18448
2009, 113, 18448–18450 Published on Web 10/07/2009
Highly Efficient Phosphorescent Light-Emitting Diodes by Using an Electron-Transport Material with High Electron Affinity Taeshik Earmme, Eilaf Ahmed, and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1750 ReceiVed: August 15, 2009
Spin-coated polymer-based phosphorescent organic light-emitting diodes (PhOLEDs) were found to be substantially brighter and more efficient when 4,9-diphenylbisindenoanthorazoline (DADA) was used as an electron-transport material compared to widely used Alq3. Devices with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) dispersed in poly(N-vinylcarbazole) as the emissive layer and DADA as the electron-transport layer showed a maximum brightness of 73 600 cd/m2 and a maximum luminous efficiency of 48.1 cd/A at a brightness of 5640 cd/m2. The high electron affinity (3.67 eV) and electron mobility (3.1 × 10-5 cm2/V · s) of DADA explain its effectiveness as an electron-transport material in PhOLEDs. These results demonstrate the use of an electron-transport material having a high electron affinity as a promising strategy for improving the charge injection and overall performance of PhOLEDs. Organic light-emitting diodes (OLEDs) have been extensively investigated over the past two decades.1-13 In the past several years, intensive efforts in utilizing phosphorescence by metal complex phosphorescent emitters in PhOLEDs have led to nearly 100% internal quantum efficiency.2,3 However, balanced charge carrier injection from both cathode and anode and balanced charge transport to the emissive layer remain key challenges in developing more efficient organic light-emitting devices.4,5 Since hole mobility is typically orders of magnitude larger than electron mobility in most current OLED systems, the introduction of an electron-transport layer can dramatically reduce the hole current and thereby result in more efficient OLEDs.6 Materials suitable for use as an electron-transport layer (ETL) in combination with high-work-function metal cathodes such as Al (e.g., 4.3 eV) and Ag are expected to have a sufficiently high electron affinity (EA > 3.0 eV) to minimize the energy barrier for the electron injection. In addition, it is necessary that they have a good electron mobility to move the charge recombination region away from near the cathode and improve the exciton generation rate within the emissive layer.6 Alq3 has been widely investigated and used as such an ETL in OLEDs and PhOLEDs.7 Heterocyclic molecules, oligomers, and polymers have recently emerged as a promising class of electron-transport materials for OLEDs.6,8-10 Those containing imine nitrogen atoms were found to provide efficient electron injection and transport in part because of interface reactions and the coordination of the evaporated metal cathode (e.g., Al or Ag) to the nitrogens.9,10 In this Letter, we report highly efficient and bright polymeric phosphorescent OLEDs achieved by means of a new electron-transport material (ETM), 4,9-diphenylbisindenoanthrazoline (DADA). By virtue of its high electron affinity (3.67 * To whom correspondence should be addressed. E-mail: jenekhe@ u.washington.edu.
10.1021/jp907913d CCC: $40.75
Figure 1. The structure of 4,9-diphenylbisindenoanthrazoline (DADA) and its normalized optical absorption and PL spectra in dilute solution and as a thin film.
eV) and efficient π-stacking, DADA as an ETL offers new features that can potentially enhance the performance of OLEDs. It was placed between the Al cathode and the polymeric emissive layer (EML) to facilitate electron injection and transport from the high-work-function metal. The chemical structure of the heptacyclic molecule, DADA, is shown in Figure 1a. DADA was synthesized by Friedlander condensation using diphenyl phosphate (DPP)8a as an acid catalyst to obtain a high yield product, whose detailed synthesis procedure is described in the Supporting Information. Normalized optical absorption and photoluminescence (PL) spectra of DADA in dilute tetrahydrofuran solution (∼10-6 M) and as a thin film are shown in Figure 1b. The absorption spectrum in solution has a well-resolved vibronic structure with peaks at 301, 377, 398, and 421 nm. The PL emission spectrum of 2009 American Chemical Society
Letters DADA in solution displays a vibronic structure, showing two peaks at 438 and 464 nm and a shoulder at 490 nm. The PL quantum yield of DADA in dilute (5 × 10-6 M) toluene solution was determined to be 52% using diphenylanthracene as a standard.5 In thermally evaporated films, the featureless PL emission is centered at 545 nm, which is 90 nm red shifted from the solution spectrum. The large bathochromic shift of the solid-state emission from the solution PL spectrum and the known π-stacking of the conjugated molecule in thin film implicate excimers as the origin of the thin film emission.11 Thermogravimetric analysis of DADA showed a high decomposition temperature of 420 °C, indicating excellent thermal stability and good potential as a robust ETM that can resist any Joule heating during OLED operation. We fabricated PhOLEDs using DADA as the ETM inserted between the emissive layer and the LiF/Al cathode. The polymeric emissive layer consisted of a blend of poly(Nvinylcarbazole) (PVK, Aldrich) and 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD, Aldrich) (PVK/PBD ) 70: 30 wt/wt) as a host and 2.4 wt % fac-tris(2-phenylpyridine)iridium (Ir(ppy)3, Luminance Technologies Co., Taiwan) as the phosphorescent dopant. For the hole injection layer, a solution of PEDOT:PSS (poly-(ethylenedioxythiophene)polystyrenesulfonate, H.C. Starck, Clevios P VP Al 4013) in water was spin-coated to make a 40 nm thick layer onto a precleaned ITO glass and annealed at 150 °C under vacuum. The 100 nm thick EML was obtained by spin-coating the PVK/PBD/Ir(ppy)3 blend in toluene onto the PEDOT:PSS layer and vacuum-dried overnight at 50 °C. A 20 nm thick DADA was thermally evaporated in a vacuum (
