Green Phosphorescent Dendrimer for Light-Emitting Diodes

Sample dahlia-SWNH eth-SWNH-10 eth-SWNH-50 gl-SWNH-50 H2O2-ox-SWNH H2O2-ox-SWNH-50 O2-ox-SWNH O2-ox-SWNH-50 Specific surface area at [m2 g±1] Total pore volume Vt [cm3 g±1] Volume of mesopores Vme [cm3 g±1] Micropore volume Vmi [cm3 g±1] 308 430 590 385 830 850 1006 1175 0.40 0.47 0.52 0.33 1.10 0.67 1.10 0.87 0.29 0.33 0.32 0.17 0.75 0.31 0.64 0.42 0.11 0.14 0.20 0.16 0.36 0.36 0.46 0.45 in ethanol benefits a remarkable development of microporosity upon compression. Thus, the micropore volume of ethSWNH-50 is twice that of pristine material. For the oxidized SWNHs (O2-ox-SWNH and H2O2-ox-SWNH) the surface area increases slightly after compression. The dependence of the mesopore volume on the liquid used for the SWNHs dispersion is complex. There is a significant decrease in the mesopore volume for the compressed gl-SWNH. However, the compression of eth-SWNHs does not change the mesopore volume, although it alters the mesopore size distribution as shown above. Compression of oxidized SWNHs affects predominantly the mesopore volume, which decreases significantly after compression. On the contrary, no effect of the compression on the micropore volume of the oxidized SWNHs is observed. In summary, the compression of dahlia-SWNHs after preliminary treatment by sonication in liquid significantly increases the packing density of SWNH aggregates and modifies their pore structure. Thus, after compression of the powdered SWNHs, the apparent volume of the material decreases almost 20 times. The compression of pristine SWNHs increases remarkably the microporosity. The spherical bundle structure, which is preserved after compression, should be important for microporosity development. The simple approach to compact SWNHs reported here could provide a useful route in the development of advanced materials for gas storage application. We predict that development of more efficient methods of microporosity enhancement of SWNHs, combined with our method for compression, will provide the desired adsorbent for supercritical gas adsorption. Received: February 8, 2002 Final version: April 8, 2002 ± [1] [2] [3] [4] [5] [6] S. Iijima, Nature 1991, 56, 354. S. Iijima, T. Ichihashi, Nature 1993, 363, 603. S. Bandow, M. Takizawa, K. Hirahara, M. Yudasaka, S. Iijima, Chem. Phys. Lett. 2001, 48, 337. T. W. Ebessen, P. M. Ajayan, Nature 1992, 358, 220. A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. Hee Lee, S. Gon Kim, A. G. Rinzler, D. T. Colbert, G. Scuseria, D. Tomµnek, J. E. Fischer, R. E. Smalley, Science 1996, 273, 483. Z. F. Ren, Z. P. Huang, J. W. Xu, J. W. Wang, P. Bush, M. P. Siegal, P. N. Provencio, Science 1998, 282, 1105. Adv. Mater. 2002, 14, No. 13±14, July 4 [7] W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zou, R. A. Zhao, G. Wang, Science 1996, 274, 1701. [8] S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai, K. Takahashi, Chem. Phys. Lett. 1999, 165, 309. [9] I. W. Chiang, B. E. Brinson, A. Y. Huang, P. A. Willis, M. J. Bronikowski, J. L. Margrave, R. E. Smalley, R. H. Hauge, J. Phys. Chem. B 2001, 105, 8297. [10] A. C. Dillon, T. Gennett, K. M. Jones, J. L. Alleman, P. A. Parilla, M. J. Hebben, Adv. Mater. 1999, 11, 1354. [11] NEC Electronic News; http://www.ee.nec.de/News/Releases/pr283-01.html, Aug. 31, 2001. [12] K. Murata, K. Kaneko, D. Kasuya, K. Takahashi, F. Kokai, M. Yudasaka, S. Iijima, in Proc. Tsukuba Symposium on Carbon Nanotubes, Tsukuba, Japan, October 2001, p. 49. [13] E. Bekyarova, K. Kaneko, D. Kasuya, K. Takahshi, F. Kokai, M.Yudasaka, S. Iijima, in Proc. Int. Symp. Nanocarbons 2001 (ISNC), Nagano, Japan, Nov. 14±16 2001, p. 141. [14] E. Bekyarova, D. Kasuya, K. Kaneko, K. Murata, M. Yudasaka, S. Iijima, Langmuir 2002, 18, 4138. [15] K. Murata, K. Kaneko, W. A. Steele, F. Kokai, K. Takahashi, D. Kasuya, K. Hirahara, M. Yudasaka, S. Iijima, J. Phys. Chem. B 2001, 105, 10 210. [16] E. Dujardin, T. W. Ebesen, H. Hiura, K. Tanigaki, Science 1994, 265, 1850. [17] The surface tension (c) of ethanol at 298 K is 22 mN m±1 and c of glycerol at 423 K is 51 mN m±1. The sample dispersed in ethanol was dried at room temperature, while SWNHs dispersed in glycerol were dried at 423 K as the vapor pressure of glycerol at room temperature is negligible. [18] S. J. Gregg, K. S. W. Sing, in Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London 1982. [19] The pore size distribution of the as-grown and ethanol-treated SWNHs were computed from the adsorption branch of the isotherm, while for the compressed sample the desorption branch conveys a more accurate information and it is more appropriate for the pore size distribution evaluation. [20] S. C. Tsang, Y. K. Chen, P. J. F. Harris, M. L. H. Green, Nature 1994, 372, 159. [21] K. Kaneko, C. Ishii, H. Kanoh, Y. Hanzawa, N. Setoysama, T. Suzuki, Adv. Colloid Interface Sci. 1998, 76±77, 295. Green Phosphorescent Dendrimer for Light-Emitting Diodes** By Shih-Chun Lo, Nigel A. H. Male, Jonathan P. J. Markham, Steven W. Magennis, Paul L. Burn,* Oleg V. Salata, and Ifor D. W. Samuel Organic light-emitting diodes (OLEDs) are likely to play an increasingly competitive role in the flat panel display market.[1] Within the OLED field, displays based on molecular materials are the most mature and there are already a number of displays on the market. However, molecular materials are processed by evaporation and it is generally thought that in the longer term, solution processing will offer a lower cost ap- ± [*] Dr. P. L. Burn, Dr. S.-C. Lo The Dyson Perrins Laboratory, University of Oxford South Parks Rd, Oxford, OX1 3QY (UK) E-mail: [email protected] Dr. N. A. H. Male, Dr. O. V. Salata Department of Materials, University of Oxford Unit 8, Begbroke Science and Business Park, Sandy Lane, Yarnton Oxford OX5 1PF (UK) J. P. J. Markham, Dr. S. W. Magennis, Prof. I. D. W. Samuel Organic Semiconductor Centre School of Physics and Astronomy, University of St. Andrews North Haugh, St. Andrews, Fife, KY16 9SS (UK) [**] We are grateful to Opsys Ltd, SHEFC, and the Royal Society for financial support. Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1307-0975 $ 17.50+.50/0 975 COMMUNICATIONS Table 1. Pore structure parameters estimated from the nitrogen adsorption isotherms. COMMUNICATIONS proach to large-area full color displays. Conjugated polymers have so far dominated solution-processed OLEDs, and devices with good efficiencies have been reported.[2] However, more recently solution-processible dendrimers have been developed for use as the light-emitting layer in OLEDs.[3,4] Light-emitting dendrimers consist of surface groups, dendrons, and cores. The light-emitting dendrimers fall into two main classes: those in which the dendrons are fully conjugated,[3] and those in which the chromophores are attached to dendrons that contain non-conjugated moieties.[4] Conjugated dendrimers have a number of potential advantages over conjugated polymers for use in OLEDs. First, they can be produced via a modular synthesis giving a greater flexibility over controlling the properties.[5] Second, the processing and electronic properties can be optimized independently. For example, the core can be chosen to determine the key electronic properties such as light emission, and the surface groups can be selected to give the desired solubility. Third, the generation of the dendrimer gives molecular control over the intermolecular interactions that are vital to OLED performance.[6] Finally, both fluorescent and phosphorescent[7] dendrimers are easily accessible. The advantage of phosphorescent light-emitting materials is that they harness emission from both singlet and triplet excited states so that it is theoretically possible to achieve OLEDs with 100 % internal quantum efficiency. In contrast, in fluorescent materials, triplet formation leads to a substantial loss of efficiency.[8] In spite of the impressive scope for molecular engineering that dendrimers offer, the efficiency of dendrimer LEDs made so far has been low. In this communication, we show how dendrimers can be used to make highly efficient organic LEDs. Our work draws on recent advances in devices made from phosphorescent iridium complexes, which have been used to make highly efficient green, red, and blue LEDs.[9±11] These devices are prepared by evaporation and typically contain a hole-transporting layer, the iridium complex blended with a wide bandgap host, and one or more electron-transporting/hole-blocking layers. There have also been reports of an OLED produced by spin-coating an iridium complex as a guest in a polymer host, but this approach gave much lower efficiencies than the devices prepared via evaporation.[12] We have developed a methodology for the preparation of solution-processible fac-tris(2-phenylpyridine) iridium-cored dendrimers and have recently reported the results of single layer OLEDs based on these materials.[13] In this paper we show that very high efficiencies can be achieved in two-layer OLEDs that contain a solution-processed light-emitting dendrimer layer and a hole-blocking/electron-transporting layer. The dendrimer 1 (IrppyD) consists of a fac-tris(2-phenylpyridine) iridium core, phenylene dendrons, and 2-ethylhexyloxy surface groups (Fig. 1). It is a first generation dendrimer as it has one level of branching. We have found that the attachment of the dendron para to the carbon iridium bond does not change the optical and electronic properties appreciably when compared with the parent molecular fac-tris(2-phenylpyri- 976 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 Fig. 1. Structure of IrppyD. dine) iridium [Ir(ppy)3)]. The potentials (E1/2) of the first quasi-reversible reduction and oxidation were found to be ±2.91 V and 0.26 V, respectively, versus the ferrocene/ferrocenium couple. These are essentially the same potentials as those reported for Ir(ppy)3.[14] To determine the energy of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals from the redox potentials of the materials used in this work we have compared the redox potentials with that of 4,4¢-bis(N-3-methylphenyl-N-phenyl)biphenyl (TPD) for which the HOMO has been determined by ultraviolet photoelectron spectroscopy (UPS).[15] We have successfully used this procedure for the determination of energy levels of another dendrimer family and shown that the determined energy levels are the same as those calculated by modeling the barrier to charge injection in a device.[16] In the cases where the materials showed only either a chemically reversible reduction or oxidation, we have calculated the energy of the irreversible process by adding or subtracting where appropriate the energy of the first absorption maximum observed in the solution UV-vis absorption spectrum. We believe that this is a reasonable approximation and have found in a number of other dendrimers that the difference in energy between the first oxidation and reduction determined by cyclic voltammetry corresponds to the first peak absorption energy observed in the UV-vis spectrum. The oxidation and first reduction of IrppyD are electrochemically quasi-reversible and by comparison with TPD we have determined the HOMO and LUMO to be 5.6 eV and 2.5 eV respectively. The photoluminescence quantum yield (PLQY) of IrppyD was found to be 57±5 % in chloroform but decreasing to 22±2 % in the solid state. The PLQY in solution of Ir(ppy)3 has been reported to be between 40 % and 50 % which is similar to IrppyD.[17] However, we are not aware of reports of the PLQY of Ir(ppy)3 in the solid state but believe it would be lower than that of the dendrimer due to the dendritic architecture controlling the intermolecular interactions of the core and hence luminescence quenching. When the dendrimer was blended with 4,4¢-bis(N-carbazolyl)biphenyl (CBP), a commonly used host material, in a ratio of 20 wt.-%, the emission 0935-9648/02/1307-0976 $ 17.50+.50/0 Adv. Mater. 2002, 14, No. 13±14, July 4 Fig. 2. EL spectra from a device with a neat dendrimer layer and with IrppyD in a CBP host. Two different host materials were chosen for comparison, namely the traditional bipolar CBP 2, and the newer 4,4¢,4²tris(N-carbazolyl)triphenylamine (TCTA)[20] 3, which contains three carbazole units around a central triphenylamine. The triphenylamine center is considered to impart greater hole-transport character for TCTA when compared to CBP. For CBP we determined the HOMO and LUMO energies to be 5.9 eV and 2.3 eV. The HOMO level of CBP lies about 0.3 eV lower in energy than the dendrimer and the LUMO about 0.2 eV higher in energy. This means that both holes and electrons would be trapped on the dendrimer in the blend. For TCTA we found that the HOMO energy was closer to that of the IrppyD at 5.7 eV, which suggests that the hole density should be more evenly distributed across the blended layer. This is different from the reported HOMO energy for Adv. Mater. 2002, 14, No. 13±14, July 4 the TCTA/Ir(ppy)3 combination where the HOMO of TCTA was reported to be 5.9 eV.[20] However, the LUMO energy of TCTA was 2.0 eV, which is 0.5 eV higher in energy than IrppyD, indicating that electrons would be trapped on the dendrimer in the blend. The configuration of the first blend devices studied was ITO/ IrppyD:CBP/BCP/LiF/Al. The IrppyD:CBP layer was deposited by spin-coating and the devices were completed by evaporation of BCP and the cathode materials. The IrppyD:CBP layer was approximately 30±40 nm thick and the thickness of the BCP layer was determined to be 60 nm. It is important to note that CBP itself cannot be spin-coated from solution to form good quality thin films. We have found that, providing the concentration of the IrppyD in CBP was greater than 10 wt.-%, the blend formed reasonably uniform thin films. When the IrppyD concentration was below this level the CBP host was prone to crystallization in the as-formed films and the devices shorted. This illustrates the power of the dendritic architecture for enhancing the processing properties of materials, and in particular for spin-coating. As in the case of molecular materials, CBP was used as the host because of its long triplet lifetime, when compared to Ir(ppy)3, and the fact that it can transfer this energy to the iridium complex for emission.[21] The synergy of energy levels is one of the important factors that gives rise to efficient Irppy-based devices. We expected that as the emissive component of the IrppyD was the same as Ir(ppy)3 the CBP would play a similar role in our devices. Controlling the concentration of Ir(ppy)3 guest in the CBP host has been shown to be important.[9] Too high a concentration leads to concentration quenching but too low a level gives poorer energy transfer and reduced emission. Therefore, the first step in our study was to determine the optimal ratio of IrppyD to CBP. For evaporated molecular Ir(ppy)3:CBP devices the optimal efficiency was found for a »6±8 wt.-% blend of guest in host. This corresponds to a 4.5±6.0 mol-% of Ir(ppy)3 in CBP. The best device efficiencies were observed with an IrppyD/ CBP ratio in the range of 15±30 wt.-%. This corresponds to a 4.0±9.0 mol-% of IrppyD in CBP. For a ratio of Ir(ppy)3 in CBP of 9 mol-%[9] there was a significant decrease in the efficiency of the devices. We believe that the flexibility in concentration of guest in host for the dendritic material is in part due to the more uniform mixing of the guest in the host. This is in addition to the dendritic architecture also controlling the intermolecular interactions of the Ir(ppy)3 cores of the dendrimers. Greater flexibility in doping ratio has also been observed in devices formed by evaporation and using a sterically encumbered pinene derivatized iridium complex.[22] For the device configuration ITO/IrppyD:CBP/BCP/LiF/Al we found that the average maximum power efficiency was 8 lm W±1 (17 cd A±1) at brightnesses around 1065 cd m±2 and voltages of 7.5 V. The turn-on voltages were between 3.5 V and 4.2 V and peak brightnesses reached 16 000 cd m±2 at 12 V. This is a significant improvement in the device efficiency when compared with the devices containing a neat IrppyD layer and BCP electrontransport/hole-blocking layer and demonstrates that the host is as important for the dendritic materials as the molecular phos- Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1307-0977 $ 17.50+.50/0 977 COMMUNICATIONS was only observed from the dendrimer and the PLQY of the film was 79±6 %. This indicates that efficient energy transfer occurs from the CBP host to the dendrimer and the CBP increases the spacing of the phosphorescent chromophores, avoiding triplet±triplet annihilation. All the devices prepared used ITO as the anode and LiF/Al as the cathode. The first device we prepared contained a neat solution-processed iridium dendrimer emissive layer with an evaporated 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) electron-transport/hole-blocking layer. For electrophosphorescent devices BCP has been widely used as the electron-transport/hole-blocking layer.[9,18] We were unable to determine the redox potentials of BCP by cyclic voltammetry but the HOMO has been reported to be 6.4 eV and LUMO 2.9 eV.[19] Therefore, the electrons and holes would be expected to recombine in the IrppyD layer. The neat dendrimer device showed bright green luminescence with a turn-on voltage of 4.5 V. The peak brightness was found to be 1680 cd m±2 at 12 V but the maximum efficiency of the device was low only reaching 0.14 lm W±1 (0.47 cd A±1) at 9.5 V. The EL spectrum of the device containing the neat IrppyD emissive layer is shown in Figure 2. The red tail to the spectrum may be due to excimer emission, which illustrates that the dendritic structure has not stopped the interactions of the emissive cores. This is not surprising given that the dendrimer is a first-generation material. COMMUNICATIONS phorescent materials. The EL spectrum of the ITO/IrppyD:CBP/BCP/LiF/Al is shown in Figure 2. It can be seen, as expected, that the red tail observed in the spectrum from the device with the neat IrppyD emissive layer has gone. This is due to the homogeneous nature of the blend with the host CBP separating the emissive IrppyD macromolecules. The EL maximum was at 518 nm and the Commission Internationale de L'Eclairage (C.I.E.) co-ordinates of the IrppyD:CBP blend device are x=0.31 and y=0.63, which are essentially the same as reported for Ir(ppy)3.[9] The fact that the color of emission of IrppyD and Ir(ppy)3 are the same is a further indication that the attachment of the dendron para to the carbon±iridium bond on the phenyl ring does not affect the electronic properties of the complex. The choice of host and of electron-transporting/hole-blocking layer can have a strong effect on device performance, and we therefore investigated alternatives to each of these components. The first refinement was to use TCTA instead of CBP as the host. We were concerned that even though thin films could be spin-coated with CBP as the host they could still be prone to crystallization and form poorer films. TCTA is dendritic in nature with three dendrons emanating from a central nitrogen. We considered that this more dendritic structure should give rise to better film formation in conjunction with our IrppyD than CBP. In addition, as already described, the HOMOs of TCTA and IrppyD are almost the same, and hence with the holes being less trapped on the dendrimer we might expect a difference in the device properties. As in the case of the CBP host, the first step was to determine the optimal ratio of IrppyD to TCTA. We again found that the ratio of IrppyD to TCTA was fairly flexible over a range of 13±26 wt.-% (5.0±11.0 mol-%) and there was little variation in the efficiency of the devices. However, when the ratio of IrppyD guest to TCTA host reached 38 wt.-% (18 mol-%) there was a noticeable decrease in device efficiency and this continued as the guest-to-host ratio was increased to 50 wt.-% (26 mol-%). It is interesting to note that the best device efficiencies for IrppyD occur at similar molar ratios for both TCTA and CBP. For a device configuration that contained ITO/IrppyD:TCTA/BCP/LiF/Al with a 30± 40 nm IrppyD/TCTA layer and 60 nm BCP layer we saw an increase in the efficiency compared to when CBP was used as a host. When TCTA was used as the host, the average maximum efficiency was found to be 23 lm W±1 (30 cd A±1) at a brightness of around 645 cd m±2 and voltage of 5.7 V. The maximum luminance achieved was 18 500 cd m±2 at 12 V. We believe that the improvement in efficiency is due only in a small part to the better film-forming properties of the TCTA/ IrppyD blend. Two other factors that can be involved in giving a large improvement in efficiency are: first, as the HOMO energies of the TCTA and IrppyD are similar, the holes are more evenly distributed across the emissive layer and so avoid charge trapping on the emissive species. Second, although the molar ratio of CBP and TCTA are the same for the best devices the number of carbazole units is different, three for TCTA and two for CBP. This suggests that the molar ratio of 978 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 carbazole units to emissive Ir(ppy)3 may be important for improved devices. The EL spectrum and C.I.E. co-ordinates did not change in moving from CBP to TCTA as the host. However, we found that for these devices repeated driving generally gave a decrease in efficiency when compared to the first observation of the device. We believed that this was at least in part due to the instability of the molecular BCP layer.[18] Therefore, as a final variation in our device structure we followed our dendritic theme and moved to 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) as the electron-transport/ hole-blocking layer.[23] TPBI has been used as an electron-transporting layer for OLEDs based on fluorescent emitters[23] and as host for phosphorescent emitters.[10] The TPBI HOMO and LUMO were found to be 6.7 eV and 2.7 eV, respectively. The LUMO energy level of TPBI is higher than that of BCP and so it would be expected that better electron injection into the emissive layer should occur with TPBI. We prepared a number of devices with ITO/IrppyD:TCTA/TPBI/LiF/Al with the ratio of IrppyD and TCTA being 13 wt.-%. The thicknesses of the emissive and TPBI layers were 30±40 nm and 60 nm, respectively. The typical efficiencies of the devices were around 32 lm W±1 (43 cd A±1) at 4.2 V and 169 cd m±2. The best devices (characteristics shown in Fig. 3) were found to have a maximum efficiency of 40 lm W±1 (55 cd A±1) at 4.5 V and 400 cd m±2. Turnon voltages were normally 3.0 V and a maximum brightness of 12 000 cd m±2 at 7.0 V has been observed. This is a signifi- Fig. 3. Characteristics of an ITO/IrppyD:TCTA (13 wt.-%)/TPBI/LiF/Al device. cant improvement in the power efficiency over the devices using the BCP electron-transport/hole-blocking layer and must be in part due to the difference in barrier heights to electron injection. In addition, repeat driving showed little decrease in the efficiency of the devices suggesting that the TPBI was a more stable layer. Taking into account light extraction losses and the PLQY of IrppyD in a carbazole-containing host, these device efficiencies correspond to balanced charge injection in this device configuration. It should be noted that the efficiencies achieved in a two-layer device with our spin-coatable emissive layer are comparable to the more complex Ir(ppy)3 evaporated devices that contain a holetransport and often a second electron-transport layer. A final 0935-9648/02/1307-0978 $ 17.50+.50/0 Adv. Mater. 2002, 14, No. 13±14, July 4 Fig. 4. Efficiency versus luminance, showing the relatively constant efficiencies at high brightnesses. In conclusion, we have shown that the dendritic architecture can be used to solubilize luminescent chromophores and form uniform films of blends where the second component does not normally form films by spin-coating. The simple device structures with a solution-processed dendrimer containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Finally, solution processing opens up the possibility of patterning by, e.g., inkjet printing of phosphorescent materials. Experimental Solution photoluminescence quantum yields (PLQYs) were measured by a relative method using quinine sulfate in 0.5 M sulfuric acid as a standard [24]. The dendrimers were dissolved in chloroform and freeze-thaw degassed. Photoluminescence spectra were recorded using a JY Horiba Fluoromax 2 fluorimeter, with the dendrimer solutions excited at 325 nm; the optical density (OD) of sample and standard were similar and small (£ 0.1). Film PLQYs were measured in an integrating sphere [25] purged with flowing nitrogen and the sample was excited at 325 nm. ITO substrates were patterned by photolithographic methods, cut into squares 1²”1² and cleaned sequentially in detergent, NH3/H2O2, 1:1 and deionized water for 1 h in an ultrasonic bath before drying in a stream of dry nitrogen. The dry substrates were transferred into a dry N2 atmosphere glove box where they were subjected to O2 plasma treatment (Emitech K1050X plasma unit) at 60 W for 4 min. Films of IrppyD-doped CBP or TCTA were deposited on the substrates by spin-coating inside the glove box. Spin-coating was performed using solutions in CHCl3 (CBP and TCTA) or toluene (TCTA) at a concentration of 5 mg mL±1 with spin rate 2000 rpm for 1 min. The dried spin-coated films were then transferred to the chamber of a vacuum evaporator without exposure to air for vacuum deposition of subsequent organic charge-transport layers and/or metal electrodes at low pressure (< 10±6 torr). The thickness of the evaporated layers was monitored by an in-situ quartz crystal microbalance and material was deposited at a rate of 0.1±0.5 nm s±1. Layer thicknesses were calibrated using a Dektak3 ST surface profiler. Electrical and optical measurements were made in air under ambient conditions with no protective encapsulation of the devices. EL emission spectra were recorded with an Oriel Spectrograph MS125 coupled to a CCD camera (spectral resolution ~1 nm). 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Send us your manuscripts ONLINE at http://www.advmat.de Received: February 1, 2002 Final version: March 27, 2002 Adv. Mater. 2002, 14, No. 13±14, July 4 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1307-0979 $ 17.50+.50/0 979 COMMUNICATIONS important observation of these dendritic devices is that the efficiency of the devices stay reasonably constant over a range of brightnesses (Fig. 4). We believe this is due to balanced charge injection and the uniform distribution of the IrppyD in the host layer.