resistivity by the absorption experiment (see Supporting Information in detail). As shown in Figure S3 (see Supporting Information for details), the absorption of F-PI film shows no distinct discrepancy before and after spin-washing with toluene three times. This peculiar selective solubility of F-PI is the prerequisite to fabricate the multilayer PLEDs. In the sequential spin-coating processes of PEDOT:PSS, F-PI, and GPFO, it is ensured that the solvent used to spin-coat the overlying layer does not dissolve or attack the dried underlying layer. The common approach to construct multilayer PLEDs adopts typical “orthogonal” solvents.24 It is difficult to modify the PEDOT:PSS layer with conjugated polymers since toluene (the solvent of the EML) may dissolve and destroy the modifying layer. Here, the unique solubility of F-PI in organic solvents provides the feasibility to modify the PEDOT:PSS layer via simple spin-coating process. The EL characteristics of the PLEDs without and with varied thicknesses of F-PI layer are displayed in parts a-d of Figure 2. It can be seen that, compared to the control device without F-PI buffer layer, the driving voltages of the PLEDs with F-PI buffer layer are lowered by approximate 2 V for achieving the same luminance, along with the distinctly increased currents at the same driving voltages. The maximum luminous efficiency is improved from 6 cd/A of the control device to 12.5 cd/A of the device with 0.9 nm F-PI buffer layer. Because of the dual contributions of the largely lowered driving voltage and the improved luminous efficiency, the promotion of the maximum power efficiency of the PLEDs is more obvious, that is, from 1.95 lm/W of the control device to 11.2 lm/W of the device with 0.9 nm F-PI buffer layer. As shown in Figure 2b, it is noted that the overall current is distinctly increased for the F-PI modified PLEDs compared to the control device. The carrier injection and transport are the two key factors to determine the final V-J characteristics of the devices. Obviously, the carrier transport is impossible to be enhanced by introducing an ultrathin insulating F-PI layer. Therefore, it is speculated that the hole injection from PEDOT: PSS to the EML is improved, which favors enhancing the

7900

J. Phys. Chem. C, Vol. 113, No. 18, 2009

Zhang et al.

Figure 3. (a) The J-V curves of the hole-only devices in the structures of ITO/PEDOT/with or without F-PI layer/GPFO/ MoO3 (10nm)/Al. (b) Photovoltaic characteristics of the devices with the configurations of ITO/PEDOT:PSS/without or with F-PI layer (varied thicknesses)/ GPFO/Ca/Al.

Figure 2. The EL characteristics of the GPFO-based control devices and F-PI modified devices with varied thicknesses of F-PI films.

performance of the PLEDs. The HOMO and the lowestunoccupied molecular orbital (LUMO) level of the green emissive GPFO are 5.8 and 3.5 eV, respectively, which were obtained by cyclic voltammetry method. For a PLED with a configuration of ITO/PEDOT:PSS/GPFO/Ca/Al, the electron injection is dominated since the HIB at the PEDOT:PSS/GPFO interface is much larger than the electron injection barrier at the GPFO/Ca interface. The hole- and electron-only device measurements (not shown here) also confirmed that the hole current is dramatically lower than the electron counterparts. Accordingly, when the PLEDs are biased, electrons are first injected and then percolate across the whole EML and finally recombine with holes in the vicinity of PEDOT:PSS/GPFO interface to decay radiatively. The injection efficiencies of minor holes actually determine the light turn-on voltage (Vlight-turn-on) as well as the light-emitting efficiency.25 It can be seen that the Vlight-turn-on of the PLEDs is largely lowered from 4.2 to 2.5 V after the F-PI modification, along with the approximate 2-fold

enhancement of maximum luminous efficiency as shown in parts a and c of Figure 2. To verify whether the HIB at the PEDOT:PSS/GPFO interface is really reduced by modifying the PEDOT:PSS with F-PI, we further fabricated the so-called hole-only devices with a structure of ITO/PEDOT:PSS/ with or without F-PI layer/ GPFO/molybdenum trioxide (MoO3) (10nm)/Al and the corresponding V-J curves are shown in Figure 3a. In comparison to the control device, the hole currents are largely increased at the same driving voltages by inserting an ultrathin layer of F-PI between the PEDOT:PSS and the GPFO layer, indicating that the HIB at the PEDOT:PSS/GPFO interface is reduced. Photovoltaic characterization has been widely used to evaluate the carrier injection barrier changes at the both sides of electrode interfaces in PLEDs,26,27 since the build-in potential of a device can be directly obtained from the VOC measurement. We also measured the photovoltaic characteristics of the PLEDs under 100 mW/cm2 white light illumination and the illuminated V-J curves of the PLEDs are shown in Figure 3b. The VOC of the PLEDs is increased from 1.9 to 2.1 V after inserting the F-PI buffer layer with the thicknesses ranging from 0.9 to 5.4 nm. Since the build-in potential of a device is correlated to the work functions of the two electrodes, the increase of VOC in the F-PI modified PLEDs should be attributed to the increase of the PEDOT:PSS work function, which induces the decrease of the HIB at the PEDOT:PSS/GPFO interface and enhances the hole injection. In addition to the lowered HIB, the insulating F-PI is also favorable for enhancing the hole injection. It was reported that the electron space charges close to the PEDOT:PSS surface are critical to the hole injection because the buildup of electrons at this region promotes the local electric field and then favors the hole injection.3,28,29 Murata et al.3 simulated that 2-nm-thick blocking layer on the anode side can effectively restrain electrons from escape and penetration, resulting in a recombination efficiency of 99.9%. Therefore, with the help of the insulating F-PI, the local electric field should be enhanced, which further assists the hole injection, and the negative effects stemming from the penetration of electrons into PEDOT:PSS

Enhancement of Polymer Light-Emitting Diodes

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7901

Figure 4. PL (solid symbols) and EL (open symbols) comparison for the samples and PLEDs without an F-PI layer (a, d) and with 0.9 nm (b, e) or 2.8 nm (c, f) of an F-PI layer, respectively.

layer are to some extent suppressed. The optimal thickness of F-PI film (1-3 nm) is consistent with the numerical modeling results.3 The evolutions of PL and EL spectra of the samples and PLEDs with F-PI modification were also investigated and shown in Figure 4. It is noted that the EL spectrum of the control device is somewhat red-shift and broad compared to its PL spectrum. It implies that, while the exciton recombination region dominates at the PEDOT:PSS/EML interfaces under electric excitation, the strong interaction between PEDOT:PSS and EML results in some low-energy states7 that causes this discrepancy. When the ultrathin F-PI interlayer is inserted at the interfaces, EL spectra of PLEDs become blue-shift and narrow with respect to that of the control device. The PLEDs with 0.9 and 2.8 nm F-PI layers show identical EL spectra, indicating that even 0.9 nm F-PI layer can effectively suppress the interaction between PEDOT:PSS and EML. Furthermore, AFM measurements (see Supporting Information for details) verified that the spin-coated F-PI layer covers the underlying PEDOT:PSS homogeneously, with slight lower root mean square (rms) roughness compared to the as-deposited PEDOT:PSS, which is consistent with the spectra changes. XPS measurements were performed to investigate the interactions at the PEDOT:PSS/F-PI interface for determining the origin of the lowered HIB. Two samples with 2.5-nm- and 20nm-thick F-PI coated on PEDOT:PSS/Si substrates are prepared. The evolutions of C1s and F1s binding energies of F-PI are shown in parts a and b of Figure 5, respectively. For the sample of PEDOT:PSS/F-PI(20 nm), the C 1s peaks at 284.6, 288, and 292.2 eV are attributed to the carbon in benzene ring, OdCsN and sCF3 of F-PI, respectively. It is observed that the binding energy peaks of C1s and F1s of the sample of PEDOT:PSS/F-PI (2.5 nm) shift to higher binding energies by ca. 0.5 and 0.3 eV, respectively, compared with those of the sample of PEDOT:PSS/F-PI (20 nm). Since the positively charged atoms show higher binding energy of electrons,30 these results indicate that the F-PI at the PEDOT:PSS/F-PI interface actually behaves as an electron-donating species. The S2p signals from the PEDOT:PSS layer with or without a 2.5 nm F-PI cover layer are shown in Figure 5c. After covering 2.5nm-thick F-PI layer, the S2p signals from PEDOT component (S2p1/2, 164.7 eV) show no obvious shifts, and the S2p signals from PSS(S 2p1/2, 168.6 eV) show 0.3 eV positive shifts to higher binding energy (S2p1/2, 168.9 eV). It is confused that the negatively charged species along with negative binding energy shifts are not observed. It should be noted that the F-PI and PSS components among the three species (F-PI, PSS, and PEDOT) in this immediately contacted region are positively shifted. Therefore, PEDOT is the only possible electronwithdrawing component and, in principle, will reveal negative shifts for S2p. The p-type doped PEDOT chains possess

Figure 5. The evolutions of XPS spectra with F-PI modifications: (a) C1s peaks originating from (A) Si/PEDOT:PSS(50nm) sample, (B) Si/PEDOT:PSS (50nm)/F-PI (2.5nm) sample, and (C) Si/PEDOT: PSS(50nm)/F-PI (20nm) sample; (b) F1s peaks originating from (B) and (C) samples, respectively; (c) S2p peaks originating from (A) and (B) samples, respectively. The arrows in the curves show the amounts of the positive shifts of the corresponding elements, when XPS spectra of sample B were compared with that of samples A or C.

delocalized positive charges throughout several segments and easily undergo reductions4,10 by withdrawing electrons from the environment. Herein, it is speculated that the electron-donating F-PI layer may induce the immediately contacted PEDOT to undergo analogous partial dedoping. The XPS information depth about S2p core levels can be estimated to about 10 nm,13 and the resulting S2p spectra actually record the total counts of photoelectrons within this region. The interactions between F-PI and PEDOT:PSS are believed to be limited within several nanometers close to the PEDOT:PSS/F-PI interface. Therefore, we infer that the negatively shifted S2p signals originated from the dedoped PEDOT are screened by the dominated signals stemming from the unaffected bulk PEDOT species. It is mentioned that analogous broadening and positive shifts (ca. 0.3 eV) of PSS core level were observed, and the S2p shifts from PEDOT were not observed in the PEDOT:PSS layer in Ho et al.’s work, in which the graded hole injection profiles were formed by the gradual decreased doping level of PEDOT through layer-by-layer alternate polyanion-polycation electrostatic assembly process.2 The dedoped PEDOT corresponds to larger ionization potential (Ip). Herein, an ultrathin partial dedoped PEDOT layer with higher Ip close to the PEDOT:PSS/ F-PI interface is also produced by the partial reduction through withdrawing electrons from the adjacent F-PI layer. Finally, this in situ formed dedoped topmost PEDOT with higher Ip together with the underlying unaffected bulk PEDOT with lower

7902

J. Phys. Chem. C, Vol. 113, No. 18, 2009

Zhang et al. The enhancement may be attributed to the improved hole injection and the electron-blocking effects by introducing an ultrathin insulating F-PI between the PEDOT:PSS layer and EML. IV. Conclusions

Figure 6. Proposed mechanisms showing the origins of the improved device performance by F-PI modification, that is, (a) the electronblocking effect at the surface of the PEDOT:PSS owing to its intrinsic insulating property; (b) the lowered HIB via partial dedoping of PEDOT (with higher Ip) at the topmost PEDOT:PSS film surface by withdrawing electrons from adjacent F-PI and the formation of stepped increased work function of PEDOT:PSS.

The interfaces in PLEDs play very important roles in the lightemitting efficiencies and the device stability. Here, we demonstrated a simple approach to modify the PEDOT:PSS surface by using an ultrathin insulating F-PI layer via spin-coating and the device performance including the light-emitting efficiencies and device lifetime were enhanced. The F-PI modification for PEDOT:PSS increases the work function of the immediately contacted PEDOT to intrinsically reduce HIB. The insulating F-PI can also block electrons into PEDOT:PSS and strengthen the local electric field at the interface for further enhancing the hole injection. As a result, the minor carriers of holes are enhanced and the device performance is largely improved. The simplicity of depositing a F-PI layer from THF solution provides the feasibility to modify the PEDOT:PSS in polymer flat panel display in the future. Acknowledgment. The authors acknowledge the support by the National Natural Science Foundation of China (Nos. 50873100, 20834005), Science Fund for Creative Research Groups (No. 20621401), and 973 project (2009CB623602). Supporting Information Available: The methods to determine the thicknesses of F-PI films, the evaluation of toluene solvent resistivity of F-PI films, and the surface topography evolutions of PEDOT:PSS with F-PI modification were elucidated in detail. This information is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. Operation half-lifetime of the GPFO-based PLEDs with or without F-PI modification at an initial brightness of 2500 cd/m2.

References and Notes

IP forms a stepped increased work function profiles along the whole PEDOT:PSS layer, which consequently enhances the hole injection. On the basis of the above discussion, the possible underlying mechanisms responsible for the enhancement of the PLEDs are summarized in Figure 6. The resulting improvements of device performance by the ultrathin F-PI modification arise from the following dual contributions. The intrinsic bulk insulating property of F-PI endows it to block the penetrations of electrons into the PEDOT:PSS layer and to obtain the higher local electric field at the PEDOT:PSS/GPFO interface, which enhances the hole injection and suppresses the electron-attack-induced PEDOT:PSS degradations. The in situ formed step-up work function of PEDOT at the PEDOT:PSS/F-PI interface intrinsically improves the hole injection. Compared to the bulk p-doped PEDOT, the immediately contacted PEDOT at the PEDOT:PSS/ F-PI interface withdraws electrons from the adjacent electrondonating F-PI layer and undergoes the partial dedoping, resulting in an increased Ip. The step-up work function of PEDOT actually reduces the HIB. The influence of the F-PI modification on the device lifetime was also investigated. The lifetime test of the PLEDs was conducted at an initial luminance of 2500 cd/m2 with a constant current operation. As shown in Figure 7, the PLED with the F-PI buffer layer shows approximate 3-fold enhancement in half-lifetime compared to the control device.

(1) Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481. (2) Ho, P. K. H.; Kim, J. S.; Burroughes, J. H.; Becker, H.; Li, S. F. Y.; Brown, T. M.; Cacialli, F.; Friend, R. H. Nature 2000, 404, 481. (3) Murata, K.; Cina, S.; Greenham, N. C. Appl. Phys. Lett. 2001, 79, 1193. (4) Kim, J. S.; Ho, P. K. H.; Murphy, C. E.; Baynes, N.; Friend, R. H. AdV. Mater. 2002, 14, 206. (5) Crispin, X.; Marciniak, S.; Osikowicz, W.; Zotti, G.; Van der Gon, A. W. D.; Louwet, F.; Fahlman, M.; Groenendaal, L.; De Schryver, F.; Salaneck, W. R. J. Polym. Sci., Part. B 2003, 41, 2561. (6) Jukes, P. C.; Martin, S. J.; Higgins, A. M.; Geoghegan, M.; Jones, R. A. L.; Langridge, S.; Wehrum, A.; Kirchmeyer, S. AdV. Mater. 2004, 16, 807. (7) Kim, J. S.; Friend, R. H.; Grizzi, I.; Burroughes, J. H. Appl. Phys. Lett. 2005, 87, 023506. (8) Koch, N.; Elschner, A.; Rabe, J. P.; Johnson, R. L. AdV. Mater. 2005, 17, 330. (9) Ouyang, B. Y.; Chi, C. W.; Chen, F. C.; Xi, Q. F.; Yang, Y. AdV. Funct. Mater. 2005, 15, 203. (10) Chia, P. J.; Chua, L. L.; Sivaramakrishnan, S.; Zhuo, J. M.; Zhao, L. H.; Sim, W. S.; Yeo, Y. C.; Ho, P. K. H. AdV. Mater. 2007, 19, 4202. (11) Koch, N. Chemphyschem 2007, 8, 1438. (12) Lee, T. W.; Chung, Y.; Kwon, O.; Park, J. J. AdV. Funct. Mater. 2007, 17, 390. (13) Lee, T. W.; Chung, Y. S. AdV. Funct. Mater. 2008, 18, 2246. (14) Gong, X.; Moses, D.; Heeger, A. J.; Xiao, S. J. Phys. Chem. B 2004, 108, 8601. (15) Kim, J. S.; Ho, P. K. H.; Murphy, C. E.; Seeley, A. J. A. B.; Grizzi, I.; Burroughes, J. H.; Friend, R. H. Chem. Phys. Lett. 2004, 386, 2. (16) Papadimitratos, A.; Fong, H. H.; Malliaras, G. G.; Yakimov, A.; Duggal, A. Appl. Phys. Lett. 2007, 91, 042116. (17) Png, R. Q.; Chia, P. J.; Sivaramakrishnan, S.; Wong, L. Y.; Zhou, M.; Chua, L. L.; Ho, P. K. H. Appl. Phys. Lett. 2007, 91, 013511. (18) Burroughes, J. H. Presented at the Asia Display/IMID′2004, August 23-27, 2004, session 11.4.

Enhancement of Polymer Light-Emitting Diodes (19) Campbell, A. J.; Bradley, D. D. C.; Antoniadis, H. J. Appl. Phys. 2001, 89, 3343. (20) Li, W. M.; Chen, G.; Zhang, S. B.; Wang, H.; Yan, D. H. J. Polym. Sci., Part. A 2007, 45, 3550. (21) Zhu, R.; Lin, J. M.; Wang, W. Z.; Zheng, C.; Wei, W.; Huang, W.; Xu, Y. H.; Peng, J. B.; Cao, Y. J. Phys. Chem. B 2008, 112, 1611. (22) US Patent PCT/US99/07876 (WO00/46321). (23) Xia, Y. J.; Friend, R. H. AdV. Mater. 2006, 18, 1371. (24) Meerholz, K. Nature 2005, 437, 327. (25) Parker, I. D. J. Appl. Phys. 1994, 75, 1656.

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7903 (26) Zhang, Y.; Huang, F.; Chi, Y.; Jen, A. K. Y. AdV. Mater. 2008, 20, 1565. (27) Cao, Y.; Yu, G.; Heeger, A. J. AdV. Mater. 1998, 10, 917. (28) Brewer, P. J.; Lane, P. A.; Huang, J. S.; DeMello, A. J.; Bradley, D. D. C.; DeMello, J. C. Phys. ReV. B 2005, 71, 205209. (29) Brewer, P. J.; Lane, P. A.; deMello, A. J.; Bradley, D. D. C.; deMello, J. C. AdV. Funct. Mater. 2004, 14, 562. (30) Matsushima, T.; Kinoshita, Y.; Murata, H. Appl. Phys. Lett. 2007, 91, 253504.

JP810824M