Intrinsic Degradation in Alq 3 -Based OLEDs Probed by Deep-Level Optical Spectroscopy
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1212-S03-01
Intrinsic Degradation in Alq3-Based OLEDs Probed by Deep-Level Optical Spectroscopy Yoshitaka Nakano Institute of Science & Technology Research, Chubu University, Kasugai, Aichi 487-8501, Japan
ABSTRACT We have successfully investigated degradation-induced variations in electronic band-gap states in the emissive region of the Alq3-based OLEDs by a deep-level optical spectroscopy technique. Through the intrinsic degradation, both deep-level traps and near-band-edge transitions in the Alq3 emissive zone are found to be red-shifted significantly towards their corresponding bulk levels of the Alq3 single layer. These variations in the interfacial electronic states are probably induced by the intrinsic degradation and indicate that initial molecular structures characteristic of the Alq3 emissive zone are transformed into the bulk-like relaxed ones through the degradation. INTRODUCTION Much progress in performance and lifetime makes organic light-emitting diodes (OLEDs) suitable candidates for flat panel display applications. Since the first report on efficient OLEDs, the most well-known electron-transport material and hole-transport material are tris(8hydroxyquinoline) aluminum (Alq3) and N,N'-di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl4,4'diamine (α-NPD), respectively [1,2]. In the case of OLEDs based on these materials, the direct recombination of the charge carriers is principally considered to take place in the narrow region of the Alq3 layer in the vicinity of the α-NPD layer [2]. On the one hand, OLEDs are easily subject to intrinsic degradation corresponding to a long-term gradual decrease in electroluminescence (EL) efficiency during continuous operation [3,4]. Although a number of research approaches have been employed to clarify the degradation phenomena, the dominant cause of the degradation remains much less understood [3-8]. At present, the intrinsic degradation is strongly supported to be due to the formation of cationic Alq3 species acting as luminescence quenchers at/near the emissive interface [2]. Among them, Kondakov et al. have reported on the formation of nonradiative recombination centers in the Alq3 emissive zone from a viewpoint of electronic states, by using a voltammetric technique [5,7]. However, the electrical and physical properties of various deep-level traps in OLEDs still remain uncertain. In particular, in-depth data concerning deep-level traps in the emissive zone of Alq3-based OLEDs have yet to be clarified experimentally. Notably, few investigations of the deep levels, particularly before and after the intrinsic degradation of the OLEDs, have been reported. Deep-level optical spectroscopy (DLOS) is a well-known powerful technique for the characterization of electronic deep levels in the band gap of inorganic semiconductors [9,10]. This technique principally enables the measurement of changes in capacitance during monochromatic optical excitation and the detailed mapping of deep levels in the band gap. In the case of inorganic semiconductors, the measured capacitance corresp
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