Inverted OLEDs with Electrically Doped Carrier Injection and Transport Layers
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Inverted OLEDs with Electrically Doped Carrier Injection and Transport Layers Xiang Zhou, Martin Pfeiffer, Jing S. Huang, Jan Blochwitz, Ansgar Werner, and Karl Leo Institute fuer Angewandte Photophysik, Technische Universitaet Dresden, D-01062 Dresden, Germany ABSTRACT Conventional organic light-emitting diodes (OLEDs) having electrically doped carrier injection and transport layers with high conductivity exhibit extremely low driving voltages, which is due to the formation of radical anions and cations and ohmic contacts at the electrode interfaces. We report here an inverted OLEDs with indium-tin-oxide (ITO) bottom contact as cathode for electron injection. The device comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq3) emission layer sandwiched in between n- and p-doped charge transport layers. INTRODUCTION Conventional OLEDs have a transparent bottom anode, an ITO coated glass substrate with high work function for hole injection, through which the light is emitted, and an opaque metal top cathode normally with low work function for electron injection [1, 2]. For display drivers employing n-channel field effect transistors, it is desirable that the bottom contact of the OLEDs is the cathode. This requires that OLEDs have inverted structure with a cathode as the bottom contact [3, 4]. We report here an inverted OLEDs with ITO bottom contact directly as cathode without any surface modification and treatment for electron injection and a Ag/Al bilayer anode evaporated onto the top of organic layers for hole injection. In order to enhance carrier injection and reduce the driving voltage, both n- and p-type doped wide-gap organic electron and hole transport layers (ETL and HTL, respectively) were used to sandwich an intrinsic Alq3 emission layer to form a nip structure [5]. (Recently, we demonstrated extremely low voltage OLEDs (2.6 V for 100 cd/m2) with a pin-type structure. Conventional OLEDs having electrically doped carrier injection and transport layers with high conductivity exhibit low driving voltages, which is due to the formation of radical anions and cations and ohmic contacts at the electrode interfaces [5-13].) This inverted device differ from previously demonstrated surface-emitting inverted OLEDs with a layer of Al [3] or MgAg [4] evaporated on Si substrate as the electron injection electrode (cathode) and a rf sputtering-deposited transparent ITO top anode for hole injection. EXPERIMENT The inverted OLEDs were prepared in an ultra high vacuum (UHV) system (Bestec GmbH) at a base pressure of 10-8 mbar without breaking vacuum. A commercial ITO coated glass substrate, having a thickness of about 30 nm and a sheet resistance of about 80 Ω/ , was used (Merck). The routine cleaning procedure of ITO substrate included sequential sonication in organic solvent (aceton and alcohol), rinsing in de-ionized water. After dried by high purity nitrogen, the ITO substrate was loaded into UHV for device fabrication without any further surface modification and treatment.
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