© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim phys. stat. sol. (RRL) 1, No. 5, 196– 198 (2007) / DOI 10.1002/pssr.200701155 www.pss-rapid.com pss Deep-level characterization of emissive interface states in Alq 3 -based OLEDs Yoshitaka Nakano * , Koji Noda, Hisayoshi Fujikawa, and Takeshi Morikawa TOYOTA Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan Received 26 July 2007, revised 13 August 2007, accepted 13 August 2007 Published online 16 August 2007 PACS 73.20.Hb, 73.40.Lq, 73.50.Gr, 73.61.Ph, 85.60.Jb * Corresponding author: e-mail y-nakano@mosk.tytlabs.co.jp, Phone: +81 561 71 7781, Fax: +81 561 63 5328 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Great progress in performance and lifetime makes or- ganic light-emitting diodes (OLEDs) suitable candidates for flat panel display applications. Since the first report of efficient OLEDs, the most well-known electron-transport material and hole-transport material have been tris(8- hydroxyquinoline) aluminum (Alq 3 ) [1] and N,N′-di-1- naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′diamine (α-NPD) [2], respectively. In the case of OLEDs based on these ma- terials, the recombination of the charge carriers is princi- pally considered to take place in the narrow region of the Alq 3 layer in the vicinity of the α-NPD layer [2]. On the one hand, OLEDs are easily subject to the degradation through a long-term intrinsic decrease in electroluminescence (EL) efficiency during operation [3, 4]. Although the life- time and the EL efficiency of OLEDs have been recently re- ported to be extended by doping Alq 3 with quinacridone (Qd) [5], the intrinsic degradation of OLEDs is still an open issue. The causes of this intrinsic degradation remain unclear in view of emissive interface states. Therefore, it is impor- tant to clarify electronic states in the band gap at the emis- sive interface between the Alq 3 and the α-NPD layers. The aim of this paper is to present interfacial electronic states in the emissive region of OLEDs by using a deep- level optical spectroscopy (DLOS) [6, 7] technique. This technique principally enables the measurement of changes in capacitance during optical excitation and the detailed mapping of the deep levels that would be undetectable by thermal emission techniques, such as deep-level transient spectroscopy (DLTS) [8, 9] and thermally stimulated cur- rents (TSC) [10]. However, the capacitance property strongly depends on materials. In case of inorganic semi- conductors, the measured capacitance corresponds to the depletion region capacitance of Schottky or pn junctions. On the one hand, in case of organic semiconductors, the DLOS technique utilizes film capacitance because these materials show insulator-like characteristics. As for most OLEDs, injected holes have a strong tendency to accumu- late in the emissive interface region. Thus, optical irradia- tion excites the accumulated carriers to the conduction and/or valence bands and their corresponding change in device capacitance can lead to the revelation of interfacial electronic states in the emissive region of the OLEDs. In this study, we have first applied modified DLOS measure- ments to α-NPD/Alq 3 -based OLEDs, and have investigated electronic deep levels at the emissive interface between Alq 3 and α-NPD layers. Two kinds of OLEDs, α-NPD/Alq 3 /LiF/Al and α-NPD/Alq 3 : Qd/LiF/Al device samples were fabricated on indium-tin-oxide (ITO, 150 nm) coated glass substrates. First, the α-NPD (60 nm) and the Alq 3 (: Qd) (60 nm) layers were sequentially deposited by thermal evaporation, and then an Al layer (150 nm) with an ultra-thin LiF (0.5 nm) We have succesfully investigated emissive interface states in fabricated indium-tin-oxide (ITO)/N,N′-di-1-naphthyl-N,N′- diphenyl-1,1′-biphenyl-4,4′diamine (α-NPD)/tris(8-hydroxy- quinoline) aluminum (Alq 3 )/LiF/Al organic light-emitting di- odes (OLEDs) by a modified deep-level optical spectroscopy (DLOS) technique. In the vicinity of the α-NPD/Alq 3 emis- sive interface, a discrete trap level was found to be located at ~ 1.77 eV below the conduction band of Alq 3 , in addition to band-to-band transitions of carriers from α-NPD to Alq 3 .