Role of Symmetry and Charge Delocalization in Two-Dimensional

Feb 10, 2007 - Mary E. Galvin*,|. Department of ... arms are positioned in a para, ortho, or meta position, respectively, in relation to each other. C...
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Chem. Mater. 2007, 19, 993-1001

993

Role of Symmetry and Charge Delocalization in Two-Dimensional Conjugated Molecules for Optoelectronic Applications Hermona K. Christian-Pandya,† Zukhra I. Niazimbetova,‡ Frederick L. Beyer,§ and Mary E. Galvin*,| Department of Materials Science & Engineering, UniVersity of Delaware, Newark, Delaware 19716, Rohm and Haas Electronic Materials, 455 Forest Street, Marlborough, Massachusetts 01752, U.S. Army Research Laboratory, Polymer Research Branch, Building 4600, Aberdeen ProVing Ground, Maryland 20015, and Air Products and Chemicals, 7201 Hamilton BouleVard, Allentown, PennsylVania 18195 ReceiVed March 3, 2006. ReVised Manuscript ReceiVed August 8, 2006

Three novel conjugated phenylenevinylene (PV)-based isomers containing oxadiazole moieties have been synthesized and characterized for optoelectronic applications. These molecules are tetra-substituted at the central phenyl ring with two poly(phenylenevinylene) (PPV)-based arms and two oxadiazolederivatized PPV (OXAPPV) arms. In the three molecules, termed p-, o-, and m-OXA-X, the OXAPPV arms are positioned in a para, ortho, or meta position, respectively, in relation to each other. Comparing these molecules, we explore the role of symmetry and charge delocalization in this class of compounds. Despite having different linear segments, they have nearly identical photophysical properties, suggesting a similar charge delocalization mechanism. However, in a LED with the molecules as emissive layers between aluminum and ITO electrodes and with PEDOT:PSS and lithium fluoride to aid in hole and electron injection, respectively, o-OXA-X exhibited the highest external quantum efficiency (EQE) of 0.46% compared to analogous devices made of p-OXA-X and m-OXA-X that showed efficiencies of 0.28% and 0.10%, respectively. We explain these differences based on the changes in the film morphology between the three molecules. Also reported are data from cyclic voltammetry and morphological data from atomic force microscopy, NMR studies, thermal characterization, and X-ray diffraction studies.

Introduction Research in organic- and polymer-based optoelectronic semiconducting materials continues to grow due to their potential applications in areas such as light-emitting diodes (LEDs),1-5 photovoltaics (PVs),6-8 thin film transistors (TFTs),9-11 and solid-state lasers,12,13 with organic and * To whom correspondence should be addressed. E-mail: galvinme@ airproducts.com. Tel.: (610) 481-1524. † University of Delaware. ‡ Rohm and Haas Electronic Materials LLC. § U.S. Army Research Laboratory. | Air Products and Chemicals.

(1) Friend, R. H. Pure Appl. Chem. 2001, 73, 425-430. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (3) Wu, W. S.; Inbasekaran, M.; Hudack, M.; Welsh, D.; Yu, W. L.; Cheng, Y.; Wang, C.; Kram, S.; Tacey, M.; Bernius, M.; Fletcher, R.; Kiszka, K.; Munger, S.; O’Brien, J. Microelectron. J. 2004, 35, 343-348. (4) Mahler, A. K.; Schlink, H.; Saf, R.; Stelzer, F.; Meghdadi, F.; Pogantsch, A.; Leising, G.; Moller, K. C.; Besenhard, J. O. Macromol. Chem. Phys. 2004, 205, 1840-1850. (5) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R 2002, 39, 143-222. (6) Hoppe, H.; Egbe, D. A. M.; Muhlbacher, D.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 3462-3467. (7) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4647-4656. (8) Schmidt-Mende, L.; Watson, M.; Mullen, K.; Friend, R. H. Mol. Cryst. Liq. Cryst. 2003, 396, 73-90. (9) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685-688. (10) Jenekhe, S. A.; Babel, A. Abstr. Pap. Am. Chem. Soc. 2003, 226, U399-U399.

polymer LEDs recently being incorporated into commercial displays.14 Some of the advantages offered by these diodes include low power consumption, wide viewing angle, low manufacturing costs, ease of processablity, and flexibility.15 However, despite these attractive features, the materials are limited by poor charge carrier mobility.5 Charge transport can be improved if delocalization is extended over two or even three dimensions. Even in PVs and TFTs, limited charge transport can contribute to device inefficiency. Organic and polymeric systems have a facile onedimensional charge transport through the conjugated segments, with transport in other dimensions limited by hopping constraints. In some of these systems, two-dimensional charge delocalization can be achieved through π-π stacking between adjacent chains, which leads to improved ease of hopping.16-18 For example, by optimizing intermolecular interactions between neighboring P3HT chains in TFTs, (11) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413-4422. (12) Xia, R. D.; Heliotis, G.; Hou, Y. B.; Bradley, D. D. C. Org. Electron. 2003, 4, 165-177. (13) Polson, R. C.; Vardeny, Z. V. Appl. Phys. Lett. 2004, 85, 1892-1894. (14) Voges, F.; Bonrad, K.; Daubler, T. K.; Frank, T.; Pommerehne, J.; Ottermann, C.; Sprengard, R. In Light Sources 2004, Proceedings of the Tenth International Symposium on the Science and Technology of Light Sources, Toulouse, France, 18-22 July 2004; Institute of Physics: Bristol, Philadelphia, 2004; pp 183-184. (15) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. S. AdV. Mater. 2000, 12, 1737-1750. (16) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122.

10.1021/cm060524j CCC: $37.00 © 2007 American Chemical Society Published on Web 02/10/2007

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Christian-Pandya et al.

Chart 1. Structures of Tetra-substituted Two-Dimensional Molecules

Sirringhaus et al. were able to increase carrier mobilities by a factor greater than 100.19 However, strong π-π intermolecular interactions like these can also increase the tendency of molecules to crystallize, leading to poor film quality and the formation of excimers which quench photoluminescence in films. Compounds that have the potential to intrinsically delocalize charge in two dimensions, while possessing good film-forming properties, present themselves as improved emissive materials. Other groups have reported work on molecules that approximate this type of two-dimensional transport. Phenylenevinylene-based dendrimeric molecules maintain some charge delocalization along the conjugated arms but this conjugation is interrupted through the central meta linkages.20,21 Two-dimensional tetrahedral oligo(phenylenevinylene) molecules reported by Robinson et al. offer the added advantage of easy solution processablity, but in these, conjugation is nearly broken through the sp3-hybridized central carbon atom.22 Although derivatives of hexabenzocoronene have been shown to exhibit exceptionally good charge transport along their aromatic macrocyclic cores in the liquid-crystalline phase, they do not allow for much engineering of the HOMO-LUMO energy levels and have poor solubility especially as the core size is increased.18 The present work is a second report on novel PPV-based two-dimensional conjugated molecules. In the prior report,23 four solution-processable materials constructed to maintain conjugation through the core and arms, showing evidence of two- and possibly three-dimensional charge delocalization, and designed to be discotic liquid crystals were introduced. These were made with a versatile synthetic approach that (17) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mullen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2003, 15, 495-499. (18) van de Craats, A. M.; Warman, J. M.; Fechtenkotter, A.; Brand, J. D.; Harbison, M. A.; Mullen, K. AdV. Mater. 1999, 11, 1469-1472. (19) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W. Synth. Met. 2000, 111, 129-132. (20) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. 1998, 37, 643-645. (21) Halim, M.; Pillow, J. N. G.; Samuel, I. D. W.; Burns, P. L. Synth. Met. 1999, 102, 922-923. (22) Robinson, M. R.; Wang, S. J.; Heeger, A. J.; Bazan, G. C. AdV. Funct. Mater. 2001, 11, 413-419. (23) Niazimbetova, Z. I.; Christian, H. Y.; Bhandari, Y. J.; Beyer, F. L.; Galvin, M. E. J. Phys. Chem. B 2004, 108, 8673-8681.

allowed for flexibility in the structures of the core and arm segments to engineer electronic properties. In this paper, two of the previously reported tetra-substituted compounds are further discussed and a third molecule is introduced. Each of the compounds have two PPV arms for hole transport and two oxadiazole-derivatized PPV arms (OXAPPV) for better electron transport.24 In these molecules, labeled p-OXA-X, o-OXA-X, and m-OXA-X, shown in Chart 1, the positions of the OXAPPV arms change in a para, ortho, or meta position with respect to each other. Though they have the same overall chemical composition, due to the placement of the PPV and OXAPPV arms, their linear segments differ. In the p-OXA-X molecule, with the OXAPPV arms directly across from each other and two PPV segments para to each other, there is a series of 9 aromatic units (without any solubilizing alkoxy groups and with two oxadiazole moieties) by 5 phenylenevinylene units. This is in contrast to the oand m-OXA-X molecules which each have a PPV arm across from an OXAPPV arm, resulting in a series of 7 aromatic units by 7 units, each containing one oxadiazole moiety and two alkoxy groups. Additionally, with the differences in the electron densities of the PPV and OXAPPV segments coupled with the changes in structural symmetry between the molecules, dipole moments of the Xs could be affected, which in turn may affect molecular packing. With the placement of the electron-withdrawing oxadiazole groups closest to each other in the o-OXA-X molecule, this isomer is likely to have the largest net dipole moment since the angle between the oxadiazole arms is the smallest and thus the vector sum of bond polarities is the greatest in this molecule. Following the same reasoning, the net dipole moment should decrease in m-OXA-X and be the least in p-OXA-X, in which the vector sum of bond polarities should cancel each other, resulting in a value of zero. The purpose of this manuscript is to explore in detail how these differences in symmetry affect the properties of this new group of materials, including photophysical spectra, thermal stability, HOMO-LUMO energy levels, tendency to crystallize or π-π stack in films, and device characteristics. (24) Peng, Z. H.; Bao, Z. N.; Galvin, M. E. AdV. Mater. 1998, 10, 680684.

Conjugated Molecules for Optoelectronic Application

Experimental Section Synthesis and Structural Characterization of m-OXA-X. All reactions were carried out under a dry nitrogen atmosphere using standard vacuum line techniques. Selecto Scientific silica gel (60 Å; 70-230 mesh) was used for column chromatography. Mass spectra were obtained on a Bru¨ker BIFLEX III (MALDI source) spectrometer. Nuclear magnetic resonance (NMR) spectra were obtained with a Bru¨ker 360 MHz spectrometer in deuterated chloroform. The syntheses, structural characterization, photophysical characterization, and thermal analysis of p-OXA-X and o-OXA-X have been previously reported.23 Syntheses of the PPV arm and OXAPPV arm have also been previously described.23 2,4-Diiodom-xylene was synthesized according to literature procedure.25 1,5-Bis(bromomethyl)-2,4-diiodobenzene, 2. This procedure was adapted from work published by Rivera et al. and Kimura et al.26,27 To a solution of 1 (1.00 g, 2.79 mmol) dissolved in 15 mL of carbon tetrachloride, N-bromosuccinimide (1.50 g, 8.37 mmol) was added followed by benzoyl peroxide (18.4 mg, 0.076 mmol). This reaction mixture was heated to reflux (75 °C) overnight under nitrogen. The hot mixture was then filtered and the filtrate was dried by vacuum evaporation. The resulting viscous light brown residue was dissolved in hexanes, heated, and then filtered to remove precipitated succinimide. The hexanes solvent from the filtrate was evaporated and the white solid product was then recrystallized in heptane (yield 72%). 1H NMR (360 MHz, CDCl3) δ (ppm): 8.28 (s, 1H, arom.); 6.80 (s, 1H, arom.); 4.56 (s, 4H, 2 × CH2Br). Diethyl 5-[(Diethoxyphosphoryl)methyl]-2,4-diiodobenzylphosphonate, 3. This procedure was adapted from the literature.27 A mixture of 2 (0.18 g, 0.35 mmol) and triethyl phosphite (4.85 g, 29 mmol) was heated at 130 °C overnight. After the mixture was cooled, the excess triethyl phosphite was distilled out under vacuum. The white solid formed was recrystallized from an ethyl acetate and hexanes mixture (yield 72%). 1H NMR (360 MHz, CDCl3), δ (ppm): 8.30 (s, 1H, arom.); 7.47 (s, 1H, arom.); 4.07 (m, 8H, 4 × CH2O); 3.33 (d, 4H, 2 × CH2P); 1.28 (t, 12H, 4 × CH3). 1-(E)-2-[4-((E)-2-2,4-Diiodo-5-[(E)-2-(4-(E)-2-[4-methyl-2,5bis(octyloxy)phenyl]ethenylphenyl)ethenyl]phenylethenyl)phenyl]ethenyl-4-methyl-2,5-bis(octyloxy)benzene, 4. This procedure was adapted from the literature and is a standard Wittig-Horner reaction.28 To a solution of 3 (588 mg, 0.93 mmol) in 10 mL of anhydrous tetrahydrofuran (THF), sodium hydride (62 mg, 2.58 mmol) was added. The mixture was heated to reflux, and a solution of PPV arm (987 mg, 2.06 mmol), in 10% excess to favor di-substitution, was dissolved in 15 mL of distilled THF and then added dropwise. The reaction contents were left stirring at 65 °C overnight. Upon cooling, the excess sodium hydride was neutralized with concentrated phosphoric acid and the mixture was extracted with chloroform and water. The organic phase was dried with magnesium sulfate overnight. After filtration of the salt, and solvent evaporation, the crude product was purified by column chromatography using chloroform and hexanes (2:1) to give 4, a yellow solid (0.463 g, 39%). 1H NMR (360 MHz, CDCl3), δ (ppm): 8.36 (s, 1H, arom.); 7.82 (s, 1H, arom.); 7.55 (dd, 8H, arom.); 7.51 (d, 2H, Hvinyl); 7.25 (d, 2H, Hvinyl); 7.08 (d, 2H, Hvinyl); 7.07 (d, 2H, Hvinyl); 7.05 (s, 2H, arom.); 6.73 (s, 2H, arom.); 3.92-4.01 (m, (25) Merkushev, E. B.; Simakhina, N. D.; Koveshnikova, G. M. Synthesis 1980, 1980, 486-487. (26) Rivera, J. M.; Martin, T.; Rebek, J. J. Am. Chem. Soc. 2001, 123, 5213-5220. (27) Kimura, M.; Narikawa, H.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Chem. Mater. 2002, 14, 2711-2717. (28) Wadsworth, D. H.; Schupp, O. E.; Seus, E. J.; Ford, J. A. J. Org. Chem. 1965, 30, 680-&.

Chem. Mater., Vol. 19, No. 5, 2007 995 8H, 4 × CH2O); 2.24 (s, 6H, 2 × CH3); 1.79-1.86 (m, 8H, CH2CH2O); 1.25-1.55 (m, 40H, 20 × CH2); 0.84-0.93 (m, 12H, 4 × CH3). MS (MALDI): m/z 1278.2 (M+; calcd: 1278.3). 2-[4-((E)-2-4-[(E)-2-(2,4-Bis[(E)-2-(4-(E)-2-[4-methyl-2,5-bis(octyloxy)phenyl]ethenylphenyl)ethenyl]-5-(E)-2-[4-((E)-2-4-[5(4-methylphenyl)-1,3,4-oxadiazol-2-yl]phenylethenyl)phenyl]ethenylphenyl)ethenyl]phenylethenyl)phenyl]-5-(4-methylphenyl)1,3,4-oxadiazole, m-OXA-X. This procedure was adapted from the literature and is a standard Heck coupling.29 A solution of 4 (230 mg, 0.180 mmol), OXAPPV arm (160 mg, 0.439 mmol, this is added in excess to favor di-substitution), and tri-o-tolylphosphine (22 mg, 0.072 mmol) in 25 mL of dry DMF was heated to 45 °C. At this temperature tributylamine (0.1 mL, 0.414 mmol) was added to the reaction contents. The resulting mixture was further heated to 55 °C at which point palladium acetate (5 mg, 0.02 mmol) was added. The temperature of the reaction contents was slowly increased to 75 °C and kept overnight. After the reaction mixture was cooled, methanol was added to precipitate the product. The product was then purified by column chromatography using chloroform. Upon evaporation of the solvent from collected fractions, yellow solid product of m-OXA-X was obtained (69 mg, 22%, 224 °C mp). 1H NMR (360 MHz, CDCl3), δ (ppm): 8.13 (d, J ) 8.3 Hz, 4H, arom.); 8.05 (d, J ) 8.3 Hz, 4H, arom.); 7.85 (s, 2H, arom. (core)); 7.68 (d, J ) 8.3 Hz, 4H, arom.); 7.49-7.60 (m, 4H, Hvinyl); 7.59 (s, 8H, arom.); 7.57 (s, 8H, arom.); 7.55 (d, J ) 12.0 Hz, 2H, Hvinyl); 7.51 (d, J ) 16.2 Hz, 2H, Hvinyl); 7.35 (d, J ) 7.9 Hz, 4H, arom.); 7.27 (d, J ) 14.4 Hz, 2H, Hvinyl); 7.19 (d, J ) 16.6 Hz, 2H, Hvinyl); 7.14 (d, J ) 16.2 Hz, 2H, Hvinyl); 7.10 (d, J ) 16.6 Hz, 2H, Hvinyl); 7.06 (s, 2H, arom.); 6.74 (s, 2H, arom.); 3.99 (t, 4H, 2 × CH2O); 3.98 (t, 4H, 2 × CH2O); 2.46 (s, 6H, 2 × CH3); 2.24 (s, 6H, 2 × CH3); 1.77-1.89 (m, 8H, 4 × CH2CH2O); 1.21-1.59 (m, 40H, 20 × CH2); 0.90 (t, 6H, 2 × CH3); 0.88 (t, 6H, 2 × CH3). MS (MALDI): m/z 1750.1 (M+; calcd: 1750.03). Thermal Analysis of m-OXA-X. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer instrument with a TGA/7 thermogravimetric analyzer at a heating rate of 10 °C/min under a nitrogen gas flow. Differential scanning calorimetry (DSC) was conducted with a Perkin-Elmer DSC/7 differential scanning calorimeter. The sample was encapsulated in an aluminum pan and heated and cooled at a rate of 5 °C/min. The melting and crystallization temperatures of the sample were determined by extrapolation from the baseline to the slope of the onset of the transition. Photophysical Characterization. UV-vis spectra were recorded using an Agilent 8453 spectrophotometer. Emission spectra were obtained with a SPEX Fluoromax3 spectrofluorometer and analyzed using Datamax software. The wavelength at maximum absorption was used to excite samples to obtain emission spectra. Quantum efficiencies were determined in THF relative to Coumarin 314 in dilute ethanol solution according to the procedure described by Williams et al.30 Electrochemistry. Cyclic voltammograms for the 2D molecules were taken in acetonitrile (film studies) containing 0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) or in THF (solution studies) with 0.10 M Bu4NPF6. The working electrode for voltammetry was a platinum disk electrode with a diameter of 1.6 mm (Bioanalytical Systems Inc.) and the counter electrode was a platinum wire. The reference electrode used was Ag/AgNO3 (silver wire submerged in a solution of 0.01 M AgNO3 + 0.10 M Bu4NPF6 in acetonitrile) that normally had a potential of -0.19 V (29) Fischetti, W.; Mak, K. T.; Stakem, F. G.; Kim, J. I.; Rheingold, A. L.; Heck, R. F. J. Org. Chem. 1983, 48, 948-955. (30) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108, 1067-1071.

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Christian-Pandya et al. Scheme 1. Synthesis of m-OXA-X

in THF and -0.10 V in acetonitrile versus the ferroceneferrocenium couple. All potentials are reported with respect to the standard potential of the ferrocene couple, 4.8 eV.31 Solutions were purged for at least 30 min with nitrogen which was saturated with acetonitrile (film) or THF (solution). The scan rate for all experiments was 200 mV/s. Films were drop-cast from chloroform. The cell used in these experiments was BASi C-3 Cell Stand from Bioanalytical Systems Inc. The EG&G Princeton Applied Research Potentiostat/Galvanostat Model 273 was used in these studies with MSDOS EChem software to collect and analyze the data. Atomic Force Microscopy. AFM images were collected with a Digital Instruments MultiMode AFM with Nanoscope IIIa controller (Veeco Instruments) in tapping mode using BS-Tap300Al tips from Budget Sensors Inc. Minor processing was performed on height and phase images to enhance quality by applying a “flattening” operation using the AFM offline software. X-ray Diffraction Analysis. Small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques were performed to investigate morphological order in powders of the 2D molecules. SAXS data for p- and o-OXA-X molecules were collected as described in the literature.23 For the p-OXA-X sample WAXD data were collected as described previously.23 For o-OXA-X, 2D WAXD data were collected using Cu KR X-rays produced by a Rigaku Ultrax18 generator operated at 40 kV and 40 mA, monochromated using a pyrolitic graphite crystal, and collected on a Bruker Hi-star multiwire area detector. The sample-to-detector distance was 6.0 cm and the sample was scanned from 4.5° to 45° 2θ. The 2D data was azimuthally averaged to generate I(q) as a function of q, where q ) 4π sin(θ)/λ, θ is one-half the scattering angle 2θ, and λ is the X-ray wavelength, 1.54 Å. No background corrections were applied. Device Fabrication and Testing. Devices were made, starting with ITO coated on glass substrates with a sheet resistance of 20 Ω/0. The ITO was cleaned according to a protocol obtained from Bayer Chemicals suited for PEDOT:PSS deposition. A 120 nm thick PEDOT:PSS film (Baytron P VP CH 8000 grade obtained from Bayer Chemicals) was spin-coated on top of the ITO as the holeinjecting/-transporting layer. After deposition of this layer, the (31) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler, H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551-554.

substrates were dried in a vacuum oven for 5 min at 100 °C and stored under nitrogen overnight to ensure complete dryness. The emissive 2D molecules were dissolved in freshly distilled 1,1,2,2tetrachloroethane at a concentration of 20 mg/mL and spin-coated onto the PEDOT:PSS layer at a rate of 900 rpm, resulting in uniform films of ca. 100 nm thickness. Thickness values of the organic layers were measured with a DekTak Profilometer. To aid in electron injection, a thin 8-10 Å layer of lithium fluoride (LiF) was evaporated, at a rate of 0.1 nm/s, over the emissive layer. Finally, a 200 nm thick aluminum cathode was shadow-evaporated at a rate of 1 nm/s onto the substrate defining pixels of ca. 0.025 cm2 area. Current-voltage properties were measured using a HP 4155B Semiconductor Parameter Analyzer with a Newport 818UV silicon photodetector. External quantum efficiencies (EQE) were determined as the ratio of the photocurrent to device current, with a correction factor of 1.43 for detector sensitivity at 540 nm. Only light transmitted through the transparent substrate was collected on the detector. No other corrections were made. Luminance measurements were made using a calibrated TRICOR Inc. Model 820 Video Photometer and analyzed using Eyeppearance 3.67 software. The devices were tested and stored in a dry nitrogen-filled glovebox at room temperature where the water and oxygen levels were