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Synthesis, Electrochemistry, and Electroluminescence of Novel Red-Emitting Poly(p-phenylenevinylene) Derivative with 2-Pyran-4-ylidene-Malononitrile Obtained by the Heck Reaction Joo Hyun Kim and Hoosung Lee* Department of Chemistry, Sogang University, 1-1 Shinsoo-Dong, Mapo-Gu, Seoul 121-742, Korea Received November 1, 2001. Revised Manuscript Received March 7, 2002
A novel red-emitting polymer, poly{1,4-phenylenevinylene-(4-dicyanomethylene-4H-pyran)2,6-vinylene-1,4-phenylenevinylene-2,5-bis(dodecyloxy)-1,4-phenylenevinylene} (PM-PPV), was prepared by the Heck coupling reaction between 2-{2,6-bis[2-(4-bromophenyl)vinyl]pyran4-ylidene}-malononitrile and 1,4-bis(dodecyloxy)-2,5-divinylbenzene. The EL device based on a single-layer structure (ITO/PM-PPV/Al) showed EL emission with a maximum at 652 nm and an external quantum efficiency of 0.004% at 0.12 mA/mm2, which is higher than that of MEH-PPV (0.0005%, at 0.15 mA/mm2) measured under the same conditions. From the electrochemistry and UV-vis spectroscopy, the HOMO and LUMO energy levels of PMPPV were figured out to be -5.44 and -3.48 eV, respectively, which are lower than those of MEH-PPV at -4.98 and -2.89 eV, respectively. It is concluded that, by lowering the HOMO and LUMO levels in PM-PPV, the injection rates of the holes and electrons are more balanced than in MEH-PPV. To improve the efficiency, the PPV layer was inserted between anode and PM-PPV layer. The device based on bilayer structure (ITO/PPV/PM-PPV/Al) showed EL emission with a maximum at 646 nm and an external quantum efficiency of 0.05% at 0.18 mA/mm2. The efficiency of the bilayer device was higher than that of the single-layer device by 13 times. The CIE chromaticity coordinates of the bilayer device were x ) 0.69 and y ) 0.31, which are very close to the CIE chromaticity coordinates (x ) 0.67, y ) 0.33) of NTSC for the red color.
Introduction A light-emitting diode (LED) using poly(p-phenylenevinylene) (PPV) as the emission medium was made by Cavendish Laboratory.1 In the past decade, many research groups have focused to tune the emission color, efficiency, and stability. Cyano (CN)-containing poly(2,5-dialkoxy-1,4-phenylenevinylene) (CN-DOPPV)2-5 and poly(3-alkylthiophene) (P3AT) derivatives6,7 are well-known red-emitting polymeric materials. The CNDOPPV has high quantum efficiency based on bilayer device and emission maximum at 710 nm.2 CN groups lower the LUMO energy level of the polymer by increasing the electron affinity and extend the emission wave* Corresponding author: e-mail
[email protected]. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. (2) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature (London) 1993, 365, 628. (3) Moratti, S. C.; Cervini, R.; Holmes, A. B.; Baigent, D. R.; Friend, R. H.; Greenham, N. C.; Gruner, J.; Hamer, P. J. Synth. Met. 1995, 71, 2117. (4) Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. Synth. Met. 1994, 6, 491. (5) Liu, Y.; Yu, G.; Li, Q.; Zhu, D. Synth. Met. 2001, 122, 401. (6) Berggren, M.; Ingana¨s, O.; Gustaffusson, G.; Rasmusson, J.; Andersson, M. R.; Hjertberg, T.; Wennerstro¨m, O. Nature (London) 1994, 372, 444. (7) Andersson, M. R.; Berggren, M.; Ingana¨s, O.; Gustaffusson, G.; Gustafsson-Carberg, J. C.; Selec, D.; Hjertberg, T.; Wennerstro¨m, O. Macromolecules 1995, 28, 7525.
length to red and near-infrared. P3AT derivatives exhibit red emission, but the EL device based on singlelayer (ITO/polymer/Al) exhibits extremely low external quantum efficiency.6,7 Eastman Kodak’s dyes (DCM class) are well-known as low molecular weight redemitting material, which can be synthesized by a relatively simple procedure.8,9 All the DCM class red dyes contain 2-pyran-4-ylidene-malononitrile (PM) derivatives as electron acceptor. To synthesize new red-emitting polymeric materials with improved efficiency, we take advantages of the electron-accepting property of PM derivatives. Polymercontaining PM in the conjugated chain has never been reported and tested for LED devices before. We synthesized poly{1,4-phenylenevinylene-(4-dicyanomethylene4H-pyran)-2,6-vinylene-1,4-phenylenevinylene-2,5-bis(dodecyloxy)-1,4-phenylenevinylene} (PM-PPV), which has a repeat unit containing a PM, two phenyl rings, and a 2,5-bis(dodecyloxy)-substituted phenyl ring. We investigated fundamental optical, electrochemical, and electroluminescent properties of the polymer. The emission wavelength of the new PPV derivative is expected at a shorter wavelength compared to CN-DOPPV, and balanced injection of holes and electrons is expected as well. (8) Chen, C. H.; Klubek, K. P.; Shi, J. U.S. Patent 5 908 581, 1999. (9) Chen, C. H.; Klubek, K. P.; Shi, J. U.S. Patent 5 935 720, 1999.
10.1021/cm011553r CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002
A Novel Red-Emitting Poly(p-phenylenevinylene)
Experimental Section Materials. Tetrahydrofuran (THF) and diethyl ether were distilled over sodium/benzophenone. Acetonitrile and chloroform were distilled over P2O5 and potassium carbonate, respectively. Tributylamine was distilled over calcium hydride. All other chemicals were purchased from Aldrich Chemical Co. and were used as received unless otherwise described. MEHPPV prepared by the dehydrohalogenation route10 was soluble in common organic solvent. 2-{2,6-Bis-[2-(4-bromophenyl)vinyl]pyran-4-ylidene}Malononitrile (1). 1.85 g (10.0 mmol) of 4-bromobenzaldehyde, 0.861 g (5.00 mmol) of 2-(2,6-dimethylpyran-4-ylidene)malononitrile, and 10 drops of piperidine in 10 mL of freshly distilled acetonitrile were refluxed under nitrogen for 24 h. The reaction mixture was cooled to room temperature. The yellow precipitate was filtered and washed with 50 mL of acetonitrile. The crude product was purified by recrystallization from methanol. The yellow solid product yield was 1.906 g (75.3%). The mp was not found up to 280 °C. 1H NMR (300 MHz, CDCl3, ppm): δ 7.61-7.58 (d, J ) 8.5, 4H), 7.47-7.44 (d, J ) 8.8, 4H), 7.48-7.43 (t, J ) 15.7, 2H), 6.80-6.75 (t, J ) 15.9, 1H), 6.73 (s, 2H). Anal. Calcd for C24H14Br2N2O: C, 56.95; H, 2.79; N, 5.53. Found: C, 56.94; H, 2.73; N, 5.49. 1,4-Bis(dodecyloxy)-2,5-divinylbenzene (2). 2,5-Bisdodecyloxy-benzene-1,4-dicarbaldehyde was synthesized according to literature procedure.11 A portion of 17.5 mL of n-BuLi (21.0 mmol, 1.2 M solution in n-hexane) was slowly added to a suspension of 7.859 g (22.0 mmol) of methyltriphenylphosphonium bromide in 50 mL of anhydrous THF at 0 °C. After the completion of n-BuLi addition, the reaction mixture was stirred for 30 min at room temperature. The solution of 5.028 g (10.0 mmol) of 2,5-bisdodecyloxy-benzene1,4-dicarbaldehyde in 50 mL of anhydrous THF was added to the reaction mixture. The reaction mixture was gently refluxed for 4 h. After cooling to room temperature, portions of 100 mL of water and 200 mL of diethyl ether were added. The organic layer was separated and washed with brine followed by drying over anhydrous magnesium sulfate and evaporating the solvent in a rotary evaporator. The crude solid was purified by recrystallization from THF/methanol. The solid product yield was 4.02 g (80.6%); mp: 62-63 °C. MS [M+]: 498. 1H NMR (500 MHz, CDCl3, ppm): 7.07-7.02 (dd, J1 ) 11.1 and J2 ) 6.7, 2H), 6.99 (s, 2H), 5.75-5.71 (dd, J1 ) 16.3 and J2 ) 1.4, 2H), 5.27-5.24 (dd, J1 ) 9.8 and J2 ) 1.4, 2H), 3.98-3.95 (t, J ) 6.5, 4H), 1.81-1.77 (m, 4H), 1.49-1.45 (m, 4H), 1.361.27 (m, 32H), 0.90-0.87 (t, J ) 6.8, 6H) 6.73 (s, 2H). Anal. Calcd for C34H58O2: C, 81.87; H, 11.72. Found: C, 81.85; H, 11.77. Anal. Calcd for C34H58O2: C, 81.87; H, 11.72. Found: C, 81.85; H, 11.77. Polymerization of Poly{1,4-phenylenevinylene-(4-dicyanomethylene-4H-pyran)-2,6-vinylene-1,4-phenylenevinylene-2,5-bis(dodecyloxy)-1,4-phenylenevinylene}(PMPPV). The polymer was prepared by the Heck coupling reaction12,13,15 between compound 1 and compound 2. A mixture of 0.50 mmol of compound 1, 0.50 mmol of compound 2, 4.5 mg (0.020 mol) of Pd(OAc)2, 30.5 mg (0.10 mmol) of tri-otolylphosphine, and 0.31 mL of tributylamine in 10 mL of DMF was stirred for 24 h at 140 °C. The hot reaction mixture was poured into 500 mL of methanol, and then red precipitate was collected by filtration. The filtered polymer precipitate was redissolved in 100 mL of chloroform and then washed with 400 mL of deionized water. The water layer was decanted carefully. The black catalyst particles in the polymer solution were removed by filtration followed by evaporation of the (10) Hseish, B. R.; Yu, Y.; VanLaeken, C.; Lee, H. Macromolecules 1997, 30, 8094. (11) Chen, Z.; Meng, H.; Lai, Y.; Huang, W. Macromolecules 1999, 32, 4351. (12) Heck, R. F. Org. React. 1982, 27, 345. (13) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009. (14) Gagnon, D. R.; Capistran, J. D.; Karasz, F. E.; Lenz, R. W.; Antoun, S. Polymer 1987, 28, 567. (15) Bao, Z.; Chen, Y.; Cai, R.; Yu, L. Macromolecules 1993, 26, 5281.
Chem. Mater., Vol. 14, No. 5, 2002 2271 solvent using a rotary evaporator. The polymer was dissolved in small amount of THF and then poured into methanol. The red polymer was collected by filtration and dried under vacuum. Anal. Calcd for C58H70O3N2: C, 82.62; H, 8.37; N, 3.32. Found: C, 79.70; H, 8.49; H, 2.94. Measurements. The synthesized compounds were characterized by 1H NMR spectra obtained using a Varian 300 or 500 MHz spectrometer. The elemental analysis was performed with a Carlo Erba elemental analyzer (CarloErba 1108). The FT-IR spectrum was recorded by a Nicolet 205 FT-IR spectrometer. Thermogravimetric analysis (TGA) of the polymer was performed under a nitrogen atmosphere at a heating rate of 20 °C/min with a Thermal Analysis 2050 thermogravimetric analyzer (TA Instruments). The UV-vis spectra of the polymer were measured using a Hewlett-Packard HP8453 with photodiode array detector. Photoluminescence (PL) and electroluminescence (EL) spectra of the polymers were obtained using a spectrofluorophotometer (Photon Technology). Cyclic voltametry was performed on EG&G 362 scanning potentiostat with a three-electrode cell in a solution of Bu4NBF4 (0.2 M) in acetonitrile at a scan rate 100 mV/s. The polymer films were coated on a Pt wire by dipping the electrode into the polymer solutions in chloroform and drying in a stream of nitrogen. Pt gauze was used as the counter electrode, and an Ag/AgCl in 3 M NaCl electrodes was used as the reference electrode. Prior to each measurement the cell was deoxygenated with nitrogen. Gel permeation chromatography (GPC) measurements were conducted by a GPC system equipped with a Milton-Loy CM4000 pump, a Rheodyne 6-port sample injection valve, a Waters temperature control module, a LDC Analytical SpectroMonitor 3100 UV detector, and two Waters Styragel linear columns using polystyrene as standard and chloroform as eluent. EL Device Fabrications. EL devices were fabricated using ITO glass (sheet resistance of ITO is about 10 Ω, Delta Technology) as an anode and aluminum (Al) as a cathode. The polymer film was prepared by spin-coating from the solution in chloroform (10 mg/mL, 1500 rpm) and then dried under vacuum for 2 h. Prior to spin-coating, the polymer solution was filtered using a 0.45 µm membrane filter. Al was deposited (150-200 nm) onto the surface of the spin-coated polymer film by thermal evaporation technique at about 10-6 Torr. The typical active area of the LED device was 4 mm2. In the measurement of EL intensity, the photodiode detector was placed as close as possible to the LED (approximately 5 mm away from the glass side). The current-voltage (I-V) and EL intensity-voltage (EL-V) characteristics were measured using a current/voltage source, a calibrated photodiode (Newport 818UV/CM), and a power meter (Newport 1830-C). CIE chromaticity coordinates were measured using a MINOLTA spectroradiometer CS-1000. UV-vis, PL, and evaluation of EL devices were performed under ambient conditions.
Results and Discussion Synthesis. Scheme 1 shows synthetic procedures for the monomers and the polymer. Compound 1 was prepared by condensation between 2-(2,6-dimethylpyran-4-ylidene)-malononitrile and 4-bromoaldehyde in a yield of over 70%. Wittig condensation of 2,5-bisdodecyloxy-benzene-1,4-carbaldehyde with ylide of methyltriphenylphosphonium bromide gave compound 2 in a yield of over 80%. Compounds 1 and 2 are well characterized by 1H NMR (see Supporting Information) and elemental analyses. The polymerization reaction conducted by the wellknown palladium-catalyzed Heck coupling reaction between compounds 1 and 2 in DMF yielded a dark red solid of PM-PPV. The chemical structure of the final product, PM-PPV, was confirmed by the FT-IR spectrum shown in Figure 1. The absorption peak at 2925 cm-1 corresponds to C-H stretching vibration of satu-
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Scheme 1. Synthesis of Monomers and Polymer
rated hydrocarbon, the absorption peak at 2209 cm-1 corresponds to -CN stretching vibration, and the weak absorption peaks at 3023 cm-1 and relatively strong absorption at 965 cm-1 correspond to C-H stretching
Figure 1. FT-IR spectrum of PM-PPV film on KBr disk.
and out-of-plane bending motions of trans-vinylene, respectively.14,15 The weight-average molecular weight of PM-PPV measured by GPC was near 8000 with a polydispersity index of 1.25. PM-PPV was readily
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Figure 2. Electronic absorption and PL spectra of PM-PPV (a) solution and (b) thin film.
soluble in common chlorinated hydrocarbon solvents, and it was possible to prepare a smooth and optically clear film by spin-coating. A weight loss of 5% of PMPPV occurred at 356 °C in the TGA thermogram. Optical Properties. The UV-vis spectra and PL spectra of the polymer solution and thin film are shown in Figure 2. The concentration of the solution for the measurements of UV-vis and PL spectra was 3.2 × 10-5 M in chloroform. The thin film for UV-vis and PL measurements was prepared by spin-coating from chloroform solution (5 mg/mL, 1500 rpm) on a quartz plate followed by drying under vacuum for 2 h at room temperature. As shown in Figure 2, the solution and the film show absorption maximum at 458 and 478 nm, respectively, which correspond to the π-π* transition of the main chain. The band gap energy figured out from the UV-vis spectrum of PM-PPV film is 1.94 eV. The solution and the film emit red light with a maximum at 620 and 656 nm, respectively. Very often the shoulders appearing in the PL spectra of the PPV derivatives are attributed to the interchain interactions. It is known that interchain interactions of the polymer have decreasing effect in the PL efficiency.16 These shoulders appear in the longer wavelength region, lowering the color purity. The PM unit in PM-PPV chains acts as a kink point because of the bent structure and the steric hindrance of the two dodecyloxy substituents. We expect significantly lower interchain interactions in PM-PPV compared to PPV derivatives. In fact, in the PL spectra of PM-PPV film, no shoulder was observed. Thus, high color purity and efficiency are expected in the EL emission. The PL maximum of the film at 656 nm blueshifted compared to 710 nm of CN-DOPPV,2 It is not surprising that, compared with CN-DOPPV, decrease in CN content and incorporation of unsubstituted phenylene units in the polymer backbone causes a blue shift of the PL maximum. Electrochemical Properties of the Polymer. In the device fabrication and investigation of LED char(16) (a) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (b) Jabubiak, R.; Rothberg, L. J.; Hsieh, B. R. Synth. Met. 1999, 230, 233. (c) Shi, Y.; Liu, J.; Yang, Y. J. Appl. Phys. 2000, 87, 4254. (d) Nguyen, T.-Q.; Ignacio, B. M.; Liu, J.; Schwartz, B. J. J. Phys. Chem. B 2000, 104, 237. (e) Nguyen, T.-Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J. Appl. Phys. Lett. 2000, 76, 2454.
Figure 3. Cyclic voltammograms of (a) FOC, (b) PM-PPV (inset is compound 1), and (c) MEH-PPV.
acteristics of polymers, information on the electronic structure of the luminescent polymer is essential. Cyclic voltammetry was used to investigate the redox behavior of the polymer and to assess the HOMO and LUMO energy levels.17,18 The cyclic voltammogram (CV) was obtained with polymer film dip-coated on a Pt wire. The CVs of ferrocene/ferrocenium (FOC), PM-PPV, and MEH-PPV are shown in Figure 3 together with the oxidation and reduction potentials. The oxidation and reduction potentials are compared with a reduction potential of FOC. As can be seen in Figure 3b, in the cathodic and anodic scans, PM-PPV shows irreversible n- and pdoping processes. In the cathodic scan, the CV show two reduction peaks at -1.10 and -1.80 V vs Ag/AgCl, respectively. By comparing with the CV of MEH-PPV (Figure 3c), the peak at -1.80 V is attributed to the reduction of the PPV moiety. By comparing with the CV of compound 1 (inset of Figure 3b), the peak at -1.10 V was attributed to the reduction of PM moiety. The band gap energy of PM-PPV can be obtained from the absorption edge (1.94 eV) as well as from the difference between the onset potentials of oxidation and reduction (1.96 eV). The two band-gap values obtained by both (17) Wu, C.; Strum, J. C.; Register, R. A.; Tian, J.; Dana, E. P.; Thompson, M. E. IEEE Trans. Electron Devices 1997, 44, 1269. (18) Peng, Z.; Galvin, M. E. Chem. Mater. 1998, 10, 1785.
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Figure 4. EL spectra of the (a) single-layer (ITO/PM-PPV/ Al) and (b) bilayer (ITO/PPV/PM-PPV/Al) devices.
methods agree with each other very well. The HOMO energy level vs vacuum level was calculated from the measured onset potential of oxidation (0.64 V vs FOC) (Figure 3a) by assuming the energy level of FOC is 4.8 eV below the vacuum level,17,18 and the LUMO energy level was calculated from the HOMO energy level and the absorption edge. The HOMO and LUMO energy levels obtained from electrochemical and UV-vis spectrum are -5.44 and -3.48 eV, respectively. The HOMO and LUMO energy levels of PM-PPV are more stabilized than those of MEH-PPV (-4.98, -2.89 eV). The HOMO and LUMO energy levels deduced from the optical and electrochemical data support the argument that the injection of holes and electrons in PM-PPV is more balanced than in MEH-PPV. EL and I-V-EL Characteristics. The external quantum efficiency was calculated on the basis of the EL intensity and electric current running through the device without any other correction. The EL spectrum of the single-layer LED (ITO/PM-PPV/Al) based on the PM-PPV showed a maximum peak at 652 nm (Figure 4a), and the luminescence was easily observable under normal room light with forward bias. The external quantum efficiency was calculated to be 0.004% at 0.12 mA/mm2, which is greater than that of typical MEHPPV, by an order of magnitude, based on a single-layer EL device (0.0005%, at 0.15 mA/mm2). These results suggest that the electron injection in PM-PPV is improved in comparison to MEH-PPV. To improve the efficiency, we fabricated devices based on the bilayer (ITO/PPV/PM-PPV/Al). The PPV layer was thermally converted from the xanthate precursor as in the literature.19,20 The bilayer device based on PM-PPV showed an efficiency of 0.05% at 0.18 mA/ mm2, which is about 13 times higher than its singlelayer device. As shown in Figure 4b, the EL maximum (19) Lenz, R. W.; Han, C.; Stenger-Smith, J.; Karasz, F. E. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 3241. (20) Burn, P. L.; Kraft, A.; Baigent, D. R.; Bradley, D. D. C.; Brown, A. R.; Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Jackson, R. W. J. Am. Chem. Soc. 1993, 115, 10117.
Figure 5. (a) Current-voltage and (b) EL intensity-voltage characteristics of the single-layer (ITO/PM-PPV/Al) (filled circle) and bilayer (ITO/PPV/PM-PPV/Al) (filled triangle) devices.
of the bilayer device appeared at 646 nm, which is slightly blue-shifted compared to that of a single-layer device. These phenomena, as explained by Pinto, Hu, Karasz, and Akcelrud,21 depend on the thickness of the PPV layer, the relative location of energy bands of the PPV and emitter, and the probability of electron transfer. Figure 5 shows the current and the emitted EL intensity as a function of the bias voltage. Typical rectification characteristics were observed in all the devices regardless of the structure. The turn-on voltage of the single-layer device being 11 V was somewhat higher than that of the bilayer device, which appeared approximately at 8 V. In Figure 5, one can also notice that the current and the EL intensity in the bilayer device increase with the applied voltage at rates much higher than those observed in the single-layer device. This implies that the hole-injection and electron-hole recombination processes are facilitated by the presence of the PPV layer.2,18 The chromaticity diagram of the PM-PPV bilayer device is shown in Figure 6 together with the National Television System Committee (NTSC) standard coordinates for red color. The Commission Internationale d’E Ä nclairage (CIE) chromaticity coordinates of the device based on bilayer structure at 13 V are x ) 0.69, y ) 0.31, which are very close the CIE chromaticity coordinates (x ) 0.67, y ) 0.33) of NTSC for red color. The chromaticity coordinates did not (21) Pinto, M. R.; Hu, B.; Karasz, F. E.; Akcelrud, L. Polymer 2000, 41, 8095.
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PPV) through the well-known palladium-catalyzed Heck coupling reaction. The polymer is readily soluble in common organic solvents and has good film-forming properties on the surfaces of ITO and PPV thin film. CIE chromaticity coordinates of the EL device based on bilayer structure are x ) 0.69, y ) 0.31, which are very close to the chromaticity coordinates of NTSC for red color. The injection of holes and electrons are more balanced. Thus, external quantum efficiency of the EL device based on a single-layer structure is 0.004% at 0.12 mA/mm2, which is greater than that of typical MEH-PPV by an order of magnitude. The PPV layer is not only a good hole injection material but also a good emitter. The EL device based on bilayer structure has an efficiency of 0.05% at 0.18 mA/mm2, which is 13 times higher than that of the corresponding single-layer device.
Figure 6. CIE chromaticity diagram and chromaticity coordinates for ITO/PPV/PM-PPV/Al and NTSC coordinates for red color (filled circle, NTSC; filled triangle, PM-PPV).
change significantly in a relatively wide bias range, i.e., from 8 to 13 V. Conclusions We have synthesized a new red-emitting poly(pphenylenevinylene) derivative with PM moiety (PM-
Acknowledgment. This research was financially supported by the Korea Science and Engineering Foundation through the Center for Electro- and PhotoResponsive Molecules (CRM), Korea University. Supporting Information Available: 1H NMR spectra of 1 and 2, 13C NMR and mass spectra of 2, TGA thermogram of PM-PPV, external quantum efficiency vs current curves for ITO/PPV/PM-PPV/Al, ITO/PM-PPV/Al, and ITO/MEH-PPV/ Al devices, and calculation method for external quantum efficiency (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM011553R
